Generation and Trapping of a Cage Annulated Vinylidenecarbene and Approaches to Its Cycloalkyne Isomer

A novel cage-annulated (bis-homocubyl) vinylidenecarbene has been generated and successfully trapped without any intermediacy of its cycloalkyne isomer. The greater kinetic and thermodynamic stability of the vinylidenecarbene vis-à-vis its cycloalkyne isomer has been predicted by DFT B3LYP/6-31G* calculations. The calculated results suggest the prospects of the cycloalkyne becoming amenable for trapping, if generated under suitable experimental conditions, owing to the substantial kinetic energy barrier associated with its possible ring contraction via 1,2-shift to the vinylidenecarbene isomer and marginal ground state energy difference. However, all of our attempts to directly generate and trap the cycloalkyne yielded unsatisfactory results. Attempted generation and trapping of a C2-symmetric bis-vinylidenecarbene from a bis-vinylidenedibromide met with unexpected failure.