Title:Organoboronic Acids: A Chance for Improving Photochemistry
Volume: 26
Issue: 4
Author(s): Tomasz Kliś*
Affiliation:
- Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
Keywords:
Photocatalysis, radicals, boronic acids, alkyltrifluoroborates, alkoxyorganoborates, 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.
