Silica: An Efficient Catalyst for One-Pot Regioselective Synthesis of Dithioethers

Abstract
The development of a silica-promoted highly selective synthesis of 1,2 or 1,3-dithioethers via solvent-free one-pot tandem reactions of an allyl bromide with excess thiol at room temperature is described. The choice of silica gel, either pre-calcined or moistened with water, exhibited notable regioselectivity in the formation of dithioethers. Plausible mechanistic routes were explored and postulated.

Introduction
Organosulfur compounds are important building blocks for the synthesis of various biologically active molecules. Versatile applications of organosulfur compounds are known in fields such as the pharmaceutical, the polymer, the pesticide and the food-processing industry. For example, organosulfur compounds in garlic are often used in food-processing industries as flavouring and preservative agents and are also used as herbal medicine. Dithioethers are commonly employed as ligands in preparing metal-coordination complexes and also as spacers in metal-organic frameworks. For example, vicinal dithioether-based zirconium and titanium complexes have been used for alkene polymerization and hydroamination. Chiral dithioethers have been prepared and their iridium complexes have been employed in asymmetric hydrogenation. Vicinal dithioethers are generally synthesised either by the metal-catalyzed addition of disulfides to alkenes or by the traditional nucleophilic substitution of 1,2-dihalides with suitable thiols/thiolates. They are also prepared by consecutive hydrothiolation of alkynes, both under nucleophilic and radical-induced conditions. On the other hand, 1,3- dithioethers can be prepared by the nucleophilic substitution of compounds bearing suitable leaving groups at 1,3-positions of alkyl chains. Because of their versatile applications, a great number of procedures have been developed to synthesize bis(thioethers) with varying degrees of success and a variety of limitations. 

The full article is published online in Beilstein Journal of Organic Chemistry and is free to access.