Nitrenium, 119 nitrene

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. 

In many cases it is not certain whether the nucleofuge X is lost first, creating an intermediate nitrene or nitrenium ion, or whether migration and loss of the nucleofuge are simultaneous, as shown above.It is likely that both possibilities can exist, depending on the substrate and reaction conditions. 

Anodic oxidation of benzenesulphenanilides 56 leads to cleavage of the nitrogen-sulphur bond in the radical-cation with the formation of a nitrenium ion, which deprotonates to the nitrene. The intermediate dimerises to a phenazine [168]. 

NITRATE REDUCTASE NITRATE REDUCTASE NITRENE NITRENIUM ION Nitric oxide,... 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

Nitrenium ions have also been generated through the decomposition of azides under acidic conditions (e.g., trifluoroacetic acid-arene solvent mixtures). There are two potential pathways for the formation of the nitrenium ion from the precursors (Fig. 13.24). The first involves initial dissociation of the azide 41 to give a singlet nitrene 42, followed by proton transfer to the latter to yield the primary nitrenium ion 43. The second involves acid-induced decompostion of the azide, whereby preprotonation of the azide (44) forms the primary nitrenium ion in a direct manner. As with the hydroxylamine route, this method is limited to acidic or protic media. 

Figure 13.30. Primary nitrenium ions through azide photolysis-nitrene protonation.

Figure 13.31. Protonation of singlet nitrenes to give singlet nitrenium ions.

The next development in direct detection of nitrenium ions came from McClelland et al. ° who applied the azide method to LFP measurements. This permitted the direct detection of those arylnitrenium ions implicated in carcinogenic DNA damage. McClelland s approach proved to be particularly useful in the study of 4-aryl and 4-alkoxy substituted phenylnitrenium ions. Apparently, the corresponding singlet nitrenes are sufficiently long lived to allow for protonation in aqueous solution. Several arylnitrenium ions studied by this route are described in Table 13.6. 

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. ... 

X 10 M Combined with the experimental pK, of 14.8 in the solvent employed in this study (20% CH3CN/H2O), and the requirement that koH /kv, — KJKvj, these rate constants lead to a calculated pK of 75n of 16.0. " Although the parent N-arylnitrenium ion 75a cannot be directly detected in LFP studies, the pH dependence of yields of nitrene and nitrenium-ion-derived products led to the conclusion that the pKg of 75a s=12.4. ... 

Scheme 59 indicates that protonation of singlet nitrenes in aqueous solution is a viable route to nitrenium ions as long as (A h [H ] + (k sc + A exp)... 

The protonation of photochemically generated singlet nitrenes in aqueous solution has recently been used to study a wide range of nitrenium ions." Data on the rate constants for reaction of 75g, 75n, and many other nitrenium... 

Our understanding of the chemistry of N-arylnitrenium ions is significantly more advanced than it was a decade ago. Nevertheless, this field of research is still considerably less developed than that of carbenium ions, carbenes, or nitrenes. For example, although singlet nitrenium ions behave as one might expect that their 4-imino-2,5-cyclohexadienyl resonance contributors would in their reactions with H2O, NJ, or Cl, their reactions with carbon, nitrogen, and sulfur nucleophiles, particularly d-G, are not so easily rationalized. Except for d-G, these reactions with soft nucleophiles have not been examined systematically and the regiochemistry exhibited by these nucleophiles is incompletely understood. 

Nitrenes ([NH]) are the neutral nitrogen analogs of carbenes, while nitrenium ions ([NH2]+) are isoelectronic to carbenes. Many of the reactions which are observed for carbenes have parallels in nitrene and nitrenium ion chemistry. Like carbenes, nitrenes and nitrenium ions can exist in both singlet and triplet states. There are some interesting divergences in chemical properties and in the effects of substituents, however, which are readily understood on the basis of orbital interaction diagrams. 

When R = H in 25 the species is a protonated nitrene. Like carbenes and nitrenes, nitrenium ions can exist in singlet or triplet states.249... 

The intermolecular cycloaddition of an electron-deficient species such as a nitrene, a nitrenium ion or a carbene (or their formal equivalents) to the ir-bond of an alkene, alkyne, imine, or nitrile is a significant approach to aziridines and azirines (Scheme 2). 

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