BINAP-copper supported by hydrotalcite as an efficient catalyst for the borrowing hydrogen reaction and dehydrogenation cyclization under water or solvent-free conditions

Article abstract of DOI:10.1039/c8gc00557e

A BINAP-Cu system supported by hydrotalcite has been developed and proved to be a highly efficient catalyst for the atom-efficient and green borrowing hydrogen reaction and dehydrogenative cyclization. This BINAP-Cu complex supported by hydrotalcite is highly air-stable and can be recycled at least five times under solvent-free conditions. Notably, 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives could be synthesized from alcohols in only one step with water as the solvent for the first time. This provided a much greener and efficient catalytic method towards the synthesis of functionalized amines, ketones and 1-benzyl-2-aryl-1H-benzo[d]imidazole derivatives with high yields under water or solvent-free conditions.

Full text of DOI:10.1039/c8gc00557e

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BINAP-Copper Supported by Hydrotalcite as Efficient Catalyst for
Borrowing Hydrogen Reaction and Dehydrogenation Cyclization
under Water or Solvent-Free Conditions†
Received 00th January
20xx,
Zhaojun Xu,a Xiaoli Yu,a Xinxin Sang,*a and Dawei Wang*a
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A BINAP-Cu system supported by hydrotalcite has been developed and proved to be a highly
efficient catalyst for the atom-efficient and green borrowing hydrogen reaction and
dehydrogenative cyclization. This BINAP-Cu complex supported by hydrotalcite is highly air-
stable and can be recycled at least five times under solvent-free conditions. Notably, 1-benzyl-2-
aryl-1H-benzo[d]imidazole derivatives could be synthesized from alcohols in only one step with
water as the solvent for the first time. This provided a much greener and efficient catalytic method
towards the synthesis of functionalized amines, ketones and 1-benzyl-2-aryl-1H-benzo[d]imidazole
derivatives with high yields in water or solvent-free conditions.
Keywords: heterogeneous; copper complex; solvent-free; borrowing hydrogen; dehydrogenation
reactions and dehydrogenation cyclizations being an
atom-efficient and green transformation, a much greener
process under solvent-free or clear solvent conditions
Introduction
Borrowing hydrogen reactions and dehydrogenative
cyclizations are of great importance due to their wide
applications and utility as a convenient and efficient tool
for the material science and pharmaceutical industry.1,2
Borrowing hydrogen reactions are usually involved in the
following process: dehydrogenation of an alcohol to the
corresponding carbonyl compound, followed by
remains
a
challenging task, particularly with
inexpensive metals as catalysts, such as Fe, Co, Cu,
Mn.8
Recently, we have synthesized several iridium and
ruthenium complexes and explored their catalytic activity
for borrowing hydrogen reaction of alcohols with
amines.9 However, these iridium and ruthenium catalysts
are expensive and could not be recycled, which would be
a great waste for modern organic chemistry. To enhance
the recyclability and to develop the most economic and
(dehydrogenation is the first step of
a hydrogen
borrowing reaction) condensation and reduction of the C-
C or C-N double bond using the borrowed H2 from the
alcohol.3 Water is produced as the only byproduct for
borrowing hydrogen reactions,4 while H2 and H2O are
produced as the byproducts for dehydrogenation
cyclizations. Given this outcome, the atom efficiency
of this strategy (borrowing hydrogen reaction and
dehydrogenation cyclization) is high.5 The most
commonly used catalysts, such as palladium, ruthenium,
iridium and rhodium, are often effective due to their
efficiency in catalytic dehydrogenation and borrowing
hydrogen reactions.6,7 Despite borrowing hydrogen
promising catalysts, we investigated
a BINAP-Cu
complex supported by hydrotalcite (HT), which proved
to be an efficient catalyst for borrowing hydrogen
reaction, giving products in high yields. This BINAP-Cu
complex supported by hydrotalcite was a highly air-
stable catalyst with good recyclability for at least five
times under solvent-free conditions or with water as the
solvent.
