Nitrenium ions parent

Figure 13.39. Addition of water and alcohols to the singlet parent nitrenium ion.

Srivastava et al. showed that the parent nitrenium ion is capable of adding to simple aromatics such as benzene, toluene and anisole (91) to give aniline derivatives (92a-c, Fig. 13.47). Unfortunately, these reactions give a distribution of regio-isomers as well as products derived from competing hydride and/or hydrogen atom transfer. 

The parent nitrenium ion (NH2) is firmly established as a ground-state triplet both extensive ab initio calculations as well as PES experiments all agree that the singlet-triplet energy gap is 30 kcal/mol. There have been several investigations on its behavior in solution. Takeuchi et al. " showed that this species could be generated by photolysis of l-amino-(2,4,6-triphenylpyridinium) ion. These photolyses were carried out in the presence of various aromatic compounds. It was found that the triplet state abstracted hydrogen atoms from traps such as toluene... 

Nitrenium ions are isoelectronic to carbenes [177]. They contain a dicoordinated nitrogen atom formally with two valence electrons. The possible electronic structures of the parent nitrenium ion NHj are shown in Figure 7.9. On the basis of Hiickel MO theory one would expect that the lowest energy configuration, that is, the electronic ground... 

Takeuchi H, Higuchi D, Adachi T (1991) Efficient direct aromatic amination by parent nitrenium ion. Photolyses of 1-aminopyridinium and 1-aminoquinolinium salts and effect of crown ethers. J Chem Soc Perkin Trans 1 1525—1529... 

The formation of the parent amine 23 in these solvolysis reactions was considered to be the most dehnitive evidence for formation of a discrete nitrenium ion. Gassman and Cryberg" postulated the following, (a) Initial Cl—N bond heterolysis would occur adiabatic ally, generating the singlet nitrenium ion 21. (b) The triplet... 

Photolysis in polar media (e.g., H20-MeCN mixtures) results in the same products observed from thermal generation. In addition, however, the parent amine, which is not observed in the thermal reactions, is formed photochemicaUy. This finding suggests that there may be a competition between heterolysis and homolysis in the photochemical reaction. It has also been suggested that the amine might result from formation of the triplet nitrenium ion. In any case this competing process along with the instability of the precursors has hmited interest in this photochemical route. 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

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. ... 

Quantum-chemical calculations for a wide variety of nitrenium ions are abundant.704 For the parent H2N+ ion the triplet state was found experimentally to be more stable by 30 kcal mol 1,701 The H N—H angle in the triplet is calculated to be considerably greater (149.4 versus 107.1°).705 The infrared spectrum of the complex He—H2N+ has also been observed.706... 

The most studied kinds of explosives are nitroaromatic explosives and their metabolites. Therefore, the emphasis of this review is on properties of nitroaromatic explosives, rather than propellants, pyrotechnics, or munitions, and their interactions with soils. Nitroaromatic explosives are toxic, and their environmental transformation products, including arylamines, arylhydroxyl-amines, and condensed products such azoxy- and azo-compounds, are equally or more toxic than the parent nitroaromatic [3]. Aromatic amines and hydroxylamines are implicated as carcinogenic intermediates as a result of nitrenium ions formed by enzymatic oxidation [4], Aromatic nitro compounds... 

A series of p-aryloxy- and p-alkoxyphenylnitrenium ions have been generated in aqueous solutions by photolysis of the parent azides, whereupon the resulting nitrenes are protonated. Hydration of these cations at the para position leads via hemiacetal or halohydrin intermediates to quinone imines, which finally hydrolyse to the ultimate quinone products. In flash-photolysis studies of these reactions it was shown that nitrenium ion hydration occurs on the ps timescale, hemiacetal or halohydrin breakdown on the MS timescale, and the final imine hydrolysis over minutes. 

Fishbein, J. C., McClelland, R. A., Halide Ion Trapping of Nitrenium Ions Formed in the Bamberger Rearrangement of N Arylhydroxylamines. Lifetime of the Parent Phenylnitrenium Ion in Water, Can. J. Chem. 1996, 74, 1321 1328. 

Takeuchi H, Hayakawa S, Tanahashi T, Kobayashi A, Adachi T, Higuchi D (1991) Novel generation of parent, alkyl, dialkyl and alicyclic nitrenium ions in photolyses of pyridinium, quinolinium, bipyridinium and phenanthrolinium salts and aromatic N-substitution by nitrenium ions. J Chem Soc Perkin Trans 2 847-855... 

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