Nitrone
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 cis–''trans'' isomerism.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. The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:However, a hydroxylamine with two α 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.
Reactions
Some nitrones oligomerize: 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
As 1,3dipoles, nitrones perform 1,3-dipolar cycloaddition| cycloadditions. For example, a dipolarophilic alkene combines to form isoxazolidine:Other ring-closing reactions are known, including formal and cycloadditions.