MECHANISM OF THE OXADI-PI-METHANE AND [1,3]-ACYL SIGMATROPIC REARRANGEMENTS OF BETA,GAMMA-ENONES - A THEORETICAL-STUDY

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.