Organoboronic Acids: A Chance for Improving Photochemistry

Abstract: Photoredox catalysis as a powerful strategy for the activation of small molecules requires the use of reactants which are suitable to undergo single-electron transfer with the formation of radicals. In this perspective, we highlight the unique ability of organoboronic acid derivatives to form radicals under photoredox catalysis conditions with particular emphasis placed on the methods of activation of B-C bonds. Key to facilitate the activation relies on use of easily oxidizable organoboronate complexes: organotrifluoroborates, alkoxyorganoboronates or nitrogen-containing Lewis base adducts. The low reduction potentials of these compounds allows facile single-electron oxidation to generate non-stabilized alkyl radicals, including primary radicals, under mild conditions. The use of molecular oxygen is also a common way to activate boronic acids in photocatalytic reactions. The role of the oxygen relies on addition of the oxygen anion-radical formed in the photocatalytic cycle to the boron center affording the easily oxidizable boronate adduct. Amongst the variety of activation methods, the protocols avoiding the use of external Lewis base are especially valuable. This has been demonstrated in visible-light-mediated alkenylation of alkylboronic acids using alkenylsulfones as coupling partners. The radical species resulting from alkylboronic acid derivatives could be utilized in the formation of C–X or C–C bonds including enantioselective photoreactions. The application of boronic acids in the visible light-driven installation of side chains at dehydroalanine residues in proteins shows the increasing role of these compounds in future syntheses of complex natural products.