Aromatic carboxylates are prepared by electrochemical oxidation of the corresponding methyl benzenes or benzaldehyde dialkyl acetals in the presence of an alkanol and of a halogenated triarylamine derivative.
Description
The present invention relates to a novel process for the preparation of aromatic carboxylates by electrochemical oxidation of benzene derivatives. J. Chem. Soc. Perkin I, 1978, 708 and German Pat. No. 2,848,397 disclose that toluenes can be converted selectively to the corresponding benzaldehyde dimethyl acetals by anodic oxidation in the presence of methanol. However, electrochemical oxidation of the toluenes or of the benzaldehyde dialkyl acetals to the corresponding esters takes place with only very little selectivity, even when a very high excess current is used. We have found that aromatic carboxylates of the general formula ##STR1## where R is alkyl of 1 to 4 carbon atoms and R.sup.1 is hydrogen, halogen, alkyl, aryl, hetaryl, alkoxy, aryloxy, acyl, acyloxy or cyano, can particularly advantageously be prepared by electrochemical oxidation of a benzene derivative of the general formula ##STR2## where R.sup.2 is methyl or a radical of the formula --CH(OR).sub.2 and R and R.sup.1 have the above meanings, with an alcohol of the formula ROH, if the electrochemical oxidation is carried out in the presence of a triarylamine compound of the general formula ##STR3## where the two radicals A either are each hydrogen or together form a single bond, X is halogen, H.sub.3 COC-- or NC--, and Y and Z are each hydrogen or halogen. Surprisingly, the novel process gives the carboxylates with good selectivity. In the benzene derivatives of the formula II, R is alkyl of 1 to 4 carbon atoms, preferably methyl or ethyl. Suitable radicals R.sup.1, in addition to hydrogen and halogen, are alkyl radicals, for example those of 1 to 6 carbon atoms. Alkoxy is, for example, methoxy or ethoxy, aryl and aryloxy are, for example, phenyl and phenoxy, and acyl and acyloxy are, for example, --CO--CH.sub.3 and --COOCH.sub.3. Examples of starting materials of the formula II are toluenes, such as toluene, o-, m- and p-xylene, 4-tert.-butyltoluene, 4-methoxytoluene, 4-chlorotoluene or 4-bromotoluene, or benzaldehyde dialkyl acetals, such as benzaldehyde dimethyl acetal, benzaldehyde diethyl acetal, 4-methylbenzaldehyde dimethyl acetal, 4-tert.-butylbenzaldehyde dimethyl acetal, 4-tert.-butoxybenzaldehyde dimethyl acetal, 4-methoxybenzaldehyde dimethyl acetal, 4-bromobenzaldehyde dimethyl acetal or 4-chlorobenzaldehyde dimethyl acetal. The preferred alkanol of the formula ROH is methanol. Triarylamine compounds of the formula III are compounds of the formula ##STR4## They contain as halogen atoms, for example, F, Cl or Br. Examples of compounds of the formula III are tris-(4-bromophenyl)-amine, bis-(4-bromophenyl)-(2,4-dibromophenyl)-amine, bis-(2,4-dibromophenyl)-(4-bromophenyl)-amine, tris-(2,4-dibromophenyl)-amine, tris-(4-chlorophenyl)-amine, bis-(4-chlorophenyl)-(2,4-dichlorophenyl)-amine, bis-(2,4-dichlorophenyl)-(4-chlorophenyl)-amine and tris-(2,4-dichlorophenyl)-amine, of which tris-(2,4-dibromophenyl)-amine and tris-(2,4-dichlorophenyl)-amine are preferred. The novel process does not require any special electrolysis cell, but an unpartitioned continuous-flow cell is preferably used. The anodes employed may be of any conventional anode materials which are stable under the electrolysis conditions, such as noble metals, e.g. gold or platinum. Preferably, graphite or glass-like carbon is used. Suitable cathode materials include graphite, iron, steel, nickel and noble metals, such as platinum. The electrolyte used in the electrochemical oxidation has, for example, the following composition: from 1 to 70% by weight of a starting compound of the formula II from 30 to 96% by weight of an alkanol, with or without a cosolvent, from 0.5 to 5% by weight of a triarylamine compound of the formula III and from 0.5 to 4% by weight of a conductive salt. Suitable conductive salts are those conventionally used in organic electrochemistry, e.g. salts of tetrafluoroboric acid, of alkyl- or arylsulfonic acids, of alkylsulfuric acids and of perchloric acid. In order to increase the solubility of the electron carrier, cosolvents may be added to the electrolyte. Examples of suitable cosolvents are halohydrocarbons, such as methylene chloride, dichloroethane or 1,2-dichloropropane, and nitriles, such as acetonitrile. The cosolvents are added to the alkanol in amounts of, for example, as high as 60 parts by weight per 100 parts by weight of alkanol. Electrolysis is carried out at a current density of from 0.25 to 5, preferably from 0.5 to 3, A/dm.sup.2. The upper limit of the electrolysis temperature is determined by the boiling point of the alkanol or of the cosolvent. Advantageously, electrolysis is effected at, for example, 5.degree.-10.degree. C. below the boiling point of the electrolyte. Where methanol is used, electrolysis is carried out at, for example, no higher than 60.degree. C., preferably from 20.degree. to 60.degree. C. Surprisingly, we have found that the novel process makes it possible to achieve substantial conversion of the benzene derivatives of the formula II without having an adverse effect on the selectivity of the electrochemical oxidation. The reacted mixture from the electrolysis is worked up by a conventional method, advantageously by distillation. Excess alkanol and any cosolvent used are first distilled off, the conductive salt and the triarylamino compound are filtered off, and the aromatic carboxylates are purified by distillation. The alkanol, the cosolvent, the conductive salt and the triarylamino compound can be recycled to the electrolysis. After 2,500 regenerative cycles, no significant loss of triarylamine compound was observed. The carboxylates obtainable by the novel process are scents and intermediates for dyes and drugs.
