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Effect of 2,6-Disubstituted Aryl Groups on Acyclic Conformation:  Preference for an Antiperiplanar Orientation of the Geminal and Vicinal Hydrogens

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posted on 10.11.2006, 00:00 by Paul R. Carlier, Yiqun Zhang, Carla Slebodnick, Michael M.-C. Lo, Ian D. Williams
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.