VI. O-Thiocarbonyl Compounds

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Compounds with carbon–sulfur single bonds are substantially less reactive with tin- and silicon-centered radicals than are compounds with C–S double bonds. Among carbohydrates these double bonds are almost always part of O-thiocarbonyl groups. (The reactions of O-thiocarbonyl carbohydrate derivatives are discussed in Chapter 12.) The reaction shown in eq 6 illustrates the greater reactivity of a C–S double bond when compared to a C–S single bond because only the O-thiocarbonyl group in the 1‑thioglycoside 12 reacts even though an ethylthio group is present in the molecule.³⁹ Greater reactivity of a carbon–sulfur double bond also can be seen in the reaction shown in Scheme 13, where Bu₃Sn· reacts only with the O‑thiocarbonyl group.²⁶ A quantitative measure of the reactivity of C–S single and double bonds comes from comparing absolute rate constants for their reactions; thus, rate constants for reaction of (Me₃Si)₃Si· with C₁₀H₂₁SC₆H₅ and C₆H₁₁OC(=S)SMe are less than 5 x 10⁶ and 1.1 x 10⁹ M^-1s^-1, respectively, at 21 ^oC.⁴⁰ The reactions in Schemes 12 and 13 also illustrate the ease of fragmentation of a carbon–sulfur single bond when a radical is centered on an adjacent carbon atom.

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