Equilibria, optically active schiff bases

The optically active Schiff bases containing intramolecular hydrogen bonds are of major interest because of their use as ligands for complexes employed as catalysts in enantioselective reactions or model compounds in studies of enzymatic reactions. In the studies of intramolecularly hydrogen bonded Schiff bases, the NMR spectroscopy is widely used and allows detection of the presence of proton transfer equilibrium and determination of the mole fraction of tautomers [21]. Literature gives a few names of tautomers in equilibrium. The OH-tautomer has been also known as OH-, enol- or imine-form, while NH tautomer as NH-, keto-, enamine-, or proton-transferred form. More detail information concerning the application of NMR spectroscopy for investigation of proton transfer equilibrium in Schiff bases is presented in reviews.42-44... 

The 8N chemical shift can be used in studies of tautomeric equilibrium for optically active Schiff bases, however, it depends not only on the position of equilibrium but also on the types of substituents. Usually values higher than —200 ppm (relative to nitromethane) indicate the presence of pure NH-form or equilibrium strongly shifted towards NH tautomer. Lower values of 815N imply the presence of tautomeric equilibrium. ... 

Deuterium isotope effects in CDCI3 have been applied in studies of proton transfer equilibrium for optically active Schiff bases [24], [25] as well as their dirhodium tetracarboxylate adducts.11 The AC-2(D) values were in the range from +541 to —546 ppb for imines and from +524 to 255 ppb for their adducts. Exceptionally low values of deuterium isotope effects of - 20 —80 ppb were observed for the Schiff bases being derivatives of 3,5-dinitrosalicylaldehyde which exist in pure NH-form. This behaviour was explained by different electronic structures of the NH... 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

Cesarotti, M. Gullotti, A. Pasini, A. Ugo, R. Optically active complexes of Schiff bases. Part 5. An investigation of some solvent and conformational effects on the equilibriums between cobalt(ll) Schiff-base complexes and dioxygen. J. Chem. Soc. Dalton Trans. 1977, 8, 757-763. 

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