Low-temperature addition of hydrogen polysulfides to olefins: formation of 2,2′-dialkyl polysulfides from alk-1-enes and cyclic (poly)sulfides and polymeric organic sulfur compounds from α,ω-dienes

Abstract

Hydrogenpolysulfide ions, generated by the reaction of hydrogensulfide ions and elemental sulfur, react in a polar organic solvent with alk-1-enes and alka-α,ω-dienes under very mild conditions: 20–50 °C for 1–10 days. The reagent was prepared either in DMF with Na⁺ as counter-ion (method A) or in EtOAc–H₂O with quaternary ammonium as counter-ion (method B). Hexadec-1-ene yields both by method A and B 2,2′-dihexadecyl polysulfides via a Markownikoff-type addition. The overall selectivity of Markownikoff vs. anti-Markownikoff addition is ca. 35:1. The selectivity decreases at higher temperatures in DMF as solvent probably as the result of a sequence of elimination/readdition reactions. Alka-α,ω-dienes are in a similar way converted (method B) into polymeric polysulfides if the number of methylenes between the double bonds is 4. Monomeric cyclic sulfur compounds are obtained if the number of methylenes is < 4. Penta-1,4-diene yields mainly the cyclic disulfide 3,5-dimethyl-1,2-dithiolane whereas hexa-1,5-diene yields the cyclic monosulfide 2,5-dimethylthiolane as the only product. Hepta-1,6-diene yields cyclic mono- and di-sulfides. Proton-catalysed, simple nucleophilic and radical mechanisms for the ‘hydrosulfurization’ are excluded. A concerted addition of a hydrogenpolysulfide ion to a double bond is proposed as the actual mechanism. The formation of thiolanes and thianes is explained by the same mechanism operating in an intramolecular fashion.