Structural and Solvent Effects on the C–S Bond Cleavage in Aryl Triphenylmethyl Sulfide Radical Cations
2012
Del Giacco, Tiziana | Lanzalunga, Osvaldo | Mazzonna, Marco | Mencarelli, Paolo
Steady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4-(CH₃O)₂-C₆H₃SC(C₆H₅)₃; 2, 4-CH₃O-C₆H₄SC(C₆H₅)₃; 3, 4-CH₃-C₆H₄SC(C₆H₅)₃; 4, C₆H₅SC(C₆H₅)₃; and 5, 4-Br-C₆H₄SC(C₆H₅)₃] has been carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate in CH₃CN, CH₂Cl₂, CH₂Cl₂/CH₃CN, and CH₂Cl₂/CH₃OH mixtures. Products deriving from the C–S bond cleavage in the radical cations 1•⁺–5•⁺ have been observed in the steady-state photolysis experiments. Time-resolved LFP showed first-order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C–S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2•⁺ and 3•⁺, charge and spin densities are mainly localized in the ArS group. In the TS of the C–S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C–S bond cleavage rates decrease by increasing the polarity of the solvent.
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