Nitrone
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In organic chemistry, a nitrone is a functional group consisting of an N-oxide of an imine. The general structure is Template:Chem2, 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.[1]Template:Rp
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.[1]Template:Rp[2]Template:Rp The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:[1]Template:Rp[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]Template:Rp
In principle, N-alkylation could produce nitrones from oximes, but in practice electrophiles typically perform a mixture of N- and O-attack.[1]Template:Rp[2]Template:Rp
Reactions
Some nitrones oligomerize:[1]Template:Rp[2]Template:Rp[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.[1]Template:Rp[2]Template:Rp
Nitrones hydrolyze extremely easily to the corresponding carbonyl and N-hydroxylamine.[1]Template:Rp[2]Template:Rp
1,3-dipolar cycloadditions
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Other ring-closing reactions are known,[7] including formal [3+3] and [5+2] cycloadditions.[6]
Isomerization
Deoxygenating reagents, light, or heat all catalyze rearrangement to the amide. Acids catalyze rearrangement to the oxime ether.[1]Template:Rp[2]Template:Rp
Reduction
Hydrides add to give hydroxylamines. Reducing Lewis acids (e.g. metals, [[Sulfur dioxide|Template:Chem2]]) deoxygenate to the imine instead.[1]Template:Rp[2]Template:Rp
See also
References
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