Claisen-Tishchenko reaction

The Tishchenko reaction is more than 100 years old and, with its different variants, has grown over the decades into a niche method for the selective production of dimeric esters from two aldehyde molecules. The reaction also known as the Claisen-Tishchenko reaction, occurs via a Lewis acid-catalysed disproportionation reaction involving a hydride shift (the rate determining step) as depicted in Scheme 18.23. Catalysts with higher Lewis acidity (like pure aluminium(m) alkoxides) afford in most cases the esters. 

A reaction mechanism quite similar to the Claisen-Tishchenko and Meerwein-Pondorf-Verley reactions assumes tetravalent aluminum in an anionic complex and is more likely to correspond to the requirements. 

If, while formulating the theory, the valency angles are considered, and the alkoxy group is marked as a chain, the relationship to the six-ring complexes (4) formulated in the reaction of Meerwein-Pondorf-Verley and Claisen-Tishchenko is at once clear. 

Rates for competing processes such as self-condensation of cyclohexanone and Cannizzaro or Tishchenko reactions of benzaldehyde are negligible compared with the Claisen-Schmidt condensation of aldehyde with ketone. 

Alkali metal t-butoxides, hydrides and bis(TMS)amides efficiently catalyse Claisen-Tishchenko disproportionation of aldehydes to the corresponding carboxylic esters. Potassium bases were more effective than sodium, and 18-crown-6 further accelerates the reaction. Kinetic studies suggest that the rate-determining step is a second-order concerted hydride transfer from a potassium hemiacetal to another molecule of aldehyde. 

When this polymerization reaction was studied for monomers such as 2,3-epoxypropanal (above) or 2,3-epoxybutanal, however, an unusual Initiation step was found to take place by the Tishchenko-Claisen reaction, which is a well-known disproportionation reaction of aldehydes to esters, and an intermediate product, the dlepoxyester was identified. This intermediate product on polymerization formed, up to about 20% conversion, linear polyethers with both oxlrane and ester side groups by an epoxy ring-opening reaction, as follows ... 

A modification of the Cannizzaro reaction was discovered by Claisen [158] and later extended by Tishchenko [159]. It involves the use of sodium or aluminum alkoxides in the conversion of aliphatic and aromatic aldehydes into the corresponding esters [160]. Other catalysts, such as boric add [161], superoxide ion/crown ether [162] and metal catalysts [163] have also been applied in this oxidation reaction. 

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