Ab Initio Study of the Thio-Ene Reaction. 1. The Enophile Substituent Effect

The ene reaction between propene and various enophiles (ethene, methanal, methanethial, ethanethial, and cyanomethanethial) were examined at the MP4/6-31G//MP2/6-31G level. The transition structures are all cyclic and concerted. The activation barriers for the thiocarbonyls are significantly lower (ranging from 16.0 to 25.0 kcal mol(-1)) than for ethene (36.3 kcal mol(-1)) or methanal 31.1 kcal mol(-1)). This trend, along with the trend in activation energies among the substituted thials, is completely consistent with FMO arguments. Comparison with experiment, particularly the thiol vs sulfide production, is also in complete agreement.