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
editMany 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
editThe 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
editAllyl 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]
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
editIsothiocyanates 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.](/wiki/File:ThiazolidineSynthesis.gif)
Electrochemical reduction gives thioformamides.[10]: 340
Flavor research
editIsothiocyanates 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
editPhenyl isothiocyanate, is used for amino acid sequencing in the Edman degradation.
Coordination chemistry
editIsothiocyanate and its linkage isomer thiocyanate are ligands in coordination chemistry. Thiocyanate is a more common ligand.
See also
editReferences
edit- ^ a b c d "Isothiocyanates". Micronutrient Information Center, Linus Pauling Institute, Oregon State University. 1 April 2017. Retrieved 14 April 2019.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ 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.
- ^ Hammerich, Ole; Parke, Vernon D. (1977). "The electrochemistry of cyanates and related compounds". In Patai, Saul (ed.). The Chemistry of Cyanates and Their Thio Derivatives. Vol. Part 1. Chichester: Wiley. ISBN 0-471-99477-4. LCCN 75-6913.