Substituent Effects on the Addition Reaction Rate of Thiophenols to Ethynylbenzenes as a Model of Polyaddition. Study on the Relative Reactivity and Reaction Mechanism on the Basis of Hammett Correlation

Abstract

The addition reactions of thiophenols to ethynylbenzenes having p-methoxy, p-methyl, and p-chloro substituent groups occurred readily without appreciable side reactions. Adducts of the anti-Markownikoff’s structure were obtained in 95–99% yields. The relative reactivities of the addition reactions of benzenethiyl radical to substituted ethynylbenzenes, the rate-determining step, were found to be quite opposite to those of substituted benzenethiyl radicals to ethynylbenzene. For example, p-methoxyethynylbenzene has the highest reactivity in the ethynylbenzenes. The p-methoxybenzenethiyl radical exhibits the lowest reactivity. The relative rates of the chain transfer reaction among β-phenylthiostyrylene radical (φĊ=CHSφ) and thiophenols were also determined. That is, the electron-donating p-substituent group on the thiophenols increased the chain transfer reaction rate. These relative reactivities were correlated by three Hammett’s equations. Better correlation was found with the Brown-Okamoto’s equation and Yamamoto’s equation considering a resonance effect. The results are discussed in relation to the transition state of the addition reaction of thiophenols to ethynylbenzenes. The substituent effects on the addition reactions between thiophenols and styrenes or thiophenols and ethynylbenzenes are comparatively discussed based on the energy levels of HOMO, SOMO, and LUMO calculated by MINDO/3. Polyaddition reactivity of 1,4-benzenedithiol to 1,4-diethynylbenzene was also estimated to be four times higher than the addition of thiophenol to ethynylbenzene.