Photoinduced intermolecular hydrogen atom transfer reactions in organic synthesis

Abstract

Hydrogen-atom transfer (HAT) provides straightforward methods to generate open-shell radical intermediates from R-H (R = C, Si, etc.) bonds and offers unique opportunities for green and sustainable synthesis. Traditional HAT protocols required harsh conditions and relied on the use of harmful reagents such as Cl 2 and peroxides. An emerging strategy is photoinduced intermolecular HAT, in which transformations can be driven by photocatalysis under mild conditions. In recent years, photoinduced intermolecular HAT reactions have seen substantial development of their versatility, efficiency, and selectivity. This review summarizes recent advances (up to December 2020) in this rapidly expanding research area. The representative examples provided are classified according to the active species responsible for hydrogen atom abstraction. The reactivity, selectivity, and established transformations for each type of active species are briefly summarized. This review aims to provide guidance for the application of photoinduced HAT in R-H functionalization reactions and to inspire further progress in this research area. The direct functionalization of unactivated R-H (R = C, Si, etc.) bonds can facilitate and expedite the synthesis of natural products, pharmaceuticals, and agrochemicals. Photoinduced intermolecular hydrogen atom transfer (HAT) catalysis is an appealing approach toward this goal. There has been growing interest in photoinduced intermolecular HAT in the last decade, and this has led to the development of atom- and step-economic synthetic protocols using abundant feedstocks as starting materials. In drug discovery, late-stage C-H functionalization based on photoinduced intermolecular HAT provides access to structural analogs of targets without the need for de novo synthesis. In material science, photoinduced HAT reactions offer new handles for material synthesis and post-modification. Photoinduced HAT catalysis has become a ubiquitous and prevailing research area in synthetic chemistry. Cao et al. provide an overview of recent advances in photoinduced intermolecular hydrogen atom transfer (HAT) reactions for organic synthesis. The reactivity, selectivity, and established transformations for different hydrogen atom abstractors are summarized. General guidance on the use of photoinduced HAT catalytic systems for various R-H bond functionalization’s is provided. Applications in enantioselective synthesis, heterogeneous catalysis, drug discovery, and material science are also briefly discussed.