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Pyrolysis of (2-Phenylethyl)phenylsulfonium Ylides

In order to obtain information concerning the reaction mechanism of the pyrolysis of sulfonium ylides bearing a substituted phenyl group on the 2-position in the S-ethyl group of ethylphenylsulfonium ylide, (2-phenylethyl)phenylsulfonium bis(methoxycarbonyl)methylide (1) and dicyanomethylide(2) were subjected to pyrolysis in benzene.The reaction rates of 1 and 2 at 140 deg C were 6.0- and 3.2-times faster than those of ethylphenylsulfonium bis(methoxycarbonyl)methylide and dicyanomethylide, respectively.The activation parameters for 1 were ΔH++=125 kJ mol-1 and ΔS++=-3.8 JK-18mol-1, while those for 2 were ΔH++=124 kJ mol-1 and ΔS++=-2.5 JK-1 mol-1.Substituent efects on the β-phenyl groups in 1 and 2 afforded positive Hammett ρ-values: ρ = 0.49 (γ = 0.997) and ρ = 0.26 (γ = 0.993), respectively.From the obtained results, it was suggested that the pyrolysis proceeds through essentially a concerted intramolecular cis-elimination inclined toward a slightly carbanion-like type from an E1-like type by introducing a phenyl substituent at the 2-position of the ethyl group in the ethylphenylsulfonium ylide.

KINETIC STUDY ON PYROLYTIC ELIMINATION OF ETHYLPHENYLSULFONIUM DICYANOMETHYLIDE

Ethyl(Substituted phenyl)sulfonium dicyanomethylides (1) were prepared and pyrolyzed in sealed tubes in benzene.The rate for pyrolysis of the substrate was correlated in a good first-order kinetic equation (γ = 0.999). The rate constant was 11.4 * 10-4 s-1 at 150 deg C.Pyrolysis of (1) was found to proceed about 3 times faster than that of ethylphenylsulfonium bis(methoxycarbonyl)methylide (2).Activation parameters calculated from the Arrhenius equation were as follows: ΔH(excit.) = 128 (KJ/mol), ΔS(excit.) = 1.7 J/K/mol (150 deg C, γ = 0.999), in which the magnitude of activation enthalpy was almost the same as that of (2), while the activation entropy was considerably larger compared with that of ethyl phenyl sulfoxide (-75.1 J/K/mol).Thus, the S-Cα bond of (1) was found to be looser in the transition state.The reactivities of sulfonium ylides, sulfoxides and sulfilimines estimate the magnitude of the activation entropies of the substrates.Substituent effect on the phenyl group afforded a positive Hammet ρ-value (ρ = 0.42, γ = 0.996) vs. ?-values. From these results, it was suggested that pyrolysis of sulfonium ylides proceeds via essentially concerted intramolecular cis-elimination in which the transition state is E1-like. Key words: pyrolysis; elimination; mechanism; kinetics; sulfonium ylide.