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 followin g methods: (i) low-temperature reaction of ne)pentacyclo[6.3.0.0(2,6). 0(3,10).0(5,9)]undecane (6) with n-BuLi-THF and (ii) low-temperature r eaction of pentacyclo[7.3.0.0(2,6).0(3,10).0(5,9)]dodec-4-ene (12) wit h LDA-THF. These results, together with the corresponding results of s ite-specific C-13-labeling experiments performed on 6-(C) over bar(4)= (CBr2)-C-13 and 12-C-13(5), provide evidence for the thermodynamic sta bility of 7a vis-h-vis 7b. In addition, the results of semiempirical a nd ab initio MO calculations demonstrate the relative kinetic as well as thermodynamic preference for 7a vis-a-vis 7b.