Nitrogen-15 kinetic isotope effects, values

TABLE 15. The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects and the Hammett p values for the ion-pair and free ion Sat2 reactions between benzyldimethylphenylammonium nitrate and sodium para-substituted thiophenoxides in methanol at 20 °C... 

Important additional evidence for aryl cations as intermediates comes from primary nitrogen and secondary deuterium isotope effects, investigated by Loudon et al. (1973) and by Swain et al. (1975 b, 1975 c). The kinetic isotope effect kH/ki5 measured in the dediazoniation of C6H515N = N in 1% aqueous H2S04 at 25 °C is 1.038, close to the calculated value (1.040-1.045) expected for complete C-N bond cleavage in the transition state. It should be mentioned, however, that a partial or almost complete cleavage of the C — N bond, and therefore a nitrogen isotope effect, is also to be expected for an ANDN-like mechanism, but not for an AN + DN mechanism. 

Recently, some attempts were nndertaken to uncover the intimate mechanism of cation-radical deprotonation. Thns, the reaction of the 9-methyl-lO-phenylanthracene cation-radical with 2,6-Intidine (a base) was stndied (Ln et al. 2001). The reaction proceeds through two steps that involve the intermediary formation of a cation-radical/base complex before unimolecular proton transfer and separation of prodncts. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tnnneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves n bonding between the unshared electron pair on nitrogen of the Intidine base with the electron-deficient n system of the cation-radical. Nonclassical cation-radicals wonld also be interesting reactants for snch a reaction. The cation-radical of the nonclassical natnre are known see Ikeda et al. (2005) and references cited therein. 

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,... 

Specific acid catalysis is also restricted to biologically noncompatible conditions because amides are protonated in oxygen rather than nitrogen with pKa values in the range of <0 [63,64]. The rarity of intermolecular acid/base effects on CTI was affirmed by the kinetic deuterium solvent isotope effects (KSIE) of about 1.0, indicating that the mechanism of spontaneous isomerization does not involve a proton-in-flight [29,42,65]. 

Phillips and Lee [32] used computational methods to obtain a value of 0.997 for the equihbrium nitrogen isotope effect at N1 for the 02 protonation of 1-methylorotate and a value of 1.004 for the kinetic nitrogen isotope effect at N1 for the decarboxylation of the proposed 02 protonated intermediate. Including these values in Eq. 5 yields ... 

With C = ks/k4) l+ki/k2)=0.885, a solution for k3reasonable limits of error, and is dependent on the exact values inserted into Eq. 5 to the third decimal place for confirmation or refutation. An attempted confirmation of 3= 1.000 for a mechanism involving an N1-independent pre-decarboxyla-tion step is also dependent on exact kinetic values. Using nitrogen isotope effects to discriminate between an 02-protonation pre-decarboxylation step with fC=0.997 and an N1-independent pre-decarboxylation step with 1.000 would seem to be a formidable task. 

---