NMR Study of Spatial Structure and Internal Dynamic of Adducts of Ninhydrin-Derived Azomethine Ylide with Cyclopropenes

Using NMR spectroscopy, the structure of the new products obtained in reaction of ninhydrin-derived azomethine ylide with cyclopropenes was established and conformational mobility was studied. Complete signal assignments in H-1 and C-13 spectra on base analysis of some homo- and heteronuclear correlation spectra were made, and qualitative comparison of the cross-peak intensities in the NOESY spectrum confirmed the structure of compounds 4a-4c in which three aromatic rings a, b and c at carbons of the cyclopropane ring are facing to us and the side substitute group at carbon C-10 is situated in back direction. The hindered rotations of these aromatic rings in rigid part of molecules under study were found and values of free energy activation increment G(not equal) of these rotations were evaluated on base coalescence temperature (T-c) and frequency difference ( increment nu) data. The existence of fast in NMR time-scale conformational equilibrium in the solution associated with hindered rotation of side substitute group around C-10-C-14 bond was proved. In case of adduct 4c, the ratio 68:32 of two preferable conformers has been determined at room temperature in CDCl3 on base a comparison of calculated and experimental values of vicinal constant (3)J(10 alpha-14 beta). This scalar constant turned out to be 1.0 Hz more for adduct 4a and this reflects an increase in the population of the main conformer almost by 10%. In the NOESY spectra of all studied adducts, a very rare phenomenon of scalar relaxation was found. It manifests itself in appearance of the additional scalar input into the effective cross-relaxation rate between two protons, and in case omega(o)tau(c) MUCH LESS-THAN 1, it may lead to a change in the sign of the cross-peak to the opposite if the scalar constant between them J(H-H) turns out to be time-dependent (scalar relaxation of the first kind) or in case of quadrupole nuclear situated near from one of these protons (scalar relaxation of the second kind). Thus, the conformational mobility which is connected with fast (in NMR time scale) nitrogen inversion in five-membered ring of adducts under study was proved due to the presence of the so-called scalar relaxation effects in NOESY spectra which look as a change cross-peak sign in comparison with the usual case.