In organic chemistry, a nitrone is a functional group consisting of an N-oxide of an imine. The general structure is , where R<sup>3</sup> 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 cisâÂÂtrans isomerism.
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. The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:
However, a hydroxylamine with two ñ hydrogens may unsaturate on either side. Carbonyl condensation avoids this ambiguity... ...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.
Some nitrones oligomerize: 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 ñ and ò carbons towards reaction. The ñ carbon is an electrophile and the ò 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.
As 1,3dipoles, nitrones perform [3+2] cycloadditions. For example, a dipolarophilic alkene combines to form isoxazolidine:
Other ring-closing reactions are known, including formal [3+3] and [5+2] cycloadditions.
Deoxygenating reagents, light, or heat all catalyze rearrangement to the amide. Acids catalyze rearrangement to the oxime ether.
Hydrides add to give hydroxylamines. Reducing Lewis acids (e.g. metals, ) deoxygenate to the imine instead.