Isothiocyanate

In organic chemistry, isothiocyanate is a functional group as found in compounds with the formula R−N=C=S. Isothiocyanates are the more common isomers of thiocyanates, which have the formula R−S−C≡N.

General structure of an isothiocyanate.

Occurrence

Many isothiocyanates from plants are produced by enzymatic conversion of metabolites called glucosinolates. A prominent natural isothiocyanate is allyl isothiocyanate, also known as mustard oils.

Cruciferous vegetables, such as bok choy, broccoli, cabbage, cauliflower, kale, and others, are rich sources of glucosinolate precursors of isothiocyanates.^[1]

Structure

The N=C and C=S distances are 117 and 158 pm.^[2] By contrast, in methyl thiocyanate, N≡C and C−S distances are 116 and 176 pm.

Typical bond angles for C−N=C in aryl isothiocyanates are near 165°. Again, the thiocyanate isomers are quite different with C−S−C angle near 100°.^[3] In both isomers the SCN angle approaches 180°.

Synthesis

Allyl thiocyanate isomerizes to the isothiocyanate:^[4]

CH₂=CHCH₂SCN → CH₂=CHCH₂NCS

Isothiocyanates can be prepared by treating organic dithiocarbamate salts with lead nitrate or tosyl chloride.^[5][6]

Synthesis of phenyl isothiocyanate

Isothiocyanates may also be accessed by the fragmentation reactions of 1,4,2-oxathiazoles.^[7] This methodology has been applied to a polymer-supported synthesis of isothiocyanates.^[8]

Reactions

Isothiocyanates are weak electrophiles, susceptible to hydrolysis. In general, nucleophiles attack at carbon:

The reaction of acetophenone enolate with phenyl isothiocyanate. In this one-pot synthesis^[9] the ultimate reaction product is a Thiazolidine. This reaction is stereoselective with the formation of the Z-isomer only.

Electrochemical reduction gives thioformamides.^[10]: 340

Flavor research

Isothiocyanates occur widely in nature and are of interest in food science and medical research.^[1] Vegetable foods with characteristic flavors due to isothiocyanates include bok choy, broccoli, cabbage, cauliflower, kale, wasabi, horseradish, mustard, radish, Brussels sprouts, watercress, papaya seeds, nasturtiums, and capers.^[1] These species generate isothiocyanates in different proportions, and so have different, but recognizably related, flavors. They are all members of the order Brassicales, which is characterized by the production of glucosinolates, and of the enzyme myrosinase, which acts on glucosinolates to release isothiocyanates.^[1]

• Sinigrin is the precursor to allyl isothiocyanate
• Glucotropaeolin is the precursor to benzyl isothiocyanate
• Gluconasturtiin is the precursor to phenethyl isothiocyanate
• Glucoraphanin is the precursor to sulforaphane

Uses

Phenyl isothiocyanate, is used for amino acid sequencing in the Edman degradation.

Coordination chemistry

Isothiocyanate and its linkage isomer thiocyanate are ligands in coordination chemistry. Thiocyanate is a more common ligand.

See also

• Methyl isothiocyanate