Nitrogen-15 NMR spectroscopy: prototropic tautomerism of azoles

Abstract

9-[l-(Methylthio)ethylidene]bicyclo[4.2.1]nonane (41).This compound is prepared and isolated as described for 15; yield 68%. Extremely pure product was obtained with the aid of high-performance LC (hexane): MS, m/e 196; 1H NMR (CDC13) 3.30 (m, 1), 3.03 (m, 1), 2.20 (s, 3), 1.97 (s, 3), 2.0-1.2 (m, 12). syn -9-Acetyl-9-chlorobicyclo[4.2.1]nona-2,4,7-triene (44). To a solution of vinyl ether 13 in S02 at -78 °C was added an excess (3 equiv) of S02C1F. The reaction was instantaneous. The reaction mixture was poured onto saturated aqueous NaHC03 and extracted into Et20. After the extract was dried (MgS04) and concentrated, hydrolysis of the intermediate ester was effected by passing the reaction mixture through a column of Si02 (CHC13 as eluent): NMR (CDC13) 6.0-5.7 (m, 4), 5.30 (d, 2), 3.53 (d, 2), 2.27 (s, 3); MS, m/e 194, 196. Carbocation Generation. NMR samples were prepared by condensing S02 from a gas cylinder into an NMR tube containing the substrate and cooled in a dry ice/acetone bath. The concentration of the samples was about 100-200 mg/0.3 mL of solvent. To the cooled solution, was carefully added freshly prepared HFS03/SbF5 (5:1) via the wall of the tube. In the case of the unsaturated substrates 13, 15, and 19,1 equiv of acid was used; the cations 47a-c were generated with a twofold excess of acid. Mixing was effected by shaking the samples vigorously with the aid of a vibromixer. Samples were checked with 60-MHz NMR spectroscopy. Spectroscopic investigations were performed in the temperature range of -100 to -30 °C. Quenching was effected by pouring the samples onto saturated aqueous NaHC03. AppendixTables IV and V contain collections of various and 13C NMR spectral data, respectively.

Assignment of 15N‐NMR resonances of vitamin B12 analogues by 2D‐[15N, 1H] long‐range correlation: Fully [15N]‐labelled Co‐β‐cyano‐5‐hydroxybenzimidazoylcobamide (factor III) and derivatives

“…It should also be mentioned that dicyanocobalamin has a 59C0 resonance shielded by 770 ppm compared with cyanocobalamin [lo], an effect assigned to the special shielding property of the cyanide ion. As the 5-hydroxybenzimidazole becomes free, the observed d(N) values resemble those of 1-methylbenzimidazole [19], i.e. -218.0 ppm (N(1)) and -119.…”

“…We found that the following long-range coupling constants are of crucial importance for correct "N-resonance assignments : Table I with other easily assignable resonances are given for all the compounds studied. The differentiation between H-C( 19) and H-C(3) is sometimes prevented because of overlap of these signals with H-C(R4) or H-C(Pr2). In these cases, decoupling experiments between H-C(18) and H-C(19) are very useful.…”

15N NMR chemical shifts of NH‐pyrazoles in the solid state and in solution at low temperature

“…The standard one-dimensional CP/MAS pulse sequence" was used. General recording parameters were quadrature detection, 'H 90" pulse duration 5 ps, C P times [8][9][10][11][12][13][14][15] …”

Nitroimines: II. Structure of Nitroamino-1,2,4-triazoles

“…Taking into account previously established nitroimine structure of compound V in solution [19], we believe that compounds I-IV and VI have a similar structure. Nevertheless, rigorous conclusion on the structure of nitroamino-1,2,4-triazoles in solution requires 15 N NMR spectra to be recorded for both compounds I-III and VI and their N-methyl derivatives [30,31]. No rigorous experimental proof for the existence of such tautomerism has been reported so far.…”