Ab initio study of stereo- and regioselectivity in the Diels-Alder reaction between 2-phenylcyclopentadiene and alpha-(methylthio) acrylonitrile

Ab initio molecular orbital calculations have been used to investigate the Diels-Alder reaction between 2-phenylcyclopentadiene and alpha-(methylthio)acrylonitrile. Geometry optimizations were performed with the RHF/3-21G, RHF/3-21G*, and RHF/6-31G* calculations while the correlation energy has been calculated at MP2 and MP3 levels. Four asynchronous transition structures corresponding to the formation of different stereoisomers and regioisomers associated with the four reaction channels have been located. The observed endo/exo stereoselectivity and para/meta regioselectivity are correctly described at all levels of calculation, and the calculated activation energies reflect accurately the relative reactivities along the four reactive channels. However, the enthalpies of activation reach good accord with experimental results only when the correlation energy is included at the MP3 level. The theoretical results point out that the regioselectivity is controlled by the presence of a phenyl group on C2 carbon atom of the diene system while the stereoselectivity is controlled by a favorable secondary orbital overlap between the lone pair of the sulfur atom located in the dienophile fragment and the pi system of the diene at the endo transition structures.