Sulfilimine

In chemistry, a sulfilimine (or sulfimide) is a type of chemical compound containing a sulfur-to-nitrogen bond which is often represented as a double bond (S=N). In fact, a double bond violates the octet rule, and the bond may be considered a single bond with a formal charge of +1 on the sulfur and a formal charge of −1 on the nitrogen. The parent compound is sulfilimine H₂S=NH, which is mainly of theoretical interest.

Ball-and-stick model of the diphenylsulfimide molecule, Ph₂S=NH^[1]

Examples include S,S-diphenylsulfilimine^[2] and sulfoximines [Category] such as methylphenylsulfoximine:^[3]

Methyl-phenylsulfoximine, a sulfur(VI) compound (left), and S,S-diphenylsulfilimine, a sulfur(IV) compound (right)

In the case of a sulfoximine, the bonds can be considered single bonds, with formal charges of −1 on both the oxygen and the nitrogen, and a formal charge of +2 on the sulfur.

Preparation

Most sulfilimines are N-substituted with electron-withdrawing groups. These compounds are typically prepared by oxidation of thioethers with electrophilic amine reagents, such as chloramine-T in the presence of a base:^[4]

R₂S + ClNHTs → R₂S=NTs + HCl

An alternative route involves reactions of electrophilic sulfur compounds with amines. The imidosulfonium reagents provide a source of "Me₂S²⁺", which are attacked by amines.

In general, aliphatic sulfilimines, unsubstituted at N, are not stable above −30 °C (−22 °F).^[5]

KMnO₄ can oxidize sulfilimines to sulfoximines, but the latter are more generally produced from addition of azides to sulfoxides.^[5]

Sulfilimine bonds in proteins

Sulfilimine bonds stabilize collagen IV strands found in the extracellular matrix^[6] and arose at least 500 mya.^[7] These bonds covalently connect hydroxylysine and methionine residues of adjacent polypeptide strands to form a larger collagen trimer.