Rhodium-Catalyzed [5 + 1 + 2] Reaction of Yne-Vinylcyclopropenes and CO: The Application of Vinylcyclopropenes for Higher-Order Cycloaddition

Transition metal-catalyzed higher-order cycloadditions involving vinylcyclopropenes (VCPEs) have not been realized to synthesize challenging medium-sized rings, partially due to their poor stability and many competing side reactions. We report here a Rh-catalyzed [5 + 1 + 2] reaction of yne-VCPEs and CO for the synthesis of eight-membered carbocycles with trienone moiety, which so far can be accessed by only limited reactions. The key to this higher-order cycloaddition is that once C-C cleavage of VCPE (C5 synthon) to form a six-membered metallacycle is initiated, CO (C1 synthon) insertion happens before alkyne (C2 synthon) insertion, attributing to the special reactivity of the sp2 carbon in the vinylcyclopropene. Quantum chemical calculations have been applied to support this reaction pathway. The present [5 + 1 + 2] reaction has a broad scope, and the C2 synthon can also be extended to alkenes and allene. Of the same importance, the present reaction can be catalyzed by either [Rh(CO)2Cl]2 or a cheaper complex, RhCl3·nH2O.