Rh-Catalyzed AldehydeAldehyde Cross-Aldol Reaction under Base-Free Conditions: In Situ Aldehyde-Derived Enolate Formation through Orthogonal Activation

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. Base‐free and easy: A Rh/1,1′‐bis(diisopropylphosphino)ferrocene catalyst generated aldehyde‐derived enol boronates in a one‐pot isomerization/cross‐aldol sequence to afford aldehydealdehyde cross‐aldol adducts in good yield with syn selectivity.