Controlling Helical Chirality in Atrane Structures: Solvent-Dependent Chirality Sense in Hemicryptophane-Oxidovanadium(V) Complexes

The diastereomeric hemicryptophane oxidovanadium(V) complexes (P)-(S,S,S)-3 and (M)-(S,S,S)-4 have been synthesized. H-1 and V-51 NMR spectra in solution are consistent with the formation of Lambda and Delta forms of the propeller-like vanatrane moiety, leading to two diastereomeric conformers for each complex: that is., (P)-(S,S,S-Lambda)-3/(P)-(S,S,S-Delta)-3 and (M)-(S,S,S-Lambda)-4/(M)-(S,S,S-Delta)-4. The Lambda/Delta ratio is rather temperature-insensitive but strongly dependent on the solvent (the de of (M)-(S,S,S)-4 changes from 0 in benzene to 92% in DMSO). The solvent therefore controls the preferential clockwise or anticlockwise orientation of the propeller-like atrane unit. The energy barriers for the Lambda=Delta equilibrium were determined by NMR experiments, and the highest Delta G(not equal) value (103.7 kJ mol(-1)) was obtained for (P)-(S,S,S)-3, much higher than those reported for other atrane derivatives. This is attributed to the constraints arising from the cage structure. Determination of the activation parameters provides evidence for a concerted, rather than a stepwise, interconversion mechanism with entropies (AS) of -243 and -272 Jmol(-1)K(-1) for (P)(S,S,S)-3 and (M)-(S,S,S)-4, respectively. The molecular structure of the (P)(S,S,S-A)-3 isomer was solved by X-ray diffraction and shows a distorted structure with one of the linkers located in the CTV cavity. Complementary quantum chemical calculations were carried out to obtain the energy-minimized structures of (P)-(S,SS)-3 and (M)(S,S,S)-4. Our density functional theory Calculations suggest that the (P)-(S,S,SA)-3 is favored, in agreement with experimental data. For the M series, a similar strategy was used to extract molecular structures and relative energies. As in the case of the P diastereomer, the A form dominates over the A one.