Chemistry: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
Isothiocyanates can be prepared by treating organic dithiocarbamate salts with lead nitrate or tosyl chloride:[4][5]
Synthesis of phenyl isothiocyanate
They generally cannot be prepared from alkylation of thiocyanate salts, as the ion's S terminus is much more nucleophilic. However, stabilized carbocations can reionize and rearrange to the thermodynamically preferred isothiocyanate.[6] For example, allyl thiocyanate isomerizes to the isothiocyanate:[7]
- CH
2=CHCH
2SCN → CH
2=CHCH
2NCS
Isothiocyanates may also be accessed by the fragmentation reactions of 1,4,2-oxathiazoles, themselves a product of a nitrile oxide and a thiocarbonyl.[8][9]
In biology, the mustard oil bomb forms isothiocyanates through a variation on the Lossen rearrangement (uncatalyzed).
In the presence of thallous acetate, sufficiently strong-acid disulfide esters (e.g. bis(para‑nitrobenzoyl) disulfide) react with isocyanides to give isothiocyanates.[10]
Reactions
Isothiocyanates are weak electrophiles, susceptible to hydrolysis. They are much less reactive than structurally analogous isocyanates.[11]
In general, nucleophiles attack at carbon:
![The reaction of acetophenone enolate with phenyl isothiocyanate. In this one-pot synthesis[12] the ultimate reaction product is a Thiazolidine. This reaction is stereoselective with the formation of the Z-isomer only.](/wiki/File:ThiazolidineSynthesis.gif)
Electrochemical reduction gives thioformamides.[13]: 340
Reaction with mercuric fluoride gives the corresponding isocyanide difluoride.[14]
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.
Phenylene diisothiocyanate is an antihelminthic.
Coordination chemistry
Isothiocyanate and its linkage isomer thiocyanate are ligands in coordination chemistry. Thiocyanate is a more common ligand.
See also
References
- ↑ 1.0 1.1 1.2 1.3 "Isothiocyanates". Micronutrient Information Center, Linus Pauling Institute, Oregon State University. 2026. https://lpi.oregonstate.edu/mic/dietary-factors/phytochemicals/isothiocyanates.
- ↑ 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. doi:10.1016/j.chemphys.2008.10.024. Bibcode: 2008CP....354..186M.
- ↑ 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.
- ↑ Dains FB; Brewster RQ; Olander CP (1926). "Phenyl Isothiocyanate". Organic Syntheses 6: 72. doi:10.15227/orgsyn.006.0072.
- ↑ 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.
- ↑ Mayr, Herbert (2011). "Farewell to the HSAB Treatment of Ambident Reactivity". Angewandte Chemie International Edition 50 (29): 6483–6486. doi:10.1002/anie.201007100. PMID 21726020. Bibcode: 2011ACIE...50.6470Mnone, an excerpt from Breugst, Robert Martin (2010). A Marcus-Theory-Based Approach to Ambident Reactivity (PDF) (PhD dissertation). Ludwig-Maximilians-Universität München. pp. 344–351.
- ↑ Emergon, David W. (1971). "The Preparation and Isomerization of Allyl Thiocyanate. An Organic Chemistry Experiment". Journal of Chemical Education 48 (1): 81. doi:10.1021/ed048p81. Bibcode: 1971JChEd..48...81E.
- ↑ 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.
- ↑ 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.
- ↑ Tanaka Sakuya; Uemura Sakae; Okano Masaya (Oct 1977). "The thallium(I) salt-catalyzed formation of isothiocyanates from isocyanides and disulfides". Bulletin of the Chemical Society of Japan 50 (10). doi:10.1246/bcsj.50.2785.
- ↑ Li, Zhen; Mayer, Robert J.; Ofial, Armin R.; Mayr, Herbert (2020-04-27). "From Carbodiimides to Carbon Dioxide: Quantification of the Electrophilic Reactivities of Heteroallenes". Journal of the American Chemical Society 142 (18): 8383–8402. doi:10.1021/jacs.0c01960. PMID 32338511. Bibcode: 2020JAChS.142.8383L.
- ↑ 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.
- ↑ Hammerich, Ole; Parke, Vernon D. (1977). "The electrochemistry of cyanates and related compounds". in Patai, Saul. The Chemistry of Cyanates and Their Thio Derivatives. Part 1. Chichester: Wiley. ISBN 0-471-99477-4.
- ↑ Ulrich, H. The Chemistry of Imidoyl Halides; Plenum Press: New York, 1968; pp. 18–19.
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