Nitrenium ion

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 

Attempted generation of nitrenium ions as distinct species under long-lived stable ion conditions has thus far been unsuccessful. Protonation of nitro- 

Recently, persistent nitrenium ions—including the triazolinium ion 282, which is considered a nitrenium ion on the basis of the resonance form 282b—have been 

The ambivalent nature of an a-cyano group on a carbocationic center has been demonstrated by the solvolytic work of Gassman and co-workers.713 714 Inductively, the cyano group strongly destabilizing. However, the major portion of this effect is offset by the mesomeric nitrenium ion structure 283b. 

Olah et al.715,716 have prepared a series of a-cyanodiarylcarbenium ions 283 (R = Ar) under superacidic conditions and have evaluated their mesomeric nitrenium ion character by H, 13C, and 15N NMR spectroscopy. A subsequent one-bond 13C-13C coupling constant measurement717 also indicates significant mesomeric nitrenium ion character of 283 (R = Ar). Protonated aroyl cyanides 284, however, exist predominantly in the carboxonium ion form of 284a over the nitrenium ion form 284b.718 

Where the Z substituent is a carbonyl group, the perpendicular orientation places the other acyl substituent parallel to the vacant 2p orbital, and a rearrangement ensues if the second substituent on N is not a good ti donor [177, 182]. 

Copyright 2001 John Wiley Sons, Inc. ISBNs 0-471-35833-9 (Hardback) 0-471-22041-8 (Electronic) 

Various analogues have been investigated indicating that the nitrogen-bridge between the two aromatic rings is the target, at least for HOCl oxidation. When alter- 

Orbital Interaction Theory of Organic Chemistry, Second Edition. Arvi Rauk 

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NitrofuraZone. 2-[5-Nitro-2-furanyl)methylene]hydrazinecarboximide, the first nitrofiiran to be employed clinically, is prepared from 5-nitro-2-furancarboxaldehyde and semicarbazide (19). This product has seen clinical use topically as an antibacterial, for systemic appHcation for bacterial infections in poultry and swine, and also has been employed as a food additive. In rats, nitrofurazone is hydroxylated at the 4 position of the furan moiety (27). The involvement of nitrenium ions has also been postulated in the mechanism of action of nitrofurazone (38). 

Photolysis of 3-phenyl-2,l-benzisoxazole in 48% HBr produced reduction and substitution products via a proposed triplet state nitrenium ion intermediate (71HCA2111). Photolytic decomposition of 5-bromo-3-phenyl-2,l-benzisoxazole in 48% HBr gave 2-amino-5-bromoacetophenone and 2-amino-3,5-dibromoacetophenone (Scheme 18). A nitrenium ion intermediate was also proposed for the photolytic decomposition of 3-phenyl-2,l-benzisoxazole in concentrated HCl (Scheme 19) (7IHCA2111). 

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. 

The triplet nitrenium ion, however, lacks symmetry but, unlike the excited triplet states of other azaaromatic systems such as pyridine, is surprisingly planar.283 Theoretical dipole moments for cyclopent[c]- and cyclopent[rf]azepine, and the molecular geometry of cyclopent[rf]-azepine, have been calculated using semiempirical ab initio methods.3... 

A nitrenium ion is thought to be involved in the facile and almost quantitative (94%) conversion of 5//-dibenz[6,/]azepine to acridine in the presence of silver(I) trifluoroacetate in dichloromethane at 20 C.228... 

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

At least two types of nitrenium ions, the nitrogen analogs of carbocations, can exist as intermediates, though much less work has been done in this area than on carbocations. In one type (46), the nitrogen is bonded to two atoms and in the other... 

Among the evidence for this mechanism are the facts that other products are obtained when the reaction is run in the presence of competing nucleophiles, for example, p-ethoxyaniline when ethanol is present, and that when the para position is blocked, compounds similar to 23 are isolated. In the case of 2,6-dimethylphe-nylhydroxylamine, the intermediate nitrenium ion 22 was trapped, and its lifetime in solution was measured. The reaction of 22 with water was found to be diffusion controlled. ... 

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. 

Gonzalez, C., Restrepo-Cossio, A., Marquez, M., Wiberg, K. B., De Rosa, M., 1998, Ab Initio Smdy of the Solvent Effects on the Singlet-Triplet Gap of Nitrenium Ions and Carbenes , J. Phys. Chem. A, 102, 2732. 

The first experimental determination of a singlet-triplet energy gap (Mst) for an organic nitrenium ion was made for the 1,3-dimethylbenzotriazolium ion . A powerful ligand, 4,5-bis(diphenylphosphinoyl)-l,2,3-triazole, has been synthesized and found to possess two different modes of chelation <00ACIE3321>. 

In 1984, we demonstrated that A-alkoxy-A-acyl nitrenium ions 15 could be generated by the reaction of A-alkoxy-A-chloroamides 14 with Lewis acids such as Ag + and Zn2+ and used these to form heterocycles by intramolecular aromatic substitution reactions (Scheme 2).90 In this manner, several novel A-acyl-3,4-dihydro-2,l-benzoxazines 16a and A-acyl-4,5-dihydro-( I //,3//)-2,1-benzoxazepines 16b were made. Subsequent work91,92 and that of Kikugawa93 96 produced numerous syntheses involving alkoxynitrenium ions including formation of natural products.97 99... 

Population of the <7 n-oac orbital weakens the bond rendering it unstable, resulting in formation of a resonance-stabilised nitrenium ion (Fig. 14c). 

The arguments presented herein lend the strongest support to SN2 attack by G-N7 at the amide nitrogen of /V-acyloxy-/V-alkoxyamides and for pathway (i) in Scheme 23 rather than pathway (iii) in which, once bound to DNA, the mutagens undergo SnI formation of reactive nitrenium ion. 

IV-acyloxy-lV-alkoxyamides, 37-39 biological activity, see IV-acyloxy-IV-alkoxyamides, biological reactivity chemical reactivity, see IV-acyloxy-IV-alkoxyamides, chemical reactivity IV-alkoxy-lV-acyl nitrenium ions, 38 resonance forms in twisted or pyramidal, 55/... 

Nitrenium ions, 38, 70, see also IV-Acyloxy-IV-alkoxynitrenium ions Arylnitrenium ions... 

Novak et al. <1998JA1643> devised a novel approach to amino-substituted tctrazolo[ 1,5-tf Jpyridine which provides a really unique pathway (Scheme 34). These authors studied the possibility of formation of nitrenium ions from the pivaloylhydroxylamine 143 and found that if azide anion is present in the main reaction route is the formation of tetrazolo[l,5-tf]pyridine 146. The authors concluded that the first intermediate is the formation of the carbonium cation 144 which captures the azide anion to yield 2-azidopyridine 145, that is, the valence bond isomer of the product 146. 

Liu ZC, Uetrecht JP. Clozapine is oxidized by activated human neutrophils to areactive nitrenium ion that irreversibly binds to the cells. J Pharmacol Exp Ther 1995 275(3) 1476-1483. 

Not only is the leaving group important, but if the reaction has SnI character, the stability of the positive charge left behind is also important. In the examples above, a primary carbocation is difficult to form and therefore the reaction of busulfan with glutathione would likely be a SN2-type reaction, whereas the nitrenium ion and carbocation formed from N -acetylaminofluorene and safrole, respectively, are relatively stable and likely to be Sn 1 -type reactions. 

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