Theoretical and experimental ¹⁵N NMR study of enamine–imine tautomerism of 4‐trifluoromethyl[b]benzo‐1,4‐diazepine system

Abstract

The tautomeric structure of 4-trifluoromethyl[b]benzo-1,4-diazepine system in solution has been evaluated by means of the calculation of (15)N NMR chemical shifts of individual tautomers in comparison with the averaged experimental shifts to show that the enamine-imine equilibrium is entirely shifted toward the imine form. The adequacy of the theoretical level used for the computation of (15)N NMR chemical shifts in this case has been verified based on the benchmark calculations in the series of the push-pull and captodative enamines together with related azomethynes, which demonstrated a good to excellent agreement with experiment.

Calculating nuclear magnetic resonance chemical shifts in solvated systems

“…Yi et al used calculated shielding values to validate peak assignments in the Chinese medicinal compound emodin. Semenov et al used the large difference in calculated 15 N chemical shift between amine and imine nitrogens and the accuracy of calculated 15 N chemical shifts, along with the experimental 15 N chemical shift of 4‐trifluoromethyl[b]benzo‐1,4‐diazepine, to show that this compound exists primarily in the imine form.…”

NMR Spectroscopic and Theoretical Studies on Tautomerism and Isomerism of Perfluoroalkyl‐Substituted 1,5‐Benzodiazepines

“…The signals for the nitrogen N(1) of I and IV appear at essentially lower field than those for NH of I , II and III . Both the NH and N resonances appear in regions, which are typical for enamines and amides and also for the diazepine ring system …”

GIAO‐DFT calculation of 15N NMR chemical shifts of Schiff bases: Accuracy factors and protonation effects

“…An interest in 15 N NMR chemical shifts stemmed mainly from the early experimental papers by Witanowski and coworkers (for the most comprehensive compilations, see reviews), in particular, those dealing with protonation effects. In this connection, in continuation of our earlier and most recent interest in the calculation of nitrogen chemical shifts of diverse open‐chain and heterocyclic nitrogen‐containing compounds, in this communication, we report on the calculation of 15 N NMR chemical shifts in the representative series of Schiff bases—a specific group of imines with a functional group that contains a carbon–nitrogen double bond with the nitrogen atom connected to an alkyl or aryl group and having a general formula R 1 R 2 C═NR 3 , where R 3 = Alk or Ar (but not H).…”