2 H-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 2 H-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 2 H-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.