Abstract
Abstract The effect of pyrolysis of the acyl group substituted on a-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 bon...
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