Hydroxamic acid

In organic chemistry, hydroxamic acids are a class of organic compounds bearing the functional group R−C(=O)−N(OH)−R', with R and R' as organic residues. They are amides (R−C(=O)−NH−R') wherein the nitrogen center has a hydroxyl (−OH) substituent. They are often used as metal chelators.[1]

The general structure of a hydroxamic acid

Synthesis and reactions

Hydroxamic acids are usually prepared from either esters or acid chlorides by a reaction with hydroxylamine salts. For the synthesis of benzohydroxamic acid, the overall equation is:[2]

C6H5CO2Me + NH2OH → C6H5C(O)NHOH + MeOH

Hydroxamic acids can also be synthesized from aldehydes and N-sulfonylhydroxylamine via the Angeli-Rimini reaction.[3]

A well-known reaction of hydroxamic acid esters is the Lossen rearrangement.[4]

Coordination chemistry and biochemistry

The conjugate base of hydroxamic acids forms is called a hydroxamate. Deprotonation occurs at the NOH group. The resulting conjugate base presents the metal with an anionic, conjugated O,O chelating ligand. Many hydroxamic acids and many iron hydroxamates have been isolated from natural sources.[6]

They function as ligands, usually for iron.[7] Nature has evolved families of hydroxamic acids to function as iron-binding compounds (siderophores) in bacteria. They extract iron(III) from otherwise insoluble sources (rust, minerals, etc.). The resulting complexes are transported into the cell, where the iron is extracted and utilized metabolically.[8]

Ligands derived from hydroxamic acid and thiohydroxamic acid also form strong complexes with lead(II).[9]

Other uses and occurrences

Hydroxamic acids are used extensively in flotation of rare earth minerals during the concentration and extraction of ores to be subjected to further processing.[10][11]

Some hydroxamic acids (e.g. vorinostat, belinostat, panobinostat, and trichostatin A) are HDAC inhibitors with anti-cancer properties. Fosmidomycin is a natural hydroxamic acid inhibitor of 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXP reductoisomerase). Hydroxamic acids have also been investigated for reprocessing of irradiated fuel.

References
  1. Munson, James W. (1992). "Hydroxamic acids". In S. Patai (ed.). Acid Derivatives (1992), Volume 2. PATAI'S Chemistry of Functional Groups. pp. 849–873. doi:10.1002/9780470772508.ch15. ISBN 9780470772508.
  2. C. R. Hauser; W. B. Renfrow Jr (1939). "Benzohydroxamic Acid". Org. Synth. 19: 15. doi:10.15227/orgsyn.019.0015.
  3. Li, Jie Jack (2003). Name Reactions: A Collection of Detailed Reaction Mechanisms (2nd ed.). Berlin, Heidelberg, New York: Springer. p. 9. ISBN 978-3-662-05338-6.
  4. Wang, Zerong (2010). Comprehensive organic name reactions and reagents. John Wiley & Sons, Inc. pp. 1772–1776. ISBN 9780471704508.
  5. Hossain, M. B.; Eng-Wilmot, D. L.; Loghry, R. A.; an der Helm, D. (1980). "Circular Dichroism, Crystal Structure, and Absolute Configuration of the Siderophore Ferric N,N',N"-Triacetylfusarinine, FeC39H57N6O15". Journal of the American Chemical Society. 102 (18): 5766–5773. doi:10.1021/ja00538a012.
  6. Abraham Shanzer, Clifford E. Felder, Yaniv Barda (2008). "Natural and Biomimetic Hydroxamic Acid based Siderophores". In Zvi Rappoport, Joel F. Liebman (ed.). The Chemistry of Hydroxylamines, Oximes and Hydroxamic Acids. PATAI'S Chemistry of Functional Groups. pp. 751–815. doi:10.1002/9780470741962.ch16. ISBN 9780470512616.{{cite book}}: CS1 maint: multiple names: authors list (link)
  7. Agrawal, Y K (1979). "Hydroxamic Acids and Their Metal Complexes". Russian Chemical Reviews. 48 (10): 948–963. Bibcode:1979RuCRv..48..948A. doi:10.1070/RC1979v048n10ABEH002422. S2CID 250857281.
  8. Miller, Marvin J. (November 1989). "Syntheses and Therapeutic Potential of Hydroxamic Acid Based Siderophores and Analogues". Chemical Reviews. 89 (7): 1563–1579. doi:10.1021/cr00097a011.
  9. Farkas, Etelka; Buglyó, Péter (2017). "Chapter 8. Lead(II) Complexes of Amino Acids, Peptides, and Other Related Ligands of Biological Interest". In Astrid, S.; Helmut, S.; Sigel, R. K. O. (eds.). Lead: Its Effects on Environment and Health. Metal Ions in Life Sciences. Vol. 17. de Gruyter. pp. 201–240. doi:10.1515/9783110434330-008. ISBN 9783110434330. PMID 28731301.
  10. Marion, Christopher; Jordens, Adam; Li, Ronghao; Rudolph, Martin; Waters, Kristian E. (August 2017). "An evaluation of hydroxamate collectors for malachite flotation". Separation and Purification Technology. 183: 258–269. doi:10.1016/j.seppur.2017.02.056.
  11. Jordens, Adam; Cheng, Ying Ping; Waters, Kristian E. (February 2013). "A review of the beneficiation of rare earth element bearing minerals". Minerals Engineering. 41: 97–114. Bibcode:2013MiEng..41...97J. doi:10.1016/j.mineng.2012.10.017.

Further reading
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