Visible Light-Driven Strain-Increase Ring Contraction Allows the Synthesis of Cyclobutyl Boronic Esters

There are a limited number of ring contraction methodologies which convert readily available 5‐membered rings into strained 4‐membered rings. Here we report a photo‐induced radical‐mediated ring contraction of 5‐membered ring alkenyl boronate complexes into cyclobutanes. The process involves addition of an electrophilic radical to the electron‐rich alkenyl boronate complex, leading to an α‐boryl radical. Upon one‐electron oxidation, ring‐contractive 1,2‐metalate rearrangement occurs to give a cyclobutyl boronic ester. A range of radical precursors and vinyl boronates can be employed, and chiral cyclobutanes can be accessed with high levels of stereocontrol. The process was extended to the preparation of benzo‐fused cyclobutenes and the versatility of the boronic ester was demonstrated by conversion to other functional groups.