In organic chemistry, an episulfide is an organic compound that contain a saturated, heterocycle consisting of two carbon atoms and one sulfur atom. It is the sulfur analogue of an epoxide or aziridine. They are also known as thiiranes, olefin sulfides, thioalkylene oxides, and . Episulfides are less common and generally less stable than epoxides. The most common derivative is ethylene sulfide ().
Structure
According to electron diffraction, the and distances in ethylene sulfide are respectively 1.473 and 1.811 Angstrom. The and angles are respectively 66.0 and 48.0°.
Preparation
A number of chemists in the early 1900s, including Staudinger and Pfenninger (1916), as well as Delepine (1920) studied episulfides. In 1934 Dachlauer and Jackel devised a general synthesis of episulfides from epoxides using alkali thiocyanates and thiourea.[Sander, M. Thiiranes. Chem. Rev. 1966, 66(3), 297-339. ]
Episulfides can also be prepared from cyclic carbonate ester, hydroxyalkylthiols, , , and chloroalkylthiols.
Base converts monothiocarbonates to episulfides:
The metal-catalyzed reaction of sulfur with alkenes has been demonstrated but is of little preparative value.
Tetrasubstituted thiiranes form spontaneously from the corresponding sulfur ylide.
Reactions
Episulfides, due to their innate ring strain, often undergo ring-opening reactions, especially with nucleophiles.
Nucleophiles include hydrides, thiolates, alkoxides, amines, and carbanions.
Like ordinary thioethers, episulfides can be oxidized to sulfoxides as in the synthesis of ethylene episulfoxide:
In a related reactivity, episulfides are easily S-alkylated. With methyl bromide, the product is the ring-opened thioether:
Such reactions proceed via episulfonium ions. Episulfonium salts can be prepared using trimethyloxonium tetrafluoroborate:
Episulfide polymerize, especially in the presence of .
Applications
Thiiranes occur rarely in nature but are found in some pharmaceuticals.[ They are intermediate in the action of , a family of .][ Mustard agents: description, physical and chemical properties, mechanism of action, symptoms, antidotes and methods of treatment. Organisation for the Prohibition of Chemical Weapons. Accessed June 8, 2010.]
Related compounds
Dithiiranes consist of three-membered ring containing two sulfur atoms and one carbon. One example was prepared by oxidation of a 1,3-dithietane.
Thiirenes are unsaturated episulfides, the parent having the formula formally derived by the formal addition of sulfur atom to an alkyne. They are antiaromatic and rarely observed. S-Alkylthiirenium salts are, however, known.
