The Activation of Strong Chemical Bonds Using Photoredox Catalysis

Abstract

Due to their stability, strong chemical bonds are ubiquitous in organic molecules. Consequently, the ability to activate these bonds provides a direct avenue for derivatizing and streamlining synthesis. Radical and/or odd-electron transition-metal intermediates are often leveraged to promote the activation of such strong chemical bonds. Recently, photoredox catalysis has served as a power method for shuttling electrons and holes to promote the production of carbon centered radicals. This work describes our approach to the activation of inert C(sp3)–H bonds of alkanes and the C–OH bonds of carboxylic acids. While C(sp3)–H bond derivatizations have proven effective for installing heteroatoms, only a handful of strategies have enabled the one-step formation of C(sp3)–C bonds. Moreover, of those methods, only a limited number are capable of aliphatic bond activation using stoichiometric quantities of substrate which may be difficult to access in large quantities in late-stage synthesis. Our approach to this problem leverages the photocatalytic generation of chlorine radical from a proposed Ni(III)–Cl intermediate. This chlorine radical then serves as a potent hydrogen atom acceptor for functionalizing alkanes. Key to the generation of chlorine radical is the pairing of a nickel catalyst with an Ir-based photoredox catalyst to regulate the oxidation states of nickel. This platform was used with acyl chloride derivatives to synthesis ketone containing products. Like C(sp3)–H bonds, carboxylic acids are an abundant and difficult to activate functional group in synthesis. The generation of acyl radicals from carboxylic acids has been accomplished using multiple derivatization steps involving the reduction of acids to aldehydes or the incorporation of redox mediators. Although enabling, these multistep reactions hinder the redox economy of accessing acyl radicals directly. In this challenge, our group leveraged photoredox catalysis to access electrophilic phosphine radical cations to generate phosphoranyl radicals through nucleophilic capture using carboxylic acids. Owing to the strong thermodynamic driving force to generate phosphine oxide, these species are capable of liberating acyl radicals through a fragmentation event known as -scission. These acyl radicals were then used for the hydroacylation of olefins to generate aliphatic ketones.