Redirecting Formate Delivery toward Alkenes: Markovnikov α-Carboxylation via Cobalt/Photoredox/Bronsted Acid Catalysis

Advances in photocatalysis have significantly expanded the chemist's toolbox, enabling the use of sustainable C-1 feedstocks for the synthesis of carboxylic acids. While linear-selective carboxylation of olefins with formate salts involving CO₂^·- generation and subsequent Giese-type addition has previously been reported, Markovnikov selectivity has remained underexplored. Herein, we report a new photochemical approach for the selective synthesis of branched carboxylic acids using formate salts. The switch in regioselectivity is achieved through the synergistic integration of a metal-hydride hydrogen atom transfer (MHAT) strategy with cobalt catalysis, a Bro̷nsted acid, and photocatalysis. A bench- and air-stable <i>N,N</i>-diprotonated DABCO salt is introduced as a novel precatalyst that facilitates both catalytic cycles. This design enables a balanced equilibrium between formate activation and the MHAT cycle, allowing the persistent radical effect to operate effectively, which was explored both experimentally and computationally. The method accommodates a broad range of olefins, including bioactive compounds, and gives access to carboxylic acids bearing all-carbon quaternary centers with Markovnikov selectivity. The operational simplicity and versatility of the protocol make it well-suited for late-stage modification and isotope labeling of natural products and biologically active scaffolds.