Catalyst-Enabled Stereodivergence in Photochemical Atom Transfer Radical Addition (ATRA) of α-Iodoboronic Esters to Alkynes

The atom transfer radical addition (ATRA) to alkynes is a valuable strategy for the synthesis of allylic substituted molecules, yet it has not been applied to the synthesis of allylic boronic acids or their esters, which are important building blocks in organic synthesis. The inherent challenge for ATRA reactions is control of the geometric selectivity. By employing different alkene geometric isomerization techniques, namely, photochemical uphill catalysis and manganese-catalyzed halogen-abstraction/radical rebound processes, we are able to synthesize both isomers of iodinated allylic boronic esters in a stereoselective manner. Mechanistic investigations reveal the dual functionalities of both catalysts, acting as catalysts for both ATRA and geometric isomerization processes. The protocols feature a broad substrate scope and good functional group tolerance. Importantly, the iodo and boryl moieties within the products provide orthogonal handles for further synthetic manipulations. The strategy employed here should inspire more efforts toward stereoselective ATRA reactions of alkynes.