Why do peroxomolybdenum complexes chemoselectively oxidize the sulfur centers of unsaturated sulfides and sulfoxides? A DFT analysis

The reasons why oxo-peroxo molybdenum complexes chemoselectively oxidize unsaturated sulfides to the corresponding sulfoxides and these to sulfones without any epoxidation of the electron-rich double bond were elucidated by transition state theory and density functional calculations at the B3LYP level. For the diperoxo model complex with the structure MoO(η(2)-O-2)(2)OPH3 and for allyl methyl sulfide as a model of unsaturated sulfides, the calculations show that the oxygen transfer process from the peroxomolybdenum complex to the sulfur center requires a lower free activation energy (&UDelta; G&DDAG; = 19.0 kcalmol(-1)) than the attack on the double bond moiety (&UDelta; G&DDAG;= 26.4 kcal mol(-1)) of the unsaturated sulfide. Subsequent oxidation of allyl methyl sulfoxide at the sulfinyl group to yield the corresponding sulfone also requires a lower activation energy (&UDelta; G&DDAG; = 21.5 kcal mol(-1)) than the corresponding epoxidation process (&UDelta; G&DDAG; = 27.7 kcal mol(-1)), yielding 2,3-epoxypropyl methyl sulfoxide, These results unambiguously account for these complexes' excellent chemoselectivity towards the sulfur groups in unsaturated sulfides and sulfoxides, as has been observed experimentally. On the basis of SCRF calculations, the level of chemoselectivity is predicted to diminish with increasing solvent polarity. Charge decomposition analysis and the orbital interaction model reveal that unsaturated sulfides with reaction sites carrying lone pair and π electrons behave as nucleophiles toward the electrophilic peroxo oxygen group. The origin of the chemoselectivity is ascribed to the fact that the electronic state related to the sulfur lone pair (HOMO) lies 1.2 eV above that associated with the π electrons (HOMO-1). © Wiley-VCH Verlag GmbH & Co.112406241