Chemo-selective oxidation of hydroxybenzyl alcohols with IBX in the presence of hemicucurbit[6]uril

Article abstract of DOI:10.1039/c3nj00660c

The chemo-selective oxidation of bifunctional substrates via a supramolecular strategy has been achieved. IBX (o-iodoxybenzoic acid) oxidations of hydroxybenzyl alcohols produce the corresponding aldehyde and o-quinones, while the presence of HemiQ[6] can restrain the IBX oxidation of phenolic hydroxyl groups to afford the aldehyde product. The conversion and reaction rate are greatly affected by the structures of substrates, and the stereo effect and electronic effect play very important roles in the selective oxidation system. Various spectroscopies, including ¹H NMR (proton nuclear magnetic resonance), IR (infrared), and UV-vis (ultra violet-visible) have been employed to confirm the host-guest interaction of HemiQ[6] with hydroxybenzyl alcohols.

Full text of DOI:10.1039/c3nj00660c

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Chemo-selective oxidation of hydroxybenzyl alcohols
with IBX in the presence of hemicucurbit[6]uril†
Hang Cong,ab Takehiko Yamato*a and Zhu Taob
Cite this: NewJ.Chem., 2013,
37, 3778
The chemo-selective oxidation of bifunctional substrates via a supramolecular strategy has been
achieved. IBX (o-iodoxybenzoic acid) oxidations of hydroxybenzyl alcohols produce the corresponding
aldehyde and o-quinones, while the presence of HemiQ[6] can restrain the IBX oxidation of phenolic
hydroxyl groups to afford the aldehyde product. The conversion and reaction rate are greatly affected
by the structures of substrates, and the stereo effect and electronic effect play very important roles in
the selective oxidation system. Various spectroscopies, including 1H NMR (proton nuclear magnetic
resonance), IR (infrared), and UV-vis (ultra violet-visible) have been employed to confirm the host–guest
interaction of HemiQ[6] with hydroxybenzyl alcohols.
Received (in Montpellier, France)
18th June 2013,
Accepted 19th August 2013
DOI: 10.1039/c3nj00660c
www.rsc.org/njc
Cucurbit[n, n = 5–8, 10]uril (Q[n], Scheme 1a) are a family of
cage-like macrocyclic compounds, which are made of methylene-
Introduction
The oxidation of hydroxyl groups is a foundation in organic
synthesis, which frequently employs chromium- and ruthenium-
based reagents, Fremy’s salt [(KSO3)2NO] and TEMPO-mediated
oxidants.1 Organic oxidants of a serial of hypervalent iodine com-
pounds have been developed since the beginning of 1990s due to
the interest in their effective oxidizing properties. The Dess–Martin
reagent is an important member of the hypervalent oxidants,2 and
o-iodoxybenzoic acid (IBX) was found as a precursor, which has
received a lot of attention as a result of its ability to oxidate hydroxyl
groups in an extraordinarily efficient and selective manner.3
Actually, IBX had not been considered as a useful reagent in
organic synthesis because of its poor solubility in common
organic solvents. However, it has been found that IBX can
dissolve in DMSO to oxidize alcohols with very satisfying
conversions.4 In the last twenty years, IBX chemistry has
experienced significant development in order to prove the
oxidation activity of hydroxyl and amino groups.5 The importance
of this reagent lies in the advantage of regio-selective oxidation, for
example, the oxidation of phenol with IBX provides o-quinone as
the only product,5b,6 but there are two products (o- and p-quinones)
formed with the use of Fremy’s salt.1e Recently, the IBX oxidation of
alcohols in aqueous solution with the supramolecular catalysis of
cucurbit[8]uril has been reported by our group.7
bridged glycoluril units with hydrophilic portals and a hydrophobic
cavity.8 Based on the host–guest interactions, the application of
cucurbiturils to chemical processes includes stabilization of the
chemical structures, stereo-selective control in photochemical
dimerizations, and the improvement of reaction activity.9 The
Qs binding of the active group on substrates usually slows the
reaction,10 and the encapsulation of a guest in the cavity of
cucurbituril controls the stereostructure of the product and
improves the reactivity by changing the orientation of the guest
to form a suitable intermediate.11 Unfortunately, the virtual lack
of solubility of cucurbiturils in common organic solvents
restricts the further exploration of their applications in organic
chemistry. However, the appearance of hemicucurbiturils
(HemiQ[n, n = 6 or 12], Scheme 1b) involving two members,
HemiQ[6] and HemiQ[12], provides a new platform for the
exploitation of the supramolecular properties of the cucurbituril
family.12 Very recently, HemiQ[6] has been successfully used to
catalyze the esterification of acids in a combined solvent of
CHCl3 with CH3OH.13 Here, the chemo-selective IBX oxidation of
a Department of Applied Chemistry, Faculty of Science and Engineering,
Saga University, Honjo-machi 1, Saga 840-8502, Japan.
