Stereochemistry and tautomeric equilibria of zinc(II) complexes of the condensation products between (1R)-3-hydroxymethylenebornan-2-one and L-amino acids

Abstract

The zinc(II) complexes derived from the condensation of (1R)-3-hydroxymethylenebornan-2-one and a series of L-amino acids, [ZnL], undergo tautomeric equilibria in solution between enolimine and ketoenamine species. While the enolimine forms exhibit the expected behaviour for metal complexes of amino acid Schiff bases, the ketoenamine forms appear structurally similar to the complex derived from the cyclic amino acid L-proline, containing a chiral tetrahedral nitrogen donor. The presence of this tetrahedral nitrogen atom and the possibility for the metal atom itself to become a chiral centre can originate up to four diastereomeric ketoenamine forms of the [ZnL] complexes, while only two can be formed for the derivative of L-proline, where the nitrogen atom co-ordinates stereospecifically to the metal. The diastereomeric ketoenamine forms of the [ZnL] complexes can be interconverted through the planar enolimine form, and this tautomerization process can be followed by n.m.r. measurements, since the signals of the various isomers approach the fast exchange limit near 100 °C. For the derivative of L-proline this possibility is no longer available and the proton signals of the two diastereomeric ketoenamines are not averaged by heating the sample up to ca. 130 °C.