Patent Application
20230241597
The present invention relates in general to the field of transition metal catalysts, and more particularly, to a general platform for the recovery of transition metal catalysts.
Without limiting the scope of the invention, its background is described in connection with transition metal catalysts.
The development of new drugs is time-consuming, expensive, and risky. Despite these challenges, the pharmaceutical industry has been remarkably successful in developing a broad range of new medicines to solve ongoing and emerging public health crises. However, a major challenge to exploring novel drug targets is over-reliance of the pharmaceutical industry on precious-metal-catalysts. This limitation is seriously hampering methodological innovation, and thus development of new therapies.
Homogeneous catalysts—defined as those that exist in the same (usually liquid) phase as the reactants and products—are typically more selective than heterogeneous catalysts. Homogeneous catalysts are also significantly less affected by limitations arising from slow transport of reactants and products to and from an interface, a problem frequently seen in heterogeneous catalysis. However, the separation of homogeneous catalysts from their reaction products can be costly and is often inefficient. Therefore, it is unsurprising that with a few notable exceptions,18-19, most industrially relevant catalysts are heterogeneous. Unfortunately, compared with their homogeneous counterparts, heterogeneous catalysts often suffer from lower performance and diminished selectivity owing to poorly defined catalytic sites, thus nullifying most benefits.
Despite these advances, a need remains for novel, highly efficient catalysts that maximize the recovery of transition metals.
As embodied and broadly described herein, an aspect of the present disclosure relates to a molecule of Formula I:
wherein R is selected from methyl, ethyl, propyl, isopropyl, butyl, phenyl, cyclohexyl, fluoro, chloro, methoxy, ethoxy, butoxy, phenoxy, aryl, alkene, alkyne, and heterocyclic, wherein the catalyst is selected for an isomerization reaction catalyst, a hydrosilylation reaction catalyst, a Suzuki-Miyaura coupling catalyst, a Negishi coupling catalyst, a Sonogashira coupling catalyst or a Heck coupling catalyst; and wherein if the molecule is:
then R is not isopropyl, and optionally M is a metal. The molecule of Formula I is a ligand until the metal M is added, at which time the molecule of Formula I is a catalyst. In one aspect, the molecule of Formula I is compound 1, which is synthesized by:
In another aspect, the molecule of Formula I is compound 1 is converted into a recyclable catalyst by the addition of the metal:
In another aspect, R is selected from benzyl, O-methylbenzyl, trifluorobenzyl, or 2,4,6-methyl benzyl. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: 1st-row transition metals, 2nd-row transition metals, 3rd-row transition metals, main group metals, lanthanide series transition metals, and actinide series transition metals. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium. In another aspect, a substrate/product is:
wherein R and X are selected from:
As embodied and broadly described herein, an aspect of the present disclosure relates to a method for recycling transition metal catalysis comprising: combining the reactants in a solvent in the presence of a molecule having the formula:
wherein R is selected from methyl, ethyl, propyl, isopropyl, butyl, phenyl, cyclohexyl, fluoro, chloro, methoxy, ethoxy, butoxy, phenoxy, aryl, alkene, alkyne, and heterocyclic, wherein the catalyst is selected for an isomerization reaction catalyst, a hydrosilylation reaction catalyst, a Suzuki-Miyaura coupling catalyst, a Negishi coupling catalyst, a Sonogashira coupling catalyst or a Heck coupling catalyst; and wherein if the molecule is:
then R is not isopropyl, wherein M is a metal; under conditions that catalyze a reaction; and recycling the molecule of Formula II. In one aspect, the molecule of Formula II is compound 1, which is synthesized by
In another aspect, the molecule of Formula II is compound 1 that is a ligand made by:
In another aspect, R is selected from benzyl, O-methylbenzyl, trifluorobenzyl, or 2,4,6-methyl benzyl. In another aspect, the molecule recycles transition metals selected from at least one of: 1st-row transition metals, 2nd-row transition metals, 3d-row transition metals, main group metals, lanthanide series transition metals, and actinide series transition metals. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium. In another aspect, M is at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium. In another aspect, a substrate/product is:
wherein R and X are selected from:
In another aspect, the reaction is an isomerization reaction, a hydrosilylation reaction, a Suzuki-Miyaura coupling, a Negishi coupling, a Sonogashira coupling or a Heck coupling.
As embodied and broadly described herein, an aspect of the present disclosure relates to a tunable tridentate ligand for recyclable homogeneous transition metal catalyst comprising a molecule of Formula I:
wherein R is selected from methyl, ethyl, propyl, isopropyl, butyl, phenyl, cyclohexyl, fluoro, chloro, methoxy, ethoxy, butoxy, phenoxy, aryl, alkene, alkyne, and heterocyclic, wherein the catalyst is selected for an isomerization reaction catalyst, a hydrosilylation reaction catalyst, a Suzuki-Miyaura coupling catalyst, a Negishi coupling catalyst, a Sonogashira coupling catalyst or a Heck coupling catalyst; and wherein if the molecule is:
then R is not isopropyl, and optionally M is a metal. The molecule of Formula I is a ligand until the metal is added, at which time the molecule of Formula I becomes the molecule of Formula II, which is a catalyst. In one aspect, the molecule of Formula I is synthesized by
In another aspect, the molecule of Formula I is converted into a recyclable catalyst by:
In another aspect, R is selected from isopropyl, benzyl, O-methylbenzyl, trifluorobenzyl, or 2,4,6-methyl benzyl. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: 1st-row transition metals, 2nd-row transition metals, 3rd-row transition metals, main group metals, lanthanide series transition metals, and actinide series transition metals. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium. In another aspect, a substrate/product is:
wherein R and X are selected from:
For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figure(s) and in which:
While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.
To facilitate the understanding of this invention, a number of terms are defined below. Terms defined herein have meanings as commonly understood by a person of ordinary skill in the areas relevant to the present invention. Terms such as “a”, “an” and “the” are not intended to refer to only a singular entity, but include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments of the invention, but their usage does not delimit the invention, except as outlined in the claims.
Transition metal catalysts are ubiquitous in chemistry and are integral to many of the most challenging and useful chemical transformations in the chemical enterprise. They often contain the most expensive and most scarce elements in the periodic table. An effective method for recovering such valuable elements after the chemical reaction is complete would be transformative for the chemical industry. The molecular design, compounds, technology, and methods disclosed herein allow for the control the solubility properties of any transition metal catalyst, rendering recovery of the complex possible via simple precipitation and filtration procedures.
The present invention overcomes the limitation in the prior art by providing novel synthetic avenues that rely on recyclable transition metal catalysts that can be reclaimed and reused. In addition, the methods address a serious threat to pharmaceutical chemistry: current methods are not sustainable because they consume precious raw materials needed for catalyst manufacture and the downstream production of drugs.
Moreover, the novel solution to these problems described herein, includes a general blueprint for recycling homogeneous catalysts. In contrast to traditional approaches involving generation of a heterogeneous catalyst based upon a successful homogeneous analog, the approach described herein avoids the need to tether a homogeneous complex to a non-innocent scaffold like alumina/silica or the incorporation of significant amounts of fluorine into the catalyst structure.
Following synthesis, the extensive use of metal scavengers (typically multiple treatments) is required to reduce the levels of toxic metals to permissible levels in pharmaceutical products. A key advantage of the present invention is that it allows for recycling homogeneous catalysts, which reduces waste, conserves scarce resources (precious metals), and reduces the amounts of metal impurities in active pharmaceutical ingredients.
Unlike specific reaction and/or transformation methods used before, the present invention includes a universal platform for recovery and recycling method that can be applied to any known catalytic transformation independent of the specific reaction or transformation methods used.
The present invention is a molecule of Formula I:
wherein R is selected from methyl, ethyl, propyl, isopropyl, butyl, phenyl, cyclohexyl, fluoro, chloro, methoxy, ethoxy, butoxy, phenoxy, aryl, alkene, alkyne, and heterocyclic,
wherein the catalyst is selected for an isomerization reaction catalyst, a hydrosilylation reaction catalyst, a Suzuki-Miyaura coupling catalyst, a Negishi coupling catalyst, a Sonogashira coupling catalyst or a Heck coupling catalyst; and wherein if the molecule is:
then R is not isopropyl, and optionally M is a metal. The metal can be any metal, se. g., those metals selected from 1st-row transition metals, 2nd-row transition metals, 3rd-row transition metals, main group metals, lanthanide series transition metals, and actinide series transition metals. For example, the metal can be platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, and/or hafnium.
