Sulfenic acid

While sulfenic acids have the potential of exhibiting tautomerism, spectroscopic measurements as well as theoretical studies indicate that the structure on the left predominates almost exclusively.

A sulfenic acid is an organosulfur compound and oxoacid with the general formula RSOH, where R ≠ H. It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids and sulfonic acids, RSO₂H and RSO₃H, respectively.

Properties

In contrast to sulfinic and sulfonic acids, simple sulfenic acids, such as methanesulfenic acid, CH₃SOH, are highly reactive and cannot be isolated in solution. In the gas phase the lifetime of methanesulfenic acid is about one minute. The gas phase structure of methanesulfenic acid was found by microwave spectroscopy (rotational spectroscopy) to be CH₃–S–O–H. Sulfenic acids can be stabilized through steric effects, which prevent the sulfenic acid from condensing with itself to form thiosulfinates, RS(O)SR, such as allicin from garlic. Through the use of X-ray crystallography, the structure of such stabilized sulfenic acids were shown to be R–S–O–H. The stable, sterically hindered sulfenic acid, 1-triptycenesulfenic acid, has been found to have a pKa of 12.5 and an O–H bond-dissociation energy (bde) of 71.9 ± 0.3 kcal/mol, which can be compared to a pKa of ≥14 and O–H BDE of ∼88 kcal/mol for the (valence) isoelectronic hydroperoxides, ROOH.

Formation and occurrence

In nature

Sulfenic acids are produced by the enzymatic decomposition of alliin and related compounds following tissue damage to garlic, onions, and other plants of the Allium genus. 1-Propenesulfenic acid, formed when onions are cut, is rapidly rearranged by a second enzyme, the lachrymatory factor synthase, giving syn-propanethial-S-oxide. 2-Propenesulfenic acid, formed from allicin, is thought to be responsible for garlic’s potent antioxidant activity. Mass spectrometry with a DART ion source were used to identify 2-propenesulfenic formed when garlic is cut or crushed and to demonstrate that this sulfenic acid has a lifetime of less than one second. The pharmacological activity of certain drugs, such as omeprazole, esomeprazole, ticlopidine, clopidogrel, and prasugrel is proposed to involve sulfenic acid intermediates. Oxidation of cysteine residues in protein to the corresponding protein sulfenic acids is suggested to be important in redox-mediated signal transduction.

Organic chemistry

The conversion of certain sulfoxides to alkenes proceeds via the elimination of sulfenic acid:

RS(O)CH₂CH₂R' → RSOH + CH₂=CHR'

Other sulfenyl compounds

The prefix sulfenyl in organic nomenclature denotes the RS group (R≠H). One example is methanesulfenyl chloride is CH₃SCl. Sulfenate esters, which have the formula RSOR', are intermediates in the Mislow-Evans rearrangement of allyl sulfoxides. Sulfenamides have the formula RSNR'₂.

Cyclohexylthiophthalimide is an example of a sulfenamide, yet another derivative of sulfenic acid.