Isotope effects with nitrogen

A second example of the correlation of isotope effects with substituent properties is given by the reaction between -substituted N,N-dimethylanilines and methyl iodide illustrated in Fig. 10.8.2°-ot-D and incoming group nitrogen KIE s are collected in Table 10.7. 

The main processes involved in the biologieal utilization of nitrogen are all associated with kinetic fractionation effects, nevertheless they exhibit different fractionation factors. The isotope effect of nitrogen fixation (a = 1.000 to 1.004) is small relative to the effects of bacterial nitrification, denitrification, or anammox (a = 1.02 to 1.04) (see Montoya et al. 1994 Table 1). The results by Miyake and Wada (1971) indicate that little overall isotope fractionation occurs in the... 

A true intramolecular proton transfer in the second step of an azo coupling reaction was found by Snyckers and Zollinger (1970a, 1970b) in the reaction of the 8-(2 -pyridyl)-2-naphthoxide ion (with the transition state 12.151). This compound shows neither a kinetic deuterium isotope effect nor general base catalysis, in contrast to the sterically similar 8-phenyl-2-naphthoxide ion. Obviously the heterocyclic nitrogen atom is the proton acceptor. 

White et al.1A have obtained similar kinetic results for the acid-catalysed rearrangement of N-nitro-N-methylaniline, i.e. a first-order dependence on the nitroamine with a linear H0 plot of slope 1.19 for phosphoric acid, and a deuterium solvent isotope effect of about three, although the results have only been presented in preliminary form. Further, an excellent Hammett a+ correlation was claimed for thirteen para substituted nitroamines which gave a p value of —3.9. Since it is expected that the rate coefficients would correlate with a (rather than different basicities of the amines, the a+ correlation implies that the amino nitrogen is electron-deficient in the transition state,... 

This is the same mechanism as that given above for esters, in equation (42). The difference between esters and amides is apparent from a comparison of the two tetrahedral intermediates [5] and [17], The former contains three oxygens, any of which can be protonated, resulting in much lsO exchange being observed when the reaction takes place in 180-enriched water,275,276 but [17] contains a much more basic nitrogen, which will be protonated preferentially and lead to much less 180 exchange, as observed.274 277,278 Also, ammonium ion formation makes the overall reaction irreversible, unlike ester hydrolysis. The calculated solvent isotope effect for the Scheme 15 process is 1.00,280 exactly in accord with experimental observation.278,279... 

In the mechanism study of /V-benzyl-/V -alkyl hydroxylamines, regarding oxidation with HgO and p-benzoquinone, it has been proposed on the basis of intra- and intermolecular kinetic isotope effects that, initially, there takes place a one-electron transfer from a nitrogen atom to the oxidant, with a subsequent proton abstraction (106—108). 

The 2,2 -hydrazonaphthalene doubly labelled with nitrogen-15 for the nitrogen isotope effect experiments and the [l,l -13C2]-2,2 -hydrazonaphthalene required for measuring the carbon isotope effect were synthesized by the reaction sequence shown in Schemes 18 and 19. 

The nitrogen kinetic isotope effect of 1.0197 found using the substrate with the natural abundance of nitrogen isotopes corresponds to an isotope effect of 1.04 for the reaction of the doubly labelled compound. Thus, the nitrogen isotope effects found using two different analytical techniques to measure the isotope effect are in excellent agreement. 

Matsson and coworkers have measured the carbon-1 l/carbon-14 kinetic isotope effects for several Menshutkin reactions (equation 35) in an attempt to model the S/v2 transition state for this important class of organic reaction. These isotope effects are unusual because they are based on the artificially-made radioactive carbon-11 isotope. The radioactive carbon-11 isotope is produced in a cyclotron or linear accelerator by bombarding nitrogen-14 atoms with between 18- and 30-MeV protons (equation 36). 

---