Adaptive Photocatalytic Regimes Enabled by CO2 ·– and Two-Photon Excitation Synergy: Cyclization-Carboxylation of Aryl Halides with CO2

Abstract Activation of aryl halides across a broad reactivity spectrum remains a central challenge in photoredox catalysis, particularly due to the kinetic barriers associated with C–Cl bond reduction. Herein, we report on a photocatalytic platform that enables substrate-dependent access to distinct reductive regimes through the synergistic integration of formate-derived CO2·– and excitation pathways. Under the developed conditions, aryl bromides and chlorides, despite exhibiting demanding reduction potentials, display markedly different reactivity, governed by kinetic factors specific to the substrate and catalytic system. CO2·– mediates thermodynamically matched reduction of aryl bromides, whereas more recalcitrant aryl chlorides are activated via a two-photon reductive manifold, consistent with either a ConPET-type mechanism or solvated electron involvement. Notably, formate serves as both the precursor to CO2·– and the electron donor enabling a two-photon, strongly reductive manifold, obviating the need for sacrificial amines. This adaptive system facilitates a variety of transformations, including the efficient cyclization-carboxylation of (2-halophenoxy)allenes with CO2 under mild conditions. Mechanistic studies, including kinetic analysis, competition experiments, isotopic labelling, spectroscopic studies, and DFT calculations, support a dual-regime mechanism governed by substrate-dependent activation barriers. This work establishes a framework for kinetically gated control of reductive strength in photoredox catalysis by harnessing CO2·– as a reductant.