Dramatic rate enhancement with preservation of stereospecificity in the first metal-catalyzed additions of allylboronates.

This communication successfully challenges the perception that the additions of allylbonates to aldehydes cannot be catalyzed effectively by added Lewis acids. Using a novel class of isomerically pure, tetrasubstituted 2-alkoxycarbonyl allylboronates (1), we describe that some metals (for example, Sc(OTf)(3) and Cu(OTf)(2)) allow these reagents to add onto aldehydes to yield gamma-lactone products 2 in good yields at temperatures almost 100 degrees C lower than the corresponding uncatalyzed reactions. The large rate enhancement over the uncatalyzed reaction provides a highly improved practical approach to access aldol-like adducts with a stereogenic quaternary carbon center. The crucial role of the 2-alkoxycarbonyl group on allylboronates 1 was demonstrated with control experiments using a model allylboronate lacking such an ester group. Moreover, the stereospecificity observed in the uncatalyzed allylborations is preserved. These observations raise intriguing mechanistic issues such as the suggestion that type I allylmetal behavior is maintained in this unprecedented catalytic reaction manifold.