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.

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 degradation of dithiocarbamate salts, e.g. induced with lead nitrate.[5] A related method is tosyl chloride-mediated decomposition of dithiocarbamate salts.[6]

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:

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

References
  1. "Isothiocyanates". Micronutrient Information Center, Linus Pauling Institute, Oregon State University. 1 April 2017. Retrieved 14 April 2019.
  2. Majewska, Paulina; Rospenk, Maria; Czarnik-Matusewicz, Bogusława; Kochel, Andrzej; Sobczyk, Lucjan; Dąbrowski, Roman (2008). "Structure and polarized IR spectra of 4-isothiocyanatophenyl 4-heptylbenzoate (7TPB)". Chemical Physics. 354 (1–3): 186–195. Bibcode:2008CP....354..186M. doi:10.1016/j.chemphys.2008.10.024.
  3. Erian, Ayman W.; Sherif, Sherif M. (1999). "The chemistry of thiocyanic esters". Tetrahedron. 55 (26): 7957–8024. doi:10.1016/S0040-4020(99)00386-5.
  4. Emergon, David W. (1971). "The Preparation and Isomerization of Allyl Thiocyanate. An Organic Chemistry Experiment". Journal of Chemical Education. 48 (1): 81. Bibcode:1971JChEd..48...81E. doi:10.1021/ed048p81.
  5. Dains FB; Brewster RQ; Olander CP (1926). "Phenyl Isothiocyanate". Organic Syntheses. 6: 72. doi:10.15227/orgsyn.006.0072.
  6. Wong, R; Dolman, SJ (2007). "Isothiocyanates from tosyl chloride mediated decomposition of in situ generated dithiocarbamic acid salts". The Journal of Organic Chemistry. 72 (10): 3969–3971. doi:10.1021/jo070246n. PMID 17444687.
  7. O'Reilly, RJ; Radom, L (2009). "Ab initio investigation of the fragmentation of 5,5-diamino-substituted 1,4,2-oxathiazoles". Organic Letters. 11 (6): 1325–1328. doi:10.1021/ol900109b. PMID 19245242.
  8. Burkett, BA; Kane-Barber, JM; O'Reilly, RJ; Shi, L (2007). "Polymer-supported thiobenzophenone : a self-indicating traceless 'catch and release' linker for the synthesis of isothiocyanates". Tetrahedron Letters. 48 (31): 5355–5358. doi:10.1016/j.tetlet.2007.06.025.
  9. Ortega-Alfaro, M. C.; López-Cortés, J. G.; Sánchez, H. R.; Toscano, R. A.; Carrillo, G. P.; Álvarez-Toledano, C. (2005). "Improved approaches in the synthesis of new 2-(1, 3-thiazolidin-2Z-ylidene)acetophenones". Arkivoc. 2005 (6): 356–365. doi:10.3998/ark.5550190.0006.631. hdl:2027/spo.5550190.0006.631.
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