The low temperature addition of hydrogen polysulphides to olefins: Formation of secondary polysulphides from 1-alkenes and heterocyclic and polymeric organic sulphur compounds from a,v-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.