Solvent-dependent helix inversion in optically active poly(diphenylacetylene)s and their chiral recognition abilities as chiral stationary phases for high-performance liquid chromatography.

We report the first example of solvent-dependent helix inversion in poly(diphenylacetylene) (PDPA) derivatives. Asymmetrically substituted PDPAs bearing optically active substituents linked through amide bonds formed preferred-handed helical conformations because of the optically active substituents in the pendants, whose helix-senses were inverted upon thermal annealing in polar solvents such as N,N-dimethylformamide and dimethylsulfoxide and in nonpolar solvents such as tetrachloroethane. Unlike the solvent-dependent helix inversion reported for other dynamic helical polymers, the macromolecular helicity induced in the polymer backbone of these PDPAs upon thermal annealing was stably maintained at room temperature, independent of the solvent polarity. These diastereomeric PDPAs with opposite helix-senses generated almost mirror-imaged left- and right-handed circularly polarized light in the same solvent at room temperature. Taking advantage of this unique solvent-dependent helix inversion property, the diastereomeric PDPAs with opposite helix-senses were coated on macroporous silica gel and applied to chiral stationary phases for high-performance liquid chromatography. Despite having the same optically active substituents on the pendant phenyl rings, they showed completely different chiral recognition abilities toward many racemates depending on the helix-sense of the polymer backbone, and the elution order of the enantiomers was reversed for some racemates. The combination of the helix-sense of the polymer backbone and the chirality of the pendants, which afforded a higher chiral recognition ability, differed depending on the racemates.

Helical springs as a color indicator for determining chirality and enantiomeric excess.

Chirality plays a key role in the physiological system, because molecular functionalities may drastically alter due to a change in chirality. We report herein a unique color indicator with a static helicity memory, which exhibits visible color changes in response to the chirality of chiral amines. A difference of less than 2% in the enantiomeric excess (ee) values causes a change in the absorption that is visible to the naked eyes. This was further quantified by digital photography by converting to RGB values. This system relies on the change in the tunable helical pitch of the π-conjugated polymer backbone in specific solvents and allows rapid on-site monitoring of chirality of nonracemic amines, including drugs, and the simultaneous quantitative determination of their ee values.

Synthesis of a poly(diphenylacetylene) bearing optically active anilide pendants and its application to a chiral stationary phase for high-performance liquid chromatography.

Poly(diphenylacetylene) having optically active anilide pendants (poly-1) were synthesized by the condensation reaction of an optically active carboxylic acid with a key precursor polymer containing amino (-NH2) groups, which was prepared by the polymerization of a phthalimide-protected diphenylacetylene monomer using WCl6-Ph4Sn as a catalyst, followed by phthalimide deprotection in the resulting polymer using hydrazine monohydrate. Poly-1 formed a preferred-handed helical conformation (h-poly-1) upon thermal annealing in DMF because of chirality of the pendant group. Poly-1 and h-poly-1 showed different chiral recognition abilities from the analogous poly(diphenylacetylene)s, having the corresponding optically active amide pendants, as chiral stationary phases (CSPs) for high-performance liquid chromatography. The resolution results with the h-poly-1-based CSP were much better than those with the poly-1-based CSP owing to the preferred-handed macromolecular helicity. Among the tested racemates, the h-poly-1-based CSP exhibited superior chiral recognition ability, especially toward binaphthyl compounds and chiral metal complexes.

Helix-Sense-Selective Synthesis of Right- and Left-Handed Helical Luminescent Poly(diphenylacetylene)s with Memory of the Macromolecular Helicity and Their Helical Structures.

Symmetrically substituted poly(diphenylacetylene) (PDPA) bearing carboxy pendants was found to fold into a one-handed helix upon thermal annealing with nonracemic amines in water accompanied by chiral amplification of the helicity. The induced right- or left-handed helical PDPA was retained (memorized) after complete removal of the chiral amines, thus producing a one-handed helical circularly polarized luminescent PDPA in a helix-sense-selective manner. The helical PDPA with static helicity memory is tolerant toward modification of carboxy pendants, providing functional PDPAs with an optical activity solely due to macromolecular helicity. The PDPA and its derivatives before and after the one-handed helicity induction and its subsequent memory of the helicity exhibited well-resolved very simple 1H and 13C NMR and Raman spectra whose spectral patterns are virtually identical independent of the helical sense bias. On the basis of the 1H and 13C NMR, IR, Raman, and vibrational and electronic circular dichroism spectral measurement results combined with theoretical calculations, the key structural features (cis or trans and cisoid or transoid) of the PDPA as well as its helix inversion barrier and absolute handedness (right- or left-handed helix) and helix-sense excess of the one-handed helical PDPA and its derivatives with static helicity memory were determined. As a result, almost complete right- and left-handed helical cis-transoidal PDPAs with 98% helix-sense excess were successfully obtained using noncovalent helicity induction and memory strategy.