A complementary set of base metal catalysts have been developed for regiodivergent alkene hydrosilylations: iron complexes of amine-iminopyridine ligands that are selective for Markovnikov hydrosilylations (branched/linear up to >99:1), while the cobalt complexes bearing the same type of ligands provide excellent levels of anti-Markovnikov selectivity (linear/branched up to >99:1). Both systems exhibit high efficiency and wide functional group tolerance.
Transition metal catalyzed hydrosilylation1-5 is one of the most widely studied and applied transformations for the upgrading of commodity alkenes and alkynes. Alkene hydrosilylation is most commonly used in the industrial production of commodity silicones, which are important feedstock for silicon-based polymers and versatile building blocks in organic synthesis. The majority of catalysts capable of alkene hydrosilylation are based upon precious metals,1-2-6 and in particular, Pt-based complexes have been explored extensively (Scheme 1a).7-10 However, platinum is a rare metal with an abundance of only 0.005 ppm in the earth's crust. Furthermore, these homogeneous precious metal catalysts are difficult to recover and are often left behind as residual impurities in the organosilicon products; the annual loss of Pt resulting from industrial hydrosilylation is estimated to reach 5.6 tons.11 Given these facts, the ideal catalyst in hydrosilylation should be based upon an earth-abundant metal complex. In the past decades, tremendous progress in homogeneous base-metal catalysis has been witnessed, particularly in Fe and Co catalysis (Scheme 1b).3-5, 12-22 With rational design of ligands, excellent reactivity, regioselectivity and even enantioselectivity can be readily achieved. In particular, Huang and coworkers also demonstrated that regiodivergent olefin hydrosilylation was possible within a single ligand class through variation of the metal center.18
Schemes 1a and 1b. The development of hydrosilylation of alkenes (in industry and academia) and challenging issues.
a. Platinum catalysts in hydrosilylation Industry
b. Selected examples of cobalt and Iron catalysts In hydrosilylation of alkenes
By contrast, the present inventors used group-assisted purification (GAP) chemistry,23-28 that incorporates the phosphoramide group into a ligand platform (metalla-GAP) to generate a novel and highly selective homogeneous catalyst.29 Thus, in three synthetic steps, a series of tunable tridentate ligands are readily accessed (Scheme 2). Upon complexation with either Co(II) or Fe(II) salts, highly active systems were produced, which proved capable of catalyzing the hydrosilylation of olefins. It was found that both Co and Fe systems feature excellent substrate generality and chemoselectivity and, remarkably, regiodivergent outcomes are obtained within the same ligand class which appears to be metal-dependent. The iron catalysts reported are highly selective for Markovnikov hydrosilylations, whereas the analogous cobalt catalysts afford the opposite anti-Markovnikov regiochemistry.
Preparation of catalysts. To effect olefin hydrosilylation, catalysts were typically generated in-situ via addition of an activator NaHBEt3 or NaOtBu to a mixture of GAP ligand (L1-L4) and CoBr2 (1:1) in THF. It proved possible to isolate X-ray quality crystals of both L2-CoBr and L2-FeBr from concentrated DCM solutions layered with toluene. The solid-state structures of L2-CoBr and L2-FeBr are isostructural and exhibit monoclinic structures in which each metal center is coordinated with only one L2 ligand in a tridentate fashion (L2-CoBr shown in
Cobalt-catalyzed hydrosilylation of alkenes. The inventors used styrene (4a) as a model substrate and performed catalytic hydrosilylation reactions employing diphenylsilane (5) (Table 1). As an activator, both NaHBEt3 and NaOtBu were tested, which are proposed to produce cobalt (II) hydride complexes.30 Recent work from Thomas and coworkers have also implicated an ‘ate’ species forming from NaOtBu and silanes serves as the real activator in the reaction.30 Combination of Co(II) centers with L1-5 in the presence of NaOBu revealed moderate to good yields of hydrosilylation product (Entries 1-5, Table 1); L2 afforded both the highest yield and regioselectivity (87% and 11:1) (Entry 2, Table 1). Switching reaction solvent from THF to either toluene or 1,4-dioxane had no effect on regioselectivity and in the case of 1,4-dioxane a substantial decrease in overall yield was observed (Entries 6-7, Table 1). Upon switching activator from NaOtBu to NaHBEt3 no noticeable difference in yield (85%) was found, however a substantially enhanced regioselectivity (23:1) was achieved (Entry 8, Table 1).
aThe reaction of 4a (0.2 mmol) with 5 (0.3 mmol) was performed in the presence
bYield of isolated product.
cThe regioisomer ratio (r.r) of the product 6a was determined by 1H NMR.
Using these reaction conditions, the scope of substrate amenable to the reaction conditions (Scheme 3) were tested. The cobalt catalyst mediated hydrosilylation of a diverse array of alkenes with Ph2SiH2, furnishing linear products in high isolated yield with excellent regioselectivities. In total, 34 substrates were shown to undergo hydrosilylation in good to excellent yield (61-99%) with superb regioselectivity in most cases (from 8:1 to >99:1 of anti-Markovnikov:Markovnikov product). Initially, styrene derivatives were evaluated with the optimal conditions (6a-6o, 6r and 6u). The electronic effect of the substituents was explored by varying para-substituents of styrene (6b, 6e-6i, 6l, 6o and 6r). In general, electron-rich styrenes seems to be superior substrates (6b, 6e-6g), which produce the hydrosilylation products with higher yields comparing with the electron-deficient ones (6h, 6i, 6l, 6o and 6r). The regioselectivity of reaction was largely independent of electronic properties, and most of the products were observed with >10:1 regioselectivity, although some specific substrates gave the products with lower selectivities (6f, 6o, 6r). Meta-substitutions were well-tolerated in the transformation and maintained the reaction performances (6c, 6j, 6m, 6u). In contrast, changing from para- to ortho-substitution resulted in lower yields but higher anti-Markovnikov selectivity, presumably due to the steric effects imparted by an ortho-substituent (6d, 6k). A substrate bearing a fused aromatic ring (6n) also proved to be amenable to these reaction conditions and affords the corresponding product with satisfactory results. Next, a broad range of aliphatic alkenes were examined with modified conditions (6p, 6q, 6s, 6t and 6v-6ah). Regardless of the nature of functional groups in the substrates, a uniformly excellent regioselectivity was obtained. Specifically, a series of synthetically useful functionalities, such as, borate, epoxy, acetal and ester groups, showed good tolerance to the reaction conditions (6o-6t). It is noteworthy that chemoselective hydrosilylation was possible (6u-6aa) and silylated products were readily obtained without affecting the internal alkene groups.
To further demonstrate the robustness of the catalyst, a gram-scale reaction was conducted with 2% catalyst loading under air and higher yield (1.2 g, 75%) was observed comparing the aforementioned reaction (
Iron-catalyzed hydrosilylation of alkenes. The inventors conducted optimization by using styrene (4a) and diphenylsilane (5) as pilot substrates. After screening various ligands and solvents, the iron catalyst (L1-FeBr2) in combination with activator NatBu was identified as the optimal conditions. Remarkably, regioselectivity could be reversed when the iron analog was employed;18 Markovnikov product 7a was obtained in 97% yield and >99:1 regioselectivity. Subsequently, the substrate scope of iron-catalyzed Markovnikov hydrosilylation with the optimal conditions (Scheme 4) was investigated.18,31 Consistent with the results of cobalt-catalyzed hydrosilylation, higher efficiency (73-99% yields) was observed in the cases of electron-rich styrenes (7b-7) comparing with electron-deficient ones (7g-7j). Remarkably, this novel protocol was well compatible with the highly electron-donating NMe2 group (7f), which might disturb the coordination of iron catalyst as a coordinate site. This method was also extended to fused ring and heterocycle (7k, 71), producing the corresponding products with moderate yields and good regioselectivities. As well as para-substitution, meta- and ortho-substituted styrenes could proceed smoothly in the reaction and provided the hydrosilylation products with relative satisfactory results (7m-7p). It is important to underline the high chemoselectivity of the hydrosilylation as internal alkene was not altered in the reaction (7o). Sterically bulky substrate vinylmesitylene was amenable to the Markovnikov hydrosilylation, albeit with a diminished yield (7q). However, aliphatic alkenes failed in the reaction, which could only furnish traces of anti-Markovnikov products.
