Photocatalytic, Scalable Anti-Markovnikov Hydrocarboxylation of Unactivated Alkenes via Amine-Mediated CO2 •– Generation from Formate

Abstract

The use of formate salt as a C1 synthon offers a practical and accessible carboxylating reagent relative to the gaseous alternatives. However, photochemical transient CO2 •– generation from formate and its utilization for carboxylation reactions is mostly restricted to the activated alkenes. Herein, we report a general, photocatalytic platform for the selective anti-Markovnikov hydrocarboxylation of unactivated and activated alkenes using formate salt. The scope of the protocol is demonstrated through the selective hydrocarboxylation of a range of alkenes, independent of the chain length. The control experiments and preliminary mechanistic studies indicate synergy between the tertiary aminium radical cation-mediated hydrogen atom abstraction (HAA) and efficient hydrogen atom transfer (HAT) from the silyl hydride underpin the controlled hydrocarboxylation process. Mechanistic studies also indicate that the radical-polar crossover of the carboxylated alkyl radical can be avoided by the judicious choice of the HAT reagent, providing access to unactivated alkenes in the hydrocarboxylation reaction utilizing formate. This strategy using light energy is mild, robust, and effective for the carboxylation of various alkenes under ambient conditions. Our method enables a modular route for the controlled, steady-state release of CO2 •– using formate, providing a unique pathway for the carboxylation of organic molecules. The application of this protocol has been demonstrated through the large-scale synthesis, synthetic manipulation, and synthesis of isotopically labeled aliphatic carboxylic acids from unactivated and activated alkenes.