Ning Chen showed that photolysis of the molecule shown at the right in the figure below leads to the carbene homocubylidene. The carbene can be trapped through traditional reactions of carbenes, but it also rearranges to the remarkable bridgehead alkene homocubene, a molecule in which the 2p orbitals formally making up the pi bond of the alkene are held by the rigid cage so that there is very little overlap between them.

The figure below summarizes some of the chemistry of this system.

What is even more remarkable is the finding that the equilibrium constant for interconversion of the carbene and the bridgehead alkene is ~1. Most of the credit for this determination goes to Matt Platz and his group at Ohio State, who collaborated with us on this work.

Details? See:

Chen, N.; Jones, M. Jr.; White, W. R.; Platz, M. S. J. Am. Chem. Soc. 1991, 113, 4981.

There is other very fine work on this system and the related cubylcarbenes from the group of Phil Eaton at Chicago. Some of their work is referenced in the citation given above.