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H. Cong et al. / Journal of Molecular Catalysis A: Chemical 379 (2013) 287–293
Scheme 1. Structures of the substrates and HemiQ[n, n = 6 or 12].
product, furan-2,5-diol, is able to be stabilized in the presence of
HemiQ[6] hereinto.
2. Experimental
Fig. 1. 1H NMR spectra of (a) furan; (b) HemiQ[6]; (c) a D2O solution of furan after
heating for 8 h in the presence of HemiQ[6].
2.1. Materials and apparatus
absorption at 1683 cm−1 (Fig. S3b, ESI). On the other hand,
that furan cannot be oxidized in deoxygenated aqueous solution
(5% Na2SO3, and Ar atmosphere) in the presence or absence of
HemiQ[6] suggests an aerobic process.
HemiQ[n] (n = 6 or 12) samples were prepared and purified
according to the method reported elsewhere [18] and were char-
acterized by 1H NMR, giving resonances for HemiQ[6] (CDCl3, ı)
at 3.40 ppm (s, 24H) and 4.67 ppm (s, 12H) and for HemiQ[12]
(CDCl3, ı) at 3.36 ppm (s, 24H) and 4.67 ppm (s, 12H). Q[n] was
synthesized using methods developed in our laboratory [28]. Furan,
2-methylfuran and thiophene were obtained commercially (Tokyo
Kasei Kogyo Co. Ltd.) and used without further purification.
1H NMR spectra were recorded at 25 ◦C on a JEOL JNM-Al00
spectrometer (300 MHz) in D2O, with TMS is used as an internal
reference.
In cucurbituril chemistry, the host–guest interactions are
always characterized by changes of chemical shifts of the encap-
sulated guest in 1H NMR. That is, the shielding effect leads to an
into the core of the macrocyclic cavity. On the other hand, the
deshielding effect from the carbonyl groups on the portals makes
the 1H NMR response of the guest remaining at the portal to shift
down [3,9]. However, no chemical shift was observed for furan
in the presence of HemiQ[6], as reported previously [19], simply
broadening of the furan peaks and the appearance of a complicated
resonance of HemiQ[6] (Fig. S4, ESI). Fortunately, the improvement
and HemiQ[6] to be detected with 1H NMR analysis (Fig. 2). In the
presence of HemiQ[6] at pD 2.0, the resonance signals of all protons
on the furan ring undergo a very slight downfield shift, which sug-
gests that the guest remains in the deshielding area around the
carbonyl groups on HemiQ[6] (Fig. 2a). The trace plots of these
chemical shift changes from ı 6.28 to ı 6.35 ppm when the ratio
of CHemiQ[6]/Cfuran is up to 4.2:1 indicate a1:1 binding model with a
weak association constant Ka = (5.2 2.4) × 10−2 L mol−1. Accord-
ingly, the host–guest interaction between furan and HemiQ[6] is
investigate the protonation of this macrocyclic compound in solu-
tion. Comparing the 1H NMR spectrum of HemiQ[6] in neutral and
acidic D2O (Fig. 3a), the proton resonance appears broadened in
acidified solution (Fig. 3b). To provide clear evidence that a pro-
ton can be captured by this host, HemiQ[6] is dissolved in acidic
aqueous solution (pH = 2.0) followed by extraction with CDCl3. Sub-
sequently, the spectrum of the organic layer shows the resonance
signal of an active proton at ı 1.23 ppm (Fig. 3c). We propose that
the presence of the proton, with its positive charge, decreases the
repulsion between the electron-rich furan ring and the carbonyl
groups on the HemiQ[6] and therefore stabilizes the host–guest
interaction complex.
2.2. Catalytic oxidation experiments
The heterocyclic compounds (0.015 mmol) were added to
0.6 mL D2O and then HemiQ[6] was added to the solution in a 1:1
ratio. The solution was heated to 65 ◦C and monitored by 1H NMR
over time. The reactant conversion was directly confirmed by 1H
NMR spectral data.
The oxidation of furan in D2O has been found to take place
shown in Fig. 1, the two resonance signals from furan, at ı 6.5
and ı 7.6 ppm (Fig. 1a), become weaker as its deuterated aque-
ous solution is heated for 8 h, and a new singlet appears at ı
6.3 ppm (Fig. 1c). Comparing with Fig. 1b, the proton resonance
of Hb on HemiQ[6] shows no shift, but the proton resonance of
Ha becomes slightly split, which indicates the encapsulation of the
product by the HemiQ[6]. The IR absorption of the hydroxyl group
shows a slight blue shift, from 3436 to 3446 cm−1, and the car-
bonyl groups on the binding HemiQ[6] show obvious differences
in their IR absorption compared with the free host (Fig. S1, ESI).
The proton resonance of OH group appears at ı 1.07 ppm with
the results of hydrogen–deuterium exchange action (Fig. S2, ESI).
The above evidences show that furan is oxidized to furan-2,5-diol.
The heated solution of furan and HemiQ[6] was acidified with DCl
and neutralized with Na2CO3 followed by extraction with CDCl3.
This presence of the dione, dihydrofuran-2,5-dione (Fig. S3a, ESI)
was confirmed, with the carbonyl group identified by IR by the
To understand the role of protons in this example of supramolec-
ular catalysis, the kinetics of the aerobic oxidation with 1 equiv.