Communication
ChemComm
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be higher than that of the (R)-enantiomer owing to preferential
inclusion in the cavity of H2N-b-CD adsorbed on the Au surface.
Finally, we note that some results could not be explained
through the simple mechanism proposed above. First, the equili-
brium constant for the complexation of 1b with HO-b-CD (KS/KR =
0.83)46 did not explain the faster oxidation of (S)-1b than that of
(R)-1b (entry 2, Table 1). Second, the s factor did not decrease to 1.0
and the conversion did not change even when a competitive guest 3
was added in great excess with respect to substrates 1a (Fig. 2b) and
1b (data not shown). Although we have no clear explanation for
these discrepancies at this moment, a different mechanism from
that based on chiral recognition by the CD cavity may be operative
for enantioselective oxidation. One possibility is that the binding
properties of the H2N-CD cavity in the presence of Au:PVP are
significantly different from those of free HO-CD cavity because of
the difference in the substitution groups and/or complex interac-
tions between them. Another possibility is that the exterior surface
of H2N-b-CD also acts as a chiral selector since CDs have multiple
stereocenters not only in the inner cavity, but also at the exterior
surface.
¨
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In summary, in situ modification of Au:PVP clusters (1.8 ꢀ
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resolution of racemic secondary alcohols featuring naphthyl
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CD with different cavity sizes and introducing the competitive
guest to the b-CD cavity. It is proposed that inclusion of the
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sible for enantioselective oxidation, although the possibility of
another mechanism cannot be excluded at present.
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This research was supported financially by the Elements
Strategy Initiative for Catalysts & Batteries (ESICB). The syn-
chrotron radiation experiments were performed under the
approval of the Japan Synchrotron Radiation Research Institute
(JASRI) (proposal no. 2019A1146).
¨
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Conflicts of interest
There are no conflicts to declare.
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15036 | Chem. Commun., 2019, 55, 15033--15036
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