Episulfides are a class of organic compound

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s that contain a saturated

Saturation (chemistry)

In chemistry, saturation has six different meanings, all based on reaching a maximum capacity...

 heterocyclic ring consisting of two carbon

Carbon

Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 atom

Atom

The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s and one sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 atom. It is the sulfur analogue of an epoxide

Epoxide

An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...

 or aziridine

Aziridine

Aziridines are organic compounds containing the aziridine functional group, a three-membered heterocycle with one amine group and two methylene groups...

. They are also known as thiiranes, olefin sulfides, thioalkylene oxides, and thiacyclopropanes.

The parent episulfide is thiirane

Thiirane

Thiirane, more commonly known as ethylene sulfide, is the cyclic chemical compound with the formula C2H4S. It is the smallest sulfur-containing heterocycle. Like many organosulfur compounds, this species has a stench. Thiirane is also used to describe any derivative of the parent ethylene...

. Most preparations of episulfides utilize a two-step method, converting an olefin to an epoxide before its conversion to the episulfide using either thiocyanate or thiourea. Common uses of episulfides in both academic and industrial settings most often involve their use as monomers in polymerization reactions.

Preparation

A number of chemists in the early 1900s, including Staudinger and Pfenninger (1916), as well as Delepine (1920), were the first to prepare and report episulfides. However, both of these accounts were isolated incidents on single substrates. It was not until 1934 that Dachlauer and Jackel devised the first general synthesis of episulfides from epoxides using alkali thiocyanates and thiourea, which is still one of the most common methods for preparing episulfides today.

While the above method represents the most common method of preparation, episulfides can also be prepared from cyclic carbonates, hydroxy mercaptans, hydroxyalkyl halides, dihaloalkanes, and halo mercaptans, as well as a number of other preoxidized starting materials. In addition, a reaction analogous to epoxidation has been reported involving the metal-catalyzed reaction of sulfur with alkenes.

alkene + “S” → episulfide

Reactions

Episulfides have an innate ring strain due to the nature of three-membered rings. Therefore, most reactions of episulfides involve the anionic nucleophilic opening of the heterocyclic ring. However, the nucleophilic opening is typically only regioselective in the case of a terminal episulfide, in which case the nucleophilic attack heavily favors attack at the primary carbon over the secondary carbon. Nucleophiles that can be used include anionic hydride, sulfur, oxygen, nitrogen, and carbon.

In large part, episulfides are less common than epoxides simply due to the more common appearance of oxygen in natural products. However, episulfides have also been found to be much less stable than epoxides, limiting their use. This destabilization is due to the increased nucleophilic character of sulfur, which often leads to undesired polymerization reactions.

The few industrial processes involving episulfides take advantage of their extreme reactivity and mainly involve the polymerization of episulfide monomers. For instance, one process has been patented which describes the use of episulfide monomers to produce high-refractive-index plastic lenses. While the exact polymerization reaction is unreported in the patent, a similar reaction involving the polymerization of episulfide monomers using [PPN]Cl/(salph)Cr(III)Cl as the catalyst has been reported.