S. Wilsey et al., THE ROLE OF DEGENERATE BIRADICALS IN THE PHOTOREARRANGEMENT OF ACYLCYCLOPROPENES TO FURANS, Journal of the American Chemical Society, 118(18), 1996, pp. 4469-4479
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.