A Model Study of the Mechanism of the Type B (Di-π-methane) and Lumiketone Rearrangement in Rotationally Constrained α,β-Enones

The ground and excited state potential energy surface topology along the 1,3(n-π*) reaction path for the type B (structurally equivalent to the di-π-methane rearrangement) and the 3(π-π*) lumiketone rearrangements of rotationally constrained α,β-enones (e.g. 2-cyclohexenones) have been modeled by CAS-SCF computations of a geometrically constrained 2(Z)-pentenal molecule and 2(Z),5-hexadienal. For the 1,3(n-π*) type B reaction, the computations indicate that funnels for intersystem crossing (ISC) and internal conversion (IC) occur on the product side of the excited state reaction path after the sigmatropic migration has taken place. This surface crossing must be a feature that does not depend on the nature (alkyl or benzyl) substitution. For the 3(π-π*) lumiketone reaction path, funnels for ISC exist on both the reactant and product side of the 3(π-π*) reaction path. The ground state reaction path will take place only if ISC on the reactant side is made efficient by increasing the spin-orbit coupling in polar solvents.