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(nπ*) and S0) and triplet
(3(nπ*) and
3(ππ*)) 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 (S1/S0 and
T1/S0 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.