Nitrenes singlet

On the basis of ab initio MO studies on the nitrenium ion 23 derived from 1//-azepine, it has been concluded that the singlet nitrene is a planar, fully conjugated C2v system with substantial 67r-aromatic character. 

Thermolysis of ethyl azidoformate in a mixture of ethylbenzene and dichloromethane, a known singlet nitrene stabilizer, failed to improve the yield of the 1//-azepine.148... 

Early attempts to induce the thermal ring expansion of arenes with (arylsulfonyl)nitrenes were disappointing in that primary sulfonamides, formed by hydrogen abstraction by the singlet nitrene, and A-phenylsulfonamides, were the major products.157 l-(Arylsulfonyl)-l//-azepines were produced in low yields and were isolated only as their [4 + 2] eycloadducts with ethene-... 

Photolysis of aryl azides in amine solution, with a tertiary amine as cosolvent to promote stabilization of the singlet nitrene, has met with some success. For example, the yield of 2-piperidino-3 W-azepme. obtained by the photolysis of phenyl azide in piperidine, is increased from 35 to 58% in the presence of A A /V. /V -tetramethylethylenediamine (TMLDA).180 Also, an improved yield (36 to 60 %) of A,(V-diethyl-3W-azepin-2-amine (38, R = Et) can be obtained by irradiating phenyl azide in triethylamine, rather than in dicthylaminc, solution.181 Photolysis (or thermolysis) of phenyl azide in TMEDA produces, in each case, 38 (R = Et) in 40% yield.181 In contrast, irradiation of phenyl azide in aniline with trimethylamine as cosolvent furnishes jV-phenyl-377-azepin-2-amine (32, R = Ph) in only low yield (2%).35... 

The [l,2,3]triazolo[2,l- ]benzotriazole (74 2,3-benzo-l,3a,4,6a-tetraazapentalene) heterocyclic ring system is obtained by an electrophilic attack of singlet nitrene, generated by heating the corresponding nitrophenyl triazole 251 in trialkyl phosphite, on the triazole nitrogen (Equation 39) <1998JOC3352>. 

Pyrazolo[l,2- ][l,2,3]triazole mesomeric betaines are generally available by an electrophilic attack of singlet nitrenes on the pyrazole nitrogen atom. When phthalazone derivative 252 is used and the nitrene is generated by reduction with triethyl phosphite, 59% yield of mesomeric betaine 253 is obtained (Equation 40) <2000T5523>. 

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. 

Matrix isolation photolytic studies on tetrazolo[l,5- ]pyridazine 25 have been reported by Hill and Platz <2003PCP1051> (Scheme 5) and formation of the l-cyano-3-diazopropene 27, triazacycloheptatetraene 28, and cyano-cyclopropene 29 was detected. Upon the absence of electron spin resonance absorptions at 7 K, the authors concluded that triplet nitrene was not formed but, instead, the resulting singlet nitrene rapidly underwent further ring openings. 

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. 

The states can be approximated as the combination of a triplet carbene with a nitrene in which its ct- and -rr-electrons have opposite spins (Fig. 14). If the latter configuration is taken to correspond to an open-shell singlet nitrene (as... 

Photolysis of alkyl azides bearing pendant aryl groups does not lead to intramolecular trapping of a nitrene by the aryl group. However, intramolecular capture of the putative nitrene is observed upon pyrolysis of the azide precursor. These observations convinced Kyba and Abramovitch" ° that 1,2-migration is concerted with loss of nitrogen from the excited state of the azide, but that a free singlet nitrene is formed upon thermal decomposition. The chemistry of acyl azides will be shown later to exhibit the opposite pattern. 

It is presumed that the barrier-to-1,2-fluorine migration in the singlet nitrene substantially exceeds the analogous barrier-to-hydrogen shift. This difference allows a portion of the nascent singlet nitrene to relax to the lower energy triplet in the matrix. 

Polyfluorination does seem to suppress rearrangements of alkyl azides and to extend the lifetimes of the corresponding singlet nitrenes. Photolysis of 5 in cyclohexane produces insertion adduct 6." ... 

Each of these azides contains an internal sensitizing group. Upon LFP the triplet states of all three azides are observed. However, in the case of 17 and 18, only singlet nitrene derived products are formed. Photolysis of 17 and 18 at 8 K in fluorolube fails to produce persistent triplet EPR spectra. It seems clear that these aroyl nitrenes also have singlet ground states. As mentioned previously, photolysis of 14 does produce a triplet nitrene EPR spectrum. 

Why does the nitrene ester have a triplet ground state in contrast to the acetyl and benzoyl analogues This author speculates that the N—O bonding interaction in the singlet nitrene ester is weaker than that in the singlet acetylnitrene because such an interaction destroys resonance within the ester group. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

For most aryl azides, the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same. Thus, in all these phenyl-nitrenes cyclization to substituted benzazirines is the rate-limiting step of the process of isomerization to ketenimine, as is the case for the parent phenylnitrene. The only known exception, o-fluorophenylnitrene, will be discussed in the next section. 

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

Laser flash photolysis of a series of fluorinated aryl azides produces the transient spectra of the corresponding singlet nitrenes. ° With the exception of singlet o-fiuorophenylnitrene (39s), the rate of decay of the singlet nitrene was equal to the rate of formation of the reaction products, for example, didehydroazepines and triplet nitrenes. Values of fejsc and the Arrhenius parameters for azirine formation are summarized in Table 11.5. 

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