Long-Range Chiral Induction in Chemical Systems with Helical Organization. Promesogenic Monomers in the Formation of Poly(isocyanide)s and in the Organization of Liquid Crystals

Abstract

The preparation of optically active poly(isocyanide)s derived from chiral promesogenic monomers is reported. Remarkably, the stereogenic carbon atom in the monomer is able to pass its chiral “information” to the growing polymer backbone which is at least 14 atoms (approximately 16 Å) remote from it. The sense of helical induction in these conformationally rigid polymers is compared to the helical sense of the cholesteric phases, as well as to the helical senses of chiral smectic C phases, induced by the monomers in nematic and smectic C phases, respectively. Both relate to the odd−even rules for chiral sense changes in liquid crystalline phases. The role of noncovalent interactions in the polymerization has been proven by performing the reactions at various concentrations and in different solventsthe chiral induction from monomer to polymer is greatest in most concentrated reactions and in solvents with a balance between high dipolarity−polarizability and low hydrogen bond accepting and cavitational terms, as determined by an LSER analysis.

Long‐range effects of chirality in aromatic poly(isocyanide)s

“…A linear solvation energy relationship analysis 42 of the optical activities of the polymers prepared in a wide variety of solvents revealed the physicochemical features of the medium, which influence the diastereoselectivity. 29 The results suggest that as the important dipolarity-polarizability term increases, so there is greater induction of chirality, backing up the proposed noncovalent association between polymer and incoming monomer. On the other hand, the higher the hydrogen bond accepting ability of the solvent, the lower the induction of chirality, possibly because of competing coordination of the solvent and the monomers with the nickel ion catalyst.…”

“…29 For both these compounds the sense of the chiral induction is the same for like enantiomers. By way of example, the circular dichroism spectrum of monomer 2 and poly-2 are shown in Figure 2.…”

“…One of the two helical conformations is most likely preferred when a stereogenic center is placed in the vicinity, as we have seen in the solid state for other systems with two rings. 36 The phenyl benzoate moieties come into close proximity in the polymer, as shown by the high field shifts in the protons attached to them in the NMR spectra, 29 and therefore they must be close just before the bond forming the polymer backbone is formed, since the conformations are trapped kinetically.…”

“…29 This effect was shown by performing polymerization reactions at different initial concentrations of the monomer and catalyst (at fixed stoichiometry and under otherwise identical conditions). The resulting polymers have an optical activity whose magnitude increases with the concentration of the reaction: Polymers formed under dilute conditions have relatively less intense Cotton effects than those formed in concentrated solutions.…”

Consequences of Subtle Chiral Effects: From ‘Majority-Rules’ to ‘Minority-Rules’

“…In a series of helical polyisocyanides, Amabilino and coworkers observed that the helical sense preference, expressed as the magnitude of the Cotton effect, diminished rapidly when the stereocentre was positioned further away from the polymer backbone. [11] This reduced induction was attributed to the increased rotational freedom of the stereogenic centre when more intervening methylene units were present. In helical poly(N-propargylamides), in which the helical superstructure is stabilised by hydrogen bonds, Masuda and coworkers introduced methylene units between the stereogenic methyl group and the amide bond.…”