Nitrone Explained

In organic chemistry, a nitrone is a functional group consisting of an N-oxide of an imine. The general structure is, 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.

Structure

Nitrones, as a tetrasubstituted double bond, admit cistrans isomerism.[1]

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] The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:[3] However, a hydroxylamine with two α hydrogens may unsaturate on either side. Carbonyl condensation avoids this ambiguity...[4] ...but is inhibited if both ketone substituents are bulky.

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

Reactions

Some nitrones oligomerize:[5] 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.

Nitrones hydrolyze extremely easily to the corresponding carbonyl and N-hydroxylamine.

1,3-dipolar cycloadditions

See main article: Nitrone-olefin 3+2 cycloaddition. As 1,3dipoles, nitrones perform [3+2] cycloadditions.[6] For example, a dipolarophilic alkene combines to form isoxazolidine:

Other ring-closing reactions are known,[7] including formal [3+3] and [5+2] cycloadditions.

Isomerization

Deoxygenating reagents, light, or heat all catalyze rearrangement to the amide. Acids catalyze rearrangement to the oxime ether.

Reduction

Hydrides add to give hydroxylamines. Reducing Lewis acids (e.g. metals, ) deoxygenate to the imine instead.

See also

Notes and References

  1. Hamer . Jan . Macaluso . Anthony . 1964-08-01 . Nitrones . Chemical Reviews . en . 64 . 4 . 473–495 . 10.1021/cr60230a006 . 0009-2665. subscription .
  2. Delpierre . G. R. . Lamchen . M. . 1965 . Nitrones . Quarterly Reviews, Chemical Society . en . 19 . 4 . 329 . 10.1039/qr9651900329 . 0009-2681.
  3. Thiesing . Jan . Mayer . Hans . 1957 . Cyclische Nitrone, II. Über die Polymeren des 2.3.4.5-Tetrahydro-pyridin-N-oxyds und verwandte Verbindungen . . 609 . 46-57 . 10.1002/jlac.19576090105.
  4. Exner. O.. A New Synthesis of N-methylketoximes. ChemPlusChem. 1951. 16. 258-267. 10.1135/cccc19510258.
  5. Thiesing . Jan . Mayer . Hans . 1956 . Cyclische Nitrone I: Dimeres 2.3.4.5-Tetrahydro-pyridin-N-oxyd . . 89 . 9 . 2159-2167 . 10.1002/cber.19560890919.
  6. Yang . Jiong . 2012 . Recent Developments in Nitrone Chemistry . . 23 . 2293-97 . 10.1055/s-0032-1317096.
  7. Murahashi . Shun-Ichi . Imada . Yasushi . 15 March 2019 . Synthesis and Transformations of Nitrones for Organic Synthesis . Chemical Reviews . 119 . 7 . 4684–4716 . 10.1021/acs.chemrev.8b00476 . 30875202 . 80623450.