Controlling Helical Chirality in Atrane Structures: Solvent-Dependent Chirality Sense in Hemicryptophane-Oxidovanadium(V) Complexes
Résumé
The diastereomeric hemicryptophane oxidovanadium(V) complexes (P)-(S,S,S)-3 and (M)-(S,S,S)-4 have been synthesized. 1H and 51V NMR spectra in solution are consistent with the formation of Λ and Δ forms of the propeller-like vanatrane moiety, leading to two diastereomeric conformers for each complex: that is, (P)-(S,S,S-Λ)-3/(P)-(S,S,S-Δ)-3 and (M)-(S,S,S-Λ)-4/(M)-(S,S,S-Δ)-4. The Λ/Δ 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 Λ⇄Δ equilibrium were determined by NMR experiments, and the highest ΔG≠ 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 (ΔS≠) of −243 and −272 J mol−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-Λ)-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,S,S)-3 and (M)-(S,S,S)-4. Our density functional theory calculations suggest that the (P)-(S,S,S-Λ)-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 Λ form dominates over the Δ one.