Potassium Thiocyanate1


[333-20-0]  · CKNS  · Potassium Thiocyanate  · (MW 97.19)

(reagent for the thiocyanation and isothiocyanation of many compounds;1 converts oxiranes to thiiranes stereospecifically11)

Physical Data: mp 173 °C; d 1.886 g cm-3.

Solubility: very sol H2O (217 g/100 mL at 20 °C); sol acetone (21 g/100 mL at 22 °C); sol EtOH.

Form Supplied in: colorless or white crystals; also available as volumetric standard in water (0.1 N). Drying: by heating to 165 °C under vacuum.

Purification: recrystallized from water or alcohol.

Handling, Storage, and Precautions: should be stored in a dark bottle.

Substitution Reactions.

Many early substitution reactions of metal thiocyanate salts were plagued by the ambident character of thiocyanate. Substitution via S affords a thiocyanate while attack via N yields an isothiocyanate. Often both products were formed and the efficiency of a synthesis of one product was measured by the rate ratio for the formation of both possible products, kS/kN. Reactions of KSCN with primary and secondary halides and sulfonates usually afforded products with a high kS/kN ratio, while the reaction of tertiary halides gave a ratio close to 1.2 The use of other metal thiocyanates (Ba or Ca(SCN)2) on an inorganic solid support provides a method for switching the S or N nucleophilicity for reactions with tertiary bromides (eq 1).3 Solid-supported KSCN gives exclusive S attack of primary and simple benzylic halides but the kS/kN ratio remains moderate for tertiary systems.4 Phase transfer catalysts enhance the smooth transformation of halides to thiocyanates with KSCN.1,5 Alkyl isothiocyanates can be prepared in good yield by preparing the analogous thiocyanate and heating, preferably in the presence of metal salts, acids, or excess thiocyanate.6 The isothiocyanate is usually the thermodynamically preferred product.1 Various metal thiocyanates have been used to prepare acyl, phosphoryl, and sulfonyl isothiocyanates, probably by S attack of thiocyanate on a reactive acid derivative and then isomerization.1

Potassium thiocyanate is an effective reagent for the substitution on unsaturated systems to form thiocyanates. Nitrogen of diazonium salts is replaced by thiocyanate, but use of a cocatalyst is recommended to improve the yield and the kS/kN ratio. Thiocyanates have also been formed from the substitution reaction of KSCN with imidoyl chlorides,1 allenyl chlorides,7 some chloroalkenes,1 and nitrophenyl halides.1 Thiocyanate supplants the halide of some phenyl iodides and bromides when these are treated with charcoal-supported CuSCN, a reagent which was prepared with the aid of KSCN (eq 2).8 KSCN performs a selective alkyne substitution on alkynyl(p-phenylene)bisiodonium ditriflates (eq 3).9 KSCN is the preferred reagent for the thiocyanation of saccharides.10

Sulfur Heterocycles.

Potassium thiocyanate is one of several reagents that converts oxiranes to thiiranes. KSCN seems to be the reagent of choice for a broad range of substituted oxiranes,11 but reagents such as Thiourea,11 N,N-Dimethylthioformamide,12 or 3-methylbenzothiazole-2-thione13 may have advantages in specific cases. With KSCN, various ring fused thiiranes can be formed (eq 4). In its reaction with substituted oxiranes, KSCN reacts by a stereospecific mechanism in that the substituent geometry of the oxirane is preserved in the thiirane. However, the reaction of KSCN with bromohydrins derived from trans-alkenes affords cis-thiiranes.14 Hence, starting with a single 1,2-disubstituted alkene, a thiirane of either substituent geometry can be obtained. KSCN supported on silica is also effective for thiirane synthesis (eq 4).15

The reaction of cyclic carbonates with KSCN has been shown to be a method for the synthesis of some thiiranes16 and thietanes,17 depending on the number of carbons and nature of substituents in the starting material (eq 5).7

Related Reagents.

Sodium Thiocyanate.

1. (a) Guy, R. G. In The Chemistry of Cyanates and their Thio Derivatives Part 2; Patai, S. Ed.; Wiley: New York, 1977; p 819. (b) Bacon, R. G. R. In Organic Sulfur Compounds, Kharasch, N., Ed.; Pergamon: New York, 1961; Vol. 1, p 306. (c) Drobnica, L.; Kristián, P.; Augustín, J. in Ref. 1(a), p 1003.
2. Watanabe, N.; Okano, M.; Uemura, S. BCJ 1974, 47, 2745.
3. Kimura, T.; Fujita, M.; Ando, T. CC 1990, 1213.
4. Ando, T.; Clark, J. H.; Cork, D. G.; Fujita, M.; Kimura, T. JOC 1987, 52, 681.
5. Kondo, S.; Takeda, Y.; Tsuda, K. S 1988, 403.
6. Fava, A.; Iliceto, A.; Bresadola, S. JACS 1965, 87, 4791.
7. Kitamura, T.; Miyake, S.; Kobayashi, S.; Taniguchi, H. BCJ 1989, 62, 967.
8. Clark, J. H.; Jones, C. W.; Duke, C. V. A.; Miller, J. M. CC 1989, 81.
9. Kitamura, T.; Furuki, R.; Zheng. L.; Fujimoto, T.; Taniguchi, H. CL 1992, 2241.
10. Witczak, Z. J. Adv. Carbohydr. Chem. Biochem. 1986, 44, 91.
11. (a) Zoller, U. In Small Ring Heterocycles; Hassner, A. Ed.; Wiley: New York, 1983; Part 1, p 333. (b) Fokin, A. V.; Allakhverdiev, M. A.; Kolomiets, A. F. RCR 1990, 59, 405.
12. Takido, T.; Kobayashi, Y.; Itabashi, K. S 1986, 779.
13. Calõ, V.; Lopez, L.; Marchese, L.; Pesce, G. CC 1975, 621.
14. Van Ende, D.; Krief, A. TL 1975, 2709.
15. Brimeyer, M. O.; Mehrota, A.; Quici, S.; Nigam, A.; Regen, S. L. JOC 1980, 45, 4254.
16. Searles, S. Jr.; Hays, H. R.; Lutz, E. F. JOC 1962, 27, 2832.
17. Searles, S. Jr.; Hays, H. R.; Lutz, E. F. JOC 1962, 27, 2828.

Adrian L. Schwan

University of Guelph, Ontario, Canada

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.