title: Effect of 2,6-disubstituted aryl groups on acyclic conformation: preference for an antiperiplanar orientation of the geminal and vicinal hydrogens.
abstract: 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.
authors.affiliation: Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24060, USA. pcarlier@vt.edu
authors.forename: Paul R
authors.initials: PR
authors.lastname: Carlier
authors.forename: Yiqun
authors.initials: Y
authors.lastname: Zhang
authors.forename: Carla
authors.initials: C
authors.lastname: Slebodnick
authors.forename: Michael M-C
authors.initials: MM
authors.lastname: Lo
authors.forename: Ian D
authors.initials: ID
authors.lastname: Williams