Patent Application
20060173219
1. Field of Invention
This invention relates to aldol condensations, especially to condensations of two different aldols.
2. Description of Prior Art
In order to produce new compounds two aldol compounds are often reacted in the known manner. If one of these compounds does not have an alpha-carbon with hydrogen on it (benzaldehyde, formaldehyde, tert-pentaldehyde) the reaction produces one product. However, if both aldols have hydrogen on the alpha-carbon specifity is lost and all four possible products result.
For example, equiniolar amounts of acetaldehyde and propionaldehyde react to produce equimolar quantities of 2-butenal, 2-pentenal, 2-butene-2-methyl-al, and 2-pentene-2methyl-al.
Attempts have been made to enhance specifity by increasing the amount of one of the reactants to 30:1 (U.S. Pat. No. 6,028,231) but this still produces byproducts and waste and is impractical where both aldols are approximately equal in cost.
Also there is currently no method for a ketone to add to an aldehyde to produce an aldehyde product. When the compounds are mixed the more reactive aldehyde always adds to the ketone producing a ketone as a product.
Several objects and advantages of the present invention are:
In accordance with the present invention a protective group consisting of an acetal or an imine, which may contain electron-withdrawing functions, for crossed aldol condensations: whereby the carbonyl oxygen of the protected aldol is rendered unreactive to allow the carbonyl oxygen of the unprotected aldol to react with the alpha-carbon hydrogens of the protected aldol, in the presence of a base of sufficient strength, to produce one product compound with no side reactions.
The ability of a compound (nitroparafins, alkyl cyanides, other aldehydes and ketones) to react with aldehydes and ketones is a function of the acidity of the hydrogen on the carbon alpha to the functional group. This acidity is described by the term pKa. In the case of acetone (pKa 20 in water), as typical of aldehydes and ketones, the ability to delocalize the positive charge of an alkali metal base is largely due to the conjugation of the carbonyl bond. Resonance between the ketonic and enolic structures of acetone allow the molecule to absorb the positive charge of the alkali metal ion.
When an acetal is formed to render the carbonyl oxygen unreactive so other portions of the molecule may be modified, the double bond is replaced by two single bonds, conjugation is lost and acidity of the alpha hydrogens decreases markedly (pKa increases).
Alkali metal hydroxides in water serve only to condense compounds with a pKa up to about 30. Above this value stronger bases must be used to initiate the reaction by abstracting a proton from the target compound. Sodium ethoxide in ethanol will serve to condense compounds with a pKa up to about 33, sodium di-tert-butylamine in di-tert-butylamine will initiate condensation with compounds with a pKa up to about 38, and butyl-lithium in hexane will abstract a proton from a hydrogen with a pKa up to about 45.
Even without conjugation, the two ether linkages on the functional carbon of an acetal are fairly powerful electron-withdrawing groups. This electron-withdrawing function can be increased, increasing the acidity of the alpha-hydrogens, by using alcohols and glycols that possess electron-withdrawing groups to form the acetal, these being typified by perfluoro-tert-butanol and perfluoro-pinacol.
By using these two techniques of employing stronger bases and electron-withdrawing alcohols and glycols, either singly or in tandem, an acetal can be made to condense with an aldehyde or ketone in the same manner as an unprotected aldehyde or ketone without side reactions occurring.
Alternately, conjugation may be preserved by forming an imine from the target aldehyde or ketone with a primary amine. Although the imine possesses a double bond, nitrogen is a more electropositive element than oxygen. This may be offset by using a primary amine possessing electron-withdrawing groups, typified by tri-fluoro methylamine and per-fluoro tert-butylamine. The imine group is less preferred to the acetal group because under certain conditions it may polymerize over time. Once the imine is formed it may be reacted with another aldehyde or ketone in the presence of a base of sufficient strength in the same manner as an unprotected aldehyde or ketone without side reactions occurring.
