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

• Published in: Angewandte Chemie International Edition 2020-04, Vol.59 (16), p.6525-6528
• Main Authors: Davenport, Raffael, Silvi, Mattia, Noble, Adam, Hosni, Zied, Fey, Natalie, Aggarwal, Varinder K.
• Format: Article
• Language: English
• Subjects: Boron chemistry; Contraction; Cyclobutanes; Esters; Functional groups; Metalate rearrangement; Oxidation; Photoredox catalysis; Ring contraction
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201915409

There are a limited number of ring‐contraction methodologies which convert readily available five‐membered rings into strained four‐membered rings. Here we report a photo‐induced radical‐mediated ring contraction of five‐membered‐ring alkenyl boronate complexes into cyclobutanes. The process involves the 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 benzofused cyclobutenes and the versatility of the boronic ester was demonstrated by conversion to other functional groups. React to contract: Easily accessible cyclic vinyl boronate complexes undergo a strain‐increase ring contraction through a reaction with electrophilic radicals generated by visible light. A variety of cyclobutyl boronic esters can be obtained in high yields and with excellent stereocontrol, including those containing contiguous quaternary stereocenters and also substantially more strained benzofused cyclobutenes.