Results and discussion
Catalyst characterization
a.The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education,
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122,
China. E-mail: wangdw@jiangnan.edu.cn, sangxx@jiangnan.edu.cn
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
A crystal of the [Cu(binap)I]2 complex was grown from a
saturated acetonitrile/n-hexane solution at ambient
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temperature (please see Supporting Information for the tBuOK offered lower efficiency. Moreover, it was observed
synthesis of [Cu(binap)I]2 complex for details). In order to that the reaction did not proceed efficiently in the absence of
confirm the structure of the complex, single crystal X-ray catalyst (Table 1, entry 8). In addition, we conducted a series
diffraction analysis was performed (Figure 1). A summary of of control experiments for the behavior of the free catalyst in
crystallographic data and important bond lengths or bond the absence of carrier HT (see Table S4 in Supporting
angles are listed in the SI (see Supporting Information for the Information). The results revealed that the reaction only
cif file containing complete single crystal data).
produce moderate yield even after reaction screening.
Table 1. Screening of reaction conditions a
Entry
Catalyst
Base
Yield[%]b
Figure 1. The structure of [Cu(binap)I]2 and ORTEP diagram
with thermal ellipsoids shown at the 30% probability level.
1
2
3
4
5
6
7
8
[Cu(binap)I]2 @HT
[Cu(binap)I]2 @HT
[Cu(binap)I]2 @HT
[Cu(binap)I]2 @HT
[Cu(binap)I]2 @HT
[Cu(binap)I]2 @HT
[Cu(binap)I]2 @HT
-
Na2CO3
Cs2CO3
K2CO3
tBuOK
NEt3
42
53
68
37-
7
Hydrotalcite (HT) has a potential use for the dispersion of
small nanoparticles as a catalyst support. Herein, we utilized
HT as a support for the [Cu(binap)I]2 complex. The detailed
synthetic procedure for [Cu(binap)I]2@HT is presented in the
experimental section. It was clear that the [Cu(binap)I]2
complex was successfully loaded on HT following several
modes of characterization. [Cu(binap)I]2@HT was
characterized by FT-IR (Fig. S2, see Supporting Information
for details), SEM and TEM (images of [Cu(binap)I]2@HT
were shown in Fig 2; small particles irregularly dispersed on
the surface of flaky hydrotalcite, see Fig. S7 for enlarged
EDX image), TGA and N2 adsorption-desorption isotherms
(Figs. S4-S5). The powder XRD pattern of the synthesized
catalyst was shown in Fig. S6.
K3PO4
KOH
30
82
<5
KOH
aReagents and conditions: 1a (2.0 mmol), 2a (4.0 mmol), catalyst
(2 mol%Cu, 5 % loading, w/w), base (1.0 mmol), 100 oC, 12 h,
under solvent-free conditions.b Isolated yield.
Figure 2. (a) SEM images of [Cu(binap)I]2@HT (b) TEM
micrographs of [Cu(binap)I]2@HT.
Figure 3. Effect of temperature on the model reaction process.
Catalytic activity for borrowing hydrogen reaction
In order to study the influence of temperature, the reaction
profiles were explored. As illustrated in Figure 3, Only 53%
yield of the corresponding product was achieved in the
presence of [Cu(binap)I]2@HT (5% loading, w/w) under
grinding condition at room temperature. Whereas by
increasing the temperature to 60 oC and then 80 oC,
significant improvement was observed and yield of the
product increased significantly (Figure 3). Furthermore, an
Initially, the borrowing hydrogen reaction between aniline
with benzyl alcohol was selected as the model to examine this
newly supported heterogeneous [Cu(binap)I]2@HT catalyst
under solvent-free conditions. It was very pleasing to find that
N-benzylaniline was obtained in low yield (Table 1, entry 1).
In order to achieve a high yield, a series of screening
experiments were explored by varying bases for the
representative reaction. KOH produced the best result, while
other bases such as Cs2CO3, K2CO3, NEt3, K3PO4, KOH and
o
increase in temperature to 100 C and 120 oC had nearly no
o
significant change in the yield of the product. Thus, 80 C
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was chosen as the optimal reaction temperature for future
investigations.
