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Structural effects responsible for stability and solvolytic reactivity of sulfonium ions

The solvolysis rates of X-substituted benzhydryltetrahydrothiophenium ions (1) in pure and aqueous alcohols were determined at 25 °C and compared with the rates of the corresponding benzhydryldimethylsulfonium ions (2). The linear free energy relationship equation log k = sf(Ef + Nf) has been used to relate quantitatively the leaving group abilities of tetrahydrothiophene (THT) and dimethyl sulfide (Me2S). It has been demonstrated that although generating a stronger base by heterolysis, substrates 1 solvolyze over lower barriers than 2. Steric and electronic influences that determine the relative reactivities of sulfonium salts have been examined computationally at B3LYP level of theory by calculating the energy of exchange of electrofuges with different substituents between THT and dimethyl sulfide. Because of more efficiently delocalized positive charge in THT moiety, tetrahydrothiophenium ions are more stable than the corresponding dimethylsulfonium ions, regardless of an electrofuge. The Hammond–Leffler coefficient is negative (α < 0) for the rate determining heterolysis of sulfonium salts 1and2.

dimethyl sulfide; leaving group; nucleofugality; solvolysis; sulfonium salts; tetrahydrothiophene

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