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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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80 NITRILE OXIDES<br />

PhCH 2Co(dmgH) 2py<br />

[PhCH 2CH=N(O)-N(O)=CHCH2Ph]<br />

− HON(O)=CHCH2Ph<br />

PhCH2<br />

PhCH2ONO<br />

N<br />

hn<br />

N O<br />

PhCH 2CNO<br />

[PhCH2NO] [PhCH=NOH]<br />

CH 2Ph<br />

Scheme 1.44<br />

Δ<br />

[PhCH2CNO]<br />

The benzyl lig<strong>and</strong> of benzylbis(dimethylglyoximato)pyrid<strong>in</strong>e cobalt complex<br />

has been selectively converted to 3,5-dibenzyl-1,2,4-oxadiazole by a reaction<br />

with alkyl nitrite <strong>in</strong> the presence of light (426). The reaction proceeds by the <strong>in</strong><br />

situ formation of an oxime <strong>and</strong> a nitrile oxide (Scheme 1.44).<br />

Ion-molecule reactions of ionized nitrile oxide, R-C≡N + -O, with several<br />

neutral nitriles, R ′ CN, were studied, us<strong>in</strong>g both t<strong>and</strong>em mass spectrometric techniques<br />

<strong>and</strong> ab <strong>in</strong>itio MO calculations (427). Ionized oxygen atom transfer <strong>and</strong><br />

formal substitution of nitric oxide by the neutral reagent <strong>in</strong> the radical cation<br />

are the ma<strong>in</strong> processes. Whereas the former reaction yields the correspond<strong>in</strong>g<br />

ionized nitrile oxide, R ′ -C≡N + -O, the second process gives an electron species<br />

tentatively ascribed, follow<strong>in</strong>g high-k<strong>in</strong>etic energy collisional activation experiments,<br />

to an aromatic azir<strong>in</strong>ium cation. All the experimental data po<strong>in</strong>t to a<br />

two-step reaction sequence where the primarily formed <strong>in</strong>termediate ions competitively<br />

dissociate by the loss of nitrile or of nitric oxide, respectively, giv<strong>in</strong>g<br />

nitrile oxide ions <strong>and</strong> azir<strong>in</strong>ium ions.<br />

The mechanism of the simplest reaction HCNO + +HCN→ cyclo-HCCHN + +<br />

NO has been explored at the MP2/6–31G(d) level of theory. The most favorable<br />

reaction proÞle <strong>in</strong>volves the formation of a C—N bond between the positively<br />

charged carbon atom of HCNO + <strong>and</strong> the nitrogen atom of hydrocyanic acid giv<strong>in</strong>ganHCNO<br />

+ /HCN <strong>in</strong>termediate which isomerizes <strong>in</strong>to an ionized nitrosoazir<strong>in</strong>e<br />

before los<strong>in</strong>g NO.<br />

Photolysis of nitrile oxide 377 (R = CNO) gives acylnitrene 377 (R = CON),<br />

which undergoes <strong>in</strong>tramolecular <strong>in</strong>sertion reactions to give products 378 <strong>and</strong> 379<br />

(428).<br />

Me<br />

R<br />

CD3<br />

NH<br />

O<br />

CD3<br />

Me<br />

D D<br />

377 378 379<br />

ND<br />

O<br />

−H·

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