After extensive screening, the effect of catalyst loading was
also studied (Table 2). Increasing the catalyst loading to 10%
showed 85% yield of the desired product could be obtained,
whereas the yield decreased slightly by increasing the amount
of the catalyst to 20%. Thus, the optimal reaction conditions
are as follows: [Cu(binap)I]2@HT (5% loading, w/w) as the
catalyst, KOH as the base under solvent-free conditions at 80
oC for 12 h.
Table 2. Effect of catalyst loading on the model reaction. a
Temp.
Time
Catalyst loading
[%, w/w]
Entry
Yield[%]b
[
oC
]
[h]
aConditions: 1 (2.0 mmol), 2 (4.0 mmol), [Cu(binap)I]2@HT (2
mol%Cu), KOH (1.0 mmol), solvent-free conditions, 12 h, 80 oC.
bIsolated yields based on 1. c(rac)[Cu(binap)I]2/HT was used.
1
2
3
4
5
6
7
8
5
80
80
80
80
80
80
80
80
12
12
12
12
12
12
6
84
85
81
70
56
<5
77
85
10
20
2
Following the optimization conditions described above, we
next examined the reaction scope in terms of other aromatic
amines with alcohols and the results were summarized in
Table 3. It was observed that a wide range of diverse
substrates and functional groups could be tolerated, including
fluoro, chloro, methyl, methoxy, bromine and ester
substituents at the different positions of aromatic amines and
alcohols. Nearly all amines were converted into the desired
products in good yields under optimized conditions with the
newly developed [Cu(binap)I]2@HT as the catalyst.
Meanwhile, the racemic catalyst was also tested for this
transformation and the result revealed that almost the same
yield of 3a was achieved.
1
0
5
5
24
aReagents and conditions: 1a (2.0 mmol), 2a (4.0 mmol), catalyst
( [Cu(binap)I]2@HT, 2 mol%Cu), base (1.0 mmol), under solvent-
free conditions. b Isolated yield.
Additionally, we explored similar reaction conditions to the
borrowing hydrogen reaction of ketones with alcohols. The
results were summarized in Table 4. We were pleased to find
that ketone derivatives were also achieved in good yields. In
general, most of the substrates (ketones) were converted
completely to produce the corresponding functionalized
ketones. Moderate to good yields could be achieved
regardless of the electron withdrawing or electron donating
nature of the substituent groups. When two halogen atoms
were introduced to this reaction, it was observed that this
transformation could proceed well without the loss of the
halogen atoms (5q, 5t, 5u). Moreover, iodine could be well
tolerated with none of the dehalogenation products being
observed (5m, 5q).
Table 3. Substrate expansion of the borrowing hydrogen
reaction of amines with alcohols a,b
Table 4. Substrate expansion of the ketones with alcohol a,b
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Table5. Substrate expansion of 1-benzyl-2-aryl-1H-
benzo[d]imidazole derivatives a,b
Me
Me
Me
N
N
N
N
N
Me
N
Me
Me
7i
, 80%
7g, 78%
7h, 82%
MeO
S
Br
N
N
N
N
N
Br
S
N
MeO
7j, 88%
7k
, 85%
7l
, 81%
aConditions: 4 (2.0 mmol), 2 (4.0 mmol), [Cu(binap)I]2@HT (2
mol%Cu), K2CO3 (1.0 mmol), 8 h, 100 oC. bIsolated yields based on
F
F
Cl
4.
