Study of a Vinylidenecarbene−Cycloalkyne Equilibrium in the D 3-Trishomocubyl Ring System

The same cage-annulated vinylidenecarbene, 7a, could be generated and subsequently trapped in situ by cyclohexene via either of the following methods: (i) low-temperature reaction of 4-(dibromomethylene)pentacyclo[6.3.0.02,6.03,10.05,9]undecane (6) with n-BuLi−THF and (ii) low-temperature reaction of 5-bromopentacyclo[7.3.0.02,7.03,11.06,10]dodec-4-ene (12) with LDA−THF. These results, together with the corresponding results of site-specific 13C-labeling experiments performed on 6-C(4)13CBr2 and 12-13C(5), provide evidence for the thermodynamic stability of 7a vis-à-vis 7b. In addition, the results of semiempirical and ab initio MO calculations demonstrate the relative kinetic as well as thermodynamic preference for 7a vis-à-vis 7b.