Annulation of Allylsilanes and Bicyclo[1.1.0]butanes with Lewis Acids

Bridged bicyclic scaffolds are of increasing importance as bioisostere of benzenes in medicinal chemistry. Herein, we describe Lewis acid-catalyzed [2π+2σ] and [4π+2σ] formal cycloadditions between allylsilanes or 2-vinyl allylsilanes and BCBs, affording functionalized bicyclo[2.1.1]hexanes (BCHs) and bicyclo[4.1.1]octanes (BCOs). This reaction features broad substrate scope, operational simplicity and mild conditions. Mechanistic studies revealed a stepwise process, which was initiated by nucleophilic addition of allylsilanes to Lewis acid activated BCBs to form a zwitterionic intermediate followed by a cyclization step. Notably, due to the straightforward transformation of silyl groups to hydrogen and other functionalities under mild conditions, this reaction provides a formal cycloaddition of simple alkenes and BCBs. Consequently, this reaction significantly expands the synthetic access to bridged bicycles.