The elaboration of Markovnikov product (7a) was then conducted to demonstrate its synthetic utility (Scheme 5).31 Potential applications of Markovnikov-selective olefin hydrosilylations include the conversion of readily available olefin substrates into synthetically important secondary alcohols. Thus, the inventors explored the Fleming-Tamao oxidation as a classical route to oxidize silanes to alcohols; by varying equivalents of reagents and reaction temperature the extent of oxidation could be controlled. At lower equivalents of KF and KHCO3 a silanol product (8) could be obtained at room temperature, while the full oxidative hydrolysis product 1-phenylethan-1-ol (9) could be accessed via use of increased equivalents KF and KHCO3 and at an elevated temperature. Finally, a transition-metal-free fluorination of silane was exploited, which provides mild route to fluorosilane (10).
Mechanistic Study. By way of explanation, and in no way a limitation of the present invention, extensive studies of the mechanism of these transformations using both experimental and computational techniques were used. To gain insight into the hydrosilylation mechanism, deuterium labeling and kinetic isotope effect (KIE) experiments were conducted (Schemes 6a, 6b). As shown in the deuterium labeling experiments, extensive deuterium incorporation (100%) at the C2 position was detected in the anti-Markovnikov product 6a-d2,5 (Scheme 6a). Iron-catalyzed hydrosilylation of 4a with deuterated diphenylsilane 5-d2 was also explored. In contrast to the results of cobalt-catalyzed hydrosilylation, product 7a-d2 was obtained without deuterium incorporation at the C1 position. These observations strongly argue for the formation of a cobalt deuteride/hydride and against a similar species in the case of iron in the catalytic cycle. The presence of such intermediates would lead to facile H/D exchange between M-D and C—H bonds through reversible olefin insertion into the M-D bond.18, 32-33 The KIE study of the cobalt- and iron-catalyzed hydrosilylation showed sharp differences in the ratio of kH/kD (2.3/1 vs 1/1). This result further illustrates that distinct pathways are involved in the regioselectively-divergent transformations.
Schemes 6a and 6b. Deuterium labeling experiment and KIE studies.
On the basis of the KIE and deuterium labeling experiments and related mechanistic studies,18,21,31 divergent catalytic pathways were proposed to rationalize the complementary regioselectivity (
The inventors prepared a series of iron and cobalt alkene hydrosilylation catalysts bearing amine-iminopyridine ligands which incorporate a phosphoramide (GAP) group. It proved possible to switch the regioselectivity of olefin hydrosilylations from >99:1 to <1:99 by simply changing the metal center from cobalt to iron. Non-precious metal-based catalysts such as these deliver a number of advantages over traditional (precious metal) systems: highly efficient, mild reaction conditions, and broad functional group tolerance with a divergent and high level of regioselectivity.
General procedure for cobalt-catalyzed hydrosilylation. Inside an argon-atmosphere glovebox, a borosilicate glass vial was charged with CoBr2 (3.2 mg, 0.015 mmol) and L2 (8.1 mg, 0.015 mmol), with anhydrous THF (1 mL) as solvent. Subsequently, NaBHEt3 (30 μL) (1 mol/L in THF) was added as activator. The resulting mixture was stirred at room temperature for 5 hours to form the active catalyst. Subsequently, diphenylsilane 5 (84 μL, 0.45 mmol, 1.5 equiv.) was added and the mixture was stirred for 30 min before adding styrene 4a (34 μL, 0.3 mmol). The reaction mixture was removed from the glovebox and stirred at 40° C. under an argon atmosphere for 24 hours. The reaction solvent was evaporated in vacuum and the resulting residue was purified by silica gel column chromatography (hexane followed by diethyl ether/hexane=1/200) affording 73 mg (84% yield) product 6a as colorless oil.
General procedure for iron-catalyzed hydrosilylation. Inside an argon-atmosphere glovebox, a borosilicate glass vial was charged with FeBr2 (3.2 mg, 0.015 mmol), 3a (5.3 mg, 0.015 mmol), picolinaldehyde (1.4 μL, 0.015 mmol) and tBuONa (4.3 mg, 0.045 mmol), with anhydrous THF (1 mL) as solvent. The resulting mixture was stirred at room temperature for 5 hours to form the active catalyst. Subsequently, diphenylsilane 5 (167 μL, 0.9 mmol, 3.0 equiv.) was added and the mixture was stirred for 30 min before adding styrene 4a (34 μL, 0.3 mmol). The reaction mixture was removed from the glovebox and stirred at 40° C. under an argon atmosphere for 24 hours. Then the reaction solvent was evaporated in vacuum and the resulting residue was purified by silica gel column chromatography (hexane followed by diethyl ether/hexane=1/200) affording 84 mg (97% yield) product 7a as colorless oil.
Stereoselective synthesis of trisubstituted alkenes is a long-standing challenge in organic chemistry, due to the small energy differences between E and Z isomers of trisubstituted alkenes (compared with 1,2-disubstituted alkenes). Transition metal-catalyzed isomerization of 1,1-disubstituted alkene can serve as an alternative approach to trisubstituted alkenes, but it remains underdeveloped owing to issues relating to reaction efficiency and stereoselectivity. The inventors have developed a novel cobalt catalyst that overcomes these challenges and provides an efficient and stereoselective access to a broad range of trisubstituted alkenes. This protocol is compatible with both mono- and dienes and exhibits a good functional group tolerance and scalability. Moreover, it is a useful tool to construct organic luminophores and a deuterated trisubstituted-alkene. A preliminary study of the mechanism suggests a cobalt-hydride pathway is involved in the reaction. The high stereoselectivity of the reaction is attributed to both a n-n stacking effect and the steric hindrance between substrate and catalyst.
Trisubstituted alkenes are widely distributed in molecules of great interest: natural products, pharmaceuticals and organic materials.1 Moreover, they are versatile building blocks in synthetic organic chemistry.2 Consequently, the stereoselective synthesis of trisubstituted alkenes has received a great deal of attention.3 The Wittig reaction is regarded as a classical and direct approach towards olefin synthesis, however typically only limited stereoselectivity can be achieved in the case of trisubstituted alkenes.4 In this context, a more general method to access (stereoselectively) trisubstituted olefins is still highly desirable.
Transition metal-catalyzed alkene isomerization has emerged as a fundamental and atom-economic transformation, which can readily convert terminal alkenes to internal alkenes.5 Prior reports of alkene isomerization usually rely on noble metals, such as, ruthenium, rhodium, palladium, iridium and platinum.6 Owing to the low abundance and high toxicity of these precious metals, attention has recently been shifting towards base-metal alternatives.5i The past few decades have witnessed tremendous progress in this field, particularly in cobalt-catalyzed olefin isomerization. For example, impressive work in Z-selective alkene isomerization was disclosed by Holland and Weix (Scheme 7a).7 In the same year, a radical triggered isomerization and cycloisomerization was reported by Shenvi employing a Co-salen catalyst (Scheme 7b).8 Subsequently, Hilt and coworkers unveiled a Co-dppp catalyzed monoisomerization of 1-alkenes to (Z)-2-alkenes (Scheme 7c).9 In another example of radical isomerization, a novel catalyst Co(dmgBF2)2(THF)2 was reported by Norton and co-workers (Scheme 7d).10 More recently, a kinetically controlled isomerization was achieved by the Liu group, affording the target alkenes in good to excellent regioselectivity for cyclic alkenes (Scheme 7e).11 Despite these impressive developments, the stereoselective alkene isomerization geared towards the synthesis of acyclic trisubstituted alkenes remains a challenging transformation.
