Research Theme

Supramolecular Catalyst Using Modified Cyclodextrin

Cyclodextrins (CDs) have been widely employed as substrate-recognition moieties in enzyme models. CDs accelerate the hydrolysis of activated esters, such as p-nitrophenyl acetate. However, the accelerated reactions have been limited to the degradation of the activated aryl esters in the presence of excess amounts of CDs. We found that CDs selectively form inclusion complexes with some lactones (the starting materials of the polyesters) to promote or suppress the hydrolysis of lactones.

Takashima, Y.; Kawaguchi, Y.; Nakagawa, S.; Harada, A.
Inclusion Complex Formation and Hydrolysis of Lactones by Cyclodextrins
Chem. Lett. 2003, 32, 1122-1123.

We speculated that if lactones are heated with CDs in bulk without water, they might form polymers because hydrolysis cannot occur. CDs were found to initiate ring-opening polymerizations of lactones selectively to give polyesters in high yields in bulk without water. This polymerization system requires neither a conventional metal catalyst, nor CDs with substituent groups. The products were found to be a polymer chain attached to the hydroxyl group of CD via an ester bond.

Takashima, Y.; Osaki, M.; Harada, A.
Cyclodextrin-Initiated Polymerization of Cyclic Esters in Bulk: Formation of Polyester-Tethered Cyclodextrins
J. Am. Chem. Soc. 2004, 126 (42), 13588-13589.

Osaki, M.; Takashima, Y.; Yamaguchi, H.; Harada, A.
Polymerization of Lactones Initiated by Cyclodextrins: Effects of Cyclodextrins on the Initiation and Propagation Reactions
Macromolecules 2007, 40 (9), 3154-3158.

Cinnamoyl CD has been found to initiate polymerization of lactone to give a polymer in high yield. The polymerization activity could be switched by the photoisomerization of the cinnamoyl group attached to the rim of CD. Specific monomer recognition and polymerization in the active site of the CD cavity is changed by the photoisomerization.

Osaki, M.; Takashima, Y.; Yamaguchi, H.; Harada, A.
Switching of Polymerization Activity of Cinnamoyl-alpha-Cyclodextrin
Org. Biomol. Chem. 2009, 7, 1646 - 1651.

In the polymerization, the propagating polymer chain was included by the CDs. The polymer chain was found to elongate in the solid state, whereas the polymer chain without threading CDs formed random coil conformation. CDs have two roles. One CD at the end of the polymer chain initiates the polymerization. Other CDs threaded onto the polymer chain are immune to the initiation directly, but have an essential role to fold the polymer chain in a proper way as an artificial chaperone.

Osaki, M.; Takashima, Y.; Yamaguchi, H.; Harada, A.
An Artificial Molecular Chaperone: Poly-pseudo-rotaxane with an Extensible Axle
J. Am. Chem. Soc. 2007, 129 (46), 14452-14457.

Harada, A.; Osaki, M.; Takashima, Y.; Yamaguchi, H.
Ring-Opening Polymerization of Cyclic Esters by Cyclodextrins
Acc. Chem. Res. 2008, 41 (9), 1143-1152.

CD-based nanosphere initiated the oligomerization of lactone on the surface of the nanosphere to give oligo(lactone)-tethered CD nanosphere in bulk. The addition of other CDs to the nanosphere led to the formation of poly-pseudo-rotaxane on the surface of the nanosphere. the poly-pseudo-rotaxane repropagated upon the addition of lactone. These behaviors are reminiscent of the function of a spherical virus, which forms an ordered spherical structure and releases RNA chains from the capsid surface.

Osaki, M.; Takashima, Y.; Yamaguchi, H.; Harada, A.
Nanospheres with Polymerization Ability Coated by Polyrotaxane
J. Org. Chem. 2009, 74 (5), 1858-1863.