Thermal [1,3] sigmatropic rearrangements of bicyclic and tricyclic vinylcyclobutanes: a gray zone between the concerted and stepwise extremes

Four typical thermal [1,3] sigmatropic rearrangements of bicyclic and tricyclic vinylcyclobutanes and one fancied analogous reaction (R2 in Scheme 1) were examined using CASSCF, CASPT2 and CAS+1+2 methods to discern the reaction mechanisms. Computed results indicate that it is difficult to simply designate these reactions as traditional single-step concerted or stepwise mechanisms, but a situation locating between these two extremes seems to be reasonable. The extent the reaction exhibits as a single-step concerted or stepwise path is much dependent on the geometrical constraints of reactant. For example, the system with three-member ring will tend to behave like a single-step concerted process, where only one rotation movement around C–C bond could be found when the bridged C–C is broken. However, the species with four-member ring will be much closer to the stepwise mechanism involving diradical varieties, because there are two different rotation movements exist when the bridged C–C is broken. Our calculation will also rationalize that only suprafacial retention product could be yielded for the thermal [1,3] sigmatropic rearrangement of tricyclic vinylcyclobutane.