Potassium salts catalyzed oxidative coupling of alkanethiols with sulfur: the effect of solid-liquid phase transfer of crown ether

Potassium salts (KA, A = anion) that catalyzed the oxidative coupling of alkanethiols (RSH) to dialkyl disulfide (R₂S₂) were investigated by using sulfur as oxidant and 18-crown-6 ether as phase transfer agent under solvent-free conditions at atmospheric pressure. Crown ether (15-crown-5 ether, 18-crown-6 ether, dibenzo-18-crown-6 ether) showed excellent solid-liquid phase transfer effect in solid KA catalyzed reaction. RS⁻ is the key active species of the KA catalyzed reaction. The catalytic activity of KA is affected by the pKa of conjugated acid (HA) of anion A⁻ in KA. The larger the pKa of the corresponding HA, the more active for KA to activate RSH to generate RS⁻. Among the KA investigated, the yield of (n-C₈H₁₇)₂S₂ for the K₂CO₃, K₃PO₄·3H₂O and K₂SnO₃·3H₂O catalyzed reaction reached more than 98.0%. The yield of R₂S₂ can reach more than 95.0% by 0.1 mol% 18-C-6 (based on sulfur) and 0.1 mol% K₂CO₃ (based on sulfur) catalyzed the oxidative coupling of different alkanethiols.