Nitrenium ions lifetimes

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. ... 

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. 

A major concern in nitrenium ion research has been to characterize the hfetimes and reaction rates of these species in solution. Two techniques have been the mainstays of this research. The oldest and most generally applicable method to characterize arylnitrenium ion lifetimes in solution is to measure the relative yields of the stable products. The ratio of rate constants can then be inferred from the product ratios. 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

The first nitrenium ion to be examined using any flash photolysis method was the 4-dimethylaminophenylnitrenium ion (131). However, the workers who carried out these experiments apparently did not consider this species to be a nitrenium ion. It was generated by pulsed Xe lamp photolysis from the corresponding azide (132, Fig. 13.64), and was detected through its absorption at 325 nm. As the resonance structure 131 implies, this nitrenium ion (or quinonediimine) is especially stable. In fact, its lifetime exceeds 100 ms at neutral pH, and it decays through hydrolysis of the C=N bond to give iminoquinone (133). 

The study of more typically reactive nitrenium ions had to await the application of faster methods. Nearly two decades later, nanosecond LFP methods were applied to the ring opening of anthranilium ions (Fig. 13.29). " A series of alkylarylnitre-nium ions were characterized in this manner and the results are compiled in Table 13.5. These species in general had much shorter lifetimes (<50ps)... 

Olah generated similar dicationic species upon treatment of 4-hydroxy- or 4-methoxy- nitrosobenzenes with 1 1 HSO F-SbFs in SO2 (Scheme 6) These species were stable in the superacid media and were characterized by NMR spectroscopy. No nitrenium ion species were detected. Although Olah was unable to detect simple monoarylnitrenium ions, a few stabilized diarylnitrenium ions were detected during cyclic voltammetry experiments on the corresponding diarylamine in CHsCN, For example, the di-para-anisylnitrenium ion and its conjugate acid (Scheme 7) were both observed. This ion had a lifetime of about 1 s in CH CN. ... 

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... 

Fishbein and McClelland examined the roles of ion pairs in the decomposition of 0-aroyl-N-acetyl-N-(2,6-dimethylphenyl)hydroxylamines (90, Scheme 42). These esters give rise to a nitrenium ion that has an estimated lifetime of ca. 0.25-0.50 ns based on the lifetime of 75m (1.5 ns) and an assumed 3- to 6-fold decrease in that lifetime caused by substituting NAc... 

Reactions of nitrenium ions with lifetimes in aqueous solution ns. It is clear from the work presented to date that these species react predominately by ion-pair or preassociation mechanisms, but the detailed processes are far from clear. The possible transition to a true bimolecular substitution mechanism (Sn2) has also not been systematically investigated. 

Reactions of highly stabilized nitrenium ions. Just as little is known of the chemistry of very short-lived nitrenium ions, very little has been reported on the chemistry of ions with lifetimes Ims. Recently, results have been reported for some benzidine nitrenium ions that fall... 

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