Tischtchenko reactions

The mechanism of the Tischtchenko reaction was discussed by Lin and Day who proposed that in the initial stages, the aluminium atom of aluminium alkoxide coordinates with the carbonyl group of the aldehyde (Eq. 2.329),... 

Mixed Tischtchenko reactions were also studied by Nord. In these reactions he isolated benzyl isovalerate and isoamyl benzoate from benzaldehyde and isovaleralde-hyde as well as benzyl acetate, ethyl acetate, and benzyl benzoate from benzaldehyde and acetaldehyde. Orloff also reported the synthesis of benzyl isobutyrate and isobutyl benzoate from benzaldehyde and isobutyraldehyde. However, the mechanism of mixed Tischtchenko reactions is rather complex. For example, in the reaction of benzaldehyde and -butyraldehyde, either of the carbonyl groups may coordinate with the aluminium alkoxide. In fact the carbonium ion formed in butyraldehyde is more active than that in benzaldehyde and further reaction may proceed with the carbonium ion of the former type. The overall mechanism may be explained by the following equations (2.331)-(2.334) ... 

However, in actual practice the yield of n-butyl benzoate was appreciably low. Thus on the basis of experimental data, it appears that the aldehyde which readily undergoes Tischtchenko reaction, forms the ester in larger amounts. It may be due to the fact that either of the mixed esters predominates over the other and thus eliminates the possibility of ester exchange between two simple esters. It was also observed that the yields of mixed ester could be increased by maintaining a lower concentration of the faster reacting aldehyde. However, with the slower reacting aldehyde such as an a,fi-unsaturated aldehyde, if the faster reacting aldehyde is added at a considerably slower rate, the polymerization of unsaturated aldehyde predominates in the reaction. Lin and Day, however, also observed that the formation of mixed ester was more effective in the presence of aluminium isopropoxide catalyst. 

The formation of esters by the Tischtchenko reaction was also demonstrated by Nakai who showed that magnesium dimethoxide or aluminium triethoxide were effective catalysts. 

Alnminium alkoxides have long been used as catalysts for the reduction of aldehydes or ketones by secondary alcohols and recently lanthanide alkoxides were reported to act similarly in addition to catalysing the epoxidation of allylic alcohols with tert-butyl hydroperoxide. Aluminium alkoxides have also been used to catalyse the conversion of aldehydes to alkylesters (Tischtchenko reaction). A review gives a fascinating account of the use of heterometal alkoxides in asymmetric catalysis. 

Tischtchenko, Meerwein-Ponndorf-Verley, and Oppenauer oxidation reactions (Section 4.15)... 

Metal alkoxides are known to act as potential catalysts (Chapter 7, Section 4) and this property has been utilized successfully in organic syntheses. Before dealing with the Tischtchenko, Meerwein-Ponndorf-Verley, and Oppenauer oxidation reactions, it is worth briefly mentioning the Cannizzaro reaction which appears to be the main basis... 

Claisen in 1887 modified the Cannizzaro reaction by using sodium ethoxide in place of sodium hydroxide, and found that benzaldeyde was converted to benzyl benzoate. In 1906 Tischtchenko observed that sodium alkoxides can be used for the formation of esters from both aliphatic and aromatic aldehydes. Nord et al so jqi... 

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