Chiral memory reversibility

Randazzo R, Mammana A, D Urso A et al (2008) Reversible chiral memory in ruthenium tris(phenanthroline)-anionic porphyrin complexes. Angew Chem Int Ed Engl 47 9879-9882... 

Yashima and co-workers reported the memory of macromolecular helicity of poly((4-carboxyphenyl)acetylene) (poly-98). Poly-98 itself possesses a large number of short helical units with many helix-reversal points, and is therefore achiral. However, in the presence of optically active amine 99, which can interact with the polymer s carboxyl groups, one-handed macromolecular helicity is induced in the polymer. When achiral amino alcohol 100 is added to the helical complex, chiral amine 99 bound to poly-98 is replaced by stronger base 100. Nevertheless, the newly formed complex still shows a one-handed helical conformation. Even after the removal of 99 by gel permeation chromatography, the poly-98-100 complex retains a one-handed helical conformation without a loss of helical intensity. Thus the helicity of poly-98 induced by complexation with a chiral amine was memorized after replacement by an achiral one. The half-life of the chiral memory is as long as four years at room temperature.48... 

Finally, we feel it is worthwhile to stress one more time the importance of the kinetic inertia in the (reversible) chiral transfer and memory processes of our porphyrin systems. Inertia provides evidence that the system is trapped in an energy minimum. In the above examples the minimum is local the real minimum is that reached from the achiral system whose formation involves the same enthalpic contribution of the chiral one but a more favourable entropic contribution. In particular, the network of electrostatic interactions ensures a quite deep local energy minimum (that is a high value of EA). 

There is a lot to explain here. It looks very odd that syn-66 is preferred and even more so that alkylation of the enolate 67 occurs on the same face as the undoubtedly large /-butyl group. Both these issues matter as the original chiral centre in proline is destroyed in 67 and only the newly introduced chiral centre in 66 retains the stereochemical information from proline. This centre acts as a relay for the stereochemical information. Others call this a memory effect. The acid-catalysed formation of the N, O-acetal 66 is under thermodynamic control (acetal formation is reversible) and the conformation 66a shows that the molecule folds about the necessarily cis ring junction and the /-butyl group prefers to be on the outside (or exo- face).8 The enolate 67 has a flattened conformation 67a (probably more flattened than this ) and its alkylation is under kinetic control. Attack is preferred on the outside, exo-face. Note that this happens to restore the original configuration at the ex-proline chiral centre. 

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