Mechanistic Investigation of Intermolecular Reductive C–H PCET via Isotopic Fractionation

Abstract

In this thesis, preliminary experimental evidence in support of a possible concerted intermolecular C–H Multi-site Proton-Coupled Electron Transfer (MS-PCET) elementary step is described. While oxidative C–H MS-PCET reactions have been investigated recently in various contexts for homolytic cleavage of strong C–H bonds, many limitations still exist for the application of such an underutilized elementary step. Previous work by the Knowles lab featured a photoredox C–H alkylation reaction featuring an oxidative C–H MS-PCET step between the substrate and a noncovalent complex between the Ir photocatalyst and the phosphate base. However, reductive C–H MS-PCET as an elementary step has been uncommon especially in an intermolecular context, as a result of the precise orientation of the proton required for efficient C–H bond formation. In this thesis, the mechanism of photocatalyzed decarboxylation of potassium triphenyl acetates is investigated, with the emphasis on distinguishing between stepwise electron transfer/proton transfer (ET/PT) and intermolecular MS-PCET for C–H bond formation via isotopic fractionation. Isotopic fractionation is a highly sensitive method to distinguish between concerted MS-PCET and stepwise ET/PT, as the fractionation slope of ET/PT is independent of the redox potential of the photocatalyst. Previous research in the Knowles group identified that in decarboxylation of triphenyl acetate anions, the H/D isotopic fractionation slope was dependent on the identity of the Ir photocatalyst. In this thesis, the dependence is further explored with a range of photoredox catalysts covering a wide range of E°Red (PC/PC-), where a transition between a slope of 1.7 ± 0.1 and 2.64 ± 0.07 with decrease in magnitude of E°Red (PC/PC-) providing support for a transition from ET/PT to a concerted MS-PCET mechanism. Thus, this thesis provides preliminary mechanistic evidence in support of the viability of intermolecular reductive C–H MS-PCET as a novel elementary step.