AB-INITIO STUDY OF THE THIO-ENE REACTION .1. THE ENOPHILE SUBSTITUENTEFFECT

The ene reaction between propene and various enophiles (ethene, methan al, methanethial, ethanethial, and cyanomethanethial) were examined at the MP4/6-31G//MP2/6-31G* level. The transition structures are all c yclic 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 th ials, is completely consistent with FMO arguments. Comparison with exp eriment, particularly the thiol vs sulfide production, is also in comp lete agreement.