E-mail: yamatot@cc.saga-u.ac.jp; Fax: +81 952288548; Tel: +81 952288679
b Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou
Province, Guizhou University, Guiyang, Guizhou 550025, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj00660c
Scheme 1 Structures of (a) cucurbit[n]urils and (b) hemicucurbit[n]urils.
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Table 1 Product distribution of the IBX oxidation of hydroxybenzyl alcohols
after 1.0 hour, in the presence or absence of HemiQ[6]a
hydroxybenzyl alcohols, a serial of substrates with bifunctional
groups, in the presence of HemiQ[6] is described.
Products
Amount of
Ratio of IBX
Results and discussion
2
3a 3b 3c
Entry Substrate HemiQ[6] (mol%) to substrate
1
2
3
1a
1b
1c
0
25
50
50
0
25
50
50
0
1 : 1
1 : 1
1 : 1
2 : 1
1 : 1
1 : 1
1 : 1
2 : 1
1 : 1
1 : 1
1 : 1
2 : 1
31 26
29 14
—
—
—
—
25
13
—
36
—
—
—
—
—
—
—
—
For hydroxybenzyl alcohols, there are two kinds of hydroxyl groups
on every substrate, both of which can be oxidized by IBX in the
absence and presence of different amounts of HemiQ[6] to provide
different products, aldehyde and quinones (Scheme 2). The pro-
ducts formed by the IBX oxidation of different hydroxybenzyl
alcohols have been collected in Table 1.
25
—
21 19
40 13 23
43
55
3
—
10
—
52 16 21
63
56
51
87
—
—
—
—
14
10
—
13
Pioneering work found that the IBX oxidation of phenols always
produces o-quinones,5b,6 where two o-quinones [3-(hydroxy-
methyl)-o-quinone and o-benzoquinone] have been found to form
from the oxidation of 2-hydroxybenzyl alcohol, with yields of 25.8%
and 24.7%, respectively, and the product 2-hydroxybenzaldehyde
has also been detected with 30.7% yield. For the purpose of the
regio-selective oxidation of a bifunctional substrate via a supra-
molecular strategy, HemiQ[6] has been introduced into this
system. The yields of the aldehyde product and quinones decrease
with increasing amounts of HemiQ[6], namely, the yields of the
quinones decreased by half in the presence of 25 mol% HemiQ[6],
and the oxidation of the phenolic hydroxyl group has been resisted
completely in the presence of HemiQ[6] to afford 2-hydroxy-
benzaldehyde as the only product, with a slightly decreasing yield
when using a HemiQ[6] : 2-hydroxybenzyl alcohol ratio of 0.5 : 1,
and an increase in the amount of IBX to two times that of the
substrate improves the yields of quinones, but the yield of the
aldehyde remains almost constant (entry 1, Table 1).
For the 3-hydroxybenzyl alcohol substrate, there are two active
sites on the benzene ring for the oxidation of the phenolic hydroxyl
group, producing 12.6% 3-(hydroxymethyl)-o-quinone and 23.4%
4-(hydroxymethyl)-o-quinone, which reveals a steric hindrance
effect – that the oxidation at the 4-position is easier than at the
3-position, and brings about the double yield of 3-(hydroxymethyl)-
o-quinone. In the mean time, 39.5% of 3-hydroxybenzaldehyde has
been found as the product of the oxidation of benzyl alcohol. With
the increasing addition of HemiQ[6], the oxidation of the phenolic
hydroxyl group has been cut down and finally resisted with a 0.5 : 1
ratio of HemiQ[6] to the substrate, while the yield of the aldehyde
product is increased from 39.5% to 55.3%. Using double the
25
50
50
a
The yield of each product was directly confirmed by 1H NMR spectral
data.
amount of IBX improves the conversion of 3-hydroxybenzyl
alcohol to 88.9%; however, further increasing the amount of
oxidant does not achieve more aldehyde product, instead the
yields of the o-quinones are increased (entry 2, Table 1).
