Thiocyanogen1

(SCN)2

[505-14-6]  · C2N2S2  · Thiocyanogen  · (MW 116.18)

(mono- or dithiocyanation of organic compounds by electrophilic or radical chemistry1)

Physical Data: mp -3 to -2 °C.

Solubility: sol alcohol, CS2, benzene, ether, acetic acid, halogenated solvents.

Preparative Methods: prepared for reaction in various solvents by treating dry Pb(SCN)2 with Bromine at rt, sometimes followed by filtration in an inert atmosphere. It can also be obtained by reacting Potassium Thiocyanate and Br2 at -70 °C or by the electrolysis of alkali or ammonium thiocyanates.

Handling, Storage, and Precautions: the neat material is explosive at >20 °C. Stable in acetic acid/anhydride as 0.1 M solution over 8 days. Use of polar protic solvents, including the stabilizing EtOH in CHCl3, may give erratic results and lower yields. Electrophilic reactions should be carried out in the dark or under diffuse light conditions. Use under N2 or Ar.

Substitution.

The electrophilic reactivity of the pseudohalogen thiocyanogen lies between those of Bromine and Iodine. Therefore, in reactions with aromatic compounds, anilines and phenols readily undergo para thiocyanation, but less activated aryl rings require a Friedel-Crafts catalyst.1 Polyaromatic species such as anthracene react readily with thiocyanogen, and the reaction may give a dithiocyanato derivative.1 Electron-rich heteroaromatic species such as pyrroles undergo thiocyanation readily (eq 1),1,2 but thiocyanation of thiophene requires a catalyst. Numerous other heterocycles react with thiocyanogen.1 Ipso substitution of silylated aromatics is also possible (eq 2).3 The polarization of thiocyanogen and the electrophilic nature of the mechanism prevent formation of any isothiocyanato compounds, products which may arise from nucleophilic substitution reactions with a metal thiocyanate.1

Other nucleophiles also attack thiocyanogen to effect substitution. Thiocyanation occurs at the nitrogen of primary and secondary amines,4 at the sulfur of divalent sulfur compounds1 and sulfinate anions,5 and at the carbon of enols,1 some enamines,6 and related nucleophiles.1 Organomercury compounds undergo thiocyanation at their metalated site.1 Grignard reagents do not tend to react well,1 but the reaction of organolithiums with Zn(SCN)2 and then N-Chlorosuccinimide affords the corresponding thiocyanate.7 Radical chain substitution proceeds well with primary and secondary benzylic hydrogens of phenyl alkanes. The reaction of tertiary benzylic hydrogens affords significant isothiocyanate.8

Electrophilic Addition.

Thiocyanogen may add across the p-bond of alkenes or alkynes by an electrophilic or radical mechanism. The electrophilic mechanism is similar to the electrophilic halogenation of alkenes: an initially formed cyanosulfonium ion (1) is attacked by thiocyanate or solvent to complete the addition process. Hence additions to simple alkenes occur with trans stereospecificity and no regiospecificity. On the other hand, with aryl alkenes the intermediate is not cyclic (2) and the overall addition occurs with regiospecificity but not stereospecificity.1

The interference of radical species is troublesome in the addition reaction. Other problems may occur through the formation of isothiocyanato thiocyanates which originate from nucleophilic attack on the intermediate cation by the nitrogen of the thiocyanate ion. The best yields of a,b-dithiocyanates are obtained when the reactions are performed (i) in the dark or with a radical inhibitor present (up to 15 mol %),9 (ii) with a metal or metal salt present,9,10 (iii) with a filtered thiocyanogen solution,9,10 and (iv) in polar or dipolar aprotic solvent9 (eq 3).10 A radical chain mechanism is also suitable for the synthesis of a,b-dithiocyanates from simple alkenes.1,11 The addition of thiocyanogen across alkynes should be done under radical conditions; the trans-isomer usually predominates.12 In all addition reactions of thiocyanogen with alkenes or alkynes, prolonged stirring may lead to the thermodynamically more stable isothiocyanate isomers.1,12


1. (a) Wood, J. L. OR 1946, 3, 240. (b) Guy, R. G. In The Chemistry of Cyanates and their Thio Derivatives; Patai, S., Ed.; Wiley: New York, 1977; Part 2, p 834. (c) Bacon, R. G. R. In Organic Sulfur Compounds; Kharasch, N., Ed.; Pergamon: New York, 1961; Vol. 1, p 306.
2. Bolós. J.; Pérez-Beroy, &AAacute;.; Gubert, S.; Anglada, L.; Sacristán, A.; Ortiz, J. A. T 1992, 48, 9567.
3. Christopfel, W. C.; Miller, L. L. JOC 1984, 49, 5198.
4. Kuhn, M.; Mecke, R. CB 1960, 93, 618.
5. Goerdeler, J.; Rosenthal, P. TL 1964, 3665.
6. Tokumitsu, T.; Hayashi, T. JOC 1985, 50, 1547.
7. Takagi, K.; Takachi, H.; Hayama, N. CL 1992, 509.
8. Bacon, R. G. R.; Irwin, R. S. JCS 1961, 2447.
9. Maxwell, R. J.; Silbert, L. S.; Russell, J. R. JOC 1977, 42, 1510.
10. Block, E.; Yencha, A. J.; Aslam, M.; Eswarakrishnan, V.; Luo, J.; Sano, A. JACS 1988, 110, 4748.
11. Guy, R. G.; Thompson, J. J. T 1978, 34, 541.
12. Guy, R. G.; Cousins, S.; Farmer, D. M.; Henderson, A. D.; Wilson, C. L. T 1980, 36, 1839.

Adrian L. Schwan

University of Guelph, Ontario, Canada



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