N
N
N
N
F
Cl
N
N
Catalytic activity for dehydrogenation cyclization
F
7n, 82%
7m
7o, 82%
, 75%
The 1H-benzo[d]imidazole skeletons are useful building
blocks, which have been found in pharmaceuticals, natural
products and plant alkaloids.10 Synthesis of these bioactive
molecules is an interesting and promising venture in organic
chemistry, especially under green conditions, which is a
challenging task. Recently, we discovered that homogeneous
catalysts couldn’t promote the synthesis of 1H-
benzo[d]imidazole derivatives under water or solvent-free
conditions and the catalyst couldn’t be recovered yet.9b
Herein, the synthesis of 1H-benzo[d]imidazole derivatives
were carried out in the presence of [Cu(binap)I]2@HT in
water for the first time and the experiments were summarized
in Table5. Clearly, aromatic alcohols, including methylbenzyl
alcohol, ethyl alcohol, tert-butyl alcohol, methoxybenzyl
alcohol, chlorobenzyl alcohol, and furfuryl alcohol could be
reacted smoothly, and different 1-benzyl-2-aryl-1H-
benzo[d]imidazole derivatives were obtained in moderate to
Cl
N
N
N
N
N
Cl
N
7q, 80%
7r
, 88%
7p, 86%
F
F
F
S
F
N
N
N
N
N
N
F
F
S
F
F
7t
, 79%
7s
, 87%
7u,
80%
Et
Et
tBu
N
N
N
N
Et
Et
tBu
N
N
7w
, 88%
7x, 82%
7v
, 85%
aConditions: 6 (0.5 mmol), 2 (1.1 mmol), [Cu(binap)I]2@HT (2
good yields ranging from 71
[Cu(binap)I]2@HT as a catalyst.
%
to 88
%
using
o
b
mol%Cu), K2CO3 (0.75 mmol), H2O (2 mL), 12 h, 90 C. Isolated
yields based on 6.
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Scheme 1. The control experiments for the synthesis of 1-
Mechanism studies
benzyl-2-aryl-1H-benzo[d]imidazole.
Control experiments and activity exploration
For a better understanding of this catalyst system and these
transformations, further exploration of the reaction
mechanism is necessary. However, we expected that the
supported heterogeneous [Cu(binap)I]2@HT catalyst could
overcome the difficulties of recovery and reusability
described earlier. As expected, the [Cu(binap)I]2@HT was
Proposed reaction mechanism
The reaction mechanism of the borrowing hydrogen
reaction between amine with alcohol or ketone with alcohol
was clear, while dehydrogenative cyclization is more
complicated. Therefore, it is necessary to account for this,
easily recovered. Importantly, it was found that this catalyst giving way to the plausible mechanism for the synthesis of 1-
possessed good catalytic activity. This was attributed to
synergistic effect of the BINAP ligand with carrier HT,11
which was similar to the synergistic effect of phosphine and
triazole.9b Another possible reason is that the reaction occurs
on the surface of the supported heterogeneous
[Cu(binap)I]2@HT catalyst, while homogeneous copper
catalysts couldn’t be dissolved in water (In fact, the
dispersivity of homogeneous copper catalyst was extremely
poor), thus homogeneous copper catalysts such as CuI or
[Cu(binap)I]2 only produced low yield or no conversion in
water or solvent-free conditions (Scheme 1). As expected, the
heterogeneous [Cu(binap)I]2@HT catalyst revealed much
higher catalytic activity than the homogeneous homogeneous
copper catalysts for these transformations described in this
report.
benzyl-2-aryl-1H-benzo[d]imidazole as proposed and shown
in Scheme 2. In the beginning, aldehyde was generated with
the help of [Cu(binap)I]2@HT, then imine B was formed via
the condensation of amine and aldehyde A, and detected by
MS analysis.9b,13 After the intermolecular electron transfer,
intermediate C was formed. Finally, through cyclization and
rearrangement, the desired product 7a was released.
Compared to other catalytic methodologies, Cu catalysis
shows some interesting advantages over Pd, In, Ru or other
metals. Firstly, Cu is cheaper than several other metals used
in catalysis, and is a good choice in large- and industrial-scale
applications. Secondly, Cu-based catalysts have proven to
exhibit exceptional activity in the cleavage of C-O bonds and
formation of C-N and C-C bonds.12a Finally, Cu-catalyzed
reactions work well with a variety of ligands, and the ligands
required are usually structurally quite simple and inexpensive
while ligands for Pd, In, Ru chemistry are often complicated,
unstable in air and expensive. Copper(I) phosphine halide
complexes are common precursors in catalysis.12b However,
there was still no research on the catalytic activity of
copper(I) complexes in borrowing hydrogen reactions and
dehydrogenative cyclizations in water and therefore
[Cu(binap)I]2 complex was prepared. Furthermore, to enhance
the recyclability and to develop the most economic and
promising heterogeneous catalysts, the BINAP-Cu complex
supported by hydrotalcite (HT) was synthesized, which
proved to be an efficient catalyst for borrowing hydrogen
reaction, giving products in high yields.
