Axially chiral thioamides of acrylic acid: correlated and uncorrelated internal rotations

In acrylic thioamides (Scheme 1), two intramolecular motions are possible: thiocarbonyl-nitrogen (=C-N) and alkenyl-carbonyl (=C-C=) rotations. Since the two mobile molecular fragments can interact by steric and by resonance effects, we intended to demonstrate the existence of correlated in addition to the above uncorrelated motions. For each of the three thioamides chosen, at least two of the four stereoisomers were enriched by crystallization and by liquid chromatography on nonracemic sorbents. Thereby axial chirality of acrylic thioamides was proven for the first time. Thermal equilibrations were monitored quantitatively by time-dependent H-1 NMR spectroscopy in the presence of a nonracemic additive, a method which, to our knowledge, has not previously been described. These kinetic results were evaluated by a simulation program with reference to uncorrelated and correlated rotations (Figs. 4-6). We have shown that all of these motions occur in thioamide 14. The enantiomers of thioamide 13 do not interconvert directly. However, indirect, two-step enantiomerizations in 13 have been proven one of the two steps consisting of correlated rotations. The latter are, therefore, possible in acrylic thioamides, a class of compounds which differ considerably from the molecules for which correlated motions were hitherto known. (C) 1998 Elsevier Science Ltd. All rights reserved.