The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules. However, a thermal six-electron process normally does take the direct route, as in for example the Diels-Alder cycloaddition as Houk and co have recently showed using molecular dynamics. So can one contrive a six-electron cycloaddition involving dichlorocarbene?
- K. Black, P. Liu, L. Xu, C. Doubleday, and K.N. Houk, "Dynamics, transition states, and timing of bond formation in Diels-Alder reactions", Proceedings of the National Academy of Sciences, vol. 109, pp. 12860-12865, 2012. http://dx.doi.org/10.1073/pnas.1209316109