Pyrazole

Pyrazole is an organic compound with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms. Pyrazole is a weak base, with pKb 11.5 (pKa of the conjugated acid 2.49 at 25 °C).[2] Pyrazoles are also a class of compounds that have the ring C3N2 with adjacent nitrogen atoms.[3] Notable drugs containing a pyrazole ring are celecoxib (Celebrex) and the anabolic steroid stanozolol.

Pyrazole
Full structural formula
Skeletal formula with numbers
Ball-and-stick model
Space-filling model
Names
Preferred IUPAC name
1H-Pyrazole[1]
Systematic IUPAC name
1,2-Diazacyclopenta-2,4-diene
Other names
1,2-Diazole
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.005.471
KEGG
UNII
Properties
C3H4N2
Molar mass 68.079 g·mol−1
Melting point 66 to 70 °C (151 to 158 °F; 339 to 343 K)
Boiling point 186 to 188 °C (367 to 370 °F; 459 to 461 K)
Basicity (pKb) 11.5
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Preparation and reactions

Pyrazoles are synthesized by the reaction of α,β-unsaturated aldehydes with hydrazine and subsequent dehydrogenation:[4]

Substituted pyrazoles are prepared by condensation of 1,3-diketones with hydrazine (Knorr-type reactions).[5] For example, acetylacetone and hydrazine gives 3,5-dimethylpyrazole:[6]

CH3C(O)CH2C(O)CH3   +   N2H4   →   (CH3)2C3HN2H   +   2 H2O
Novel pyrazole ligands

History

The term pyrazole was given to this class of compounds by German Chemist Ludwig Knorr in 1883.[7] In a classical method developed by German chemist Hans von Pechmann in 1898, pyrazole was synthesized from acetylene and diazomethane.[8]

Conversion to scorpionates

Pyrazoles react with potassium borohydride to form a class of ligands known as scorpionate. Pyrazole itself reacts with potassium borohydride at high temperatures (~200 °C) to form a tridentate ligand known as Tp ligand:

3,5-Diphenyl-1H-pyrazole

3,5-Diphenyl-1H-pyrazole is produced when (E)-1,3-diphenylprop-2-en-1-one is reacted with hydrazine hydrate in the presence of elemental sulfur[9] or sodium persulfate,[10] or by using a hydrazone in which case an azine is produced as a by-product.[11]

Occurrence and uses
Celecoxib, a pyrazole derivative used as an analgesic

In 1959, the first natural pyrazole, 1-pyrazolyl-alanine, was isolated from seeds of watermelons.[12][13]

In medicine, derivatives of pyrazole are widely used[14]

The pyrazole ring is found within a variety of pesticides as fungicides, insecticides and herbicides, including chlorfenapyr, fenpyroximate, fipronil, tebufenpyrad, tolfenpyrad, and tralopyril.[15]. Pyrazole moieties are listed among the highly used ring systems for small molecule drugs by the US FDA [16]

See also

References
  1. "Front Matter". Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 141. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4.
  2. Dissociation constants of organic acids and bases
  3. Eicher, T.; Hauptmann, S. (2003). The Chemistry of Heterocycles: Structure, Reactions, Syntheses, and Applications (2nd ed.). Wiley-VCH. ISBN 3-527-30720-6.
  4. Schmidt, Andreas; Dreger, Andrij (2011). "Recent Advances in the Chemistry of Pyrazoles. Properties, Biological Activities, and Syntheses". Curr. Org. Chem. 15 (9): 1423–1463. doi:10.2174/138527211795378263.
  5. Nozari, M., Addison, A., Reeves, G.T, Zeller, M., Jasinski, J.P., Kaur, M., Gilbert, J. G., Hamilton, C. R., Popovitch, J. M., Wolf, L. M., Crist, L. E., Bastida, N., (2018) Journal of heterocyclic Chemistry 55, 6, 1291-1307. https://doi.org/10.1002/jhet.3155
  6. Johnson, William S.; Highet, Robert J. (1963). "3,5-Dimethylpyrazole". Organic Syntheses.; Collective Volume, 4, p. 351
  7. Knorr, L. (1883). "Action of ethyl acetoacetate on phenylhydrazine. I". Chemische Berichte. 16: 2597–2599. doi:10.1002/cber.188301602194.
  8. von Pechmann, Hans (1898). "Pyrazol aus Acetylen und Diazomethan". Berichte der deutschen chemischen Gesellschaft (in German). 31 (3): 2950–2951. doi:10.1002/cber.18980310363.
  9. Outirite, Moha; Lebrini, Mounim; Lagrenée, Michel; Bentiss, Fouad (2008). "New one step synthesis of 3,5-disubstituted pyrazoles under microwave irradiation and classical heating". Journal of Heterocyclic Chemistry. 45 (2): 503–505. doi:10.1002/jhet.5570450231.
  10. Zhang, Ze; Tan, Ya-Jun; Wang, Chun-Shan; Wu, Hao-Hao (2014). "One-pot synthesis of 3,5-diphenyl-1H-pyrazoles from chalcones and hydrazine under mechanochemical ball milling". Heterocycles. 89 (1): 103–112. doi:10.3987/COM-13-12867.
  11. Lasri, Jamal; Ismail, Ali I. (2018). "Metal-free and FeCl3-catalyzed synthesis of azines and 3,5-diphenyl-1H-pyrazole from hydrazones and/or ketones monitored by high resolution ESI+-MS". Indian Journal of Chemistry, Section B. 57B (3): 362–373.
  12. Fowden; Noe; Ridd; White (1959). Proc. Chem. Soc.: 131. Missing or empty |title= (help)
  13. Noe, F. F.; Fowden, L.; Richmond, P. T. (1959). "alpha-Amino-beta-(pyrazolyl-N) propionic acid: a new amino-acid from Citrullus vulgaris (water melon)". Nature. 184 (4688): 69–70. Bibcode:1959Natur.184...69B. doi:10.1038/184069a0. PMID 13804343.
  14. Faria, Jéssica Venância; Vegi, Percilene Fazolin; Miguita, Ana Gabriella Carvalho; dos Santos, Maurício Silva; Boechat, Nubia; Bernardino, Alice Maria Rolim (1 November 2017). "Recently reported biological activities of pyrazole compounds". Bioorganic & Medicinal Chemistry. 25 (21): 5891–5903. doi:10.1016/j.bmc.2017.09.035. ISSN 0968-0896. PMID 28988624.
  15. FAO
  16. Taylor, R. D.; MacCoss, M.; Lawson, A. D. G. J Med Chem 2014, 57, 5845.

Further reading

A. Schmidt; A. Dreger (2011). "Recent Advances in the Chemistry of Pyrazoles. Part 2. Reactions and N-Heterocyclic Carbenes of Pyrazole". Curr. Org. Chem. 15 (16): 2897–2970. doi:10.2174/138527211796378497.

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