Heterogeneous Cobalt-Catalyzed H2S-Reagent-Free Dialkylpolysulfane Synthesis from Alkenes, Elemental Sulfur, and Hydrogen

Herein, we report a heterogeneous cobalt-catalyzed H2S-reagent-free synthetic method for 1,2-bis-(2,4,4-trimethylpentan-2-yl)-polysulfanes, which have been used as extreme-pressure additives in the field of tribology and have been produced worldwide in the tens of thousands of tonnes per year. The current industrial synthetic processes typically use hydrogen sulfide as the reductant, but improvements are needed in terms of the toxicity and the cost associated with safety control. The present reaction with Co3O4 and Na-X zeolite as catalysts provides the corresponding polysulfanes with three to eight sulfur atoms from diisobutylene, hydrogen, and elemental sulfur in up to 88% yield under neat reaction conditions. Experimental and density functional theory (DFT) mechanistic investigations provide insights into the roles of Na-X zeolite and cobalt catalysts. The DFT calculations suggest that heterolytic H-2 activation with cobalt polysulfide species and subsequent protonation of alkenes and the C-S bond formation take place in a single step by a terminal SSH group in the cobalt polysulfide species, which provides a reasonable interpretation of the observed Markovnikov selectivity.