Rhodium-catalyzed arylative and alkenylative cyclization of 1,5-enynes induced by geminal carbometalation of alkynes.

A rhodium(I)-catalyzed addition-cyclization of 1,5-enynes with aryl- and alkenylboronic acids has been developed. The reaction allows for efficient C-C coupling of multiple reactive components while accomplishing a net R,H-addition in a single step under mild conditions. In the presence of [Rh(OH)(COD)]2 catalyst and triethylamine base in methanol solvent, a range of 1,5-enynes undergo an intermolecular addition with a wide variety of aryl- and alkenylboronic acids and concomitant endo-selective cyclization to yield 1-aryl and alkenyl-substituted cyclopentene derivatives as the products. Deuterium labeling studies suggest that the reaction involves formation of a rhodium vinylidene complex with the terminal alkyne of the enyne substrate. The subsequent migration of the aryl or alkenyl group from the rhodium center to the alpha-carbon of the vinylidene ligand gives a vinyl rhodium complex, a formal 1,1-carbometalation process of the alkyne. This vinyl rhodium then adds to the pendent alkene, and the protodemetalation of the resulting rhodium enolate affords the product.