Nitrenes formation from carbenes

If the carbene centre is next to a heterocyclic ring carrying a nitrogen atom, nitrene formation takes place. Nitrene formation from carbenes is favoured by the greater stability of the nitrene, even when a competing carbene route is available. ... 

The triplet state is usually the ground state for non-conjugated structures, but either species can be involved in reactions. The most common method for generating nitrene intermediates, analogous to formation of carbenes from diazo compounds, is by thermolysis or photolysis of azides.246... 

The nitrene transfer from PhI=NTos to alkenes catalyzed by the CpFe(CO)2+ fragment gave better results (85% for styrene) [25], but the characteristics of the chemistry of the cationic intermediates as postulated by the reaction mechanism are closely connected to the alternative formation of aziridines by a carbene transfer... 

Thermolysis or photolysis of azides gives nitrenes with expulsion of nitrogen. This method is analogous to the formation of carbenes from diazo compounds. 

Alkyl nitrenes can also be obtained by the photolysis of isocyanates with the expulsion of carbon monoxide, a method analogous to carbene formation from ketenes. 

Pyridines are observed in some of these reactions, and the substituent pattern is that expected from nitrene-carbene interconversion Small yields of azepine have been recorded in the photolysis of 158 i ). The bicyclic intermediate 44 [Eq. (64)] has been postulated frequently over the last twenty years. A wavelength dependence of the formation of azepines has been observed i ) the yield of azepine increases with the energy of the light and with the concentration of the trapping agent (diethylamine). At the same time, the yields of competitive processes of the nitrene (carbazole formation from 2-biphenylylnitrene p-cyanophenyl-hydrazine from p-cyanophenylnitrene) decrease. The rate of the azepine-forming... 

Azide anion is a good nucleophile. Many azide reactions proceed via formation of an active intermediate, nitrene, analogous to carbene. This results from the photolysis of hydrazoic acid ... 

There follows a base-induced elimination of HBr from nitrogen to form a nitrene intermediate, which is analogous to the formation of a carbene (Section 14-7B) ... 

Disubstituted adamantanes are most frequently obtained from internal cyclization reactions of a bridgehead substituent. Thus, photolysis of the azido-formate 79 gives 80 by nitrene insertion 279). 80 may be readily hydrolyzed to 2-amino-1-adamantanol (81) 75 Subsequent oxidation gives l-hydroxy-2-ada-mantanone (82) from which other derivatives may be prepared 280>2S1). An analogous carbene insertion is initiated by the thermal decomposition of the... 

It was pointed out in the discussion of the lower part of Figure 11.1 that leaving groups cannot be dissociated from O or N atoms, respectively, if these dissociations resulted in the formation of oxenium or nitrenium ions, respectively. The same is true if a nitrene (R—N ) would have to be formed. These three sextet systems all are highly destabilized in comparison to carbenium ions and carbenes because of the high elec-... 

On pyrolysis, 1-arylimidazoles rearrange to 2-arylimidazoles. In other systems, pyrolysis causes more deep-seated changes. 1-Arylbenzotriazoles on pyrolysis or photolysis give carbazoles via intermediate nitrenes (see Section 3.4.1.2.1). 1-Phenyl-1,2,4-triazole 789 is converted by pyrolysis into isoindole 790 via a carbene intermediate and another example of the participation of A-phenyl groups is found in the formation of benzimidazoles from tetrazoles (see Section 3.4.1.2.1). In the oxadiazolinone series, a nitrene intermediate 791 is also probably formed, which then ring closes. 

Scheme 5.11 shows a general blueprint that serves as a useful framework for considering reaction mechanisms that involve biradical intermediates B. The scheme also applies to carbenes (Section 5.4.1) and nitrenes (Section 5.4.2). When formed by thermal or photochemical homolytic bond cleavage from the reactant molecule M, biradicals will initially have the multiplicity of their immediate precursors. Singlet biradicals ( B ) tend to be short-lived because spin-allowed bond formation leading back to the reactant M or to a... 

The formation of fluorene from 2-biphenylcarbene is completely analogous to the carbazole formation described above. Contrary to earlier beliefs, the facile gas-phase rearrangement of diphenylcarbene to fluorene (Scheme 51) is not a direct process (path a),208 Other carbocyclic analogs of this reaction have been reported,10,329 and it has allowed the preparation of a number of heterocyclic compounds in high yields as well as a deeper understanding of the carbene-nitrene rearrangement.210 232... 

Diazopyrazole (368) undergoes gas-phase thermal extrusion of two molecules of nitrogen and formation of the azirine 369 the same product is formed from the azide 370. This indicates the occurrence of the carbene-nitrene rearrangement 371 -> 372 (Scheme 72)432 Carbenes like 371 can be trapped in solution.432... 

Evidence for the formation of a nitrene is also derived from the isolation of the azo compound 376b (27%) from the 3-diazoindazole 373b (Scheme 72).433 The carbene-nitrene interconversion 375a 374a had already been... 

---