Promotion of organic reactions by interfacial hydrogen bonds on hydroxyl group rich nano-solids

Article abstract of DOI:10.1039/b801361f

Surface hydroxyl group rich nano-structured solids dramatically increase the rate of several organic reactions; such effect is attributed to the formation of interfacial hydrogen bonds between the surface hydroxyl groups and the reactants; this catalytic

Full text of DOI:10.1039/b801361f

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COMMUNICATION
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Promotion of organic reactions by interfacial hydrogen bonds on
hydroxyl group rich nano-solidsw
Fang Niu,a Chang-Chang Liu,b Zhi-Min Cui,a Jin Zhai,a Lei Jianga and Wei-Guo Song*a
Received (in Cambridge, UK) 24th January 2008, Accepted 12th March 2008
First published as an Advance Article on the web 11th April 2008
DOI: 10.1039/b801361f
Surface hydroxyl group rich nano-structured solids dramatically
increase the rate of several organic reactions; such effect is
attributed to the formation of interfacial hydrogen bonds
between the surface hydroxyl groups and the reactants; this
catalytic effect is versatile and applicable for a broad range of
reaction conditions.
In this communication, we will demonstrate that some
organic reactions can be catalyzed by the interfacial hydrogen
bonds between the reactants and surface hydroxyl groups on
nano-iron hydroxide and other hydroxyl group rich solids.
The function of these nano-solids is as hydrogen bond donors
to provide hydroxyl groups for forming hydrogen bonds with
reactants, as illustrated in Scheme 1. In several organic reac-
tions, we observed a dramatic increase of reaction rate due to
the presence of these nano-solids. Nano-solid catalysts are
versatile and applicable to a broader range of reaction condi-
tions than homogeneous systems.
Promotion of some organic reactions by hydrogen-bond donors
have attracted considerable attentions.1 One fascinating example
is the acceleration of Diels–Alder reaction rate by the hydrogen
bond donors, which was reported by Kelly et al.2 The catalytic
effect is due to the presence of biphenylenediol, an organic
hydrogen bond donor. Other examples of catalytic effect of
hydrogen bond donors include asymmetric catalysis by chiral
hydrogen bond donors3 and organo-catalysis mediated by
thiourea derivatives4 in homogenous systems. It is proposed
that the essence of hydrogen bond catalysis is both energetic and
conformational: the hydrogen bond catalyzes the Diels–Alder
reaction by lowering the LUMO energy of the carbonyl-contain-
ing dienophiles and stabilizing the charged transition state of the
reaction; while in asymmetric catalysis, hydrogen bonds may
help the reactants to retain preferred conformations for asym-
metric reactions. However, such catalytic effect by hydrogen
bond donors has not yet been fully exploited using heteroge-
neous hydrogen bond donors. Using inorganic solids to catalyze
organic reactions may not only expand our understanding of
interfacial chemistry, but also benefit the environment due to
certain advantages of a heterogeneous system.
Iron hydroxide nano-particles with large surface area and
abundant surface hydroxyl groups can catalyze the coupling
reaction between p-benzoquinone and 2-methylindole (reac-
tion (1)) at room temperature and dramatically increase the
reaction rate (See ESIw for experimental details). Soon after
iron hydroxide nano-particles and a reaction mixture consist-
ing of THF, reactants 1 and 2 were mixed and stirred, a
characteristic violet colour appeared, indicating the formation
of product 3.
ð1Þ
After 24 h of reaction, a 72% yield for product 3 was achieved.
No by-product was detected by GC. In sharp contrast, when
no iron hydroxide nanoparticles was used, no product was
detected (Table 1), suggesting very low reaction rate for
reaction (1) by itself.
In previous studies, we synthesized several nano-structured
metal oxide nano-particles, including iron oxide,5 titanium
oxide6 and cerium oxide.7 These materials have large surface
areas and abundant surface hydroxyl groups. When a nano-
solid particle is immersed in a solution, its external surface is
exposed to the molecules in the solution. The surface hydroxyl
groups can then form interfacial hydrogen bonds with organic
molecules in the solution, the same way as the hydrogen bonds
can form in homogeneous solutions. Inspired by the catalytic
effect of hydrogen bonds in homogeneous system, we specu-
lated that similar catalytic effect by interfacial hydrogen bonds
will also promote the organic reactions.
As a control experiment, water was added to the reaction
mixture, forming a homogeneous solution. The presence of water
also helped, and the yield was 5% after 24 h and 21% after 120 h.
a Beijing National Laboratory for Molecular Sciences (BNLMS),
Institute of Chemistry, Chinese Academy of Sciences, 100190
Beijing, P. R. China. E-mail: wsong@iccas.ac.cn;
Fax: (+86)10-62557908
b Department of Chemistry, University of Science and Technology of
China, 260000 Hefei, P. R. China
w Electronic supplementary information (ESI) available: The prepara-
tion of metal hydroxides and metal oxides, molecular ion mass
patterns of H/D exchange. See DOI: 10.1039/b801361f
Scheme 1 Reaction promoted by hydrogen bonds between the
reactant and hydrogens on hydroxyl rich nano-particles.
