Nitrene equilibrium

Important synthetic paths to azirines and aziridines involve bond reorganization, or internal addition, of vinylnitrenes. Indeed, the vinylnitrene-azirine equilibrium has been demonstrated in the case of trans-2-methyl-3-phenyl-l-azirine, which at 110 °C racemizes 2000 times faster than it rearranges to 2-methylindole (80CC1252). Created in the Neber rearrangement or by decomposition of vinyl azides, the nitrene can cyclize to the p -carbon to give azirines (Scheme 4 Section 5.04.4.1). 

The equilibrium interconversion between an ethylene phosphite and a bicyclic spirophosphorane is shown to proceed by the insertion of the phosphite into the labile O-H bond of the hydroxyethyl ester. The mechanism is similar to the insertion of carbenes or nitrenes. Energy relationships of reaction intermediates were studied by MO RHF, MP2(full), MP4SDTQ, and DFT calculations. In most cases, they predicted that hydroxyethyl ethylene phosphates were more stable than the strained spirophosphoranes, which is not supported by the experimental evidence. The best correspondence to experimental data was obtained by DFT calculations with Perdew-Wang correlation functions <2003JST35>. 

Thermal decomposition of a doubly labelled azidotriazole gave rise to the open-chain triazine (117) in which the labels were not scrambled. Laser flash photolysis of the triazole leads to (117) within 20 ns with no observable intermediate. Ab initio calculations indicate that a dynamic equilibrium between the open-chain triazine (117) and a cyctic nitrene (118), as originally proposed, is highly disfavoured. 

As mentioned previously, unique kinetic results were obtained upon LFP of o-fluorophenyl azide,in that the singlet nitrene decays faster than the ketenimine is formed. This finding requires the presence of an intermediate, presumably ben-zazirine 40, between the singlet nitrene and ketenimine 42. The data could be interpreted by assuming that azirine 40 reverts easily to singlet nitrene according to the scheme below.The equilibrium constant is equal to the ratio of [40]/[39s] and was deduced to be 0.5 with AG 350 caFmol. Younger and Bell have also reported a system in which a benzazirine and ketenimine interconvert. 

These results suggest the intriguing possibility that the azirine may be in thermal equilibrium with the vinyl nitrene. The vinyl nitrene can then react further to give the thermodynamically more stable nitrile or, in some cases, the indole. However, the intermediacy of the vinyl nitrene in the 1-azirine pyrolysis reactions has not been determined. 

Thus, the equilibrium between nitren with unpaired electron and biradical with a conjugated system bond is supposed. 

Aniline cannot participate in the reaction in the initial stages since the rate of nitrogen elimination (in phenyl azide) is the same in nitrobenzene or aniline. It has been suggested that phenyl nitrene is in equilibrium with an ionic form which can lead to azacyclopropene, viz. 

Norcaradiene formation from a cycloheptatriene corresponds to a 1,3-ring-closure. On the basis of this reaction, aminocyclopropane 317 was obtained as a solid in 94% yield from piperidine and the tropylium ion 315 (equation 74). A rapid equilibrium between 316 and 317 was postulated in solution. Electrocyclic 1,3-bond connections also were involved in the fluctual behaviour of 9-azabarbaralanes " , in the formation of homoazepines (from nitrenes and cycloheptatriene" " ) and in a 6-azabenz[10]annulene system . ... 

The decomposition of 2-azido-4,6-dimethylpyrimidine (186) (in equilibrium with the tetrazolo compound 185) in the presence of copper acetylacetonate in cyclohexane at 140° gave the hydrogen abstraction product (187) (46%) and C—H insertion product (188) (8-5%) The same products were obtained in essentially the same yields by thermolysis of 186 at 185° °, (see section III.B.5). It was felt that the reaction proceeded via a copper-nitrene complex. The copper-catalysed decomposition of 186 and also 2-azidopyridine (in... 

When the azomethine group is part of an electron-deficient ring, such as pyridine, pyrimidine or thiazole, the compounds exist as tetrazoles in the solid state, and at equilibrium with the azido form in solution . The equilibrium constants depend on the solvent, the nature of the substituents and the temperature . 2-Azido-4,6-dimethylpyrimidine (288a) thus exists in equilibrium with tetrazolo-pyrimidine (288b). Its chemical behaviour is, however, in accord with the azide structure 288a, including dipolar addition reactions and nitrene reactions . 

---