Optical isomerism Twist mechanisms

Mechanisms for optical and geometrical isomerization reactions similar to those employed for substitution reactions can be envisaged. Additionally possible is a twist mechanism, involving distortion of the polyhedral framework in the activated state but in which no ligands depart or join the coordination sphere. 

The case is selected in which the ligands are unsymmetrical so as to also show whether geometrical isomerization accompanies optical inversion. The twist mechanisms have been suggested by some authors as possible modes of racemization. For some jS-diketonates and other chelates inferential evidence has suggested that isomerizations occur principally by bond rupture plus bond reorganization rather than by bond reorganization without bond... 

The n.m.r. spectra of E/Z mixtures of twelve compounds were assigned and the angles of twist about the aryl-imino-carbon bond were determined.Subsequently, the pure (E)- and (Z)-isomers were obtained by preparative t.l.c. at 258 K or by stereospecific syntheses from (E)- and (Z)-thioamides. The chemical shifts of the protons in the methyl groups were measured and the (E) (Z) ratios and the barriers to isomerization were determined by n.m.r. spectroscopy. The conformational states of some AW-di(primary alkyl)-amides and -thioamides were studied by dynamic H n.m.r. spectroscopy, and the barriers to the interconversion of enantiomers that were thus found were in good agreement with molecular mechanics calculations. Lr. and n.m.r. data for optically active thioamides R CSNR CHMeR indicate that there are mixtures of (E) and (Z) conformers, and i.r. and P n.m.r. studies of phosphorylated thiobenzamides provide evidence of a tautomeric equilibrium [reaction (4)], the amide form being favoured in more... 

---