Direct a-Chlorination of Aldehydes and Ketones

The mechanism for the a-chlorination of aldehydes catalyzed by the C2-symmetric (2R,5R)-diphenylpyrrolidine 3h is probably different from that out- 

Ketones have also been enantioselectively a-chlorinated, although the catalysts used for the a-chlorination of aldehydes were found not to be optimal for the functionalization of these substrates. Poor yields and moderate enantiomeric excess were observed with, for example, (2P,5P)-diphenylpyrrolidine 3h and i-proline amide 3i due also to poly-chlorination of the starting material [28]. A catalyst screening revealed that the C2-symmetric 4,5-diphenylimidazoline 3j constitutes, in combination with 2-nitrobenzoic acid as a rate-accelerating additive, an efficient catalytic system for the a-chlorination of both cyclic and acyclic ketones 

30 using NCS 29 as the chlorinating reagent (Eq. 7) [28]. The scope of the direct enantioselective a-chlorination showed that the optically active a-chloro ketones 

31 were obtained in 86-98% ee, with the highest enantioselectivity for the cyclic ketones. 

Various transformations of the optically active a-chloro ketones were also presented [28] optically active 2-chloro cyclohexanone was oxidized by a Baeyer-Villiger oxidation to the corresponding lactone in 81% yield without reduction in enantiomeric excess, and the ketone functionality was selectively reduced to the corresponding syn-a-chloro alcohol with a high diastereomeric ratio. 

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