A MODEL STUDY OF THE MECHANISM OF THE TYPE-B (DI-PI-METHANE) AND LUMIKETONE REARRANGEMENT IN ROTATIONALLY CONSTRAINED ALPHA,BETA-ENONES

The ground and excited state potential energy surface topology along t he (1,3)(n-pi) reaction path for the type B (structurally equivalent to the di-pi-methane rearrangement) and the (3)(pi-pi) lumiketone rea rrangements of rotationally constrained alpha,beta-enones (e.g. 2-cycl ohexenones) have been modeled by CAS-SCF computations of a geometrical ly constrained 2(Z)-pentenal molecule and 2(Z),5-hexadienal. For the ( 1,3)(n-pi) type B reaction, the computations indicate that funnels fo r 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 th at does not depend on the nature (alkyl or benzyl) substitution. For t he (3)(pi-pi) lumiketone reaction path, funnels for ISC exist on both the reactant and product side of the (3)(pi-pi) reaction path. The g round 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.