Scalar coupling constants across the intramolecular NHN hydrogen bond of symmetrically and non-symmetrically substituted 6-aminofulvene-1-aldimines
✍ Scribed by Mariusz Pietrzak; Hans-Heinrich Limbach; Marta Pérez-Torralba; Dionísia Sanz; Rosa María Claramunt; José Elguero
- Publisher
- John Wiley and Sons
- Year
- 2001
- Tongue
- English
- Weight
- 237 KB
- Volume
- 39
- Category
- Article
- ISSN
- 0749-1581
- DOI
- 10.1002/mrc.937
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✦ Synopsis
Multinuclear liquid state magnetic resonance experiments have been performed on two seven-membered 15 N-labeled H-chelates of the 6-aminofulvene-1-aldimines type in order to characterize the strong intramolecular NHN hydrogen bonds as a function of the molecular symmetry. In particular, the symmetrically substituted N,N'-diphenyl-6-aminopentafulvene-1-aldimine-15 N 2 (1) and its asymmetric analog 3,4-triazol)-6-aminopentafulvene-1-aldimine-15 N 5 (2) have been studied. For 1, an NN coupling constant across the hydrogen bridge of 2h J. 15 N, 15 N/ = 10.6 Hz was determined indirectly by 13 C NMR at two different Larmor frequencies, 125.76 and 67.93 MHz; this coupling constant is characteristically enhanced compared with the value of 8.6 Hz obtained previously for 2. Because of a fast degenerate proton tautomerism the hydrogen bond proton in 1 is coupled with both nitrogen atoms with a coupling constant of -40.8 Hz. f 15 Ng tickling experiments were performed on 2 in order to determine the relative signs of the coupling constants of the NHN hydrogen bridge. We find that 2h J. 15 N, 15 N) and 1h J. 1 H, 15 N/ = +4.4 Hz exhibit the same sign, i.e. the opposite sign compared with 1 J. 15 N, 1 H/ = -88.6 Hz. This finding proves that 1h J. 1 H, 15 N/ corresponds to an intrinsic coupling, which is not induced by a tautomerism absent in 2 because of the large difference in basicities of the aniline and the amino-1,3,4triazole substituents. Therefore, these observations indicate a sign change of J. 15 N, 1 H/ when the proton is transferred successively from one nitrogen to the other, as observed previously for FHF hydrogen bonds. The relation between the values of the coupling constants and the hydrogen bond geometries is discussed in terms of the valence bond order model, as are the implications for obtaining equilibrium constants of tautomerism from coupling constant data.