NMR Studies of Chiral Discrimination Relevant to the Liquid Chromatographic Enantioseparation by a Cellulose Phenylcarbamate Derivative

Chromatographic enantioseparation of 1,1‘-bi-2-naphthol
(2) and its mono- and di-O-methylated
derivatives
(3a, 3b), 2,2‘-dihydroxy-6,6‘-dimethylbiphenyl
(4), and 10,10‘-dihydroxy-9,9‘-biphenanthryl
(5) has been performed
on cellulose tris(5-fluoro-2-methylphenylcarbamate) (1)
as a chiral stationary phase for high-performance liquid
chromatography (HPLC). The complete base-line separation of
2 and 4 was achieved with the elution order
of
enantiomers such that the (R)-isomers eluted first followed
by the (S)-isomers. The resolution of 5 and
the
O-methylated 3 was difficult on 1.
The cellulose derivative 1 dissolved in chloroform also
exhibited a chiral
discrimination for 2 and 4 in 1H and
13C NMR spectroscopies as well as in HPLC. The hydroxy
and some aromatic
protons and carbon resonances of 2 and 4 were
clearly separated into a pair of peaks due to enantiomers in
the
presence of 1. The binding geometry and dynamics
between 1 and the enantiomers of 2 were
investigated on the
basis of spin−lattice relaxation time, 1H NMR titrations,
and intermolecular NOEs in the presence of 1.
These
NMR data are fully consistent with the chromatographic elution order.
These results, combined with molecular
modeling, reveal the chiral discrimination rationale.