Regio- and stereoselectivity in the Diels-Alder reaction of vinylallenes with acrolein: an ab initio study

To explore the regio- and stereoselectivity in Diets-Alder reactions of vin ylallenes with acrolein, a parent vinylallene/acrolein system and a methyl- substituted vinylallene/acrolein system were studied. Ab initio calculation s were used to identify eight transition state structures for each of the t wo Diels-Alder reactions at various computational levels (RHF/6-31G(d), RHF /6-311G(2d), B3LYP/6-311G(2d), and MP2/6-311G(2d)). The relative energies o f the endo and exo transition states along with the regioselectivity have b een determined from these calculations. In the parent vinylallene/acrolein system, the endo s-cis transition structure is the preferred stereoselectiv ity at all levels of theory, however, there is no regioselectivity. In the methyl substituted vinylallene/acrolein system, the endo s-trans transition state tends to compete with the endo s-cis transition state at the RHF lev els of theory and is 1 kcal/mol more stable at the B3LYP/6-311G(2d) level o f theory. Also, in the methyl-substituted system, there is now a definite p reference for one regioisomer over the other. Both Diels-Alder reaction sys tems are asynchronous with the methyl-substituted system being more pronoun ced. (C) 1999 Elsevier Science B.V. All rights reserved.