An Exciton-Coupled Circular Dichroism Protocol for the Determination of Identity, Chirality, and Enantiomeric Excess of Chiral Secondary Alcohols

Abstract

Chiral mono-ols are among the most sought after targets in asymmetric synthesis, and therefore, their chemical characterization and associated enantiomeric excess (ee) values are commonly reported. A simple optical method for determining alcohol identity and ee 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, ee calibration curves allowed the determination of the ee 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.