Recent Advances in the Activation of Boron and Silicon Reagents for Stereocontrolled Allylation Reactions

Despite the popularity of boron and silicon allylation reagents in stereocontrolled synthesis, they suffer from a number of inherent limitations that have slowed down their development as synthetic tools for nucleophilic additions to carbonyl compounds and imine derivatives. These limitations are the low reactivity and diastereoselectivity of allyl trialkylsilane reagents, and the lack of catalytic systems for the activation and substoichiometric control of enantioselectivity in the additions of allyl boron reagents. To develop more efficient and general methods for the control of absolute stereochemistry in the resulting homoallylic alcohols, new approaches aimed at solving the problem of activation of allylic boron and silicon reagents are needed. This Minireview describes a number of recent approaches that have been devised to address this problem. The activation of allyl boron and allyl silicon reagents is a subject of much current interest. Recent strategies are based on increasing the reactivity of the reagents by varying the remaining substituents on the boron or silicon center, or by using a Lewis acid or a Lewis base additive to accelerate reactions with carbonyl compounds or imine derivatives to make homoallylic alcohols and amines in a stereocontrolled fashion (see scheme).