Inorganic chemistry, anitrone is afunctional group consisting of anN-oxide of animine. The general structure isR1R2C=N+(−O−)(−R3), where R3 is not ahydrogen. Their primary application isintermediates inchemical synthesis. A nitrone is a1,3-dipole used incycloadditions, and acarbonyl mimic.
Nitrones, as a tetrasubstituteddouble bond, admitcis–trans isomerism.[1]: 474
Typical nitrone sources arehydroxylamine oxidation orcondensation withcarbonyl compounds. Secondary hydroxylamines oxidize to nitrones in air over a timescale of several weeks, a processcupric salts accelerate.[1]: 476 [2]: 332–333 The most general reagent used for the oxidation of hydroxylamines is aqueousmercuric oxide:[1]: 476 [3]
However, a hydroxylamine with twoα 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 fromoximes, but in practiceelectrophiles typically perform a mixture ofN- andO-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 otherunsaturated functional groups, nitrones activate theα and β carbons towards reaction. The α carbon is an electrophile and the β carbon a nucleophile; that is, nitronespolarize like carbonyls and nitriles but unlikenitro 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
As1,3‑dipoles, nitrones perform[3+2] cycloadditions.[6] For example, a dipolarophilicalkene combines to formisoxazolidine:
Otherring-closing reactions are known,[7] including formal [3+3] and [5+2]cycloadditions.[6]
Deoxygenating reagents,light, or heat all catalyze rearrangement to theamide. Acids catalyze rearrangement to theoxime ether.[1]: 489–490 [2]: 345–347
Hydrides add to givehydroxylamines.ReducingLewis acids (e.g.metals,SO2)deoxygenate to theimine instead.[1]: 490 [2]: 343