Control of asymmetric biaryl conformations with terpenol moieties: Syntheses, structures and energetics of new enantiopure C 2-symmetric diols

New enantiopure, C(2)-symmetric biphenyl-2,2'-diols based on (-)-menthone (BIMOL), (-)-verbenone (BIVOL) and (-)-carvone (BICOL and hydrogenated BIMEOL), are accessible via short, synthetic routes. All diols form intramolecular hydrogen bonds and hence can be employed as chelating ligands for catalyst design, as it demonstrated for the (-)-fenchone based BIFOL. The sense of asymmetry of the biphenyl axes is controlled by the chiral terpene units and is conformationally surprisingly stable. X-ray analyses reveal M biphenyl conformation for BIMOL and P biphenyl conformation for each of BIVOL, BICOL and BIMEOL. The origins of the conformational biphenyl preferences are confirmed by computational ONIOM evaluations of the diols and their diastereomeric conformers. The experimentally observed biphenyl conformations are all energetically preferred, i.e. with 1.3 kcal/mol for (M)-BIMOL, with 5.1 kcal/mol for (P)-BIVOL, with 5.8 kcal/mol for (P)-BICOL, and with 5.4 kcal/mol for (P)-BIMEOL.