Photoredox-Neutral Platform for Direct Decarboxylative Cyclopropanation via Acridine-Iron Dual Catalysis

A photoredox-neutral approach to cyclopropanes from in situ generated non-stabilized carbenes via a new acridine-iron dual catalytic system is reported. This strategy merges the oxidative decarboxylation of bench-stable carboxylic acids with the reduction of an iron porphyrin under mild photochemical conditions. Bespoke acridine photocatalysts efficiently shuttle electrons between carboxylic acids and iron complexes, obviating the need for a sacrificial reductant. Mechanistic studies reveal that, contrary to prevailing assumptions, decarboxylation does not proceed through a concerted PCET pathway but instead via a stepwise proton transfer followed by a highly exergonic electron transfer and decarboxylation event. Density functional theory calculations further support the formation of a key Fe(IV)-carbenoid intermediate, which engages alkenes through a stepwise, radical-type carbene transfer mechanism.