Sulfenamides (also spelled sulphenamides) are a class of

organosulfur compound Organosulfur compounds are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur derivatives, e.g., saccharin. Nature abounds with organosulfur compounds—sulf ...

s characterized by the general formula RSNR'₂, where R and R' are H,

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloal ...

, or

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...

.Capozzi, G., Modena, G., Pasquato, L. in "Chemistry of Sulphenyl Halides and Sulfenamides" ''The Chemistry of Sulphenic Acids and their derivatives''. Ed. Saul Patai. John Wiley & Sons Ltd. Chapter 10. 403-516, 1990. Sulfenamides have been used extensively in the

vulcanization Vulcanization (British: Vulcanisation) is a range of processes for hardening rubbers. The term originally referred exclusively to the treatment of natural rubber with sulfur, which remains the most common practice. It has also grown to include ...

of rubber using sulfur. They are related to the oxidized compounds sulfinamides (RS(O)NR'₂) and

sulfonamide In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactiv ...

s (RS(O)₂NR'₂).

Preparation

Sulfenamides are usually prepared by the reaction of

sulfenyl chloride In organosulfur chemistry, a sulfenyl chloride is a functional group with the connectivity , where R is alkyl or aryl. Sulfenyl chlorides are reactive compounds that behave as sources of . They are used in the formation of and bonds. According ...

s and

amine In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent su ...

s: :RSCl + R'₂NH → RSNR'₂ + HCl The S-N bond formation generally obeys standard bimolecular nucleophilic substitution rules, with the basic nitrogen centre being the nucleophile. Primary sulfenamide formation as shown above occurs with the reaction of the

sulfenyl halide In organosulfur chemistry, a sulfenyl chloride is a functional group with the connectivity , where R is alkyl or aryl. Sulfenyl chlorides are reactive compounds that behave as sources of . They are used in the formation of and bonds. Accordin ...

with

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogeno ...

. Additionally primary as well as secondary and tertiary

amines In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

form sulfenamides through reaction with,

thiols In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

, disulfides, and

sulfenyl In chemistry, a sulfenic acid is an organosulfur compound and oxoacid with the general formula . It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids () and sulfonic acids (), respectively. The base mem ...

thiocyanates Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in py ...

. In one illustrative synthesis, triphenylmethanesulphenyl chloride and butylamine react in

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen ato ...

at 25 C: :Ph₃CSCl + 2BuNH₂ → Ph₃CSN(H)Bu + BuNH₃Cl Many other routes to sulfenamides are known, starting from

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl gro ...

s and

disulfide In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In ...

s.I V Koval' "Synthesis and Application of Sulfenamides" Russian Chemical Reviews, 1996, Volume 65, :RSSR + 2R'₂NH + Ag⁺ → RSNR'₂ + AgSR + R'₂NH₂⁺ Captan Structural Formula V

Structure

Sulfenamides have been characterized by

X-ray crystallography X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angle ...

. The S-N bond in sulfenamides is a

chiral Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object. An object or a system is ''chiral'' if it is distinguishable from i ...

axis that leads to formation of

diastereomeric In stereochemistry, diastereomers (sometimes called diastereoisomers) are a type of stereoisomer. Diastereomers are defined as non-mirror image, non-identical stereoisomers. Hence, they occur when two or more stereoisomers of a compound have di ...

compounds. The existence of these distinct

stereoisomers In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in ...

is due to the formation of a partial double bond between either

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seve ...

’s lone pair and the other atom's

antibonding orbitals In chemical bonding theory, an antibonding orbital is a type of molecular orbital that weakens the chemical bond between two atoms and helps to raise the energy of the molecule relative to the separated atoms. Such an orbital has one or more no ...

. Additionally bulky substituent groups and

lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC '' Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. L ...

repulsion can contribute resistance to interconversion. The resulting torsional barriers can be quite large and vary from 12-20 kcal/mol.{{cite journal , last1 = Craine , first1 = Leslie , last2 = Raban , first2 = Morton , year = 1989 , title = The Chemistry of Sulfenamides , journal = Chemical Reviews , volume = 89 , issue = 4 , page = 669 , doi = 10.1021/cr00094a001 The interactions are thought to be dependent on the torsional preferences (also known as the gauche effect). The nitrogen atom is usually pyramidal, but cyclic and strongly steric hindered acyclic sulfenamides can display a planar arrangement of bonds around the nitrogen atom.

Reactions

The S-N bond in sulfenamides are labile in a variety of ways. The sulfur atom tends to be the more

electrophilic In chemistry, an electrophile is a chemical species that forms bonds with nucleophiles by accepting an electron pair. Because electrophiles accept electrons, they are Lewis acids. Most electrophiles are positively charged, have an atom that car ...

center of the S-N bond. Nucleophillic attack on sulfur can occur by amines, by

, and by alkyl-magnesium

halides In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...

which leads to either new sulfenamide compounds or back to starting compounds such as

sulfides Sulfide ( British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds ...

and disulfides respectively. Both the nitrogen and sulfur atoms comprising the S-N bond in sulfenamides have lone pairs of electrons in their outer shells, one and two for nitrogen and sulfur respectively. These lone pairs allow for the possibility of forming either higher order bonds(double, triple) or adding new substituent groups to the compound For instance the nitrogen in the S-N bond of 2-hydroxysulfenanilides can oxidized to an

imine In organic chemistry, an imine ( or ) is a functional group or organic compound containing a carbon–nitrogen double bond (). The nitrogen atom can be attached to a hydrogen or an organic group (R). The carbon atom has two additional single bo ...

species with sodium dichromate. Sulfenamides react with amino-azaheterocycles to form heterocyclic systems (often used as amino protecting groups in various other synthesis reactions). Chlorocarbonylsulfenyl chloride (ClCOSCl) also readily forms S-N bonds with 2-amino-azaheterocycles, but always of a cyclical nature. A novel variant of the Appel reaction has been noted for sulfenamides. Reaction of o-nitrobenzenesulfenamide with PPh₃ and CCl₄ leads the formation of o-nitro-N-(triphenylphosphorany1idene)-benzenesulfenamide. In this variant reaction the triphenyl phosphine forms a double bonded linkage with nitrogen in the sulfenamide instead of oxygen as is customary in the Appel reaction. Additionally in the traditional Apple reaction the R-OH bond is cleaved leaving oxygen attached to triphenylphosphine. In this variant the S-N bond is not cleaved.

Applications

Sulfenamides, e.g.

cyclohexylthiophthalimide Cyclohexylthiophthalimide (abbreviated CTP) is an organosulfur compound that is used in production of rubber. It is a white solid, although commercial samples often appear yellow. It features the sulfenamide functional group, being a derivative ...

, are used extensively in the

rubber Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds. Thailand, Malaysia, a ...

. The sulfenamides are used to accelerate the process via the transient formation of labile S-N bonds. The substituents on the sulfenamides determine the point at which they will become active. Temperature dependent activation of sulfenamide accelerants is useful in the vulcanization process because the temperature at which the rubber polymerizes determines the length of the sulfur chains, and properties such as the elasticity of the final product.

References

Functional groups