Schemes 7a to 7e. Cobalt-catalyzed alkene isomerization of the prior art
a. Holland and Weix's Work
b. Shenvi's Work
c. Hilt's Work
d. Norton's Work
e. Liu's work
Low reaction efficiency and poor stereoselectivity are significant problems in alkene isomerization. These issues can be rationalized using the two major proposed reaction pathways5i,12 a) H radical initiated isomerization (Scheme 8a); b) metal-hydride involved isomerization (Scheme 8b). In the radical pathway, hydrogen atom transfer (HAT) should be a facile step due to formation of a stable radical, however, hydrogen abstraction can occur with less stereo-control under a radical pathway as shown in the work of Shenvi and Norton (Scheme 8a).8,10 In contrast, the alkene insertion step, in the metal-hydride triggered isomerization, can be sluggish owing to the high-energy barrier associated with formation of a quaternary metal-carbon intermediate (Scheme 8b).13 Moreover, thermodynamically, the energy differences between F and Z isomers of trisubstituted alkene are much smaller compared with 1,2-disubstituted alkenes.14 In this context, developing a stereoselective metalloradical catalyst or a robust metal-hydride catalyst would prove key in mitigating these problems.
Schemes 8a and 8b. Typical mechanisms for olefin isomerization of the prior art compared to the strategy of the present invention.
a. H′ Radical Initiated Isomerization
b. Metal-Hydride Involved Isomerization
c. Our work
The present inventors have further development of a novel Co-hydride catalyst by incorporating phosphamide group16 into the ligand (Scheme 8c). The in-situ generated metal-hydride catalyst could readily facilitate challenging isomerizations to afford a wide range of trisubstituted alkenes and dienes (44 examples) with up to 98% yield and 130/1 (EZ) stereoselectivity. Interestingly, some of the trisubstituted alkenes were observed to display aggregation-induced emission enhancement. Thus, the present approach can provide material scientists a potent tool to construct functional trisubstituted-alkenes. Additionally, a deuterium labelled trisubstituted-alkene can be readily obtained with this protocol, which might be useful in pharmaceuticals.17
Catalyst Screening. At the outset, α-ethylstyrene (1a) was selected as a model substrate with which to optimize the isomerization conditions (Table 2). It was found that the best results were obtained when L1 was employed as a ligand with CoBr2 as metal salt, tBuONa as activator and BH3·NH3 as a hydride source (Entry 1). Any deviation from the optimal conditions shut down the reaction (Entries 2-5). These results indicate that all these components are crucial for the reaction. In a control experiment, it was demonstrated that tBuONa does not promote the reaction (Entry 6), which precludes the possibility of a base-mediated isomerization. Replacing L1 with L2 or L3 led to lower stereoselectivity (Entries 7-8). Inferior results were obtained by varying hydride source and solvents (Entries 9-10). The important role of BH3·NH3 and toluene might be attributed to the poor solubility of BH3·NH3 in toluene, which enables a slow release of hydride. Use of tBuOK in place of tBuONa afforded moderate yield and E/Z selectivity.
tBuOK was used as
aThe reaction of 1a (0.5 mmol) was performed in the presence of CoBr2 (0.015
bIsolated yield.
cThe ratio of E/Z was determined by the 1H NMR of crude product.
To place the performance of the cobalt complex in the proper context, a comparison was carried out with other cobalt catalysts known to effect olefin isomerization. All cobalt-based catalysts were examined using a common substrate, 1a (Table 3). As shown in Table 2, low catalytic efficiency was observed in the cases of Cat. 1, Cat. 2 and Cat. 3 (Entries 1-3), although an excellent E/Z selectivity (>100/1) was detected in the case of Cat. 1. In contrast, Cat. 4 and Cat. 5 served as efficient catalysts and afforded the isomerized product 2a with good yields (94% and 92%, respectively, Entries 4-5). However, both catalysts suffered from reduced stereoselectivity, especially in the case of Cat. 4 (10/1). Thus, it seemed like Cat. 4 and Cat. 5 were promising alternatives. However, in moving from the model substrate to α-benzylstyrene (data in parentheses of entries 4-6), it was found that the isomerization proceeded with a lower stereoselectivity (6/1) and lower yield (54%) in the presence of Cat. 4 and Cat. 5, respectively. These results herein highlight the unique advantage of the novel cobalt catalyst (Entry 6) of the present invention.
aIsolated yield.
bThe ratio of E/Z was determined by the 1H NMR of crude product.
cThe data in parentheses was obtained using α-benzylstyrene as a substrate.
Substrate Scope. With these optimized conditions in hand, various geminal disubstituted alkenes were synthesized and screened in isomerization chemistry (Scheme 9). Initially, a series of alkyl groups (R2) were investigated (1a-1g) and the corresponding products were obtained with slightly lower yields and ≥15/1 E/Z selectivities. Remarkably, 2-norbornaneacetic acid and 5β-cholanic acid derived alkenes underwent the reaction smoothly to afford the products (2f-2g) with high stereocontrol. Subsequently, various α-ethylstyrenes were explored under optimized conditions, and uniformly good results were observed regardless of the electronic properties or substitution patterns (2h-2n). It was particularly noteworthy that a bromine substituent was well tolerated under this protocol without giving rise to any dehalogenation products (2n). To further evaluate the substrate scope, the inventors tested a large number of diaryl-substituted alkenes (2o-2ah), which could provide more conjugated trisubstituted alkenes. Gratifyingly, increased reaction performance was observed among these substrates in terms of both yields (up to 98%) and stereoselectivity (up to 112/1) (2o-2ah). Moreover, some reactive functional groups (2af-2ag), such as, heteroarenes, amines and ester groups proved to be suitable in the reaction, albeit with a lower yield and selectivity. However, the ester group did lead to a reduced reaction performance (2ah). This might be attributed to the relative ease of reduction of the ester under reaction conditions which generate a highly reactive metal-hydride species. Finally, cyclic alkenes were also found to be amenable substrates (2ai-2ak). Specifically, high regioselectivity (29/1) was observed in the case of substrate 1ak and the thermodynamically stable product was obtained as the major product.
Compared to the isomerization of monoenes, dienes were considered as more challenging substrates in prior reports, due to undesired radical-initiated cyclization.8,10 Under modified conditions, excellent yields and stereoselectivities were observed in both conjugated and non-conjugated diene products (2al-2ao, Scheme 10). The configuration of product 2ao was assigned using X-ray diffraction. Additionally, m- and p-phenylenediacetic acid derived dienes were also explored (2ap-2aq). Using 5 mol % catalyst loading, isomerization reactions proceeded smoothly to afford the products which containing more freely-rotating groups. Notably, a chemoselective isomerization was observed in the case of substrate tar and the sterically hindered terminal double bond was untouched in the reaction (2ar).
Intriguingly, some of these trisubstituted alkenes displayed promising aggregation-induced emission (AIE)19 properties. The fluorescence images of selected examples are shown in
To further demonstrate the synthetic utility of the approach, a deuterated substrate 1a-D2, which was readily prepared through H/D exchange and Wittig reaction, was employed in the reaction to construct deuterated trisubstituted-alkene (Scheme 11a). As expected, an intramolecular deuterium transfer proceeded smoothly affording product 2a-D2 with an exclusive site-selectivity (Scheme 11a). Regioselective deuteration of sp3 C—H and sp2 C—H is commonly regarded as a long-standing challenge.21 The approach herein approach serves as an alternative means to access this synthetically challenging product. Moreover, the gram-scale isomerization of 1a was performed with a lower catalyst loading (1 mol %). An isolated yield of 1.24 g (2a) was obtained with a slightly lower E/Z selectivity (64/1, Scheme 11b). Furthermore, a catalyst recycling experiment was also explored and is enabled by virtue of the poor solubility of the catalyst in hexanes.16 The catalyst could be precipitated from the reaction mixture by treatment with hexanes. A moderately diminished yield and stereoselectivity were observed upon the use of the recycled catalyst (Scheme 11c).
