Nitrenium ions singlet-state reactions

Gonzales and co-workers extended their work on simple nitrenium ions, examining the reactions of several small, monosubstituted nitrenium ions (XNH+, where X = H, F, Cl, CN, and Me) with water. These computational studies (QCISD(T)/6-311-1—1- G ) treated only the singlet states of the nitrenium ion. According to these calculations, each singlet nitrenium ion adds to water to form an O-protonated hydroxylamine intermediate (R2N—OHj). No activation barrier was detected for... 

The role of singlet and triplet nitrenium ions in these reactions has been evaluated.228,229 Convincing evidence for the participation of singlet ground-state 2-acetylphenyl nitrenium ions during the photodecomposition of 3-methylanthranils in concentrated sulfuric acid has been obtained.229 For example, in the presence of arenes or anisole, in addition to the usual products, substantial amounts of diphenylamines and 3- and 5-(p-methoxy-phenyl)-2-aminoacetophenones, respectively, are formed by electrophilic attack of the nitrenium ion on the added substrate. 

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. 

By the late 1980s it was clear that a significant number of thermal and photochemical reactions of arylhydroxylamines and their derivatives, N-chloroanilines, aryl azides, anthranilium salts, and other compounds could be explained in terms of nitrenium ions or transition states that resembled nitrenium ions. Since no monoarylnitrenium ion had been directly observed, and data on the lifetimes and quantitative reactivity/selectivity of these species were not available, it was not possible to assess whether the reactions that had been observed were due to free ions, or ion pairs, or preassociation processes. In many cases Sn2 reactions could not be ruled out because appropriate kinetics experiments had not been performed. Most authors had attributed the presence of reduction products in thermal and photochemical reactions to triplet ions, but calculations suggested that the triplet species may not be accessible in thermal processes. It was clear that singlet ions could be reduced under certain conditions, so the presence of the... 

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. 

Due to its short lifetime, the singlet phenylnitrene 52 has never been trapped chemically, the only exception being protonation with the formation of phenyl nitfenium ion 54.200-202 jj. predicted theoretically, that 54 has a singlet ground state which is favored by 21.2kcal/mol over the lowest triplet state.20" Protonation of 52 competes with its isomerization to 51 only at low pH < I.200 McClelland and co-workers performed product analysis and LFP stndies of the photolysis of phenyl azide in water at pH = 0-2 and proposed a mechanism of reactions of nitrenium ion 54 (Scheme 11.29).2 2o> The nitreninm ion 54 reacts with water or other nucleophiles yielding substituted anilines.2 2 Note, that lifetime of 52 in water (25-50 ps) was estimated to be much shorter than in hydrocarbons ( lns). 

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