Arylnitrenes

In contrast to the acyl- and sulfonylnitrenes described in this section, arylnitrenes produced thermally or photolytically from aryl azides, including those bearing strongly electron-withdrawing substituents (e.g., CN, N02, CF3), fail to promote ring expansion of arenes to 1H-azepines, although intermolecular substitution of electron-rich substrates, e.g. mesitylene and A.TV-dimethylaniline, have been noted.167... 

Whereas the production of arylnitrenes by the deoxygenation of nitrosobenzenes or nitro-benzenes by trivalent phosphorus reagents and their subsequent intramolecular ring expansion to 3//-azepines are well-known processes, the corresponding intermolecular reactions to form 1//-azepines have been exploited only on rare occasions and appear to be of little preparative value. For example, the highly electrophilic pentafluorophenylnitrene, obtained by deoxygenation of pentafluoronitrosobenzene with triethyl phosphite in benzene solution, produced a low yield (2-10%) of l-(pentafluorophenyl)-l//-azepine, which was isolated as its [4 + 2] cycloadduct with ethenetetracarbonitrile.169 With anisole as the substrate l-(pentafluorophenyl)-l//-azepin-2(3//)-one (16% bp 128 —130 C/0.4 Torr) was obtained. 

The structure of the tar, formed in many azide decompositions, consists of polyazepine units,103 and arises by attack of arylamine, formed from triplet arylnitrene, on the azepine precursor 30 to give the l//-azcpin-2-amine 31, which acts as a nucleophile towards more azepine precursor to yield ultimately the polyazepine 95. 

Before discussing recent studies on the ring expansion reactions of phenylcarbene and phenylnitrene, we will describe some of the earlier experimental and theoretical work on these molecules. Our purpose here is to give a brief overview, in order to provide a context for the discussion of more recent results. For detailed descriptions of the chemistry of arylcarbenes and arylnitrenes, we refer the reader to the many excellent reviews in this area.1,2... 

As shown in Table 3, triplet lb is computed to be 25-26 kcal/mol lower in enthalpy than triplet lc.77 Table 3 also shows that radicals 8b and 8c, formed by adding a hydrogen atom to lb and lc, respectively, differ in enthalpy by only 1-3 kcal/mol. Therefore, the large enthalpy difference between 3lb and 3lc is not due to a difference between the abilities of the phenyl and pyridyl groups to stabilize an unpaired tt electron. Instead it must reflect an intrinsic enthalpy difference between arylnitrenes and arylcarbenes. Table 3 also shows that aniline (9b) and fl-picoline (9c) are also predicted to have very similar enthalpies, thus providing further evidence that the large enthalpy difference between lb and lc is, indeed, due to the fact that lb is a nitrene, while lc is a carbene. 

Ring expansion of alkyl-substituted arylnitrenes has also been studied. Sund-berg and co-workers found that generation of several 2-alkyl-substituted arylnitre-... 

X-ray irradiation of a matrix containing 2,6-difluorophenylazide in methylene chloride allows fixing the corresponding arylnitrene cation-radical, although such a particle is extremely unstable (Carra et al. 2006). 

These paramagnetic, air-stable complexes could equally well be formulated as Os " arylnitrenes. Both crystal-field and charge-transfer bands have been observed in the diffuse reflectance spectrum of [OSF5] . Calculation of the spin-orbit coupling constant and crystal-field parameters indicated that the degree of covalency is high for a fluorocomplex, and comparable with that found for quadrivalent [MFg] species of the first transition series. ... 

H.J. Wester, K. Hamacher, S. Guhike, G. Stdcklin, A simple and fast F-labeling of proteins by coupling with photogene rated [ F]arylnitrene, J. Nucl. Med. 35 (1994) 73P. 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

Fragmentation of arylsulfonyl nitrenes to form an arylnitrene and SO2 is also known. Presumably, this process involves a prior rearrangement to a species analogous to 21. 

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. 

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. 

Understanding the chemistry of arylnitrenes proceeded more slowly than that of arylcarbenes because product studies were less informative. Unlike carbenes, much... 

The strongest chemical evidence for the existence of singlet arylnitrenes comes not from the parent system, but from intermolecular capture of ortho-substituted aryl azides,and intermolecular capture of certain highly electron-deficient aryhii-... 

The electronic absorption spectra of 33s and 33t are very similar (Figs. 2 and 4), but all of the calculated and experimental bands of 33s exhibit a red shift compared to those of 33t. This result is very reasonable because both these species have very similar open-shell electronic configurations ( 2 and 2). The absorption maxima of several singlet arylnitrenes are given in Table 11.1. 

An o-cyano group has little influence on kisc, but two o-cyano groups slightly accelerate intersystem crossing. ° ° Singlet arylnitrenes with electron-withdrawing groups in the para-position have little influence on the rate constant of ISC. 

Abramovitch et al. " and Banks et al. discovered that unlike most aryMtrenes, polyfluorinated arylnitrenes have a bountiful bimolecular chemistry. Perfluorophen-ylnitrene reacts with diethylamine to form a hydrazine, with tetramethylethylene to form an aziridine and forms robust adducts with benzene and even cyclohexane. Polyfluorinated arylnitrenes are useful reagents in synthetic organic chemistry, photoaffinity labeling, and for the covalent modification of polymer surfaces. ... 

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

In Bamberger s time the chemistry of arylnitrenes was unknown so he did not know of the characteristic reactions that could be used to preclude their intermediacy. The formulation of the mechanism in terms of a nitrenium ion is due to Heller, Hughes, and Ingold who showed in 1951 that the reaction rate is proportional to the concentration of protonated hydroxylamine, reaching a constant value in sufficiently acidic solutions. They also discovered that the reaction rate is independent of [CP] in HCl solutions even under conditions in which 5b and 6b are major products. Other workers showed through hydrolysis in 0-H20 that 4 is produced in an interm olecular process, and that no is incorporated into 3 during the reaction. ... 

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