The Ei Reaction of Substituted threo- and erythro- 1-Phenylethyl Phenyl Sulfoxides

Article abstract of DOI:10.1246/bcsj.62.1891

Substituted (RS,SR)-1-phenylethyl phenyl sulfoxides (threo) (XC6H4S(O)CH(CH3)C6H4Y) and some substituted (RR,SS)-sulfoxides (erythro) were prepared and kinetic investigation for the thermal decomposition was carried out at 80.0, 90.0, and 100.0 deg C in dioxane.Hammett plots for threo-XXC6H4S(O)CH(CH3)C6H5 gave positive ρ-values (ρ_x=0.60-0.64 at three temoeratures), while those for threo- and erythro-C6H5S(O)CH(CH3)C6H4Y showed V-shape lines with bottoms at the m-OCH3 substituent though the effects of the substituents were small.Meanwhile, large kinetic isotope effects for threo- and erythro-C6H5S(O)CH(O)CH(CH3)C6H4Y (Y=H, p-OMe, m-Cl) (k_H/k_D=4-6) were observed at all temperatures.The activation energies were in the range of 104-121 kJ mol^-1 for all sulfoxides, while the activation entropies were relatively large (7-37 JK^-1mol^-1) and were correlated with Hammett ?-values to give small negative trend.Reactions of all erythro-isomers examined were 2-3 times faster than those of corresponding threo-isomers.From these results, it is suggested that the pyrolysis of 1-arylethyl aryl sulfoxides proceeds via a concerted mechanism in which the transition state is variable from an Ei'1-like to a conjugated one.In the latter transition state, conjugation of the phenyl group bearing the electron-withdrawing substituent with developing ?-bond electron acidifies the β-proton.