Determination of absolute configuration of helicenes and related biaryls from calculation of helical twisting powers by the surface chirality model

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Abstract
The sign and the magnitude of the helical twisting power β for a series of helicenes have been calculated by the Surface Chirality model. The principal contribution to β derives from the helicity in the direction perpendicular to what can be defined as the main molecular plane; the cholesteric axis is also predicted to be along this direction, in agreement with a view of the cholesteric induction by helical molecules based on empirical observations. In the case of non-rigid biphenanthryl derivatives, the value of β is predicted to vary with the dihedral angle between the phenanthryl moieties, changing sign at about 90° where the conformation passes from s-cis to s-trans, i.e. when the stereochemical descriptor of the biaryl moiety exchanges the M and P indices. Comparison between experimental and calculated values indicates an s-trans conformation for the flexible dopants, in agreement with a previous conclusion drawn from empirical correlations. The Surface Chirality model appears to be a promising technique to assess the absolute configuration of rigid molecules by the comparison of experimental and calculated β values. For flexible molecules, the quality of the information depends critically on the degree of knowledge of their conformational freedom.

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