The results of the IBX oxidation of 4-hydroxybenzyl alcohol in
the absence of HemiQ[6] are simpler than the above cases, and
only two products, 4-hydroxybenzaldehyde and 4-(hydroxy-
methyl)-o-quinone, have been obtained with yields of 62.7%
and 14.4%, respectively. The presence of HemiQ[6] causes the
phenolic hydroxyl groups to become chemically inert, that is, the
corresponding quinone has vanished and only 4-hydroxybenzal-
dehyde has been observed with 50 mol% HemiQ[6], moreover,
the yield of the aldehyde product has been improved by 36% in
company with 13.1% o-quinone by the addition of more than 1
equiv. IBX (entry 3, Table 1), which reveals that the ternary
complex of the substrate, IBX and HemiQ[6], and additionally
the reaction rate of the IBX oxidation would be slower than the
dynamic interaction of 4-hydroxybenzyl alcohol with HemiQ[6].
The IBX oxidation of 2-hydroxybenzyl alcohol was carried
out with 50 mol% HemiQ[6], and the 1H NMR spectra traces
suggest that only the corresponding aldehyde product has
been formed (Fig. 1a). Comparing this result with that of the
reaction without the protection, the oxidation conversion of
Scheme 2 IBX oxidation of hydroxybenzyl alcohols in the absence and presence of HemiQ[6].
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Fig. 1 (a) 1H NMR spectra of the IBX oxidation of 2-hydroxybenzyl alcohol in the presence of 50 mol% HemiQ[6], with the proton resonances of the aldehyde product
highlighted in red; (b) kinetics plot of the IBX oxidation of 2-hydroxybenzyl alcohol to afford 2-hydroxybenzaldehyde in the presence of HemiQ[6].
2-hydroxybenzyl alcohol in the presence of HemiQ[6] is less, hydroxyl group has been resistant to oxidation, so no quinone
1
which provides only 25.3% aldehyde in 1.0 hour, and the product has been found, as observed from the H NMR spectra
conversion is improved by 30.5% over the next 2.0 hours. The (Fig. 2a). On the other hand, the reaction achieves 55.3%
kinetic plots of the oxidation of 2-hydroxybenzyl alcohol in the conversion in 1.0 hour, which is more than double the conver-
presence of 50 mol% HemiQ[6] has been displayed in Fig. 1b, sion of 2-hydroxybenzyl alcohol, and the non-linear fitting of the
which could be described by the following first-order formula (1): kinetic plots with formula 1 gives a corresponding rate constant
of k = 4.1 hÀ1 (Fig. 2b), which is almost twice that in the case of
Ct = C0  [1 À exp(Àk  t)]
(1)
2-hydroxybenzyl alcohol. Accordingly, both the above thermo-
dynamic and kinetic evidence suggests that the IBX oxidation of
3-hydroxybenzyl alcohol in the presence of HemiQ[6] is more
effective than for 2-hydroxybenzyl alcohol.
As in the above cases, the phenolic hydroxyl group on
4-hydroxybenzyl alcohol could be protected against IBX oxidation
by the addition of 50 mol% HemiQ[6], and 4-hydroxybenzaldehyde
where C0 is the initial 2-hydroxybenyl alcohol concentration (%)
and Ct is the concentration (%) at time t, k is the corresponding
reaction rate constant, which has been found as k = 2.2 hÀ1 in
this case. In the presence of 50 mol% HemiQ[6], the alcoholic
hydroxyl group of the 3-hydroxybenzyl alcohol has been oxidized
by IBX to produce 3-hydroxybenzaldehyde, while the phenolic
Fig. 2 (a) 1H NMR spectra of the IBX oxidation of 3-hydroxybenzyl alcohol in the presence of 50 mol% HemiQ[6], with the proton resonances of the aldehyde product
highlighted in red; (b) kinetics plot of the IBX oxidation of 3-hydroxybenzyl alcohol to afford 3-hydroxybenzaldehyde in the presence of HemiQ[6].
