Nitrenes substituents

The 1,2,4-triazole ring survives oxidative destruction of aromatic substituents and rings fused to it but destabilization by a nitrene intermediate (45) permits the oxidative cleavage of 4-aminotriazoles (46 Scheme 13) (70TL3S51). Another instance of destabilization of the ring by a nitrene substituent (Scheme 14) is the thermal decomposition of azides (47) to... 

Another example of the analogy between pyrazole and chlorine is provided by the alkaline cleavage of l-(2,4-dinitrophenyl)pyrazoles. As occurs with l-chloro-2,4-dinitrobenzene, the phenyl substituent bond is broken with concomitant formation of 2,4-dinitrophenol and chlorine or pyrazole anions, respectively (66AHC(6)347). Heterocyclization of iV-arylpyrazoles involving a nitrene has already been discussed (Section 4.04.2.1.8(i)). Another example, related to the Pschorr reaction, is the photochemical cyclization of (515) to (516) (80CJC1880). An unusual transfer of chlorine to the side-chain of a pyrazole derivative was observed when the amine (517 X = H, Y = NH2) was diazotized in hydrochloric acid and subsequently treated with copper powder (72TL3637). The product (517 X = Cl, Y = H) was isolated. 

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

The characteristic reaction of an alkyl nitrene is migration of one of the substituents to nitrogen, giving an imine. 

This is the best known of the reactions of sulphonyl nitrenes and a large number of examples were studied by Curtius and his coworkers. The first attempt to establish semi-quantitative correlations between the nature of the substituent and orientation of the entering group was made by... 

In remembrance of the isolation of carbene 78 (Section m.C) it seems acceptable that 141, formed from 140 by a [l,2]-migration of the substituent, functions as a second intermediate. Carbene 141 can subsequently fragment into a nitrene 143 and carbon monoxide or open to an isocyanate 144, which alternatively may also be generated from an acid azide via acylnitrene 142. In comparison to calculated reaction pathways 75 78 and 82 81 (Schemes... 

Our prediction, that 2-fluorophenylnitrene (lOf) should preferentially cyclize away from the fluorine, was subsequently verified experimentally by Leyva and Sagredo.89 They photolyzed 2-fluorophenyl azide in diethylamine and isolated the trapping product expected from cyclization of the nitrene away from the fluoro substituent (Scheme 20).89... 

The photochemical reaction of azide-functionalized tetrazole derivatives such as 38 leads to the formation of the 5-5 bicyclic ring system 40 (Scheme 5) in very moderate yields <1999JHC863>. This reaction is believed to proceed via the singlet nitrene intermediate 39. Attack at the aromatic substituent in ortho position leads to product 40 <1974JOG2546> by subsequent cyclization. This intermediate is deprotonated during the workup conditions to the mesoionic tricyclic derivative 41. 

A series of transformations via nitrene formation similar to the previously discussed case was also found under flash vacuum thermolytic (FVT) conditions by the same team as shown in Scheme 8 <2003JOC1470>. 9-Phenyltetrazolo[l,5- ]quinoline 29 underwent nitrene 30 and cyclic carbodiimide 31 formation, and this intermediate - similar to the previous case - could open up to the isoquinoline nitrene 32 in which, however, proximity of the nitrene to the phenyl substituents allowed the ring closure to the stable tetracyclic ring system 33 which was obtained in 73% yield. 

This property is relatively rare in the very large number of reactions for which substituent effects were evaluated quantitatively106. It seems to be common, however, for all dediazoniations of arenediazonium ions and of related compounds, e.g. of substituted phenyl azides forming nitrenes, as well as for additions of carbenes to alkenes. 

Diphenyl- -triazole is aminated by reaction with hydroxylamine-0-sulfonic acid. The I - and 2-aminotriazoles are formed in approximately equal amounts. Intramolecular amination of 1- and 2-aryltriazoles is achieved by generating a nitrene intermediate in the ortho position of the aryl substituent for example, in the thermolysis (Scheme 42) of the azide (17). ... 

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