2H‑Chromene-3-carboxylic Acid Synthesis via Solvent-Controlled and Rhodium(III)-Catalyzed Redox-Neutral C–H Activation/[3 + 3] Annulation Cascade

An efficient and redox-neutral synthesis of 2H-chromene-3-carboxylic acids from N-phenoxyacetamides and methyleneoxetanones has been realized via a solvent-controlled and rhodium­(III)-catalyzed C–H activation/unusual [3 + 3] annulation sequence. This transformation represents the first example of using an α-methylene-β-lactone unit as the three-carbon source in transition-metal-catalyzed C–H activations through selective alkyl C–O bond cleavage. Synthetic applications and mechanistic details, including further derivatization of 2H-chromene-3-carboxylic acids, the isolation and identification of a five-membered rhodacycle, as well as the theoretical studies for reasoning a plausible Rh­(III)–Rh­(V)–Rh­(III) process, have also been discussed.