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.