S. Wilsey et al., MECHANISM OF THE OXADI-PI-METHANE AND [1,3]-ACYL SIGMATROPIC REARRANGEMENTS OF BETA,GAMMA-ENONES - A THEORETICAL-STUDY, Journal of the American Chemical Society, 118(1), 1996, pp. 176-184
The oxadi-pi-methane ([1,2]-acyl shift) and [1,3]-acyl shift rearrange
ments of a simple beta,gamma-enone (but-3-enal) have been investigated
using MC-SCF computations in a 6-31G basis set. The excited state re
action pathways and decay funnels for this model compound can be used
to explain the direct and triplet-sensitized photochemistry of beta,ga
mma-enones in general. Our calculations show that the ''classical'' bi
radical intermediates proposed for both reactions correspond to decay
funnels at which four states (S-1(n pi), T-1(pi pi*), T-2(n pi*), and
S-0) are degenerate. Both efficient internal conversion (IC) and effi
cient intersystem crossing (ISC) can occur at these points, and the gr
ound state reaction path is therefore the same regardless of the state
initially populated. The ratio of products formed on photolysis is go
verned by the relative heights of the barriers leading to these decay
funnels, and these will be sensitive to substituent effects on the rea
ctant molecule. The oxadi-pi-methane rearrangement is found to occur v
ia a three-step process, where the four-level decay funnel corresponds
to the first of two floppy intermediates on S-0. There are two possib
le mechanisms leading to the [1,3]-acyl shift product: one involving t
he four-level decay funnel which corresponds to a ''tight'' intermedia
te in a quasi-concerted pathway, and a second which involves dissociat
ion and recombination.