Enantioconvergent carbenoid insertion into carbon−boron bonds

Boron-mediated homologation can potentially access almost any kind of chiral centers from readily available boronates via asymmetric carbenoid insertion, followed by versatile transformations of the carbon−boron bonds. However, the current asymmetric boron homologation strategies exhibit limitations, and enantioselective insertion of diversely substituted carbenoids remains challenging. Here we report an enantioconvergent approach for direct insertion of carbon-, oxygen-, nitrogen-, sulfur-, and silicon-substituted carbenoids into carbon−boron bonds. The excellent enantioselectivity is enabled by a new class of chiral oxazaborolidines derived from inexpensive α-amino esters. Computational studies revealed that the non-C2-symmetric oxazaborolidine features a puckered geometry and the cooperative effects of multiple substituents create an asymmetric environment for effective enantioinduction. This method is scalable, and each chiral center can be independently controlled by the chiral oxazaborolidine without being influenced by nearby stereocenters. Besides forming singular chiral centers, iterative operations of this asymmetric homologation simplify synthesis of complex molecules with multiple stereocenters.