Nitrone

Nitrones, as a tetrasubstituted double bond, admit cis–trans isomerism.{{Cite journal |last=Hamer |first=Jan |last2=Macaluso |first2=Anthony |date=1964-08-01 |title=Nitrones |url=https://pubs.acs.org/doi/abs/10.1021/cr60230a006 |journal=Chemical Reviews |language=en |volume=64 |issue=4 |pages=473–495 |doi=10.1021/cr60230a006 |issn=0009-2665|url-access=subscription }}{{Rp|page=474}}

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.{{Rp|page=476}}{{Cite journal |last=Delpierre |first=G. R. |last2=Lamchen |first2=M. |date=1965 |title=Nitrones |url=http://xlink.rsc.org/?DOI=qr9651900329 |journal=Quarterly Reviews, Chemical Society |language=en |volume=19 |issue=4 |pages=329 |doi=10.1039/qr9651900329 |issn=0009-2681|url-access=subscription }}{{Rp|pages=332-333}} The most general reagent used for the oxidation of hydroxylamines is aqueous mercuric oxide:{{Rp|page=476}}{{cite journal |last1=Thiesing |first1=Jan |last2=Mayer |first2=Hans |year=1957 |title=Cyclische Nitrone, II. Über die Polymeren des 2.3.4.5-Tetrahydro-pyridin-N-oxyds und verwandte Verbindungen |journal=Justus Liebigs Ann. Chem. |volume=609 |page=46-57 |doi=10.1002/jlac.19576090105}}

File:NitrMech1.png

However, a hydroxylamine with two α hydrogens may unsaturate on either side. Carbonyl condensation avoids this ambiguity...{{cite journal|last=Exner|first=O.|title=A New Synthesis of N-methylketoximes|journal=ChemPlusChem|year=1951|volume=16|page=258-267|doi=10.1135/cccc19510258}}center

...but is inhibited if both ketone substituents are bulky.{{Rp|page=477}}

In principle, N-alkylation could produce nitrones from oximes, but in practice electrophiles typically perform a mixture of N- and O-attack.{{Rp|page=479}}{{Rp|page=334}}

Some nitrones oligomerize:{{Rp|page=483}}{{Rp|location=334,337-338}}{{cite journal |last1=Thiesing |first1=Jan |last2=Mayer |first2=Hans |year=1956 |title=Cyclische Nitrone I: Dimeres 2.3.4.5-Tetrahydro-pyridin-N-oxyd |journal=Chem. Ber. |volume=89 |issue=9 |page=2159-2167 |doi=10.1002/cber.19560890919}} centerSyntheses 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 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.{{Rp|page=483}}{{Rp|pages=338-340}}

Nitrones hydrolyze extremely easily to the corresponding carbonyl and N-hydroxylamine.{{Rp|page=491}}{{Rp|page=344}}

{{Main|Nitrone-olefin 3+2 cycloaddition}}

As 1,3{{Nbh}}dipoles, nitrones perform [3+2] cycloadditions.{{cite journal |last=Yang |first=Jiong |year=2012 |title=Recent Developments in Nitrone Chemistry |journal=Synlett |volume=23 |page=2293-97 |doi=10.1055/s-0032-1317096}} For example, a dipolarophilic alkene combines to form isoxazolidine:

File:Nitrone cycloaddition.pngOther ring-closing reactions are known,{{cite journal |last1=Murahashi |first1=Shun-Ichi |last2=Imada |first2=Yasushi |date=15 March 2019 |title=Synthesis and Transformations of Nitrones for Organic Synthesis |journal=Chemical Reviews |volume=119 |issue=7 |pages=4684–4716 |doi=10.1021/acs.chemrev.8b00476 |pmid=30875202 |s2cid=80623450}} 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.{{Rp|pages=489-490}}{{Rp|pages=345-347}}

Hydrides add to give hydroxylamines. Reducing Lewis acids (e.g. metals, Sulfur dioxide) deoxygenate to the imine instead.{{Rp|page=490}}{{Rp|page=343}}

• N-Oxoammonium salt
• Nitronate

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Category:Functional groups