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)(2)S-2 for the K2CO3, K3PO43H(2)O and K2SnO33H(2)O 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.