Nitrone

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.

Nitrones, as a tetrasubstituted double bond, admit cis–trans isomerism.^{[1] : 474}

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.^{[1] : 476 [2] : 332–333} The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:^{[1] : 476 [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.^{[1] : 477}

In principle, N-alkylation could produce nitrones from oximes, but in practice electrophiles typically perform a mixture of N- and O-attack.^{[1] : 479 [2] : 334}

Some nitrones oligomerize:^{[1] : 483 [2] : 334,337-338 [5]}

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

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.^{[1] : 483 [2] : 338–340}

Nitrones hydrolyze extremely easily to the corresponding carbonyl and N-hydroxylamine.^{[1] : 491 [2] : 344}

As 1,3‑dipoles, 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.^[6]

Deoxygenating reagents, light, or heat all catalyze rearrangement to the amide. Acids catalyze rearrangement to the oxime ether.^{[1] : 489–490 [2] : 345–347}

Hydrides add to give hydroxylamines. Reducing Lewis acids (e.g. metals, SO₂) deoxygenate to the imine instead.^{[1] : 490 [2] : 343}

• N-Oxoammonium salt
• Nitronate

• Functional groups

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• Short description is different from Wikidata