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

摘要

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.