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

The equilibrium geometries and the relative stabilities of all the possible isomers of phosphole, 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.