Synthetic Applications of Photochemically Generated Radicals from Protic C(sp³)−H Bonds

Abstract

The utilization of photo‐induced processes in C–H functionalization via radical pathways has emerged as a highly promising strategy for the preparation and modification of complex organic compounds. While current methods for generating carbon‐centred radicals from C–H bonds primarily focus on hydridic C–H bonds to yield nucleophilic radical species, the reactivity and potential applications of electrophilic radicals derived from protic C–H bonds remain largely unexplored. In this review, we aim to shed light on the seminal findings regarding the activation of protic C(sp3)–H bonds while also showcasing noteworthy examples of this radical formation process. Mechanistically diverse modes of activation are discussed, unified by proton‐coupled electron transfer (PCET) concepts.

Reduction of CO to Methanol with Recyclable Organic Hydrides

“…An alternative pathway to a formal hydride transfer is the reversible transfer of an electron followed by a consecutive proton-coupled electron transfer (ET-PCET) according to eq Ru − CO 2 + + BIH .25ex2ex k ET ⇌ k ‐ ET Ru − CO + + BIH • + .25ex2ex k PCET ⇌ k ‐ PCET Ru − CHO + + BI + where k ET and k –ET are the forward and reverse second-order rate constants for electron transfer from the organohydride to Ru–CO 2+ , respectively, and k PCET is the rate constant for proton-coupled electron transfer from the BIH •+ radical cation to the monoreduced cis -[Ru(bpy) 2 (CO) 2 ] + (abbreviated herein as Ru–CO + ). Examples of formal hydride transfer that have been characterized by the ET-PCET mechanistic pathway are abundant in the literature. , Experimental evidence combined with DFT calculations suggests an ET-PCET mechanism. For example, when the reaction was carried out in air, no formyl product was generated and instead decomposition products were observed.…”

“…Examples of formal hydride transfer that have been characterized by the ET-PCET mechanistic pathway are abundant in the literature. 63,64 Experimental evidence combined with DFT calculations suggests an ET-PCET mechanism. For example, when the reaction was carried out in air, no formyl product was generated and instead decomposition products were observed.…”