Diastereoisomeric charge-transfer complexes. Measurement of thermodynamic constants by 1H nuclear magnetic resonance spectroscopy

Abstract

The addition of (–)-7,7′-bismethoxycarbonyl[6]helicene to a solution of (±)-2-(2,4,5,7-tetranitrofluoren-9-ylideneamino-oxy)propionic acid methyl ester (TAMP) results in the formation of two diastereoisomeric charge-transfer complexes. In the 1H n.m.r. spectrum, this shifts the TAPM signals upfield and simultaneously splits them into separate groups of peaks. The chemical shifts relative to pure TAPM of solutions containing different helicene: TAPM ratios led to the determination of the equilibrium constants for the formation of the two complexes at room temperature. These measurements were then extended to a ca. 100 °C temperature range, and from that the enthalpies and enthropies of a complex formation were obtained. It is shown that while at low temperatures, the (–)(+)-complexes is more stable than the (–)(–)-one, this is reversed at higher temperatures with a crossover point at 16 °C. These results are discussed by comparison with the chromatographic separation of helicene enantiomers.