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Absolute Configuration Assignment from Optical Rotation Data by Means of Biphenyl Chiroptical Probes
Authors:Dr. Stefania Vergura  Dr. Patrizia Scafato  Dr. Sandra Belviso  Prof. Dr. Stefano Superchi
Affiliation:Department of Science, Università degli Studi della Basilicata, Via dell'Ateneo Lucano, 10, 85100 Potenza, Italy
Abstract:A non-empirical approach for the assignment of the absolute configuration of chiral 2-alkyl-substituted carboxylic acids and primary amines by [α]D measurements has been developed. The method requires the conversion of the chiral acids or amines into the corresponding 4,4′-disubstituted biphenylamides or biphenylazepines, respectively. In these derivatives a central-to-axial chirality transfer induces a preferred torsion in the biphenyl moiety revealed by the sign of the biphenyl A band in the ECD spectrum. By 4,4′-substitution on the biphenyl moiety a redshift of the A band is obtained, leading to an increase of its relative contribution to optical rotation. This allows to reliably establish a direct correlation between the [α]D sign, the biphenyl twist and, then, the substrate absolute configuration. This approach thus constitutes a really practical and reliable method to assign the absolute configuration of chiral carboxylic acids and primary amines by simple and straightforward [α]D measurement, readily obtainable by a routine instrumentation like the polarimeter.
Keywords:absolute configuration  biphenyls  chiroptical probes  circular dichroism  optical rotation
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