Mechanistic Insights into the Ru(II)-Catalyzed Intramolecular Formal [3 + 2] Cycloaddition of (E)-1,6-Enynes

Design of a unique reaction pathway in transition-metal-catalyzed 1,6-enynes cyclization to construct valuable synthetic motifs is a significant challenge in organic chemistry. Herein, we report a Ru(II)-catalyzed formal [3 + 2] cycloaddition as an efficient method to prepare unprecedented bicyclo[3.3.0]octenes from readily available (E)-1,6-enynes. Mechanistic studies based on the deuterium labeling experiments and the DFT calculation disclose a reasonable mechanistic pathway, where a ruthenacyclopentene generated by an ene–yne oxidative cyclization undergoes a sequential ß-hydride elimination and intramolecular hydroruthenation to form a ruthenacyclohexene, producing the desirable bicyclo[3.3.0]octenes.