Triplet nitrene intermediate

Irradiation of tetrazolo[l,5- ]pyrimidine 1 in benzene in the presence of trifluoroacetic acid was investigated by Takeuchi and Watanabe <1998JP0478> (Scheme 10). Formation of 2-anilinopyrimidine 57, 2-aminopyrimidine 58, and biphenyl was observed. The authors assumed that 57 was formed via a singlet nitrene, whereas formation of the other products may result from the triplet nitrene intermediate. In contrast to this early assumption, Hill and Platz <2003PCP1051> exclude the formation of a triplet nitrene upon spectroscopic evidence as discussed in Section 11.18.5.1. 

A novel tricyclic mesoionic tetrazolium derivative of 1,3,5-triazocine 9 was the major product of photochemical conversion of 5-azido-l-mesityl-3-phenyltetrazolium tetrafluoroborate 32 (Scheme 4, Section 14.08.5.2.1 <2000JHC1129>). The proposed mechanism of the transformation included benzylic hydrogen abstraction by the triplet nitrene intermediate to produce biradical species, which captures the solvent acetonitrile to afford 9. 

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... 

Similar products are obtained from the photosensitized decomposition of the tertiary azides, suggesting that decomposition may result from the triplet azides under both direct and sensitized photolysis/461 Additional evidence for a discrete nitrene intermediate comes from the observation that this intermediate can be trapped by decomposition of the azides in the presence of good hydrogen donors such as tri- -butyItin hydride and jec-butyl mercaptan. Triarylamines result ... 

ESR spectra for a number of nitrene intermediates produced by photolysis at 77°K have been reported.<50) Analysis of these spectra has resulted in their assignment to triplet ground state nitrenes. 

A photolytic study with a 5-azidotetrazole derivative also led to the formation of a tetrazolo[l,5-/z]quinazoline compound, although the yield was fairly low. Araki et al. published findings that irradiation of 110 in aqueous medium resulted in formation of the mesoionic enolate 112 in 8% yield <2000JHC1129> (Scheme 20). The authors concluded that the formation of the tricyclic structure can be rationalized by an intramolecular insertion of the triplet nitrene formed from 110 to a C-H bond of an ortho-methyl group to give at first intermediate 111, which was converted under the applied reaction conditions to produce 112. 

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

The addition of a singlet nitrene to a double bond is supposed to occur concert-eddy, while the triplet nitrene is supposed to add in two discrete steps via a 1.3-di-radical intermediate. The rate of ring closure of the diradical is supposed to be smaller than that for the rotation around the C—C-bond, therefore the stereochemistry of the olefin will be lost. 

Such ESR characterization of 1,3-diradical intermediates is of considerable interest to mechanistic chemical studies. Some of the ESR results are given in Table 17. The ground-state triplet nitrene and its derivatives... 

Zero-Field Parameters of Some Intermediate Ground-State Triplet Nitrenes Produced by Photolysis3... 

The vapor-phase pyrolysis of phenyl azide yields azobenzene in 72 % yield . The pyrolysis in aniline solution leads to the production of dibenzamil , identified as an azepine by modern techniques in benzene and p-xylene, the major products are azobenzene and aniline, presumably originating from reactions of PhN in hydrocarbon solvents the thermolysis produces aniline, alkyl-anilines, azobenzene and polymer . It has been shown that aniline and iV-alkylanilines arise from triplet state nitrene reactions with the solvent. In the gas-phase thermolysis of phenyl azide at low pressures aniline and azobenzene are the major products whereas at higher pressures the formation of l-cyano-1,3-cyclopentadiene (or 2-anilino-7H-azepine in the presence of aniline) predominates . It was deduced that aniline and azobenzene arise from triplet nitrene reactions and that at high pressures, a hot singlet nitrene is formed, which undergoes intramolecular insertion to form an azocyclopropene intermediate, viz. 

Photolysis of the aryl-alkyl azide CH CPhaNa showed that the migratory aptitudes of the methyl and phenyl groups were almost identical and this and the formation of triphenylmethyl amine from irradiation of triphenylmethyl azide in the presence of efficient hydrogen donors were taken to confirm the existence of discrete nitrene intermediates. Although the occurrence of a triplet-sensitized decomposition from alkyl azides and triphenylmethyl azides (the... 

In considering a mechanistic rationale for vinyl azide chemistry one is strongly tempted to invoke a vinyl nitrene intermediate in either its singlet, 131 or triplet, 132, state. The existence of nitrenes has been... 

Unsubstituted organic azides absorb appreciably in the near-UV region (<380 nm). Their direct irradiation leads to the extrusion of molecular nitrogen to form singlet nitrene (Section 5.4.2), which can intersystem cross to triplet nitrene (Scheme 6.175).1112,1114,1162 Triplet nitrene can also be obtained by photosensitization. In general, nitrenes are reactive intermediates that can undergo various reactions. 

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