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,γ-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.