Schemes 11a to 11c. Synthetic utility of the reaction and catalyst-recycling experiment
a. Synthesis of Deuterium Labeled Alkene
b. Gram Scale Reaction:
c. Catalyst Recycling Experiment:
Mechanistic studies. There are four main pathways5i invoked in transition-metal catalyzed olefin isomerization: a) radical mechanism (Scheme 12a); b) metal hydride mechanism (Scheme 12b); c) 1,3-hydrogen shift mechanism (Scheme 12c); d) oxidative cyclization mechanism (Scheme 12d). These mechanisms can be classified into two types of pathways according to the means of H-transfer: stepwise (pathways a and b) and concerted (pathways c and d).
Schemes 12a to 12d. Four main pathways of the transition-metal catalyzed olefin isomerization
(a) Radical Mechanism (Stepwise H-Transfer)
(b) Metal-Hydride Mechanism (Stepwise H-Transfer)
(c) 1,3-H Shift Mechanism (Concerted H-Transfer)
(d) Oxidative Cyclization Mechanism (Concerted H-Transfer)
To distinguish the reaction pathway, a series of experiments were carried out with the goal of ruling out certain mechanisms (Schemes 13a and 13b). A crossover experiment using mixed 1a-D2 and is was investigated and the crossover deuterated product 2s was observed (Scheme 13a). This result ruled out the concerted H-transfer pathways (pathway c and d).5i Subsequently, a common radical scavenger 1,1-diphenylethylene was included in the reaction of 1a, which caused negligible effects on the reaction yield and stereocontrol (Scheme 13b). This observation excluded a possible H radical reaction pathway. Taken together, the metal-hydride pathway emerges as the most plausible reaction mechanism.
Schemes 13a and 13b. Control experiments to distinguish reaction pathway
a. Crossover Experiment
b. Radical Trapping Experiment
Kinetic isotope effect studies of the isomerization were further studied by using mixed 1a and 1a-D2 (Scheme 14a). The proportion of deuterium in the product suggested 1/1 ratio of kH/kD, indicating the cleavage of C—H is not the rate-determining step in the reaction. Additionally, mixed isomers of product (Z/E=4.4/1) were tested under more optimized conditions to explore the origin of stereoselectivity in the reaction (Scheme 14b). Although an increase in the ratio of E isomer was detected in the product, the stereoselectivity of the reaction should mainly stem from the isomerization of terminal to internal alkenes, not from the isomerization of E/Z isomers of trisubstituted alkene product. The reaction of las produced product 2a with far less E/Z selectivity (20/1) compared with the reaction of in. This result further supports the conclusions on the origins of stereoselectivity. Additionally, n-n stacking effects between the substrate and catalyst were explored by using the aliphatic alkene but-1-en-2-ylcyclohexane (1at) (Scheme 14c). A significantly reduced conversion (45%) and stereoselectivity (3/1) was observed. By way of explanation and not a limitation of the present invention, this observation suggests that the presence of π-π stacking interactions are possibly crucial for both reaction efficiency and selectivity.
Schemes 14a to 14c. KIE study and stereoselectivity origin study
a. KIE Study:
b. Stereoselectivity Origin Study
c. π-π Stacking effect investigation
By way of explanation, and in no way a limitation of the present invention, based on the experimental observations and the single crystal stricture of the precatalyst, a Co-hydride catalytic pathway is proposed as shown in
This example shows a stereoselective olefin isomerization catalyzed by a novel cobalt-hydride catalyst. The mild reaction conditions and high efficiency allows a scalable access to acyclic trisubstituted alkenes with high stereoselectivity. Moreover, a preliminary catalyst-recycling experiment shows that the catalyst is recyclable. The interesting ALE property of products arising from diene substrates makes this protocol useful in novel material preparation. The synthetic utility of this reaction is also demonstrated by the synthesis of a deuterated trisubstituted alkene. Additionally, a plausible catalytic cycle is proposed to rationalize the mechanistic observations.
This example demonstrates the use of catalytic systems which take advantage of a metalla-GAP strategy (GAP=Group-Assisted-Purification) in homogeneous catalysis for the first time. This discovery allows simple reclamation and re-use of homogeneous metal catalysts. Homogeneous catalysis has many appealing properties, such as high chemo-, regio- and stereoselectivity, an abundance of suitable catalysts to choose from, and generally mild reaction conditions. However, many homogeneous catalyst systems cannot be used in more practical, industrially relevant settings mostly arising from issues relating to poor separability between products and catalyst. The m-GAP strategy is used in palladium-catalyzed Suzuki-Miyaura cross-coupling reactions. Excellent recyclability and synthetic utility are observed in these palladium catalyzed transformations. Introduction
Homogenous catalysis is widely used due to the abundance of compounds and complexes that are soluble in organic solvents.1,2 A key drawback of this approach is separability, with catalysts typically being lost to waste at the end of a reaction as the recovery process is simply not cost effective.3 As an alternative, heterogeneous catalysts can be employed, as the catalyst and products are in different phases and are easily separated; this allows reclamation of the catalyst to be reused in subsequent reactions, increasing the overall efficiency of the catalytic system.4 In contrast to homogeneous systems, heterogeneous catalysts typically operate under more extreme reaction conditions (elevated temperature, pressure, etc.) and are often less selective as a result.5 By making homogenous catalysts recyclable or by designing systems that facilitate better separability this key advantage of heterogeneous catalysis can be incorporated into homogenous systems. Group-assisted purification (GAP) chemistry and metalla-GAP (m-GAP) chemistry were used by the inventors to expand the use of ligands that contain a phosphoramide motif in homogenous catalysis and to further explore their potential as recyclable catalytic systems.
Carbon-carbon bond forming (cross-coupling) reactions are an incredibly important transformation and have wide-ranging applications in many fields including medicinal chemistry and pharmaceutical synthesis.10 The Suzuki-Miyaura reaction is the most predominately used cross-coupling reaction in industry.11, 12 The Suzuki-Miyaura reaction was first reported by Akira Suzuki and co-workers in 1979 is one of the most widely used cross coupling reactions for the formation of biphenyls, due to the high functional group tolerance and scalability for use in industrial processes.11,13,14 Most Suzuki-Miyaura coupling reactions employ palladium catalysts which are highly reactive but expensive due to their scarcity.15,16,17 Palladium is a non-abundant platinum group metal, as such palladium salts are expensive and natural deposits will eventually be depleted.18,19 In an effort to improve the sustainability of palladium-catalyzed cross-coupling reactions and make the reactions more environmentally friendly, ideally the palladium content should be recovered at the end of the reaction.20,21 Extensive work has focused on methods recovering palladium content from spent catalysts to reduce the excessive costs.22 Catalyst recycling methodologies have the potential to improve the sustainability of palladium-catalyzed reactions and make the processes “greener”.23, 24 Recycling of palladium catalysts is highly desirable as “active” Pd species can be recovered and re-used and previous reports have described separable heterogeneous and immobilized Suzuki catalysts.25-32 The present inventors focused on the use of GAP, initially reported by Li and co-workers as ligand architectures for metal catalysts.8 By way of explanation, it is hypothesized that this could provide an alternative to fully heterogeneous or immobilized catalysts as the transformation occurs in a homogeneous manner with all the advantages associated with homogeneous catalysis. To further extend the use of m-GAP ligands, the inventors applied the approach to palladium-catalyzed Suzuki-Miyaura cross couplings (Scheme 15).
In the initial studies, 4-Iodotoluene and phenylboronic acid were used for substrate screening with m-GAP ligands and palladium acetate, to determine the optimal reaction conditions (Scheme 15, Table 4, see also
Homogeneous Catalysis. The Suzuki-Miyaura coupling between phenylboronic acid and 4-iodotoluene was used as a model reaction with which to evaluate optimal reaction conditions. GAP ligands L1 and L2 were screened under various conditions in the presence of palladium acetate and potassium tert-butoxide as base in an effort to optimize reaction conditions.8,9 Under ambient temperature and using THF as a solvent, the catalyst system derived from L1 was found to afford both slightly higher conversion and higher selectivity for cross-vs. homo-coupling products than the catalyst system derived from L2 (Table 4, Entries 1 and 2). Raising the temperature to 60° C. resulted in lower overall conversion albeit with an improved product selectivity (Table 4, Entry 3). Improved results could be obtained using L2 if the reaction time was extended to 48 h and the catalyst mixture was allowed to stir overnight prior to use in the coupling chemistry (Table 4, entry 6). A ligand-free reaction was also performed and revealed that even in the absence of any ligand the reaction still proceeds albeit in diminished conversion. After consideration of the reaction time, solvent employed and temperature, the inventors chose to employ L1, THF and room temperature as perfectly cromulent reaction conditions.
