Tishchenko aldehyde-ester disproportionation

The Tishchenko Reaction is a disproportionation reaction that allows the preparation of esters from two equivalents of an 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 ... 

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. 

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. 

As mentioned above, hydrogen atoms, removed from the alcohol substrate, can return to form the product however, if the final hydrogenation step could not occur, a product that is more oxidized than the starting material is obtained. The formation of esters from alcohols and of amides from alcohols and amines concern the most representative and studied reactions of this type. In these cases, aldehydes, formed on the first oxidation stage from alcohols, undergo Tishchenko- and Cannizzaro-type reactions, where esters or carboxylates and alcohols are formed upon fusion or disproportionation of aldehydes, respectively. 

A similar strategy is employed when using the Evans-Tishchenko reaction [81]. The Tishchenko reaction that is first described in 1906 uses a Lewis acid-mediated disproportionation of two carbonyl moieties to form an ester. In 1990, Evans and Hoveyda reported on their variant, which involves a preformed p-hydroxy ketone and an aldehyde... 

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