15N-NMR Spectroscopy

Analogies for such a mechanism in diazotization are found in the nitrous acid-catalyzed nitration of A,A-dimethylaniline, mesitylene, 4-nitrophenol, and some related compounds, which were investigated by 15N NMR spectroscopy in Ridd s group (Ridd and Sandall, 1981 Ridd et al., 1992 Clemens et al., 1984a, 1984b, 1985 Johnston et al., 1991 review Ridd, 1991). Ridd and coworkers were able to demonstrate clearly that not only the nitration proper, but also the preceding C-nitrosation, is accompanied by a marked 15N nuclear polarization. This was at-... 

However, we have to criticize more specifically the paper by Lown et al. (1984), who characterized alkanediazonium ions, as well as (E)- and (Z)-alkanediazoate ions, by 15N NMR spectroscopy. They also report NMR data on the (E)- and (Z)-benzenediazohydroxides as reference compounds, describing the way they obtained these compounds in only three lines. Obviously the authors are not familiar with the work on the complex system of acid-base equilibria which led 30 years earlier to the conclusion that the maximum equilibrium concentration of benzenediazohydroxide is less than 1 % of the stoichiometric concentration in water (see Ch. 5). The method of Lown et al. consists in adding 10% (v/v) water to a mixture of benzenediazonium chloride and KOH in dimethylsulfoxide. In the opinion of the present author it is unlikely that this procedure yields the (Z)- and CE>benzenediazohydroxides. Such a claim needs more detailed experimental evidence. 

G. E. Martin, M. Solntseva and A. J. Williams, Applications of 15N NMR spectroscopy in alkaloid chemistry, in Modern Alkaloids Structure, Isolation, Synthesis and Biology, E. Fattorusso and O. Taglialatela-Scafati (eds.), Wiley, New York, 2008, pp. 409-476. 

The 77Se NMR spectra can be used to analyse the composition of a complex mixture of Se-N compounds. For example, 77Se NMR provides a convenient probe for analysing the decomposition of selenium(IV) diimides RN=Se=NR (e.g., R=tBu).30 An elegant application of 77Se NMR spectroscopy, in conjunction with 15N NMR spectroscopy, involved the detection of the thermally unstable eight-membered rings (RSeN)4.31... 

Analysis of nitronates by 14N and 15N NMR spectroscopy has an auxiliary character (see Table 3.12). The 14N NMR signals are often broadened and, hence, are difficult to observe and are poorly informative, although magnitudes of their chemical shifts could in principle help in distinguishing between covalent nitronates and salts. It is difficult to observe 15N NMR signals in natural-abundance NMR spectra of nitronates, while an introduction of a label is an expensive procedure. 

Nuclear magnetic resonance spectroscopy for /V-acyloxy-/V-alkoxyamides 13C NMR spectroscopy, 56-58 15N NMR spectroscopy, 58-59 dynamic 1H NMR spectroscopy, 59 Nucleophilic substitution (SN2) reactions, /V-acyloxy-/V-alkoxyamidcs, 70-90 alcoholysis reactions, 89-90 with aromatic amines, HERON reactions, 70-74... 

Structure elucidation of three related derivatives of ring system 2 ( temozolomide 9a, mitozolomide 9b, and the related acid derivative 9c) has been carried out by 13C and 15N NMR spectroscopy <1995J(P1)249> (Scheme 2). The 1SN NMR chemical shifts measured in dimethyl sulfoxide (DMSO) solutions are listed in Table 1. For compound 9a, Lowdin charges of the nitrogen atoms have also been calculated and found to have a linear relationship with the experimentally determined chemical shifts of these atoms. The NMR data of 9a have been correlated with those of a series of heterocycles of related structure by the same team <2002MRC300>. 

Asakura, T., Yoshimizy, H., and Yoshizawa, F. (1988). NMR of silk fibroin. 9. Sequence and conformation analysis of the silk fibroins from Bombyx mod and Philosamia cynthia ricini by 15N NMR spectroscopy. Macromolecules 21, 2038-2041. 

NMR spectroscopy appears to be a very suitable technique to detect and assign failures in the outermost layer of the dendrimer structure at each generation. All three nuclei present in the dendrimers have been used including 1H-, 13C- and 15N-NMR spectroscopy [5-7],... 