To 0.1 moles of acetaldehyde-perfluoropinacol-acetal (32.9 grams) in 250 ccs of diethylamine At 0 C is added 0.1 moles of sodium diethylamine (9.5 grams) in 100 ccs of diethylamine at 0 C with stirring. Five minutes are allowed for the sodium anion of the acetaldehyde-perfluoropinacol-acetal to form, whereupon 0.1 moles of propionaldehyde (6 grams) at 0 C is added over 3 minutes with stirring to form the sodium salt of 3-hydroxy pentanal-perfluoropincol-acetal. The diethylamine is removed under reduced pressure and the solid salt is gradually added to dilute hydrochloric acid at 0 C to form 2-pentene-al-perfluoropinacol-acetal. The reaction mixture may now be heated to 100 C to decompose the acetal yielding pure 2-pentene-al
To 0.1 moles of 1,3 propion-dialdehyde-perfluoro-tert-butyl alcohol-acetal (99.2 grams) in 500 ccs of ethanol at 0 C is added 0.1 moles of sodium ethoxide (6.8 grams) in 50 ccs of ethanol at 0 C with stirring. Five minutes are allowed for the sodium anion of 1,3 propion-dialdehyde-perfluoro-tert-butyl alcohol acetal to form whereupon 0.1 moles of cyclohexanone at 0 C is added over 3 minutes with stirring to form the sodium salt of 1-hydroxy cyclohexane 1-(2) 1,3 propion-dialdehyde-perfluoro-tert-butyl alcohol-acetal. The ethanol is removed under reduced pressure and the solid salt is gradually added to dilute hydrochloric acid
at 0 C to form delta-2 cyclohexane, 1,3 propion-dialdehyde-perfluoro-tert-butyl alcohol-acetal. The reaction mixture may now be heated to 100 C to decompose the acetal yielding pure delta-2 cyclohexane 1,3 propion-dialdehyde.
To 0.1 moles of acetaldehyde-perfluoro-tert-butylamine-imine (24.9 grams) in 250 ccs of di-tert-butylamine at 0 C is added 0.1 moles of sodium di-tert-butylamine (15.6 grams) in 100 ccs of di-tert-butylamine at 0 C with stirring. Five minutes are allowed for the sodium anion of acetaldehyde-perfluoro-tert-butylamine-imine to form whereupon 0.1 moles of acetone (7 grams) at 0 C are added over 3 minutes with stirring to form the sodium salt of 3-hydroxy, 3-methyl butanal-perfluoro-tertbutylamine-imine. The di-tert-butylamine is removed under reduced pressure and the solid salt is gradually added to dilute hydrochloric acid at 0 C to form 3-methyl butene-al-perfluoro-tert-butylamine-imine. The reaction mixture may now be heated to 100 C to decompose the imine to yield pure 3-methyl butene-al.
To 0.1 moles of acetaldehyde-ethylene glycol-acetal (8.7 grains) in 250 ccs of hexane at −20 C is added 0.1 moles of butyl-lithium (6.4 grams) in 100 ccs of hexane at −20 C with stirring. Five minutes are allowed for the lithium anion of acetaldehyde-ethylene glycol-acetal to form whereupon 0.1 mole of 1,5 dimethyl, 5-hexene-al (10.9 grams) in 50 ccs of hexane at −20 C is added over 3 minutes with stirring to form the lithium salt of 3,7 dimethyl, 3-hydroxy, 7 octene-al-ethylene glycol-acetal. The hexane is removed under reduced pressure and the solid salt is added gradually to dilute hydrochloric acid at 0 C to form citral-ethylene glycol-acetal. The reaction mixture may now be heated to 100 C to decompose the acetal to yield pure citral.
Accordingly, the reader will see the acetal and imine protective groups for crossed aldol condensations are more convenient than current methods. Furthermore the process has additional advantages in that:
Yields are increased
By-products and waste are reduced
Separation from by-products is easier
Allows deactivation of aldehyde carbonyl oxygen so a ketone may add to the aldehyde's alpha-carbon hydrogen's to produce an aldehyde as a product.
Although the description contains specifities, these should not be construed as limiting the scope of the invention, but merely providing illustrations of some of the presently preferred embodiments of the invention. For example other alcohols, glycols and amines may be used, and other bases, solvents and acids may be used. Thus the scope of the invention should be determined by the appended claims and their legal equivalents rather than by the examples given.
