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.