[a]Pd(OAc)2(0.015 mmol), Ligand (0.015 mmol), Iodotoluene (0.3 mmol), Phenylboronic acid (0.6 mmol), Potassium tert-butoxide (0.6 mmol), Solvent = 2 ml, Reaction time = 24 hours.
[b]Catalyst prepared via overnight stirring.
[c]48 hours
[d]Spectroscopic yield calculated with tetramethylsilane as an internal standard.
[e]Ratio of heterocoupled product (4-methyl-1,1-biphenyl (2a)) to homocoupled product (4,4-dimethyl-1,1-biphenyl (2b)).
Next, the inventors turned to an examination of the scope of substrate amenable to optimized conditions, in part to ensure that thein-GAP palladium complex would exhibit a comparable reactivity profile to other Suzuki-Miyaura catalysts. Thus, Li/Pd system was deployed as a catalyst for several Suzuki-Miyaura couplings with a broad range of aryl halide and aryl triflate substrates (Table 5, Scheme 16). It was found that aryl iodide substrates (2a-2f) tended to afford good to excellent yields in coupling reactions with phenylboronic acid, the use of substrates with a second halogen atom (not iodine) (2d-2f) delivered excellent chemoselectivity as C—C bond formation occurs exclusively at the C—I bond of the substrate, with no evidence for “double coupling” observed.33,34 To probe the limits of the system, the catalyst was challenged with substrates and functional groups often found to be incompatible with Suzuki-Miyaura conditions. Substrates containing alcohol, primary amine, and indole groups (2g-2k) were less favorable coupling partners in this system. Only trace amounts of product were observed with amino and indole substrates. This is likely a result of amino group coordination and N—H oxidative addition at palladium.35-37 Poor reactivity is also observed with iodophenols under the previously optimized reactions conditions, however, increasing the reaction temperature to 70° C. gave full conversion. It was found that the Pd-GAP catalyst afforded excellent conversion of the heterocycle 2-iodothiophene (2) to coupled product under the conditions taught herein. Having investigated the scope of Suzuki coupling reactions with aryl iodides the inventors then turned to aryl bromides (2l-2t). To facilitate catalytic turnover the reaction temperature was increased to 70° C. Functional group tolerance of the aryl bromides was consistent with the results obtained for aryl iodides (2l-20) and good to excellent yields were obtained. 4-bromoacetophenone (2p) proved to be a competent substrate, efficient and highly selective C—C bond formation occurred with no undesired reaction of the ketone group observed.38 No reaction was observed with 2-bromoacetophenone (2q) which is nominally an sp3-sp2 coupling reaction.39 2-bromopyridine, 2,6-dibromopyridine and 4-tbutylbromobenzene (1r-1t) gave good conversion with modest yields. The suitability of aryl chloride substrates was examined, which required more forcing conditions to promote catalytic activity (90° C.) (1u-1z). Functional group tolerance remained consistent with the observed reactivity of aryl iodides and aryl bromides. Full conversion was observed with a variety of substrates with the products obtained in good yields (1u-1y). Coupling reactions between sp3 and sp2 centres (1aa-1ad) were unsuccessful. Finally, one pseudo-halide, tosylate, substrate (1ae) was studied, and the desired coupling product was obtained in a modest yield.
aPd(OAc)2 (0.015 mmol), Ligand (0.015 mmol), Aryl halide/triflate (0.3 mmol),
bTemperature = 70° C.
cTemperature = 90° C.
Reclamation and Recycling Studies. Next, the inventors investigated the viability of the GAP-Pd catalyst as a reusable Suzuki-Miyaura catalyst. Catalyst and product separability was found to be excellent in hexane or pentane, and hence the recovered catalysts were able to be re-used in subsequent catalytic cycles. Recyclability studies were conducted, and the results are detailed below (
It was observed that while initially catalytic conversion is modest (Cycle 1) repeat reactions with the recycled palladium catalyst give improved reactivity and selectivity. The initial recycling experiment (Cycle 2) gives almost complete conversion, and no significant loss of activity is then observed in subsequent cycles. The catalyst was reused nine times (ten catalytic cycles in total) which is excellent for a homogenous catalyst and activity remains consistent and comparable to previously reported heterogeneous Suzuki-Miyaura catalysts.21, 29-31 It is possible that the first catalytic cycle serves to attenuate the precatalyst or convert the system into a more active form of the catalyst which results in both higher conversion and increased selectivity for cross-coupled (versus homo-coupled) product.
The importance of the GAP ligand to recyclability was probed by attempting a “ligand free” recycling experiment. Surprisingly, a catalytic system simply employing palladium acetate and potassium tert-butoxide did exhibit limited recyclability. However, elevated temperatures were required, and the system exhibited limited recyclability before becoming inactive. This demonstrated the superiority of the m-GAP system for recyclability applications. Moreover, the nature of the catalyst system was not entirely clear, though strong evidence suggests it is heterogeneous in nature. On the homogeneity of the species present in the reaction it can be remarked that recent literature studies suggest that the active catalytic species present in Suzuki-Miyaura coupling reactions are homogenous in nature, even if nanocomposite structures form in situ.40-42 Further mechanistic investigations into these reactions will be conducted in due course to determine the active species present.
The Pd-GAP complexes were studied computationally via Density Functional Theory (DFT) to determine their optimized structures and hence the coordination chemistry of the complexes.43,44
This example shows that Palladium-catalyzed Suzuki-Miyaura couplings are fully recyclable with the GAP ligands taught herein over ten catalytic cycles. The m-GAP palladium catalysts are excellent catalysts for the couplings of a wide range of aryl halides with simple boronic acids. Further work will be conducted to expand the scope of m-GAP catalysis, mechanistic studies and the isolation of key reactive intermediates will be reported on in due course.
Prophetic new approach to palladium-catalyzed reactions, a mainstay of pharmaceutical chemistry. Transition metal catalysts are crucial to a range of pharmaceutical syntheses.1-3 Palladium is particularly important, as Pd-catalyzed processes underpin many of the key syntheses in the $1 trillion pharmaceutical industry. Remarkably, Pd-catalyzed cross-coupling reactions account for 17% of all reactions used to build New Molecular Entities (NMEs) by the pharmaceutical industry; ca. 70% of processes used to form new Active Pharmaceutical Ingredients (APIs) involve a Pd-catalyzed step. However, Pd is toxic, and Food and Drug Administration (FDA) regulations dictate that elemental impurities of Pd in drug products must be <100 μg/day for orally administered drug products. These limits create purification, and associated cost, implications for the healthcare industry.4 The present invention can be used to address an urgent evidence gap in pharmaceutical chemistry: the need for cost- and time-effective routes to remove elemental impurities in organometallic reagents and catalysts. For example, the route to manufacture Ceftolozane features a palladium-mediated C—N cross-coupling reaction that is only two steps from the final drug substance, Ceftolozane sulfate (an antibiotic). Current synthetic routes require treatment with a palladium scavenger, thioglycerol, to reduce palladium content to acceptable levels5 A major drawback of scavengers is that they introduce extra steps in reaction processing and do not allow for precious metal recycling and reuse. This prophetic example overcomes this barrier by eliminating the need for such scavengers. Instead, the inventors have developed and validated an entirely new concept in catalysis by applying mGAP methods to transition metal chemistry. The invention uses mGAP to render any homogenous catalyst recyclable and reusable, thus eliminating the need for capture and removal of toxic transition metals from APIs.
mGAP chemistry overcomes significant barriers to expanding use of homogenous catalysis by delivering recoverable and recyclable, homogeneous transition metal catalysts. The inventors have developed an innovative approach based on the application of novel N-phosphonyl imine derived molecules to use as ligands for catalytically relevant metals. GAP chemistry enables purification of reaction products via a simple filtration operation;6-13 this strategy has been successfully used in many transformations.14-17 The straightforward preparation of mGAP ligands (L) in high yield using commercially available chiral building blocks as shown in
The development of new drugs is time-consuming, expensive, and risky. Despite these challenges, the pharmaceutical industry has been remarkably successful in developing a broad range of new medicines to solve ongoing and emerging public health crises. However, a major challenge to exploring novel drug targets is the over-reliance of the pharmaceutical industry on precious-metal-catalysts. This limitation is seriously hampering methodological innovation, and thus the development of new therapies. This prophetic example provides a solution to overcoming this limitation by exploring synthetic avenues that rely on recyclable transition metal catalysts that can be reclaimed and reused. In addition, the prophetic methods taught herein address a serious threat to pharmaceutical chemistry: current methods are not sustainable because they consume precious raw materials needed for catalyst manufacture and the downstream production of drugs.
