Pyrolysis of α- and β-Heteroatoms Substituted Ethyl Phenyl Sulfoxides

Abstract

A study on the mechanism of the thermal decomposition of α- and β-heteroatoms substituted ethyl phenyl sulfoxides was carried out using 1-chloroethyl phenyl sulfoxide (1); two diastereomeric 1-acetoxyethyl (substituted phenyl) sulfoxides (2a) and (2b); and 2-chloroethyl phenyl, 2-bromoethyl phenyl, and 2-methoxyethyl phenyl sulfoxides (3, 4, 5). The rate of pyrolysis of 1 was 4.8 times faster at 160°C than that of ethyl phenyl sulfoxide used as a reference, while those of 2a and 2b were 107 and 155 times faster, respectively. The results indicate that the lone pair of electrons on the α-heteroatoms has a larger rate acceleration effect than the electronegativity of them. The substituent effects of the phenyl group of 2a and 2b gave positive Hammett ρ-values (ρ a = 0.76 and ρ b = 0.80 vs. σ). Activation parameters for 2a and 2b are as follows: 2a, ΔH ‡ = 112 kJmol−1, ΔS ‡ = −20 JK−1mol−1; 2b, ΔH ‡ = 107 kJmol−1, ΔS ‡ = −29 JK-1mol−1. Large deuterium kinetic isotope effects for 1-acetoxyethyl-2,2,2-d3 phenyl sulfoxides (2ad and 2bd) were observed (kH/kD = 3.5 ∼ 4.1). These results suggest that the pyrolysis of α-heteroatom substituted ethyl phenyl sulfoxides proceeds via a five-membered transition state deviated to E1-like in character. On the other hand, from the results of kinetics for the pyrolysis of 3, 4, and 5, no effect by the β-halogen atoms or some deceleration effect by the β-methoxy group was observed. Thus the reaction seems to proceed via an E1-like mechanism.