Mechanism and Selectivity of the Cationic Rh-Catalyzed [2+2+2] Cycloaddition and Dimerization of 1,6-Enynes with Cyclopropylideneacetamides

The cationic Rh-catalyzed [2+2+2] cycloaddition and dimerization of 1,6-enynes with cyclopropylideneacetamides have been investigated by DFT calculations. For the formation of spirocyclohexenes with retaining cyclopropane rings, the pathway involves oxidative coupling/alkene insertion of cyclopropylideneacetamides/reductive elimination. For the formation of trienes through cleavage of the cyclopropane rings, the originally proposed mechanism, which involves oxidative cyclization of cyclopropylideneacetamides with 1,6-enynes/alkene insertion/β-carbon elimination/β-hydrogen elimination/reductive elimination, is not feasible. Instead, our calculations support a mechanism that involves the generation of a crucial bicyclic intermediate based on oxidative coupling in the cycloaddition reaction. Compared with the cycloaddition pathway, the insertion of the alkene in the opposite direction leads to a selective conversion between the spirocyclohexene product and the linear triene product, followed by β-carbon elimination and ultimately C-H activation through σ-bond metathesis. The influence of substrate substituents on product selectivity is discussed. For comparison, various C-C coupling reaction pathways have been examined.