Nitrenium ions trapping

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. 

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 the presence of alcohols, the corresponding ethers are formed and added nucleophiles such as chloride ion40 or azide ion41 lead to the chloro- and azido-amine products, respectively. Rate constants are independent of the concentration of added nucleophile. Labelled 180 from the solvent is incorporated in the product42. All the evidence points to a reaction mechanism where water is lost from the O-protonated reactant to give a nitrenium ion-iminium ion intermediate which is rapidly trapped by a nucleophile (H2O in this case) to give the final product. This is shown in Scheme 7. Protonation at N- is likely to be more extensive, but there is no pathway to products from the N-protonated intermediate. 

The source of alcohol (111) was acid-catalysed hydrolysis of 103 to the nitroso-carbonylarene intermediates (112), which, like acid chlorides, react with water to give benzoic acids 113 (Scheme 20, pathway (iii)) . 112 were trapped as the Diels-Alder adducts (114) in reactions in MeCN/H20 and in the presence of cyclopentadiene. In MeCN/10% Hz O, 114 was enriched in providing unequivocal evidence for both the trapping of the nitrenium ion intermediate, 102, by solvent water molecules and subsequent hemiacetal-like hydrolysis to the nitrosocarbonylarene... 

In some cases, the initial adduct can combine with an additional nucleophile to give a diadduct. This process is particularly likely when the driving force for aro-matization of the monoadduct is weak or absent. For example, Novak has shown that heteroarylnitrenium ion 85 is trapped at first by water and then by an additional nucleophile to give the diadduct 87 (Fig. 13.45). Likewise, Bose et al. reported the dihydroxylation of the nitrenium ion 88 derived from stilbene... 

Arylnitrenium ions are likewise capable of adding to n nucleophiles. With substituted aromatics (e.g., toluene) there exists the possibility of three reactive sites on the nitrenium ion (the nitrogen, ortho- and para-ring positions), along with up to three possible sites on the arene (ortho, para, and meta in the case of a monosub-stituted trap). Thus in a typical case there is the possibility of nine distinct regio-isomers. Obviously, any synthetic utility of such chemistry relies on the ability of the reagents to react in a selective manner. 

There are several examples of arylnitrenium ion additions to alkenes (Fig. 13.54). For example, Dalidowicz and Swenton trapped A-acetyl-A-(4-methoxyphenyl)-nitrenium ion 112 with 3,4-dimethoxy-l-propenylbenzene and isolated products resulting from addition of the alkene to the ortho position of the nitrenium ion, giving cation 113, followed by its cychzation onto N (yielding 114) or the acetyl carbonyl group (yielding 115). 

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... 

X 10 M s. Direct measurements of azide trapping reactions have generally validated this assumption (see Chapter 2). Except for highly stabilized nitrenium ions, feaz. is seen to fall in a narrow range of 4—10 x 10 M s. of azide adducts relative to the competing products can be used to estimate the rate constants for formation of the latter. 

Aryl substituent para to the nitrenium ion. The trap is water. 

The aminopyridinium route has been employed in flash photolysis studies of aryl as well as diarylnitrenium ions. Several examples of nitrenium ion species, along with their absorption maxima and some trapping rate constants are given in Table 13.7. To the extent the data are comparable, there is good agreement with the behavior of nitrenium ions generated by the azide route. For example, the 4-biphenylyl systems from the azide protonation and /-aminopyridinium routes both give absorption maxima at 460 nm and live for several microseconds in water. Likewise, the 4-methoxyphenyl systems show maxima at 300 nm (from azide) and 320 nm (from aminopyridinium ion). The discrepancy in this case can be attributed to the A -methyl substituent, present in the aminopyridinium route, but absent in the azide experiment. 

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. 

The application of the azide clock methodology to nitrenium ions was made by Fishbein and McClelland who showed that NJ trapped a reactive intermediate identified as the nitrenium ion 75m, during the Bamberger rearrangement of N-(2,6-dimethylphenyl)hydroxylamine (Scheme 33)7 Kinetic studies showed that the NJ-solvent partitioning occurred after the rate-limiting step of the reaction so an Sn2 process could be eliminated. The selectivity ratio, was determined to be 7.5 M . Assuming that k is... 

The less basic purines generate different adducts. Both a C-8 adduct 107 and an 0-6 adduct 108 are produced in the presence of I, while the exclusive product of the reaction of A with 75n and 75o is the unique benzene imine 109. ° These purines also exhibit lower selectivity for trapping of the nitre-nium ions (Table 3). The pyrimidine nucleosides thymidine (T), uridine (U), and cytosine (C) showed negligible reactivity with these two nitrenium ions. ° The selectivity ratios for T, U, and C given in Table 3 are upper limits based on the decrease in the yield of the hydrolysis products at high nucleoside concentration (ca. 50mM). ° Since no adducts were isolated it is not clear that these selectivities represent nucleophilic trapping by the pyrimidines. 

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