Homogeneous catalysts—defined as those that exist in the same (usually liquid) phase as the reactants and products—are typically more selective than heterogeneous catalysts. Homogeneous catalysts are also significantly less affected by limitations arising from slow transport of reactants and products to and from an interface, a problem frequently seen in heterogeneous catalysis. However, the separation of homogeneous catalysts from their reaction products can be costly and is often inefficient. Therefore, it is unsurprising that with a few notable exceptions,18-19 most industrially relevant catalysts are heterogeneous. Unfortunately, compared with their homogeneous counterparts, heterogeneous catalysts often suffer from lower performance and diminished selectivity owing to poorly defined catalytic sites, thus nullifying most benefits.
This new approach provides a novel solution to these problems, by supplying a general blueprint for recycling homogeneous catalysts. In contrast to traditional approaches involving the generation of a heterogeneous catalyst based upon a successful homogeneous analog, this approach avoids the need to tether a homogeneous complex to a non-innocent scaffold like alumina/silica or the incorporation of significant amounts of fluorine into the catalyst structure.
The present invention includes synthesizing well-defined metal complexes using kinetics and stoichiometric studies, with a focus on reactions involving unsaturated substrates, to investigate each step in the catalytic cycle. Investigating decomposition pathways facilitates the preparation of the most robust (recyclable) catalyst architectures.
Identifying characteristics of recyclability in homogeneous complexes to evaluate their utility as reusable catalysts.
Exquisite product selectivity under mild conditions is the hallmark of homogeneous catalysis. However, the difficulty associated with separating homogeneous catalysts from their products leads to inevitable losses because of increased costs and inefficiency. This has stimulated development of strategies that facilitate the recycling of homogeneous catalysts.21-22 Groundbreaking work reported by Dioumaev and Bullock two decades ago described the use of a tungsten-based complex to catalyze the hydrosilylation of carbonyl groups.32 The key to their success was the solvent-free nature of the reaction; the depletion of (liquid) substrate and subsequent formation of product resulted in a distinct change of solubility of the tungsten complex, which precipitated from the reaction upon completion. As remarkable as this report was, the process suffered from several drawbacks: 1) limitation to liquid substrates (they provided the ‘solvent’ for the reaction), 2) limited substrate scope (five ketones and one ester were studied) combined with by-product formation and, 3) catalyst recyclability was not a result of rational design, which limits available options for improvements. Using mGAP-Zinc Complexes as Ketone Hydrosilylation Catalysts. Previous reports by Parkin and co-workers showed that double hydrosilylation of ketones can be achieved employing a zinc catalyst.33-34 Parkin's system is one of only a handful of reported homogenous catalysts that are highly selective for multiple hydrosilylations of ketones.35-37 To provide a direct comparison with Bullock's earlier work, the inventors chose to investigate the catalytic hydrosilylation of ketones with the mGAP ligated zinc catalysts. Benzophenone (1a) can be used as a model substrate with which to optimize reaction conditions using diphenylsilane as the reductant; these conditions were used to examine scope of the substrate (
mGAP-Palladium Complexes as Suzuki-Miyaura Coupling Catalysts. Carbon-carbon bond coupling reactions are incredibly important reactions in medicinal chemistry and pharmaceuticals. With Suzuki-Miyaura couplings, they are the most common reactions in industry.38-39 Most Suzuki coupling reactions use palladium catalysts because they are highly reactive, but they are also expensive and often toxic.40-41 Catalyst recycling methodologies hold potential to improve the sustainability of palladium-catalyzed reactions and make the processes “greener”.42-43 Thus, to further extend the use of mGAP ligands, the inventors used the compounds in palladium-catalyzed Suzuki cross couplings. Iodotoluene (3a) and phenylboronic acid were used for substrate screening with m-GAP ligands and palladium acetate to determine optimal reaction conditions, with subsequent study of substrate scope (Scheme 15).
Recycling mGAP-Metal Complexes. Using a simple procedure, addition of hexanes to the reaction mixture, the precipitated catalyst can be isolated via centrifugation/decanting of the mother liquors (
Thus, it was possible to incorporate the GAP moiety into a ligand architecture to impart recyclability to a homogeneous transition metal catalyst. It is now possible to manipulate ligand structure to control subsequent activity and recyclability.
Probing mGAP Ligand Properties. Key to the design of superior mGAP catalysts will be a thorough understanding of structure-function relationships. The inventors can modify the mGAP ligand as shown in
Influence of Solvent Effects. Since the recycling strategy is based upon solubility differences between catalyst and reaction products, even subtle changes in structure could play an important role in determining separability of the catalyst. More broadly, the precipitating solvent should also be considered from an environmental/toxicity perspective. Heptane is considered a ‘greener’ hydrocarbon solvent than hexane(s)45 and the substitution of hexanes with heptanes for catalyst recovery can be determined. It is also possible to explore the relative solubility of mGAP complexes in a range of typical organic solvents. Changing the “R” groups from short-chain alkyl units to longer chain alkyls or extended ether-type units can confer solubility in both non-polar and polar solvents, whereas the use of aromatic substituents may impart solubility in aromatic solvents.
Installation of GAP group on existing catalysts: A universal approach to recyclability. Using the present invention, it is possible to explore the potential to render any homogeneous catalyst recyclable via the installation of a GAP moiety (
As a result of the present invention, it is now possible to generate a universal approach to producing recoverable mGAP catalysts for any chemical transformation. Thus, alternative transformations can be accomplished, e.g., stable iron (III) oxo complexes that tend to show high reactivity in alkene hydrophosphination reactions47-48.
Novel transformations enabled by GAP-based transition metal complexes. Transition metal-catalyzed alkene isomerization has emerged as a fundamental and atom-economic transformation that can readily convert terminal alkenes to internal alkenes.50-58 Prior reports of alkene isomerization usually rely on noble metals, such as ruthenium, rhodium, palladium, iridium, and platinum.59-71 Owing to the low abundance and high toxicity of these precious metals, attention has shifted to finding base-metal alternatives.58 The past few decades have witnessed tremendous progress in this field, particularly in cobalt-catalyzed olefin isomerization. For example, Holland and Weix did impressive work in Z-selective alkene isomerization.72 In the same year, a radical triggered isomerization and cycloisomerization was reported by Shenvi employing a Co-salen catalyst.73 Subsequently, Hilt and coworkers unveiled a Co-dppp catalyzed monoisomerization of 1-alkenes to (Z)-2-alkenes.74 More recently, a kinetically controlled isomerization was achieved by the Liu group, affording the target alkenes in good to excellent regioselectivity.76
Despite these impressive developments, the stereoselective alkene isomerization towards the synthesis of acyclic trisubstituted alkenes remains a challenging transformation.77 Particularly, low reaction efficiency and poor stereoselectivity are significant problems in alkene isomerization because they limit the application of an otherwise atom-economical approach to synthetically useful target molecules.
