Synthesis of Sulfides, Sulfoxides and Sulfones
1991; Elsevier BV; Linguagem: Inglês
10.1016/b978-0-08-052349-1.00155-4
Autores Tópico(s)Asymmetric Synthesis and Catalysis
ResumoDuring the 1980s, organic sulfur compounds have become increasingly useful and important in organic syntheses. Sulfur incorporated into an organic molecule activates, by its electronegativity, the hydrogen atoms situated on the adjacent carbon atom, increasing their thermodynamic acidity. The acidity of a proton α to a sulfoxide group or to a sulfone group falls between those of a benzylic proton and a proton α to a carboxylate function. It has been demonstrated that sulfur groups exert stabilizing effects on the corresponding carbanions that are much larger than those expected from their polar contributions.1 The stabilizing resonance effect for the phenyl sulfone group was estimated to be equivalent to 33–75 kJ mol−1 (8–18 kcal mol−1), which is comparable with that of a cyano group but somewhat smaller than that of carbonyl or nitro functions. No data are available for a sulfoxide group and, to our knowledge, the extent of such acidification in β-keto sulfoxides or related compounds has not yet been determined. Another report2 also showed that the presence of a phenylthio group increases the acidity of cyclohexanone by at least 3 pK units, corresponding to a stabilizing effect of the order of 25–42 kJ mol−1 (6–10 kcal mol−1).3 This stabilizing effect was attributed to conjugative effects or polarization effects4 involving the 3d-orbitals of sulfur.
Referência(s)