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Nitrone

Inorganic 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 intermediatesinchemical synthesis. A nitrone is a 1,3-dipole used in cycloadditions, and a carbonyl mimic.

Structure[edit]

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

Generation of nitrones[edit]

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}

Reactions[edit]

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.

Carbonyl mimic[edit]

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}

1,3-dipolar cycloadditions[edit]

As1,3‑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]

Isomerization[edit]

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

Reduction[edit]

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

See also[edit]

• N-Oxoammonium salt
• Nitronate

References[edit]