Using mGAP-Cobalt Complexes as Olefin Isomerization Catalysts. As shown in the previous examples, base-metal catalysis provide a novel co-hydride catalyst supported by an mGAP ligand.44 The in-situ-generated metal-hydride catalyst could readily facilitate challenging isomerizations to afford a wide range of trisubstituted alkenes and dienes (41 examples) with up to 98% yield and 130/1 (E/Z) stereoselectivity. Various α-ethylstyrene substrates afforded uniformly good to excellent results, regardless of the electronic properties or substitution patterns. Interestingly, 2-norbornaneacetic acid and 5β-cholanic acid derived alkenes underwent the reaction smoothly to afford the products with high stereocontrol. Cyclic alkenes were also found to be amenable substrates. Specifically, high regioselectivity (29/1) was observed and the thermodynamically stable product was obtained as the major product.82
Regioconvergent Conversion of Olefin Mixtures and Regioselective Olefin Isomerization. Most building blocks in the chemical industry come from petroleum raw materials. Unrefined olefins tend to exist as isomeric mixtures which are difficult to separate; a regioconvergent olefin isomerization process would lead to isomerically pure products which are significantly more valuable (Scheme 18). Regioconvergent isomerization has been utilized in tandem with hydrosilylation to afford terminal silanes from mixtures of internal alkenes.81-84 The successful isomerization of both internal and terminal olefins provides an opportunity to integrate the mGAP into an industrially relevant transformation, the transformation of unrefined isomeric olefin mixtures, to a single alkene product. Mixtures of olefins can be used and the treatment of a 1:1:1 mixture of hexenes (E/Z mixture) with optimized isomerization conditions should give rise to a single reaction product. These regioconvergent olefin isomerizations are an attractive approach to upgrading inexpensive olefin feedstock to more valuable isomerically pure alkenes. In preliminary results, the inventors have demonstrated that mixtures of 1-, 2- and, 3-hexenes are selectively isomerized to the 2-hexene isomer exclusively. Piperidine-derived alkenes (Scheme 18) can also be reacted with the present invention, as each has at least two possible sites that may serve to direct C═C bond migration. In a similar fashion, regiodivergent transformation of both exocyclic and endocyclic trisubstituted alkenes will be examined. In a small (NMR) scale reaction, it was found that BOC-protected piperidine 5 can be exclusively isomerized to internal alkene 6 in quantitative yield. This result represents a particularly attractive conversion to a value-added product.
Beyond Olefin Isomerization. In the absence of an external radical trap, a simple alkene subjected to MHAT can take two main pathways leading to isomerization (described above) or hydrogenation. After an initial HAT, the nascent carbon-centered radical may abstract a second hydrogen atom to give a hydrogenated compound. However, in the presence of appropriate groups, C—O, C—C, C—N and C—X bond formation are also possible.75, 86-88 MHAT functionalization reactions can also be targeted. For example, the examination of tandem reactions in which olefin isomerization is paired with a second catalytic reaction is another attractive route to pursue, as was the hydrosilylation with mGAP iron and cobalt complexes are effective olefin hydrosilylation catalysts described hereinabove above.
As embodied and broadly described herein, an aspect of the present disclosure relates to a method for recycling transition metal catalysis comprising, consisting essentially of, or consisting of: combining the reactants in a solvent in the presence of a molecule having the formula:
wherein R is selected from methyl, ethyl, propyl, isopropyl, butyl, phenyl, cyclohexyl, fluoro, chloro, methoxy, ethoxy, butoxy, phenoxy, aryl, alkene, alkyne, and heterocyclic, wherein the catalyst is selected for an isomerization reaction catalyst, a hydrosilylation reaction catalyst, a Suzuki-Miyaura coupling catalyst, a Negishi coupling catalyst, a Sonogashira coupling catalyst or a Heck coupling catalyst; and wherein if the molecule is:
then R is not isopropyl, wherein M is a metal; under conditions that catalyze a reaction; and recycling the molecule of Formula II. In one aspect, the molecule of Formula II is compound 1, which is synthesized by
In another aspect, the molecule of Formula II is compound 1 that is a ligand made by:
In another aspect, R is selected from benzyl, O-methylbenzyl, trifluorobenzyl, or 2,4,6-methyl benzyl. In another aspect, the molecule recycles transition metals selected from at least one of: 1st-row transition metals, 2nd-row transition metals, 3rd-row transition metals, main group metals, lanthanide series transition metals, and actinide series transition metals. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium. In another aspect, M is at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium.
In another aspect, the reaction is an isomerization reaction, a hydrosilylation reaction, a Suzuki-Miyaura coupling, a Negishi coupling, a Sonogashira coupling or a Heck coupling.
As embodied and broadly described herein, an aspect of the present disclosure relates to a tunable tridentate ligand for recyclable homogeneous transition metal catalyst comprising, consisting essentially of, or consisting of: a molecule of Formula I:
wherein R is selected from methyl, ethyl, propyl, isopropyl, butyl, phenyl, cyclohexyl, fluoro, chloro, methoxy, ethoxy, butoxy, phenoxy, aryl, alkene, alkyne, and heterocyclic, wherein the catalyst is selected for an isomerization reaction catalyst, a hydrosilylation reaction catalyst, a Suzuki-Miyaura coupling catalyst, a Negishi coupling catalyst, a Sonogashira coupling catalyst or a Heck coupling catalyst; and wherein if the molecule is:
then R is not isopropyl, and optionally M is a metal. The molecule of Formula I is a ligand until the metal is added, at which time the molecule of Formula I becomes the molecule of Formula II, which is a catalyst. In one aspect, the molecule of Formula I is synthesized by
In another aspect, the molecule of Formula I is converted into a recyclable catalyst by:
In another aspect, R is selected from isopropyl, benzyl, O-methylbenzyl, trifluorobenzyl, or 2,4,6-methyl benzyl. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: 1st-row transition metals, 2nd-row transition metals, 3rd-row transition metals, main group metals, lanthanide series transition metals, and actinide series transition metals. In another aspect, the molecule of Formula I recycles transition metals selected from at least one of: platinum, palladium, vanadium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, ruthenium, iridium, rhodium, hafnium.
It is contemplated that any embodiment discussed in this specification can be implemented with respect to any method, kit, reagent, or composition of the invention, and vice versa. Furthermore, compositions of the invention can be used to achieve methods of the invention.
It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims.
All publications and patent applications mentioned in the specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference.
The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and/or.” Throughout this application, the term “about” is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects.
As used in this specification and claim(s), the words “comprising” (and any form of comprising, such as “comprise” and “comprises”), “having” (and any form of having, such as “have” and “has”), “including” (and any form of including, such as “includes” and “include”) or “containing” (and any form of containing, such as “contains” and “contain”) are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. In embodiments of any of the compositions and methods provided herein, “comprising” may be replaced with “consisting essentially of” or “consisting of”. As used herein, the phrase “consisting essentially of” requires the specified integer(s) or steps as well as those that do not materially affect the character or function of the claimed invention. As used herein, the term “consisting” is used to indicate the presence of the recited integer (e.g., a feature, an element, a characteristic, a property, a method/process step or a limitation) or group of integers (e.g., feature(s), element(s), characteristic(s), propertie(s), method/process steps or limitation(s)) only.
The term “or combinations thereof” as used herein refers to all permutations and combinations of the listed items preceding the term. For example, “A, B, C, or combinations thereof” is intended to include at least one of: A, B, C, AB, AC, BC, or ABC, and if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context.
As used herein, words of approximation such as, without limitation, “about”, “substantial” or “substantially” refers to a condition that when so modified is understood to not necessarily be absolute or perfect but would be considered close enough to those of ordinary skill in the art to warrant designating the condition as being present. The extent to which the description may vary will depend on how great a change can be instituted and still have one of ordinary skilled in the art recognize the modified feature as still having the required characteristics and capabilities of the unmodified feature. In general, but subject to the preceding discussion, a numerical value herein that is modified by a word of approximation such as “about” may vary from the stated value by at least ±1, 2, 3, 4, 5, 6, 7, 10, 12 or 15%.
Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Field of Invention,” such claims should not be limited by the language under this heading to describe the so-called technical field. Further, a description of technology in the “Background of the Invention” section is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.
All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.
To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims to invoke paragraph 6 of 35 U.S.C. § 112, U.S.C. § 112 paragraph (f), or equivalent, as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.
For each of the claims, each dependent claim can depend both from the independent claim and from each of the prior dependent claims for each and every claim so long as the prior claim provides a proper antecedent basis for a claim term or element.
This application claims priority to U.S. Provisional Application Ser. No. 63/304,119, filed Jan. 28, 2022, the entire contents of which are incorporated herein by reference.
This invention was made with government support under CHE-1554906 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63304119 | Jan 2022 | US |
