Tischtschenko reaction

Normally, Oppenauer oxidations are performed employing Al3+ cations as catalyst because aluminium alkoxides possess a good balance of a desired high hydride transfer capability versus a low propensity to promote undesired base-induced reactions, like aldol condensations and Tischtschenko reactions. In the reaction, as originally described by Oppenauer, aluminium t-butoxide is used as catalyst,4 because its high basicity allows a very favourable equilibrium towards the formation of the aluminium alkoxide of the alcohol whose oxidation is desired. However,... 

Tischtschenko Reaction. This involves the reaction between two molecules of an aldehyde which may or may not have of-hydro-gen, accompanied by oxidation of one carbonyl to carboxyl and the reduction of the other carbonyl to alcohol. The product is an ester. The reaction is catalyzed by alkoxides. 

Trimethylphenoxymagnesium bromide catalyzes aldol condensation of aliphatic aldehydes to form a,3 unsaturated aldehydes in excellent yield if the reaction is carried out in benzene (Scheme 1). In hexamethylphosphoric triamide (HMPA), the diol monoesters (1) and (2) are produced, also in nearly quantitative yield. Compounds (1) and (2) arise from a process known as the Tischtschenko reaction vide infra). 

Hydride transfer is also involved in the Tischtschenko reaction, wherein an aliphatic aldehyde is dimerized under the influence of aluminum tri-r-butoxide to the corresponding ester (equation 15). Tischtschenko reaction products are sometimes observed as side products in aldol reactions. Under proper conditions, rather complex esters may be prepared in excellent yield (e.g. equations 16 and 17). ... 

The redox-neutral dimerization of aldehydes to form esters known as the Tischenko (or Tischtschenko) reaction may be catalyzed by group 2 hexamethyldisilazides [M N(SiMe3)2 2(THF)2] (M = Ca, Sr, and Ba), bio-inspired magnesium thiolate complexes or with bicyclic guanidinate-stabilized magnesium amide complexes (Scheme 23) [139-142]. 

The most common side reactions during Oppenauer oxidation consist of base-induced condensations of the aldehyde or ketone, generated during the oxidation, with the carbonyl compound used as oxidant.65 This side reaction is particularly prominent during the obtention of aldehydes because they are generally more reactive in aldol condensations than ketones. Furthennore, aldehydes very often suffer Tischtschenko condensations,66 resulting in the formation of dimeric esters during Oppenauer oxidations. That is why, the Oppenauer oxidation is seldom useful for the preparation of aldehydes. 

Benzaldehyde undergoes Tischtschenko type reaction to produce benzylbezoate over alkaline earth oxides[30]. The catalytic activity correlates well with the amount of basic sites, and the basic sites are believed to be active sites. The reaction proceeds as follows. The slow step of the reaction is H- transfer from (I) to (II) In this reaction, not only basic sites (0 ions) but also acidic sites (metal cations) are participating as shown above. 

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