The Role of Degenerate Biradicals in the Photorearrangement of Acylcyclopropenes to Furans

The prototype rearrangement of acylcyclopropene to furan has been studied using CAS-SCF calculations in a 6-31G* basis set. The topology and reaction funnels of the singlet ((1)(n pi*) and S-0) and triplet ((3)(n pi*) and (3)(pi pi*)) potential energy surfaces have been characterized along four possible reaction coordinates, corresponding to mechanisms where the initial step involves (a) C-O bond formation to yield an oxahousane biradical intermediate, (b) C-C bond fission to yield a 1,5 biradical intermediate, (c) C-C bond formation to yield a bicyclic intermediate, and (d) synchronous C-O and C-C bond formation to give a tricyclic intermediate, The three biradical intermediates are shallow minima on both ground and excited state surfaces. However, the most surprising result is that the reaction funnels (S-1/S-0 and T-1/S-0 crossings) are located at very similar geometries to the biradical intermediates and form the common point where decay to the ground state is possible, and where the subsequent reaction path is essentially barrierless.