Pyrolysis of Sulfoxide Bearing Electron-Withdrawing Substituents on β-Position in S-Ethyl Group of Ethyl Phenyl Sulfoxide

Article abstract of DOI:10.1246/bcsj.60.2491

In order to obtain information concerning the reaction mechanism of the pyrolysis of a sulfoxide bearing an electron-withdrawing substituent on the β-position in the S-ethyl group of ethyl phenyl sulfoxide, 2-(substituted phenyl)ethyl aryl sulfoxides (1) and 2-cyanoethyl (substituted phenyl)sulfoxides (2) were pyrolyzed.The rate-enhancing effect of the β-phenyl group of 1 was small.The activation enthalpy and entropy of 1 were found to be 110 kJ mol^-1 and -45 JK^-1mol^-1, respectively.Hammett plots for 1 gave positive trends for the substituents, both on the β-phenyl group (ρ=0.76) and on the S-phenyl group (ρ=0.32), though the ρ-value of the latter was about half of the former.On the other hand, the Hammett plot for 2 did not give a straight line, but a concave curve in which a series of substituents from the p-OCH3 to p-Cl groups gave a negative trend (ρ=-0.49), while the p-NO2 group afforded a positive trend.The pyrolytic rate of 2 was found to be about 23-times faster than that of 1 at 100 deg C.The kinetic isotope effect for β-position hydrogen in the β-phenylethyl group of 1 was considerably large (k_H/k_D=4.3).From the obtained results, it was suggested that the pyrolysis of 1 and 2 proceeds via a nearly carbanion-like mechanism in a five-membered cyclic transition state.