ꢀc
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Table 1 Yields of product 3 in reaction (1) using different nano-solids
Added
reagent
t/h
Surface
area/m2 gꢁ1
Conversion
(%)
Yield
(%)
Fe(OH)3
Fe(OH)3
a-Fe2O3
—
Water
Water
24
24
24
24
24
120
214
15
30
N/A
N/A
N/A
72
12
0
0
5
21
72
12
NDa
ND
5
21
a
Not detected.
Note that the amount of water used was 10 times the amount of
iron hydroxide used. In two other control experiments, we
measured the reaction yields using iron hydroxide with low
surface area and hydroxyl group free a-iron oxide. While 72%
of product 3 yield was achieved using iron hydroxide nano-
particles with high surface area of 214 m2 gꢁ1, only a 12% yield
was obtained using iron hydroxide particles with low surface
area. Interestingly, hydroxyl group free a-iron oxide showed no
reactivity, and no product was detected from the same reaction.
TEM images of the iron hydroxide nano-particles (see ESIw)
shows that the average diameter of these nano-particles is about
10 nm. There are abundant free hydroxyl groups on the surface of
iron hydroxide nano-particles.8 The hydroxyl groups can insert
into the liquid at the liquid–solid interface, form interfacial
hydrogen bonds with the reactant molecules and intermediate
species, as depicted in Scheme 1, and catalyze the reaction through
lowering the energy states of key reaction components. The
number of surface hydroxyl groups on an iron hydroxide sample
is proportional to its surface area. (Thus, there are less hydroxyl
groups on the surface of micron sized iron hydroxide particles.)
Table 1 indicates that the yield of the reaction is roughly propor-
tional to the surface area and the amount of surface hydroxyl
groups of iron hydroxides. Such dependence on the amount of
surface hydroxyl groups suggests that the presence of hydroxyl
groups is the crucial factor for promoting the reaction.
Fig. 1 Time dependant yield of product 3 in reaction (1) promoted by
iron hydroxide nano-particles.
ment is that if iron hydroxide is a solid acid catalyst, the
hydroxyl group on its surface will be involved in the C–H bond
activation step; and H/D exchange is expected to form deuter-
ated product 3. However, GC-MS analyses of product 3
showed no H/D exchange (see ESIw). Exactly the same mole-
cular ion mass patterns were observed for product 3 from four
different experiments, indicating that deuterium atoms are not
incorporated into product 3 when Fe(OD)3 is used.
Similar results were obtained when D2O is added in control
experiments (Table 2). The lack of H/D exchange on surface
hydroxyl groups suggested that the main function of iron
hydroxide nanoparticles is to provide hydroxyl groups to form
hydrogen bonds with reactant 2, as illustrated in Scheme 1,
and the reaction is promoted by the interfacial hydrogen
bonds between hydroxyl groups and reactant 2.
The above results also suggest that besides iron hydroxide;
other solids with enough free surface hydroxyl groups should
also promote reaction (1). In other sets of experiments, various
solids were added to a 2-methylindole–p-benzoquinone–THF
mixture to study their catalytic activities. These solids are chosen
as pairs, one with sufficient amount of surface hydroxyl groups,
and the other without enough hydroxyl groups. Differences in
the reaction rate are expected from each pair. Table 3 shows the
details of these materials and their performances in reaction (1).
g-Al2O3 is a well studied material as it is a frequently used
industrial catalyst supporter. There are many hydroxyl groups
on the surface of g-Al2O3.10 g-Al2O3 with surface area of 240 m2
Fig. 1 showed that the yield of product 3 increases almost
linearly with reaction time. The linear increase of the product
yield suggests a zero-order reaction. Reactants 1 and 2 were
added at a 1 : 2 mole ratio, so during the reaction, both
reactants were consumed substantially and their concentra-
tions were decreasing. However, the reaction rate did not
change much with the reaction time, indicating a zero-order
reaction kinetic and a constant reaction rate. This agrees well
with the interfacial hydrogen bond promotion mechanism
shown in Scheme 1, where the reaction rate is determined by
the number of p-benzoquinone molecules that form hydrogen
bonds with the hydroxyl groups. In this study, since the
amount of surface hydroxyl groups on iron hydroxide is
constant, therefore the reaction rate is also constant.
g
ꢁ1 has a larger amount of surface hydroxyl groups and leads to
a higher product 3 yield of 56%, while a low surface area g-Al2O3
only resulted in a 5% yield. For zinc oxide and zinc hydroxide,
hydroxyl group rich zinc hydroxide results in a 27% yield; while
on zinc oxide, which has no surface hydroxyl groups, no product
is produced. These results further confirm that hydroxyl rich
solids can catalyze certain organic reactions through interfacial
hydrogen bonds.
