Thioamides and selenoamides with chirality solely due to hindered rotation about the C-N bond: enantioselective complexation with optically active hosts

Several thioformamides and selenoformamides, with chirality solely due to restricted rotation about the C-N bond, were resolved to enantiomers by inclusion crystallization with optically active diols (TADDOLs). The absolute configuration of the guest molecules was deduced from the X-ray crystal structures of the inclusion complexes. The optical activity of the resolved compounds is manifested by their CD spectra showing relatively strong Cotton effects in the region of thioamide or selenoamide n-pi* transition. The optically active thioformamides and selenoformamides are configurationally labile compounds and gradually racemize in solution but are stable in the form of the inclusion complexes. The first-order kinetics of the racemization in solution allowed us to assign the C-N rotation barriers of thioformamides by spectropolarimetric measurements. (c) 2005 Elsevier Ltd. All rights reserved.