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*Asymmetric Synthesis
Particular attention is now focused on the exploration of new helical scaffolds that are enantiomerically pure, structurally robust, and readily modifiable at their side chains. Poly(quinoxaline-2,3-diyl)s are quite unique polymer which fulfill all those requirements. However, there was no practical synthetic accesses to single-handed helical poly(quinoxaline)s. We established that living polymerization of 1,2-diisocyanobenzenes with optically active organopalladium initiators could control the screw-sense of the helices with quite high level of screw-sense induction (>90% screw-sense excess) [Ref. 1].
#ref(Graphics for Reserch/asymmetric polymerization.jpg,center)
* “Chirality-Switchable Materials” for New Chiral Science and Technology [#zdeb7b59]
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.
#ref(helical structure of PQX2.jpg,center,70%)
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*Application to Chiral Polymer Catalysts
This asymmetric polymerization system was unique in that the optically active group, which remains at the non-living terminus of the polymer chain, controls the helical chirality of the polymer main chain. Since helical conformation of the polymer does not rely on the polymer side chain, but rely on the chiral group at the polymer terminus, the side chains of the polymer is readily modifiable with functional groups, including catalytically active groups.
We’ve recently found that polyquinoxaline bearing a coordinating diphenylphosphino group serves as the chiral monodentate ligand in asymmetric hydrosilylation of styrene, affording >85% ee [Ref. 2]. The polymer ligand has a chiral group only at the polymer terminus, which is far away from the reaction sites and thus has no direct participation to the created asymmetric environment around the reaction site.
#ref(Graphics for Reserch/chiral polymer ligand.jpg,center)
*1.1. Chirality-Switchable Catalysts for Asymmetric Synthesis [#v0481224]
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).
#ref(helical structure.jpg,center)
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**References
-Ref. 1
--Highly Screw-Sense Selective Polymerization of 1,2-Diisocyano-3,6-di-'''p'''-tolylbenzene Initiated by Optically Active Binaphthylpalladium(II) Complexes Y. Ito, T. Ohara, R. Shima, M. Suginome, '''J. Am. Chem. Soc.''' ''1996'', 118, 9188–9189, [[10.1021/ja961818g>http://dx.doi.org/10.1021/ja961818g]]
--Asymmetric Synthesis of Helically Stable Poly(quinoxaline-2,3-diyl)s Having Hydrophilic and/or Hydrophobic Side-chains Y. Ito, T. Miyake, T. Ohara, M. Suginome, '''Macromolecules''' ''1998'', 31, 1697-1699, [[10.1021/ma9717855>http://dx.doi.org/10.1021/ma9717855]]
--Asymmetric Synthesis of Helical Poly(quinoxaline-2,3-diyl)s by Palladium-Mediated Polymerization of 1,2-Diisocyanobenzenes: Effective Control of the Screw-Sense by a Binaphthyl Group at the Chain-End Y. Ito, T. Miyake, S. Hatano, R. Shima, T. Ohara, M. Suginome, '''J. Am. Chem. Soc.''' ''1998'', 120, 11880-11893, [[10.1021/ja982500m>http://dx.doi.org/10.1021/ja982500m]]
--Structural Modification of Living Polymers: Synthesis of Helical Block Copolymers from a Single Monomer via Palladium-Mediated Aromatizing Polymerization of 1,2-Diisocyanobenzenes Y, Ito, T. Miyake, M. Suginome, '''Macromolecules''' ''2000'', 33, 4034-4038, [[10.1021/ma991801t>http://dx.doi.org/10.1021/ma991801t]]
--Highly Effective, Easily Accessible Screw-Sense-Determining End Group in the Asymmetric Polymerization of 1,2-Diisocyanobenzenes M. Suginome, S. Collet, Y. Ito, '''Org. Lett.''' ''2002'', 4, 351-354, [[10.1021/ol017113n>http://dx.doi.org/10.1021/ol017113n]]
-Ref. 2
--Helical Poly(quinoxaline-2,3-diyl)s Bearing Metal-Binding Sites as New Polymer-Based Chiral Ligands T. Yamamoto, M. Suginome, '''Angew. Chem., Int. Ed.''' '''in press''', [[10.1002/anie.200803719>http://dx.doi.org/10.1002/anie.200803719]]
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).
#ref(switch.jpg,center,70%)
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*1.2. New Chirality-Switchable Chiroptical Materials [#fc702245]
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.
#ref(Reflection.jpg,center,70%)
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