Reactions homo-benzoin

Enders and coworkers reported moderate to good enantioselectivity for the coupling of heteroaromatic aldehydes and highly electrophiUc trifluoromethyl ketones in the presence of chiral triazohum salt 41 (Scheme 18.5) [30]. It was shown that a reversible homo-benzoin reaction takes place initiaUy, foUowed by an irreversible cross-benzoin reaction involving the ketone (40). 

An enantioselective aza-benzoin reaction of enals with activated ketimines employs an NHC catalyst incorporation of appropriate steric hindrance in the catalyst blocks competing reaction through the homo-enolate route. ... 

The chemo- and enantioselective cross-couphng of two different aldehydes via a benzoin reaction remains a significant challenge. Four different benzoin products can be formed (two homo- and two cross-benzoin products), each as two possible enantiomers. The challenge is compounded by the commonly observed reversibility of the reaction. 

Benzaldehyde lyase (BAL) from Pseudomonas fluorescens (Table 10.4, entries 6-15) is one of the most efficient catalysts for the homo- and cross-carboligation reaction of aromatic and aliphatic aldehydes, because of its broad substrate range and its high (P)-stereoselectivity [14,57,58,65]. The ability of BAL to catalyze the cleavage of benzoins can also be instrumental in the resolution of racemic mixtures of these compounds [65]. 

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