2,5-Bis(diphenylmethylene)-3-cyclopentenone: a solvent-dependent cobalt cluster mediated propargyl radical coupling process

Protonation of (1,1-diphenyl-2-propyn-1-ol)Co2(CO)6 with HBF4 in dichloromethane generates the expected metal-stabilized propargyl cation, and also rearranges to give the tricobalt cluster Ph2CCH-CCo3(CO)9 9. In contrast, use of THF as solvent affords the radical (Co2(CO)6)[HCC-CPh2] 2, which dimerizes at the methyne position; subsequent cyclization and carbonylation yields 2,5-bis-(diphenylmethylene)-3-cyclopentenone 11. The products have been characterized by X-ray crystallography, and the scope and mechanism of the reaction are discussed.