Mechanism of the Oxadi-π-methane and [1,3]-Acyl Sigmatropic Rearrangements of β,γ-Enones: A Theoretical Study

The oxadi-π-methane ([1,2]-acyl shift) and [1,3]-acyl shift rearrangements of a simple β,y-enone (but-3-enal) have been investigated using MC-SCF computations in a 6-31G* basis set. The excited state reaction pathways and decay funnels for this model compound can be used to explain the direct and triplet-sensitized photochemistry of β,γ-enones in general. Our calculations show that the "classical" biradical intermediates proposed for both reactions correspond to decay funnels at which four states (S1(nπ*), T1(ππ*), T2(nπ*), and S0) are degenerate. Both efficient internal conversion (IC) and efficient intersystem crossing (ISC) can occur at these points, and the ground state reaction path is therefore the same regardless of the state initially populated. The ratio of products formed on photolysis is governed by the relative heights of the barriers leading to these decay funnels, and these will be sensitive to substituent effects on the reactant molecule. The oxadi-π-methane rearrangement is found to occur via a three-step process, where the four-level decay funnel corresponds to the first of two floppy intermediates on S0. There are two possible mechanisms leading to the [1,3]-acyl shift product: one involving the four-level decay funnel which corresponds to a "tight" intermediate in a quasi-concerted pathway, and a second which involves dissociation and recombination.