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Fig. 3 (a) 1H NMR spectra of the IBX oxidation of 4-hydroxybenzyl alcohol in the presence of 50 mol% HemiQ[6], with the proton resonances of the aldehyde product
highlighted in red; (b) kinetics plot of the IBX oxidation of 4-hydroxybenzyl alcohol to afford 4-hydroxybenzaldehyde in the presence of HemiQ[6].
has been found as the only product. The 1H NMR spectrum between HemiQ[6] and the hydroxybenzyl alcohols have been
1
(Fig. 3a) suggests that the IBX oxidation of this substrate is also investigated. H NMR spectroscopy is a common method used
more effective than that of 2-hydroxybenzyl alcohol. A conversion of to confirm the host–guest interaction models by observation of
58.6% has been achieved after 2.0 hours, and a further 7.4% the changes in the chemical shifts of the guest after addition of
conversion has been achieved after 3.0 hours. The degree of the host.8 In this case, the proton resonances of all of the
conversion of 4-hydroxybenzyl alcohol at different times could be hydroxybenzyl alcohols do not show any shift in the presence of
non-linearly fitted to give a kinetic constant of k = 1.1 hÀ1 with HemiQ[6] (ESI,† Fig. S1), however, all of resonances of the active
formula 1 (Fig. 3b), which is surprisingly just half of the constant in protons are fading away with the addition of the host and
2-hydroxylbenzyl alcohol system, so the oxidation of 4-hydroxy- broadened peaks appear from d 3.6 ppm–d 4.2 ppm, which
benzyl alcohol is the slowest of the studied cases.
reveals the host–guest interaction between hydroxybenyl
The oxidizing activities of the hydroxybenzyl alcohols by IBX alcohols and HemiQ[6] with hydrogen bonds. The formation
in the presence of HemiQ[6] could be comprehended fully from of supramolecular complexes was also confirmed by the different
the viewpoints of both thermodynamics and kinetics. 2-Hydroxy- IR absorptions of the carbonyl groups on binding HemiQ[6] from
benzyl alcohol is the most inert among the bifunctional sub- the those of the free host (ESI,† Fig. S2). UV-vis spectroscopic
strates, with only a 30.5% conversion of 2-hydroxybenzylaldehyde titration has also been employed to monitor the host–guest
being observed, while the oxidation of 3,4-hydroxybenzyl alcohols interactions. To avoid the solvent absorption of DMSO in this
provide more products, whose distributions are 56.8% and 66.0%, wavelength region, a mixed solvent of chloroform and methanol
respectively. It is obvious that the conversion of hydroxybenzyl in the ratio of 1 : 1 has been applied in the UV-vis titrations. The
alcohol depends on the substituent site of the phenolic group on max electronic absorption of 2-hydroxybenzyl alcohol appears at
the benzene ring, that is, when the distance between the phenolic 275 nm, which increases with the increasing concentration of
hydroxyl group and the alcoholic hydroxyl group is increased, a HemiQ[6] (Fig. 4a), and the change in the UV-vis spectra and the
more effective oxidation at the benzyl alcohol is observed, so the variable concentration of the macrocyclic compound could
stereo-hindrance effect of HemiQ[6] helps to resist the IBX oxida- be fitted to give a 1 : 1 interaction model with Ka = (4.8 Æ 1.5) Â
tion of 2-hydroxybenzyl alcohol. However, the oxidation reaction 103 L molÀ1 (Fig. 4b). For 3-hydroxybenzyl alcohol, the UV-vis
of 4-hydroxybenzyl alcohol has the smallest rate constant of absorbance at 275 nm is improved by the addition of HemiQ[6]
k = 1.1 hÀ1, and the rate constant of the IBX oxidation of (Fig. 4c), and the increase in the concentration of HemiQ[6] fits to
3-hydroxybenzyl alcohol is the largest (k = 4.1 hÀ1). According to a 1 : 1 binding model with a moderate binding constant of Ka =
the results obtained in our previous work,7b,c on the electronic (7.4 Æ 2.2) Â 104 L molÀ1 (Fig. 4d). A similar change in the UV-vis
effects of substituents on the activity of the IBX oxidation spectrum has also been observed in the host–guest interaction
of benzyl alcohols, the meta-substituent with an electron- system of 4-hydroxybenzyl alcohol and HemiQ[6] (Fig. 4e), and the
withdrawing group always favours the oxidation due to its increase in the absorbance band at 277 nm versus the increase in
strong inductive effect.
the concentration of HemiQ[6] was fitted to a 1 : 1 binding model
To understand the role of HemiQ[6] in this chemo-selective to obtain an association constant of Ka = (5.5 Æ 1.6) Â 104 L molÀ1
oxidation, the foundations of the host–guest interactions (Fig. 4f). Accordingly, the interaction of 2-hydroxylbenzyl alcohol
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Fig. 4 UV-vis titration of the host–guest interactions between HemiQ[6] and (a) 2-hydroxybenzyl alcohol; (c) 3-hydroxybenzyl alcohol; (e) 4-hydroxybenzyl alcohol.