Scheme 2. The posible reaction pathway for synthesis of 7a.
The capture of intermediates
To unambiguously confirm this reaction mechanism, the
separation of intermediates A and B were carried out. It was
pleasing to see that intermediate A could be obtained in 65%
yield
when
phenylmethanol
was
treated
with
[Cu(binap)I]2@HT, while only 12% of intermediate B was
isolated when the reaction was conducted with
[Cu(binap)I]2@HT for 2 hours. The following observation
has been supported by related results (Scheme 3).9b, 13
NH2
OH
Catalyst, K2CO3
H2O, 90 oC, 12 h
+
N
N
NH2
Scheme 3. The posible reaction pathway for synthesis of 7a.
6a
2a
7a
To validate the possibility of a radical pathway based on a
single electron transfer (SET), control experiments with
aniline and benzyl alcohol were carried out under the
optimization conditions by using radical scavenger. As seen
[Cu(binap)I]2@HT: 78% yield
Cu(binap)I]2:
CuI:
11% yield
<5% yield
<5% yield
none:
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in Scheme 4, the results revealed that the yield of the desired
product was generally unchanged when using 2,6-Di-tert-
butyl-4-methylphenol (BHT) (1.0 eq) and 2,2,6,6-tetramethyl
piperidinyloxy (TEMPO) (1.0 eq) with [Cu(binap)I]2@HT as
the catalyst. In order to explore the effect of inhibitor and its
concentration on the reaction, we carried a series of contrast
experiments (see Table S5 in Supporting Information). As the
concentration of the inhibitor increased, the yields seemed to
be not affected. As a result, a single electron transfer (SET)
should be not involved in the process.
Figure 4. Catalytic reusability of [Cu(binap)I]2@HT in the
borrowing hydrogen reaction of aniline (1a) with benzyl
alcohol (1b).
Scheme 4. Control experiments with radical scavengers.
Conclusions
Reusability of the catalyst
In summary, we established the synthesis, characterization,
and catalytic activity of novel composite [Cu(binap)I]2@HT
catalysts, which were demonstrably effective catalysts for
borrowing hydrogen and dehydrogenation reactions under
water or solvent-free conditions. This provided an efficient
methodology for the synthesis of substituted amine
derivatives, functionalized ketones and 1-benzyl-2-aryl-1H-
benzo[d]imidazole derivatives under clear conditions.
The recycling test of [Cu(binap)I]2@HT was investigated
through the borrowing hydrogen reaction between aniline (1a)
with benzyl alcohol (1b) under optimum conditions. The
catalyst could be separated easily through high speed
centrifugation, followed by washes with EtOH, deionized
water and subsequent vacuum freeze-drying for 24 h. Then
the new substrates and base were added for the next cycle. As
shown in Figure 4, no significant decreases in yield could be
seen with maintained reaction selectivity after five cycles.
To further investigate the catalyst system and measure the
extent of [Cu(binap)I]2@HT leaching after the reactions, a hot
filtration experiment was performed. The amounts of the
[Cu(binap)I]2@HT leaching into solution for the reaction of
aniline with benzyl alcohol under solvent free conditions at
80 oC was detected by checking the Cu and P leaching
amount before and after each reaction cycle through ICP
analysis. The amount of Cu leaching after the first run was
determined by ICP assessment and found to be only 0.64%
(%, w/w). After 5 repeated cycles, the leaching was found to
be 2.83%. Meanwhile, it was evident that less than 0.35% of
the P species had leached into the reaction mixture even after
five cycles. Thus, the ICP technique showed that the leaching
of [Cu(binap)I]2@HT was negligible. These experiments
demonstrated [Cu(binap)I]2@HT had excellent stability and
regenerating capability.