EXAMPLE 1 Electrochemical synthesis of methyl benzoate ______________________________________Cell: Unpartitioned beaker cell with cooling jacketAnode: Cylinder of glass-like carbon, diameter = 26 mm, height = 50 mm.Cathode: Platinum wireStarting 720 mg (1 millimole) of tris-(2,4-dibromo-materials: phenyl)-amine 920 mg (10 millimoles) of tolueneElectrolyte: 3:1 CH.sub.3 OH/CH.sub.2 Cl.sub.2 ; 1.5% by weight NaClO.sub.4 ; 0.7% by weight of tris-(2,4- dibromophenyl)-amine; 1% by weight of tolueneCurrent density: From 0.5 to 0.7 A/dm.sup.2Electrolysis: Carried out using 15.5 F/mole of tolueneTemperature: 30.degree. C.Working-up The electrolysis solution is evaporatedprocedure: down to half its volume, 20 ml of water are added and the mixture is extracted with pentane in a perforator. The organic phase is dried, the pentane is removed in a rotary evaporator and the products are isolated and purified by distillation in a bulb tube apparatus.Result:Conversion: 80%Yield of methyl benzoate: 1.037 g = 76%Selectivity: 95%.______________________________________ EXAMPLE 2 Electrochemical synthesis of methyl p-methylbenzoate ______________________________________Cell: Unpartitioned beaker cell with cooling jacketAnode: Cylinder of glass-like carbon, diameter = 26 mm; height = 50 mm.Cathode: Platinum wireStarting 720 mg (1 millimole) of tris-(2,4-dibromo-materials: phenyl)-amine 1.06 g (10 millimoles) of p-xyleneElectrolyte: 3:1 CH.sub.3 OH/CH.sub.2 Cl.sub.2 ; 1.5% by weight of NaClO.sub.4 ; 0.7% by weight of tris-(2,4-dibromo- phenyl)-amine; 1% by weight of p-xyleneCurrent density: 0.5 to 0.7 A/dm.sup.2Electrolysis carried out using 9.7 F/mol of p-xyleneTemperature: 30.degree. C.Working-up The electrolysis solution is evaporatedprocedure: down to half its volume, 20 ml of water are added and the mixture is extracted with pentane in a perforator. The organic phase is dried, the pentane is removed in a rotary evaporator and the products are isolated and purifed by distillation in a bulb tube apparatus.Result:Conversion: 95%Yield of methyl p-methylbenzoate: 1.101 g = 73%Selectivity: 77%.______________________________________ EXAMPLE 3 Electrochemical synthesis of methyl 4-tert.-butylbenzoate ______________________________________Cell: Unpartitioned beaker cell with cooling jacketAnode: Cylinder of glass-like carbon, diameter = 26 mm, height = 50 mm.Cathode: Platinum wireStarting 720 mg (1 millimole) of tris-(2,4-dibromo-materials: phenyl)-amine 1.480 g (10 millimoles) of 4-tert.-butyl tolueneElectrolyte: 3:1 CH.sub.3 OH/CH.sub.2 Cl.sub.2, 1.5% by weight of NaClO.sub.4 ; 0.7% by weight of tris- (2,4-dibromophenyl)-amine; 1.5% by weight of 4-tert.-butyltolueneCurrent density: From 0.5 to 0.7 A/dm.sup.2Electrolysis carried out using 11.1 F/mole of 4-tert.- butyltolueneTemperature: 30.degree. C.Working-up The electrolysis solution is evaporatedprocedure: down to half its volume, 20 ml of water are added and the mixture is extracted with pentane in a perforator. The organic phase is dried, the pentane is removed in a rotary evaporator and the products are isolated and purified by distillation in a bulb tube apparatus.Result:Conversion: 98%Yield of methyl 4-tert.-butylbenzoate: 1.382 g = 72%Selectivity: 73%.______________________________________ EXAMPLE 4 Electrochemical synthesis of methyl p-methylbenzoate ______________________________________Cell: Unpartitioned beaker cell with cooling jacketAnode: Cylinder of glass-like carbon, diameter = 26 mm, height = 50 mm.Cathode: Platinum wireStarting 720 mg (1 millimole) of tris-(2,4-dibromo-materials: phenyl)-amine 1.66 g (10 millimoles) of 4-methylbenzalde- hyde dimethyl acetalElectrolyte: 3:1 CH.sub.3 OH/CH.sub.2 Cl.sub.2 ; 1.5% by weight of NaClO.sub.4 ; 0.7% by weight of tris- (2,4-dibromophenyl)-amine; 1.6% by weight of 4-methylbenzaldehyde dimethyl acetalCurrent density: From 0.5 to 0.7 A/dm.sup.2Electrolysis carried out using 3.3 F/mole of 4-methyl- benzaldehyde dimethyl acetalTemperature: 30.degree. C.Working-up The electrolysis solution is evaporatedprocedure: down to half its volume, 20 ml of water are added and the mixutre is extracted with pentane in a perforator. The organic phase is dried, the pentane is removed in a rotary evaporator and the products are isolated and purified by distillation in a bulb tube apparatus.Result:Conversion: 87%Yield of methyl p-methylbenzoate: 1.28 g = 85%Selectivity: 98%.