Pryrolysis of α-Acyl Substituted Ethyl Phenyl Sulfoxide

Abstract

The effect of pyrolysis of the acyl group substituted on ä-position carbon in S-ethyl group of ethyl phenyl sulfoxide was investigated by using two kinds of substrates, l-ethoxycarbonylethyl phenyl sulfoxide (1) and l-ethoxycarbonyl-l-methylethyl phenyl sulfoxide (2) together with 2-methoxycarbonylethyl phenyl sulfoxide (4). The rate of pyrolysis of (1) was found to be about 740 ∼ 890 times faster than that of ethyl phenyl sulfoxide (5), while (2) was 5700 times faster. The rate enhancement effect on α-ethoxycarbonyl group was larger than that on β-methoxycarbonyl group. Large deuterium kinetic isotope effect (k ₁₁/k _D = 5.5 for PhS(O)C(CD₃)₂CO₂Et (3)) was observed. Activation energy for (2) is about 110 kJ/mol, which is just about the same as that of (5) (108 kJ/mol), while activation entropy lies in ca. 1 Jdeg⁻¹ mol⁻¹. Hammett plot for substituted (2) gave ρ = 0.69. These kinetic results reveal that the pyrolysis of the sulfoxides proceeds via a loose transition state involving advanced C-S bond cleavage. The remarkably large rate enhancement by α-carbonyl group might be due mainly to the conjugation of that group and developing double bond acidifying the β-proton in the transition state.

Key Words:

• Pryolysis
• elimination
• mechanism
• isotope effect
• kinetics
• sulfoxide