Experimental Section
Materials
2,2'-bis(diphenylphosphanyl)-1,1'-binaphthyl
(BINAP,
98%), Cuprous iodide (CuI, 98%), amines, alcohols and
ketones were purchased from Energy Chemical and used
without any treatment. [Cu(binap)I]2 complex was
synthesized according to the known procedure.14 Hydrotalcite
(HT, Mg/Al = 3) were purchased from Makahi reagent and
were used without further purification. Dimethyl sulfoxide,
toluene, xylene, alcohols were purchased from Sinopharm
Chemical Reagent Co., Ltd., China. All other chemicals were
of analytical grade. The used water was deionized water.
Preparation of [Cu(binap)I]2@HT
[Cu(binap)I]2 complex (50 mg) was dissolved in DMSO
(10 mL) in a 100 mL Schlenk tube under a nitrogen
atmosphere at 160 ºC. After the complex completely
dispersed and dissolved in solution, hydrotalcite (500 mg)
was added to the solution with vigorous stirring. The mixture
was continuously stirred for 24 h. Subsequently, deionized
water (5 mL) was added to precipitate more solids. After
cooling to ambient temperature, the precipitate was collected
by centrifugation and washed with deionized water and
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Y. Obora, ACS Catal. 2014, 4, 3972. (h) D. Hollmann,
ChemSusChem 2014, 7, 2411.
(a) F. G. Mutti, T. Knaus, N. S. Scrutton, M. Breuer, N. J.
Turner, Science 2015, 349, 1525. (b) C. Gunanathan, D.
Milstein, Science 2013, 341, 249. (c) A. J. A. Watson, J. M. J.
Williams, Science 2010, 329, 635.
ethanol for several times, followed by vacuum freeze-drying
for 24 h.
2
3
Characterisation techniques
Single crystal X-ray diffraction analysis was conducted on
Bruker Advance Smart 1000. The FTIR characterization of
hydrotalcite (HT), [Cu(binap)I]2 complex and hydrotalcite
Supported [Cu(binap)I]2 Catalysts were performed on Total
reflection Fourier infrared spectrometer Nicolet 6700
(Thermo Fisher Scientific Co. Ltd., USA). X-ray diffraction
(XRD) patterns of the sample powder were carried out by
BRUKER-D8 Advance diffractometer with Cu Kα radiation
(λ = 0.154 nm) over a 2θ range between 3 and 90°. The
morphologies of hydrotalcite supported [Cu(binap)I]2
complex was observed by scanning electron microscopy
(SEM) Hitachi S-4800 (Hitachi Ltd., Japan). Energy dis-
persive X-ray (EDX) was obtained by scanning electron
microscopy (SEM). The catalyst morphology structure was
investigated by a JEOL JEM-2100 Transmission electron
microscopy (TEM). The Brunauer-Emmett-Teller (BET)
measurement of the sample was analyzed by the Full
automatic specific surface area and micropore physical
adsorption instrument ASAP2020 MP (micromeritics, USA).
Thermogravimetic analysis (TGA) were measured on
TG/DSC1/1100 Met-tler Toledo, the sample were heated
from 25 to 800 ºC at a heating rate of 10 ºC min-1 under
flowing nitrogen.
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4
5
Representative procedure for the preparation of 7a
The mixture of catalyst (2 mol%Cu), H2O (2 mL), diamine
(0.5 mmol), alcohol (1.1 mmol) and K2CO3 (0.75 mmol) was
o
stirred in 20 mL colorimetric tube under 90 C for 12 h. Then
the reaction mixture was cooled down to the room
temperature and the solvents was removed to give the residue.
The residue was directly extracted by ethyl acetate and then
purified by column chromatography with petroleum
ether/ethyl acetate (petroleum ether /ethyl acetate = 10:1) as
eluent to give the desired product (7a).
6
7
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10786.
Acknowledgements
We gratefully acknowledge financial support of this work
by the National Natural Science Foundation of China
(21776111, 21401080), Jiangsu Talents Project (2013-JNHB-
027), the Fundamental Research Funds for the Central
Universities (JUSRP 51627B) and MOE & SAFEA for the
111 Project (B13025).
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