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Nitrone
Class of compounds

In organic chemistry, a nitrone is a functional group consisting of an N-oxide of an imine. The general structure is R1R2C=N+(−O−)(−R3), where R3 is not a hydrogen. Their primary application is intermediates in chemical synthesis. A nitrone is a 1,3-dipole used in cycloadditions, and a carbonyl mimic.

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Structure

Nitrones, as a tetrasubstituted double bond, admit cis–trans isomerism.1: 474 

Generation of nitrones

Typical nitrone sources are hydroxylamine oxidation or condensation with carbonyl compounds. Secondary hydroxylamines oxidize to nitrones in air over a timescale of several weeks, a process cupric salts accelerate.2: 476 3: 332–333  The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:4: 476 5

However, a hydroxylamine with two α hydrogens may unsaturate on either side. Carbonyl condensation avoids this ambiguity...6

...but is inhibited if both ketone substituents are bulky.7: 477 

In principle, N-alkylation could produce nitrones from oximes, but in practice electrophiles typically perform a mixture of N- and O-attack.8: 479 9: 334 

Reactions

Some nitrones oligomerize:10: 483 11: 334,337-338 12

Syntheses with nitrone precursors obviate the issue with increased temperature, to exaggerate entropic factors; or with a nitrone excess.

Carbonyl mimic

Like many other unsaturated functional groups, nitrones activate the α and β carbons towards reaction. The α carbon is an electrophile and the β carbon a nucleophile; that is, nitrones polarize like carbonyls and nitriles but unlike nitro compounds and vinyl sulfur derivatives.13: 483 14: 338–340 

Nitrones hydrolyze extremely easily to the corresponding carbonyl and N-hydroxylamine.15: 491 16: 344 

1,3-dipolar cycloadditions

Main article: Nitrone-olefin 3+2 cycloaddition

As 1,3‑dipoles, nitrones perform [3+2] cycloadditions.17 For example, a dipolarophilic alkene combines to form isoxazolidine:

Other ring-closing reactions are known,18 including formal [3+3] and [5+2] cycloadditions.19

Isomerization

Deoxygenating reagents, light, or heat all catalyze rearrangement to the amide. Acids catalyze rearrangement to the oxime ether.20: 489–490 21: 345–347 

Reduction

Hydrides add to give hydroxylamines. Reducing Lewis acids (e.g. metals, SO2) deoxygenate to the imine instead.22: 490 23: 343 

See also

References

  1. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  2. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  3. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩

  4. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  5. Thiesing, Jan; Mayer, Hans (1957). "Cyclische Nitrone, II. Über die Polymeren des 2.3.4.5-Tetrahydro-pyridin-N-oxyds und verwandte Verbindungen". Justus Liebigs Ann. Chem. 609: 46-57. doi:10.1002/jlac.19576090105. /wiki/Justus_Liebigs_Ann._Chem. ↩

  6. Exner, O. (1951). "A New Synthesis of N-methylketoximes". ChemPlusChem. 16: 258-267. doi:10.1135/cccc19510258. /wiki/ChemPlusChem ↩

  7. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  8. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  9. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩

  10. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  11. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩

  12. Thiesing, Jan; Mayer, Hans (1956). "Cyclische Nitrone I: Dimeres 2.3.4.5-Tetrahydro-pyridin-N-oxyd". Chem. Ber. 89 (9): 2159-2167. doi:10.1002/cber.19560890919. /wiki/Chem._Ber. ↩

  13. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  14. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩

  15. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  16. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩

  17. Yang, Jiong (2012). "Recent Developments in Nitrone Chemistry". Synlett. 23: 2293-97. doi:10.1055/s-0032-1317096. /wiki/Synlett ↩

  18. Murahashi, Shun-Ichi; Imada, Yasushi (15 March 2019). "Synthesis and Transformations of Nitrones for Organic Synthesis". Chemical Reviews. 119 (7): 4684–4716. doi:10.1021/acs.chemrev.8b00476. PMID 30875202. S2CID 80623450. /wiki/Doi_(identifier) ↩

  19. Yang, Jiong (2012). "Recent Developments in Nitrone Chemistry". Synlett. 23: 2293-97. doi:10.1055/s-0032-1317096. /wiki/Synlett ↩

  20. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  21. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩

  22. Hamer, Jan; Macaluso, Anthony (1964-08-01). "Nitrones". Chemical Reviews. 64 (4): 473–495. doi:10.1021/cr60230a006. ISSN 0009-2665. https://pubs.acs.org/doi/abs/10.1021/cr60230a006 ↩

  23. Delpierre, G. R.; Lamchen, M. (1965). "Nitrones". Quarterly Reviews, Chemical Society. 19 (4): 329. doi:10.1039/qr9651900329. ISSN 0009-2681. http://xlink.rsc.org/?DOI=qr9651900329 ↩