Nitrenium ions, direct detection

The next development in direct detection of nitrenium ions came from McClelland et al. ° who applied the azide method to LFP measurements. This permitted the direct detection of those arylnitrenium ions implicated in carcinogenic DNA damage. McClelland s approach proved to be particularly useful in the study of 4-aryl and 4-alkoxy substituted phenylnitrenium ions. Apparently, the corresponding singlet nitrenes are sufficiently long lived to allow for protonation in aqueous solution. Several arylnitrenium ions studied by this route are described in Table 13.6. 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

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. 

Falvey and co-workers used the N-ferf-butyl-3-methylanthranilium salts 130a-h as photoprecursors to the nitrenium ions 131a-h (Scheme 55). The unsubstituted 130a and related anthranilium salts and anthranils had previously been investigated as photoprecursors for nitrenium ions, but no attempts had been made to directly detect transient ions. Transients identified as 131b-f were detected by UV spectroscopy as products of the... 

X 10 M Combined with the experimental pK, of 14.8 in the solvent employed in this study (20% CH3CN/H2O), and the requirement that koH /kv, — KJKvj, these rate constants lead to a calculated pK of 75n of 16.0. " Although the parent N-arylnitrenium ion 75a cannot be directly detected in LFP studies, the pH dependence of yields of nitrene and nitrenium-ion-derived products led to the conclusion that the pKg of 75a s=12.4. ... 

Recently, the nitrenium ion 54 was directly detected by tfansient absorption spectroscopy in pure formic acid.2°2 The decay of nitrene 52 (t = 12ps) produces 54 with a broad absorption band centered at 500nm. The lifetime of 54 is 110 ps in pure formic acid.2°2 Taking into account very recent results the mechanism of phenyl azide photolysis in condensed phase could be described as shown in Scheme 11.29. 

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