Theoretical study of the Diels-Alder reaction of selenoaldehydes and selenoketones

The Diels-Alder reaction of a series of substituted selenoaldehydes (Se=CHR ; R = H, F, Me, OMe, CH2F, CF3, and CN) or selenoketones (Se=CR2; R = Me an d CN) with 1,3-butadiene or 2-methoxy-1,3-butadiene to yield 3,6-dihydro-2H -selenopyrans were examined using B3LYP with a modified 6-31G* basis set. T his method is compared with. results from a number of standard ab initio pr ocedures and compares well with post-HF results. The Diels-Alder reaction o f the selenocarbonyl compounds proceeds through a concerted, though asynchr onous, transition state. Strong electron-withdrawing groups alter the mecha nism; a charge-transfer complex is first formed, followed by a concerted TS before reaching the heterocyclic product. The transition state geometry, r egiochemistry, and dependence of the activation barrier on substituents can be understood in terms of FMO theory.