The corresponding absorbance versus different ratios of HemiQ[6] to (b) 2-hydroxybenzyl alcohol; (d) 3-hydroxybenzyl alcohol; (f) 4-hydroxybenzyl alcohol.
with HemiQ[6] is weaker than the others, which reveals that a both aldehydes and quinones, due to two hydroxyl groups
steric hindrance effect is present in the host–guest interaction.
being present on the benzene ring. However, 50 mol%
HemiQ[6] is able to protect the phenolic hydroxyl groups
against oxidation by IBX, and produces the corresponding
aldehydes as the only product. The oxidations with supra-
Conclusions
In summary, we have developed a chemo-selective oxidation molecular protection are affected by both the steric effects
reaction for bifunctional substrates, hydroxybenzyl alcohols, and electronic effects of the substrates. The conversion of
with the participation of hemicucurbit[6]uril. The IBX oxida- 2-hydroxyl alcohol is the lowest, as the two hydroxyl groups are
tions of hydroxybenzyl alcohols provide a mixture of products, very close to each other. The inductive effect of the phenolic
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hydroxyl group on 3-hydroxybenzyl alcohol makes the reaction Collaboration Project of Guizhou Province (No. [2011]7003), and
faster than the others. The investigation of the host–guest inter- the Natural Science Foundation of Guizhou Province (No. [2008]75).
1
actions of HemiQ[6] with hydroxybenzyl alcohols with H NMR
spectroscopy, IR absorption analysis and UV-vis titration suggests
that the hydrogen bonds are formed between HemiQ[6] and the
Notes and references
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3 A. Duschek and S. F. Kirsch, Angew. Chem., Int. Ed., 2011,
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benzyl alcohol guests in an ratio of 1 : 1, through the binding of
active hydrogen by the macrocyclic compound.
Experimental
Materials and apparatus
The HemiQ[6] sample was prepared and purified according to a
method reported in the literature12 and was characterized by
1H NMR. HemiQ[6] (CDCl3, d): 3.40 (s, 24H), 4.67 (s, 12H). IBX
was prepared with 2-iodobenzoic acid and oxone.14 1H NMR
(DMSO-d6, d): 7.20 (t, 1H), 7.44 (t, 1H), 7.66 (d, 1H), 7.94 (d, 1H).
Mp: 230–233 1C with explosive decomposition. Hydroxybenzyl
alcohols and hydroxybenzyl aldehydes were obtained commercially
(Tokyo Kasei Kogyo Co., Ltd) and used without further purification.
The 1H NMR spectra were recorded at 25 1C on a JEOL JNM-
Al00 spectrometer in a mixture of CDCl3 and DMSO-d6. UV-vis
absorption spectra were recorded on an Perkin-Elmer Lambda
19 UV/Vis/NIR instrument at 25 1C.
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HemiQ[6]
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Hydroxybenzyl alcohols (0.01 mmol) and IBX (0.01 mmol) were
added to a mixture (0.6 mL) of CDCl3 and DMSO-d6 (2 : 1, v/v) in a
sealed bottle. The reactions were carried out at 30 1C for 1.0 hour,
and then the solutions examined with 1H NMR directly.
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IBX oxidation of hydroxybenzyl alcohols in the presence of
HemiQ[6]
Hydroxybenzyl alcohols (0.01 mmol), HemiQ[6] (0.005 mmol)
and IBX (0.01 mmol) were added to a mixture (0.6 mL) of CDCl3
and DMSO-d6 (2 : 1, v/v) in a sealed bottle. The reaction was
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1
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UV-vis titration of the host–guest interactions
Hydroxybenzyl alcohol solutions were prepared in a mixture
of CHCl3 and CH3OH (1 : 1) with a concentration of 2.5 Â
10À4 mol LÀ1, this solution was combined with HemiQ[6] to give
guest–HemiQ[6] ratios of 0, 4 : 1, 2 : 1, 1 : 1, and 1 : 2 and so on.
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Acknowledgements
We acknowledge the financial support of the National Natural 14 M. Frigerio, M. Santagostino and S. Sputore, J. Org. Chem.,
Science Foundation of China (No. 21162003), the International
1999, 64, 4537.
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