Construction of Isochromenes via a Ruthenium-Catalyzed Reaction of Oxabenzonorbornenes with Propargylic Alcohols

The ruthenium-catalyzed cyclization of a propargylic alcohol with an oxabenzonorbornene in methanol leads to the creation of an isochromene framework. The proposed mechanism herein discussed for the formation of the product involves six major steps, the first four being oxidative cyclization, beta-hydride elimination, hydroruthenation, followed by [2+2] cycloreversion. The ruthenium carbene formed at this stage undergoes a [1,3]-alkoxide shift that provides the observed product after reductive elimination. This process, believed to occur via a cationic ruthenium species, is in competition with two other pathways, ruthenium-catalyzed [2+2] cycloaddition and cyclopropanation. Although both [Cp*Ru(CH3CN)(3)]PF6 and Cp*Ru(COD)Cl are effective catalysts, the latter gives better yield and product ratio. The reaction was also found to proceed with high regioselectivity and product selectivity when unsymmetrical alkenes bearing a coordinating functional group at the bridge junction were used.