Nitrenium complexes

Nitrenium cation is an extremely unstable species (89-91). That is why nitrenium complexes are usually prepared by the interaction of organic compounds with some sources of nitrenium cation (91, 92). Thus, the interaction of O-mesityl-sulfonylhydroxylamine with 1,10-phenanthroline results in the formation of 1 -amino- 1,10-phenanthrolinium mesitylenesulfonate (93-99). The structure of this salt has been confirmed by H and, 3C NMR spectroscopy (93,... 

Figure 5-36. Oxidation of 5.20 yields an iron(ii)-nitrenium complex 5.21 by the loss of the electron from the radical nitrogen centre. The nitrenium centre has only six electrons on the nitrogen and is electron deficient.

Amino and azo functions are annotated to form an imidazole ring by fusing the compound with a benzaldehyde at 220 An azido-imine is cyclized (via a nitrenium complex) by brief treatment at room temperature with boron triiluor-ide etherate. 

Nitrenium ions, particularly the arylnitrenium ions, have been proposed as intermediates in deoxyribonucleic acid (DNA) damaging reactions that can ultimately convert a normal cell into a cancer cell. Carcinogenesis is a complex phenomenon. 

The principal disadvantage to the aminopyridinium ion route is the accessibihty of the precursors. None are available commercially, and most require multistep syntheses giving relatively low yields. Another potential pitfall is the formation of the pyridine byproduct (54). Pyridines can function as nucleophiles, attacking the nitrenium ion and creating complex mixmres. Finally, pyiidinium ions are electron deficient and can serve as good ground-state electron acceptors. Many of the stable products generated from nitrenium ion reactions are amines and are relatively easy to oxidize. Thus, a potential problem is secondary reaction, whereby primary photoproducts are oxidized by the precursor. 

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. 

These results lead to a general mechanism for the reaction nitrenium ions with aromatic compounds (Fig. 13.71). Initial encounter leads to a 7i-complex (141). The latter is converted into isomeric a complexes (142-144) which, in turn, either tau-tomerize to give stable adducts (145-147) or else dissociate to give radicals. The relative rates of these processes depend on the reactivities of the nitrenium ion and the arene. With less reactive nitrenium ions the 7i-complex is relatively long lived. With more reactive nitrenium ions the n complex forms in a low steady-state... 

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 oxidation of 5.20 (by air, nitric acid, iron(m) etc.) results in electron loss from the nitrogen-centred radical and the formation of an iron(n) nitrenium species 5.21 (Fig. 5-36), which then undergoes deprotonation to yield an iron(n) complex of a tetraene 5.22 (Fig. 5-37). 

Boron trihalide promotes cyclization of o-azido-azo compounds under mild conditions with the formation of a benzotriazole in high yield. A nitrenium ion-BXs complex may be involved. The conversion of 2-azidoazobenzenes (p. 292) to benzotriazole is promoted by boron trichloride so that the conversion proceeds at room temperature and in high yield [3056]. 

The photochemical behavior of anthranils in acid solution is more complex. In general, mixtures of 3- and 5-substituted 2-aminoacylbenzenes are formed by conjugate addition of the acid to the initially formed resonance-stabilized nitrenium ion as outlined in Scheme 21 for 5-substitution (generally the major product).225,226... 

The experimentally observed orientations can be rationalized by considering the stability of the a-complexes (Wheland intermediate) involved. For instance, when comparing the a-complexes resulting from the addition of electrophiles to the 2-, 3- and 4-positions in pyridine, it is found that only electrophilic attack on the 3-position avoids the energy-rich nitrenium canonical form. Dications are postulated as intermediates for reactions involving pyridinium ions. Among these, the product resulting from attack on the 3-position has the most favourable electronic stability. 

If aryl azides are irradiated in acidic medium in the presence of arenes, derivatives of diphenylamine are formed." For the diphenylnitrenium ion, G-complexes with benzene have been detected by laser flash photolysis. In reactions with alkenes, nitrenium ions react more rapidly with electron-deficient olefins. Rate constants for the reaction of diphenylnitrenium with electron-rich olefins such as cyclohexanone trimethylsilyl enol ether are of the order of 1.9 x 10 M" s". In the reaction of 2-fluorenylni-trenium with 2 -deoxyguanosine, reaction at C8 of the guanine base takes place. ... 

Mclhoy, S. and Falvey, D. E., Reactions of nitrenium ions with arenes laser flash photolysis detection of a o-complex between N,N-diphenylnitrenium ion and alkoxybenzenes, /. Am. Chem. Soc., 123, 11329, 2001. 

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