Rh-catalyzed aldehyde-aldehyde cross-aldol reaction under base-free conditions: in situ aldehyde-derived enolate formation through orthogonal activation

Abstract

The chemoselective generation of aldehyde-derived enolates to realize an aldehyde-aldehyde cross-aldol reaction is described. A combined Rh/dippf system efficiently promoted the isomerization/aldol sequence by using primary allylic, homoallylic, and bishomoallylic alcohols; secondary allylic and homoallylic alcohols; and trialkoxyboranes that were derived from primary allylic and homoallylic alcohols. The reaction proceeded at ambient temperature under base-free conditions, thus giving cross-aldol products with high chemoselectivity. Mechanistic studies, as well as its application to double-aldol processes under protecting-group-free conditions, are also described.

Keywords: aldol reaction; boron; homogeneous catalysis; rhodium; synthetic methods.