Competitive Pseudopericyclic [3,3]- and [3,5]-Sigmatropic Rearrangements of Trichloroacetimidates

The
Woodward–Hoffmann rules predict whether concerted pericyclic
reactions are allowed or forbidden based on the number of electrons
involved and whether the cyclic orbital overlap involves suprafacial
or antarafacial orbital overlap. Pseudopericyclic reactions constitute
a third class of reactions in which orthogonal orbitals make them
orbital symmetry allowed, regardless of the number of electrons involved
in the reaction. Based on the recent report of eight-centered ester
rearrangements, it is predicted that the isoelectronic eight-centered
rearrangements of imidates would also be allowed. We now report that
these rearrangements occur, and indeed, an eight-centered rearrangement
is slightly favored in at least one case over the well-known six-centered
Overman rearrangements, in a trichloroacetimidoylcyclohexadienone,
a molecular system where both rearrangements are possible.