A novel optically active polymer consisting of riboflavin
units
as the main chain (poly-1) was prepared from naturally
occurring riboflavin (vitamin B2) in three steps. The riboflavin
residues of poly-1 were converted to 5-ethylriboflavinium
cations (giving poly-2), which could be reversibly transformed
into the corresponding 4a-hydroxyriboflavins (giving poly-2OH) through hydroxylation/dehydroxylation reactions. This reversible
structural change was accompanied by a visible color change along
with significant changes in the absorption and circular dichroism
(CD) spectra. The nuclear Overhauser effect spectroscopy (NOESY) and
CD spectra of poly-2 revealed a supramolecularly twisted
helical structure with excess one-handedness through face-to-face
stacking of the intermolecular riboflavinium units, as evidenced by
the apparent NOE correlations between the interstrand riboflavin units
and intense Cotton effects induced in the flavinium chromophore regions.
The hydroxylation of poly-2 at the 4a-position proceeded
in a diastereoselective fashion via chirality transfer from the induced
supramolecular helical chirality assisted by the ribityl pendants,
resulting in a 83:17 diastereomeric mixture of poly-2OH. The diastereoselectivity of poly-2 was remarkably
higher than that of the corresponding monomeric model (64.5:35.5),
indicating amplification of the chirality resulting from the supramolecular
chirality induced in the stacked poly-2 backbones. The
optically active poly-2 efficiently catalyzed the asymmetric
organocatalytic oxidation of sulfides with hydrogen peroxide, yielding
optically active sulfoxides with up to 60% enantiomeric excess (ee),
whose enantioselectivity was higher than that catalyzed by the monomeric
counterpart (30% ee). In addition, upon exposure to primary and secondary
amines, poly-2 exhibited unique high-speed vapochromic
behavior arising from the formation of 4a-amine adducts in the film.