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

Abstract

Abstract 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−1 mol−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 °C. The kinetic isotope effect for β-position hydrogen in the β-phenylethyl group of 1 was considerably large (kH⁄kD=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.