Rhodium-Catalyzed [6+2] Cycloaddition of Internal Alkynes with Cycloheptatriene: Catalytic Study and DFT Calculations of the Reaction Mechanism

A rhodium-catalyzed [6 + 2] cycloaddition of internal alkynes with cycloheptatriene is described. A series of substituted alkynes were cycloadded to cycloheptatriene through a [6 + 2] addition to give a variety of substituted bicyclic compounds in excellent yields. The optimal catalytic system for these transformations was a [Rh(COD)Cl](2) (5.0 mol %) catalyst in combination with CuI (10 mol %) and PPh3 (10 mol %). The proposed mechanism for this system includes an initial oxidative coupling reaction between the coordinated cycloheptatriene and the internal alkyne, followed by a [1,3]-shift of the Rh metal center and a reductive elimination from the Rh(III)-allyl complex to give the final product. Calculations using a model Rh(I) catalyst were also carried out to further understand this mechanism.