Energy barriers to rotation in axially chiral analogues of 4-(dimethylamino)pyridine.
The barriers to enantiomerization of a series of axially chiral biaryl analogues of 4-(dimethylamino)pyridine (DMAP) 1-10 were determined experimentally by means of dynamic HPLC measurements and racemization studies. The barriers to rotation in derivatives 1-6 (based on the bicyclic 5-azaindoline core) were lower than those in the corresponding derivatives 7-10 (based on the monocyclic DMAP core). Semiempirical (PM3), ab initio Hartree-Fock (HF/STO-3G), and density functional theory (DFT/B3LYP/6-31G*) calculations reveal that these differences in barriers to rotation are the result of differing degrees of hybridization of the non-pyridyl nitrogen in the enantiomerization transition states (TSs). The importance of heteroatom hybridization as a factor in determining nonsteric contributions to barriers to rotation in azabiaryls of this type is discussed.
Spivey
A C
AC
Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, UK. a.c.spivey@sheffield.ac.uk
Charbonneau
P
P
Fekner
T
T
Hochmuth
D H
DH
Maddaford
A
A
Malardier-Jugroot
C
C
Redgrave
A J
AJ
Whitehead
M A
MA