We see from Table 1 that the only observable nuclide for oxygen, 170, has a very low natural abundance, even in comparison with those of popular nuclides like 13C (1.108%) and 15N (0.37%). Moreover, its quadrupole moment prevents any practical utilization of polarization transfer techniques like INEPT or DEPT, now widely used in 13C and 15N NMR spectroscopies. A range of chemical shifts much wider than those of 13C and 15N is an important point in favour of utilization of 170. All these properties did not prevent important applications of 170 NMR spectroscopy in organic chemistry, even from the times of continuous wave NMR spectroscopy. Interesting examples of such pioneering works can be found both at natural abundance4,6 as well as with enriched samples7,8. However, also in the case of 170 NMR spectroscopy, FT NMR proved to be decisive for its development. 

The manzamines are novel polycyclic p-carbolinealkaloids reported first by Higa and coworkers in 1986,73 from the Okinawan marine sponge Haliclona. Manzamines have been shown to have a diverse and interesting range of bioactivity including cytotoxicity and antimicrobial and insecticidal activities.74 75 Isolation ( 0.85% dry weight) and structure elucidation, aided by 15N-NMR spectroscopy, of the known manzamine A (59), and the... 

There is a growing interest in the use of 15N NMR spectroscopy for elucidation of various structural problems of azole chemistry, especially tautomerism. For example, the mole fractions of the prototropic tautomers were obtained from the 15N chemical shifts of the NH tautomers and the corresponding N-methyl derivatives. By this method, the average mole fraction for the 2-NH tautomer of benzotriazole is 0.02 in both CDC13 and DMSO, and that of 1,2,3-triazole is 0.34 in CDC13 and 0.55 in DMSO (82JOC5132). 

Some other examples of application of 15N NMR spectroscopy include establishing the protonation site of A2-pyrazolines (87MI301-01) and assigning the structures to isomeric N-7 and N-9 substituted purines (86T5073). 

The existence of tautomers and rotamers of dehydroacetic acid (239) and derived Schiff s bases has been investigated by 1H, 13C and 15N NMR spectroscopy (82JCS(P2)513). The tautomeric form (239) proposed earlier (61AK( 17)523) was confirmed and its existence as the rotamer (240) was established by variable temperature studies. 

Scrambling of the nitrogen atoms during irradiation does not take place. This was substantiated by irradiating 2-15N-5-phenylthiatriazole to give partial transformation and analyzing the remaining material by means of 15N NMR spectroscopy. 

The first member of the series was the subject of some controversy since previous IR, UV and pKa measurements137 suggested that tautomer 31a was the most stable form, while 15N NMR spectroscopy studies suggest that an equilibrium exists between forms 31a and... 

Other Substituted Diazonium Ions. A series of aminodiazonium ions have been prepared under superacidic conditions [Eq. (4.141)]. Schmidt495 described the preparation and IR spectra of protonated hydrazoic acid 212 and methylazide as their hexachloroantimonate salts. Olah and co-workers496 have carried out a comprehensive study on aminodiazonium ions (protonated azides) by H, 13C, and 15N NMR spectroscopy. Even the electrophilic aminating ability of aromatics of 212 has been explored.496 The tetrachloroaluminate salt of 212 has also been prepared496... 

The applications of nitronium salts as a synthetic reagent530 are discussed in Chapter 5. Until recently, the nitronium ion was recognized only as a nitrating agent. However, it has been found that it possesses significant ambident reactivity. This has been recently shown in the oxidation of sulfides, selenides, and phosphines. In fact, the sulfide reaction has been monitored by 15N NMR spectroscopy wherein both nitrosulfonium and nitritosulfonium ions 88 and 89 were detected as distinct intermediates [Eq. (4.147)]. 

Olah et al.715,716 have prepared a series of a-cyanodiarylcarbenium ions 283 (R = Ar) under superacidic conditions and have evaluated their mesomeric nitrenium ion character by H, 13C, and 15N NMR spectroscopy. A subsequent one-bond 13C-13C coupling constant measurement717 also indicates significant mesomeric nitrenium ion character of 283 (R = Ar). Protonated aroyl cyanides 284, however, exist predominantly in the carboxonium ion form of 284a over the nitrenium ion form 284b.718... 

---