The above results establish that the reaction rate for reaction
(1) is increased dramatically when hydroxyl group rich iron
hydroxide nano-particles are added to the reaction. Reaction
(1) can be catalyzed by Brønsted or Lewis acids.9 In order to
check whether nano-iron hydroxides’ ability to promote reac-
tion is via solid acid catalysis or interfacial hydrogen bond
catalysis, we studied the H/D exchange during reaction (1).
Fe(OD)3 was prepared to repeat the same experiments for
reaction (1). The rationale behind this H/D exchange experi-
Table 2 H/D exchange results for reaction (1)
Added reagent
t/h
Deuterium content in product 3 (atom%)
Fe(OD)3
D2O
24
24
0a
0a
a
Based on the identical mass ion peaks in mass spectrometry.
ꢀc
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Table 3 Yields of product 3 in reaction (1) using various solids
Table 4 Yields of product 8 in reaction (3) under various conditions
Solid
t/h Surface area/m2 gꢁ1 Conversion (%) Yield (%)
Added reagent
T/1C
t/h
Conversion (%)
Yield (%)
g-Al2O3
g-Al2O3
ZnO
24 240
56
5
0
27
56
—
H2O
Fe(OH)3
Fe(OH)3
g-Al2O3
120
100
20
120
120
24
24
8
3
3
0
0
24
24
2.0
14
43
5
Trace
40
98
Trace
36
90
NDa
27
Zn(OH)2 24
99
92
a
Not detected.
solid, g-Al2O3 gives 92% yield. Reaction (3) usually requires
acid or base to initiate the reaction. g-Al2O3 is a Lewis acid,
which may help to catalyze the reaction. However, as men-
tioned earlier for reaction (1), iron hydroxide is not known to
show significant basicity or acidity. Its ability to promote
reaction (3) is very likely due to catalysis effect of interfacial
hydrogen bonds.
The ability of hydroxyl rich nano-particles to promote
organic reactions is not limited to reaction (1). We also carried
out a Diels–Alder reaction between p-benzoquinone and cyclo-
pentadiene (reaction (2)) using iron hydroxide nano-particles.
After 24 h of reaction at room temperature, the yield of product
5 reached 84%, while no product was detected under control
experiments without solid particles. So the catalytic effect of
interfacial hydrogen bonds may be a general effect.
ð3Þ
ð2Þ
In summary, dramatic reaction rate increases were observed
at the presence of hydroxyl rich nano-solids. The extent of rate
increase is determined by the amount of surface hydroxyl
groups. Solids with more hydroxyl groups lead to higher yields,
while hydroxyl group free solids do not promote the reaction at
all. H/D exchange results established that interfacial hydrogen
bonds between surface hydroxyl groups and reactant molecules
are responsible for such catalytic effect. Future work will focus
on exploiting the interfacial hydrogen bond catalysis by nano-
iron hydroxide for more organic reactions.
Such interfacial hydrogen bond catalysis again shows the
crucial, yet not fully explored roles of hydrogen bond in
chemical reactions and even biological systems.11 Besides the
hydrogen bond catalysis in homogeneous solution1–4 men-
tioned above and for the heterogeneous system reported here,
in a so-called on-water reaction, the hydrogen bonds between
water and organic molecules at the water/oil interface also
catalyze some organic reactions.12 Wang and co-workers13
studied reaction (1) in an on-water reaction. The result from
our work is compared with reaction results from other cata-
lytic systems, even though there are fundamental differences
among them. For example, in Wang et al.’s on-water reaction
study, the yield for product 3 was 82% using water after 10 h.
Apparently the rate increases from nano-iron hydroxide
promotion is slightly less than that from water.
We gratefully thank the National Natural Science Founda-
tion of China (NSFC 20673125 and 50725207), Ministry of
Science and Technology (MOST 2007CB936403) and the
Chinese Academy of Sciences for financial supports.
Notes and references
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A heterogeneous catalytic system has many advantages. For
example, in a homogeneous system, the hydrogen-bond donor
molecules must be soluble with reactants, and separating the
organic hydrogen bond donor and the product is difficult.
However, for heterogeneous systems with solid particles, such
difficulty is avoided. A simple centrifugation or filtration will
separate the solid and liquid. If combined with magnetic nano-
solids, catalyst recovery may become even easier. In addition,
reaction temperature may be limited in on-water reactions. So
far water promoted reactions must be carried out below
100 1C. With solid particles, the reaction can be carried out
at a broader temperature range. This will enable us to study
reactions that can not be promoted by water.
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10035–10045.
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As a preliminary example, an aldol condensation reaction
(reaction (3)) is studied using p-xylene as solvent. As shown in
Table 4, a control experiment without solid at 120 1C produces
no product 8 at all. Adding water does not help either, partly
because water does not stay as liquid at 100 1C, and no stable
hydrogen bonds are formed. However, when iron hydroxide is
added, a 36% yield is obtained at 20 1C in 8 h; and 90% yield
is achieved at 120 1C in 3 h. Another hydroxyl group rich
ꢀc
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Chem. Commun., 2008, 2803–2805 | 2805