Chiral induction, memory

Chiral Induction and Memory Porphyrin (Hetero-) Self-Assemblies in the Presence of Poly-Glutamate... 

Treatment of a number of covalent polymers substituted with molecular recognition capability with suitable guests leads to chiral induction. The same crown ether-amino acid complementary pair described for the rosettes above was employed in the form of crown-ether pendant cis-transoidai poly(phenylacetylene). When the achiral polymer is treated with amino acids (in the form of their hydroperchlorate salts in acetonitdle) a large induced CD signal is observed in the backbone of the polymer. The polymer is sensitive to small enantiomeric excesses in the amino acid, as little as 0.005% enantiomeric excess of alanine can be detected. In a similar vein, c/5-transoidal poly(carboxyphenylacetylene) shows induced circular dichroism when treated with nonracemic chiral amines In addition, the system displays chiral memory, in that treatment of the complex with achiral amino alcohols results in retention of the chiral polymer backbone. 

Karabatsos, G. J., Rotational Isomerism about sp2-sp3Carbon-Carbon Single Bonds, 5, 167. Kawabata, T., Memory of Chirality Asymmetric Induction Based on the Dynamic Chirality of Enolates, 23, 175. 

The noncovalent helicity induction and chiral memory concept is versatile enough to produce and maintain either a right- or left-handed helix because the helix-sense is predetermined by the chirality of the enantiomeric amines used. Consequently, the opposite enantiomeric helicity induction and the memory requires the opposite enantiomeric amine, followed by replacement with achiral amines. However, both mirror-image enantiomeric helices can be produced with a high efficiency from a helical poly(phenylacetylene) induced by a single enantiomer (Fig. 26) [129]. This dual memory of enantiomeric... 

As presented in Scheme 1, induction of chirality is a necessary and preliminary step leading to its permanent transfer (i.e. memory). For this reason we present first a system which does not show chiral memory but represents a kind of milestone in the development of our research [26]. The rationale behind this study was to investigate systems more complex than those studied in literature at that time (1998). The reason was not simply to do something different or new but to understand if and how chirality transfers and propagates from a chiral template to a first shell of achiral guests and from here to a second shell of achiral guests. 

Mammana A, De Napoli M, Lauceri R et al (2005) Induction and memory of chirality in porphyrin hetero-aggregates the role of the central metal ion. Bioorg Med Chem 13 ... 

Lauceri R, D urso A, Mammana A et al (2008) Chiral memory induction, amplification, and switching in porphyrin assemblies. Chirality 20 411 —419... 

Lauceri R, Fasciglione GF, D Urso A et al (2008) Kinetic investigation of porphyrin interaction with chiral templates reveals unexpected features of the induction and self-propagation mechanism of chiral memory. J Am Chem Soc 130 10476—10477... 

Fig. 4.32. Chiral recognition and induction of molecular memory in Ce(IV) complexes with a substituted porphyrin. Redrawn from S. Shinkai et al., J. Chem. Soc., Perkin Trans. I 3259, 1999.

Memory of Chirality Asymmetric Induction Based on the Dynamic Chirality of Enolates... 

Matsumura and co-workers reported a memory effect of chirality in the electrochemical oxidation of 95 to give 96, although the enantioselectivity was modest (Scheme 3.25). The reaction is assumed to proceed via carbenium ion intermediate Q.46 The mechanism for asymmetric induction is not clear. A possible mechanism involves chiral acid (95)-mediated deracemization of racemic 96 produced by the electrochemical oxidation of 95. However, this suggestion may be eliminated based on the finding that treatment of racemic 96 with 95 in methanol containing 5% formic acid did not produce optically active 96. 

The fast formation of the xyn-it-allyl complex with selective attack of the nucleophile at the branched position allows the induction of stereoinformation with a chiral ligand when starting from linear (E)-configured substrates. Branched racemic substrates are less suited because a significant memory effect based on different reaction rates of the diastereomeric allyliridium complexes is observed, diminishing the enantiomeric excess value of the product [145]. 

Kawabata T, Fuji K. Memory of chirality asymmetric induction based on the dynamic chirality of enolates. Top. Stereochem. 2003 23 175-205. 

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