Tishchenko-type reactions

Hydroxy ketones can be reduced with aldehydes in a Tishchenko-type reaction, (101) —> (102), using a zirconocene catalyst.165 The reaction provides a stereoselective route to a //-l,3-diols. A labelling experiment with CpZrE and MeCDO indicated that the aldehydic hydrogen ends up on the alcohol carbon. The kinetic isotope effect of ca 1.8 suggests that hydride transfer may be rate limiting. 

Tishchenko-Type Reactions of Aldehydes with 1-BU2AISR. 96... 

A new method of synthesizing thiol, selenol, tellurol esters from aldehydes via a Tishchenko-type reaction using diisobutylaluminum chalcogenoate (/-BU2AIYR, Y=S, Se, Te) has been reported it is shown in Eq. 12 [17]. 

RuH2(PPh3)4 reacts with aldehydes to give esters via Tishchenko-type dimerization. For example, benzaldehyde is converted to benzyl benzoate by RuH2(PPh3>4(eq (45)) [166-167]. This reaction involves C—H bond activation of the formyl proton followed by formation of a ruthenium acyl alkoxide complex Ru(OCH2Ph)(COPh)(PPh3)4. 

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. 

Chiral lithium diphenylbinaphtholate (2) has been found to be an effective catalyst for the enantioselective aldol-Tishchenko reaction, affording 1,3-diol derivatives with three contiguous chiral centres and high stereoselectivities (up to 99% ee) ° A direct, highly enantioselective alkylation of arylacetic acids via enediolates using a readily available chiral lithium amide (3) as a stereodirecting reagent has been developed. This approach circumvents the traditional attachment and removal of chiral auxiliaries used currently for this type of transformation. 

A related intramolecular reaction of this type is the so-called Evans-Tishchenko reaction [166]. Here, a P-hydroxy ketone is reduced in the presence of an aldehyde yielding 1,3-diol monoesters. Several metal catalysts such as samarium iodide [166, 167] and zirconocene complexes [168] are effective. The reaction is highly diastereo-... 

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