Abstract
New enantiopure, C2-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.
Highlights
Enantiopure biaryl systems with flexible chiral axes are widespread, e.g. in pharmacological natural products or as ligands in enantioselective catalyses [1]
We recently reported the synthesis and the X-ray crystal structure of (M)BIFOL [7] and its derivatives [8,9,10,11]. (M)-BIFOL exhibits, in a similar way as BINOLs, a flexible biaryl axis with M conformation, induced by the hydrogen bonded fenchol moieties, and sterically crowded aliphatic alcohol functions, as in TADDOLs
The new 1,1′-biphenyl-2,2′-bisterpenols BIMOL, BIVOL and BICOL were synthesized by addition of 2,2′-dilithiobiphenyl to (−)-menthone, (−)-verbenone and (−)-carvone and subsequent hydrolysis (Scheme 2)
Summary
Enantiopure biaryl systems with flexible chiral axes are widespread, e.g. in pharmacological natural products or as ligands in enantioselective catalyses [1].
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