An Exciton-Coupled Circular
Dichroism Protocol for
the Determination of Identity, Chirality, and Enantiomeric Excess
of Chiral Secondary Alcohols
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Abstract
Chiral mono-ols are among the most sought after targets
in asymmetric
synthesis, and therefore, their chemical characterization and associated
enantiomeric excess (<i>ee</i>) values are commonly reported.
A simple optical method for determining alcohol identity and <i>ee</i> could be widely used. Toward this end, an in situ-generated
multicomponent assembly that creates diastereomeric tris(pyridine)
metal complexes incorporating chiral secondary alcohols was explored
using exciton-coupled circular dichroism (ECCD). Qualitative models
were proposed to predict the preferential diastereomer and its twist,
and computational studies provided a rationalization of the CD spectra.
Different ECCD spectra found for diastereomers formed in the self-assembled
tris(pyridine) complexes were used to determine the absolute configurations
of chiral mono-ols. Linear discriminant analysis was successfully
employed to classify the alcohol analytes, thereby allowing identification
of the alcohols. Conformational effects imparted by heteroatoms were
also explored, further expanding the substrate scope. Finally, <i>ee</i> calibration curves allowed the determination of the <i>ee</i> of unknown samples of three chiral secondary alcohols
with an average error of 3%. The assay described here is unique because
no preparation of structurally elaborated chiral hosts is needed