CHIRAL AMPLIFICATION OF WATER-SOLUBLE POLYOL-RH CONFORMERS

Chiral assembly from achiral components on the basis of helical motifs is a topic of current interest, which has been recently reviewed (1). On behalf our report on the synthesis of water-soluble trisubstituted hybrid polyol-Cp rac-(1, 2, 4) - CpOOORh(NBD) [CpOOO = C5H3[CH(CH2)3CH)OH]3 Rhodium complex A (2), herein we describe the unexpected chiral response of the ECCD spectra observed in polar donor solvating media and consistent with negative ORD values (25αD = -12). The DFT calculated spectra allow for the interpretation of the exciton-coupled CD signal due to the auxochromic hydroxy groups, describing the metal orbitals involved in the red-shifted experimental UV absorptions. Although the synthetic pathway goes to isomers simplification for stereogeometric reasons, affording only one diastereomer, we isolated two distinct atropisomers, showing a reciprocal opposite position for the hydroxyl cyclopentanol substituents, as blades of the propeller-like CpOOO ligand. As shown by the X-ray determination, the effects due to the central and planar chiralities are canceled out by the spontaneous crystalline conglomerate architecture of the racemic mixture. Then the chiral behaviour in solution has to be exclusively ascribed to the preferred helicity, related to the A more stable conformer. Accordingly to the VT NMR experiments, showing a strong OH(1)•••OH(2) hydrogen intramolecular bond interaction, the helical structure results to be predominantly anticlockwise. Due to the low energy rotation barriers (experimentally and theoretically calculated ~ 7-8 Kcal mol-1), the DFT in vacuo calculations permit the identification of several minimum energy conformations. The solvent effect which may intercepts the privileged rotameric forms, explaining the chiral behavior of racemic complexes in the absence of chiral auxiliaries or anisotropic medium will be also discussed.