Enantioselective α-Bromination of Aldehydes and Ketones with Simple Organic Catalysts

Significance

The first organocatalytic enantioselective α-bromination of aldehydes and cyclic ketones is described. Since previously reported conditions for the α-chlorination of aldehydes (J. Am. Chem. Soc. 2004, 126, 4790-4791) proved unsuitable for the bromination using NBS, 4,4-dibromo-2,6-di-tert-butyl-cyclohexa-2,5-dienone is utilized as brominating agent. Aldehyde brominations are mediated by C ₂-symmetric (2R,5R)-diphenylpyrrolidine, and the addition of small amounts of acid increases both yield and enan­tioselectivities. Cyclic ketones are reacted under catalysis of C ₂-symmetric 4,5-diphenyl-imidazolidine. Due to the volatility of the generated α-functionalized carbonyls, the products were isolated as the corresponding alcohols after NaBH₄ reduction. An example of an enantioselective α-iodination is also given.