Thioureas

Thioureas are members of a family of organosulfur compounds with the formula SC(NR2)2. The parent member of this class of compounds is thiourea SC(NH2)2. The thiourea functional group has a planar CSN2 core.

Methylthiouracil is a thiourea derivative used as an antithyroid drug.

Structure and bonding

Thioureas have planar N2CS core. The C=S bond distance is near 1.71 Å, which is 0.1 Å longer than in normal ketones. The C-N distances are short.[1] Thioureas occurs in two tautomeric forms. For the parent thiourea, the thione form predominates in aqueous solutions.[2] The thiol form, known as an isothiourea, can be encountered in substituted compounds such as isothiouronium salts.

Synthesis

N,N′-unsubstituted thioureas can be prepared by treating the corresponding cyanamide with hydrogen sulfide or similar sulfide sources.[3] Organic ammonium salts react with potassium thiocyanate as the source of the thiocarbonyl.[4]

Alternatively, N,N′-disubstituted thioureas can be prepared by coupling two amines with thiophosgene:[5]

2 R2NH + CSCl2 + 2  (R2N)(R′2N)CS + 2 HCl

Amines also condense with organic thiocyanates to give thioureas:[6]

R2NH + R′NCS → (R2N)(HR′N)CS

Cyclic thioureas are prepared by transamidation of thiourea with diamines. Ethylene thioureais synthesized by treating ethylenediamine with carbon disulfide.[7] In some cases, thioureas can be prepared by thiation of ureas using phosphorus pentasulfide.

Ethylene thiourea is an excellent accelerant of vulcanization of neoprene and polychloroprene rubbers.

Applications

Precursor to heterocycles

Thioureas are building blocks to pyrimidine derivatives. Thus thioureas condense with β-dicarbonyl compounds.[8] The amino group on the thiourea initially condenses with a carbonyl, followed by cyclization and tautomerization. Desulfurization delivers the pyrimidine. The pharmaceuticals thiobarbituric acid and sulfathiazole are prepared using thiourea.[9] 4-Amino-3-hydrazino-5-mercapto-1,2,4-triazole is prepared by the reaction of thiourea and hydrazine.

Catalysis

Some thioureas are vulcanization accelerators. Thioureas are also used in a research theme called thiourea organocatalysis.[10]

References
  1. D. Mullen and E. Hellner (1978). "A Simple Refinement of Density Distributions of Bonding Electrons. IX. Bond Electron Density Distribution in Thiourea, CS(NH2)2, at 123K". Acta Crystallogr. B34: 2789–2794. doi:10.1107/S0567740878009243.CS1 maint: uses authors parameter (link)
  2. Allegretti, P.E; Castro, E.A; Furlong, J.J.P (March 2000). "Tautomeric equilibrium of amides and related compounds: theoretical and spectral evidences". Journal of Molecular Structure: THEOCHEM. 499 (1–3): 121–126. doi:10.1016/S0166-1280(99)00294-8.
  3. Koketsu, Mamoru; Kobayashi, Chikashi; Ishihara, Hideharu (2003). "Synthesis of N-aryl-S-alkylthiocarbamates". Heteroatom Chemistry. 14: 374–378. doi:10.1002/hc.10163.
  4. Herr, R. J.; Kuhler, L.; Meckler, H.; Opalka, C. J. (2000). "A Convenient Method for the Preparation of Primary and Symmetrical N,N′-Disubstituted Thioureas". Synthesis: 1569. doi:10.1055/s-2000-7607.
  5. Yi-Bo Huang, Wen-Bin Yi, Chun Cai (2012). "Thiourea Based Fluorous Organocatalyst". Top. Curr. Chem. 308: 191–212. doi:10.1007/128_2011_248.CS1 maint: uses authors parameter (link)
  6. Miyabe, H.; Takemoto, Y. (2008). "Discovery and Application of Asymmetric Reaction by Multifunctional Thioureas". Bull Chem Soc Jpn. 81: 785.CS1 maint: uses authors parameter (link)
  7. C. F. H. Allen; C. O. Edens; James VanAllan. "Ethylene Thiourea". Organic Syntheses.; Collective Volume, 3, p. 394
  8. Foster, H. M., and Snyder, H. R. (1963). "4-Methyl-6-hydroxypyrimidine". Organic Syntheses.CS1 maint: multiple names: authors list (link); Collective Volume, 4, p. 638
  9. Bernd Mertschenk, Ferdinand Beck, Wolfgang Bauer (2002). "Thiourea and Thiourea Derivatives". Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH. doi:10.1002/14356007.a26_803.CS1 maint: uses authors parameter (link)
  10. R. Schreiner, Peter (2003). "Metal-free organocatalysis through explicit hydrogen bonding interactions". Chem. Soc. Rev. 32: 289–296. doi:10.1039/b107298f. PMID 14518182.

Further reading
  • Patai, S., ed. (1977). The Chemistry of double-bonded functional groups. New York, NY: John Wiley & Sons. pp. 1355–1496. ISBN 0-471-92493-8.
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