Synthesis of enantiomerically pure (1R,2S)- and (1S,2R)-2-Amino-1,2-bis(pentafluorophenyl)ethanols

Effect of 2,6-disubstituted aryl groups on acyclic conformation: preference for an antiperiplanar orientation of the geminal and vicinal hydrogens

Multiple X-ray crystallographic and 1H NMR spectroscopic studies demonstrate that 2,6-disubstituted aryl groups exert a strong effect on acyclic conformation, inducing the geminal and vicinal hydrogens on the adjacent sp3-sp3 C-C bond to adopt an antiperiplanar orientation. Twenty-one examples comprising nitrile, ketone, and dithiane aldols are provided. A search of the Cambridge Structural Database uncovered an additional 11 examples of this effect. This preference causes some unanticipated remote effects on acyclic conformations: in anti-nitrile aldols, this effect causes 2,6-disubstituted aryls to generally prefer a gauche, rather than antiperiplanar, relationship to the largest vicinal groups. X-ray crystallography, 1H-1H NOESY spectroscopy, and computation demonstrate that minimization of allylic 1,3-strain and syn-pentane-like interactions work together in establishing this conformational preference.