THE ROLE OF DEGENERATE BIRADICALS IN THE PHOTOREARRANGEMENT OF ACYLCYCLOPROPENES TO FURANS

The prototype rearrangement of acylcyclopropene to furan has been stud ied 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 character ized along four possible reaction coordinates, corresponding to mechan isms where the initial step involves (a) C-O bond formation to yield a n 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 s urprising result is that the reaction funnels (S-1/S-0 and T-1/S-0 cro ssings) are located at very similar geometries to the biradical interm ediates and form the common point where decay to the ground state is p ossible, and where the subsequent reaction path is essentially barrier less.