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.
Carlier
Paul R
PR
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24060, USA. pcarlier@vt.edu
Zhang
Yiqun
Y
Slebodnick
Carla
C
Lo
Michael M-C
MM
Williams
Ian D
ID