Methyl and vinyl substituted 1H-, 2H-, and 3H-phospholes and their Diels-Alder reactions with ethylene: a semiempirical AM1 study

Abstract

The equilibrium geometries and the relative stabilities of all the possible isomers of phospho le, methylphosphole and vinylphosphole have been estimated using the semiempirical AM1 method. The Diels-Alder transition states and products between each of these isomers, with ethylene as dienophile, have been located. AM1 results show that 2H-phosphole is more stable than 3H-phosphole by 3.6 kcal mol^-1 which is in turn more stable than ¹H-phosphole by about 6.0 kcal mol^-1. The Diels-Alder activation energies of 2H-phospholes are much lower compared to those involving 1H- and 3H-phospholes. Either methyl or vinyl substitution does not alter the broad features in the activation and reaction energies compared to the parent compound.