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“Chirality-Switchable Materials” for New Chiral Science and Technology

Efficient control of molecular chirality is a critical issue in synthetic organic chemistry, primarily because biological activities of pharmaceutical as well as agricultural chemicals are sharply dependent on the molecular chirality. In addition, molecular aggregation is significantly affected by the chirality of aggregated molecules, as exemplified by the formation of chiral nematic liquid crystalline materials. Furthermore, there is an increasing demand for chiral organic materials that can manage the handedness of circularly polarized light in display and information technologies. Under these circumstances, it is desirable to establish new chiral molecular framework to make efficient control of chirality possible.

We have been interested in the molecular chirality of helical polymers, aiming at their application to new molecular material for the control of chemical and physical chirality on the basis of their unique molecular structure.

Our particular interest has been focused on poly(quinoxaline-2,3-diyl)s (PQX), which are prepared by living aromatizing polymerization of 1,2-diisocyanobenzenes. PQX features highly rigid, rod-like helical structure, which is still able to undergo helical inversion between right and left-handed helices. Below, two key functions of non-racemic PQX are summarized.

helical structure of PQX2.jpg
 

1.1. Chirality-Switchable Catalysts for Asymmetric Synthesis

Particular attention is focused on the exploration of new helical polymer scaffolds that are enantiomercally pure, structurally robust, and readily modifiable at their side chains. Poly(quinoxaline-2,3-diyl)s are unique aromatic polymer fulfilling all those requirements. They do not have pre-fixed chiral axes or stereogenic centers in the main-chains, but adopt right- or left-handed helical structure on the basis of homogeneous axial chirality between the quinoxaline rings, which is induced thermodynamically by chiral side chains or pendants (Org. Lett. 2002).

We’ve recently found that polyquinoxaline copolymer bearing both coordinating diphenylphosphino pendants and (R)-2-butoxymethyl side chains serves as the chiral ligand in asymmetric hydrosilylation of styrene, affording up to 97% ee (J. Am. Chem. Soc. 2010). In particular, use of high-molecular-weight polymer PQXphos (ca. 1000mer) allowed us to obtain insoluble polymer complex, which could be reused eight times without adding a palladium source with constantly high enantioselectivities (97-98% ee).

helical structure.jpg
 

It is also noteworthy that the helical conformation is switchable by a solvent effect (Chem. Commun. 2010). In CHCl3, it takes perfect right-handed helical structure, which affords (S)-hydrosilylation product in 97% ee, while in 1,1,2-trichloroethane it adopts left-handed helical structure, which gives (R)-hydrosilylation product in 93% ee. These results clearly indicate that use of chiral polymer scaffolds in asymmetric catalyses is attractive not only because of the high enantioselectivities, but also because of the switchability of enantioselection.

The new polymer-based chiral ligands PQXphos have also shown high enantioselectivities and bidirectional enantioselection in some other palladium-catalyzed reactions including asymmetric Suzuki-Miyaura cross-coupling (Angew. Chem. 2011) and ring-opening silaboration of meso-methylenecyclopropanes (J. Am. Chem. Soc. 2012).

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1.2. New Chirality-Switchable Chiroptical Materials

Organic materials that give right- or left-handed circularly polarized light gain considerable interests in materials and information technologies. We have recently showed that the main chain of PQX emits circularly polarized luminescence whose handedness is switchable by the solvent effect (Chem. Commun. 2014). This is the first organic molecule whose luminescence's handedness is switchable in solution reversibly.

We have recently established that a dry polymer thin film fabricated from binary or ternary random copolymers bearing (S)-2-methylbuthoxy side chains exhibited selective reflection of right- or left-handed circularly polarized light (CPL) in the visible region (J. Am. Chem. Soc. 2014). Interestingly, when the film was annealed in the vapor of CHCl3, the film reflected right-handed CPL selectively, while left-handed CPL was reflected selectively after annealing the same film by the vapor of 1,2-dichloroethane. The color of the film was fully tunable by the composition of the ternary polymers. It is noteworthy that both the color of the film and the handedness of the CPL were fully tunable by changing the ratio of the solvents used in the solvent vapor annealing process. This is the first solid thin film whose color and the CPL handedness are both switchable reversibly.

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Last-modified: 2014-08-19 (火) 14:03:04 (1192d)