______________________________________ EXAMPLE 5 Electrochemical synthesis of methyl 4-tert.-butoxybenzoate ______________________________________Cell: Unpartitioned beaker cell with cooling jacketAnode: Cylinder of glass-like carbon, diameter = 26 mm, height = 50 mm.Cathode: Platinum wireStarting 720 mg (1 millimole) of tris-(2,4-dibromo-materials: phenyl)-amine 2.24 g (10 millimoles) of 4-tert.-butoxy- benzaldehyde dimethyl acetalElectrolyte: 3:1 CH.sub.3 OH/CH.sub.2 Cl.sub.2 ; 1.5% by weight of NaClO.sub.4 ; 0.7% by weight of tris- (2,4-dibromophenyl)-amine; 2.2% by weight of 4-tert.-butoxybenzaldehyde dimethyl acetalCurrent density: From 0.5 to 0.7 A/dm.sup.2Electrolysis carried out using 4 F/mole of 4-tert.- butoxybenzaldehyde dimethyl acetalTemperature: 30.degree. C.Working-up The electrolysis solution is evaporatedprocedure: down to half its volume, 20 ml of water are added and the mixture is extracted with pentane in a perforator. The organic phase is dried, the pentane is removed in a rotary evaporator and the products are isolated and purified by distillation in a bulb tube apparatus.Result:Conversion: 96%Yield of methyl 4-tert.-butoxybenzoate: 1.86 g = 89%Selectivity: 93%.______________________________________ EXAMPLE 6 (comparative experiment) Electrochemical synthesis of methyl p-methylbenzoate ______________________________________Cell: Unpartitioned beaker cell containing 11 bipolar graphite electrodeAnode: GraphiteCathode: GraphiteElectrolyte: 3204 g of CH.sub.3 OH 360 g (2.17 moles) of 4-methylbenzaldehyde dimethyl acetal 36 g of KSO.sub.3 C.sub.6 H.sub.5Current density: 3.3 A/dm.sup.2Electrolysis carried out using 10 F/mole of 4-methyl- benzaldehyde dimethyl acetalTemperature: from 25 to 30.degree. C. The electrolyte is pumped through a heat exchanger at a rate of 200 l/h during the electrolysis.Working-up When the electrolysis is complete,procedure: methanol is distilled off under atmospheric pressure, the conductive salt is filtered off and the filtrate is subjected to fractional distillation under 2 mbar and at from 73 to 152.degree. C. This gives 56.6 g of unconverted 4-methylbenzaldehyde dimethyl acetal as well as 21 g of methyl 4-methylbenzoate.Result:Conversion: 84%Yield of methyl 4-methylbenzoate: 6%Selectivity: 8%.______________________________________
Claims
1. A process for the preparation of an aromatic carboxylate of the formula ##STR5## where R is alkyl of 1 to 4 carbon atoms and R.sup.1 is hydrogen, halogen, alkyl, aryl, hetaryl, alkoxy, aryloxy, acyl, acyloxy or cyano, wherein a benzene derivative of the formula ##STR6## where R.sup.2 is methyl or a radical of the formula --CH(OR).sub.2 and R and R.sup.1 have the above meanings, is subjected to electrolysis with an alcohol of the formula ROH in the presence of a triarylamine compound of the formula ##STR7## where the two radicals A either are each hydrogen or together form a single bond, X is halogen, H.sub.3 COC-- or NC--, and Y and Z are each hydrogen or halogen, at a current density of from 0.25 to 5 A/dm.sup.2 and at 5.degree.-10.degree. C. below the boiling point of the alcohol.
2. A process as claimed in claim 1, wherein the triarylamine compound used is tris-(2,4-dibromophenyl)-amine or tris-(2,4-dichlorophenyl)-amine.
3. A process as claimed in claim 1, wherein the electrolyte used contains from 1 to 70% by weight of a benzene derivative of the formula II, from 30 to 96% by weight of an alkanol, with or without a cosolvent, from 0.5 to 5% by weight of a triarylamine compound and from 0.5 to 4% by weight of a conductive salt.
4. A process as claimed in claim 1, wherein the alcohol used is methanol, and electrolysis is carried out at not more than 60.degree. C.
Patent Grant
4612092
