A green and recyclable chitosan supported catalyst for the borylation of α,β-unsaturated acceptors in water

Article abstract of DOI:10.1016/j.catcom.2016.08.002

We herein report a green and recyclable chitosan supported copper catalyst which is capable of catalyzing the borylation of α,β-unsaturated acceptors in water at room temperature. A broad substrate scope including chalcone derivatives, esters and nitriles have been explored. In all the cases, desired products were obtained in good to excellent yields. Remarkably, this chitosan supported catalyst could be recovered and reused for five times without any significant decrease of reactivity.

Full text of DOI:10.1016/j.catcom.2016.08.002

Catalysis Communications 86 (2016) 23–26
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
A green and recyclable chitosan supported catalyst for the borylation of
α,β-unsaturated acceptors in water
a
a,
Pengyu Xu b,1, Bojie Li a,1, Liansheng Wang a, , Caiqin Qin , Lei Zhu
⁎
⁎
a
College of Chemistry and Materials Science, Hubei Engineering University, HuBei Collaborative Innovation Center of Conversion and Utilization for Biomass Resources, Hubei 432000, China
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 April 2016
Received in revised form 19 July 2016
Accepted 1 August 2016
Available online 03 August 2016
We herein report a green and recyclable chitosan supported copper catalyst which is capable of catalyzing the
borylation of α,β-unsaturated acceptors in water at room temperature. A broad substrate scope including
chalcone derivatives, esters and nitriles have been explored. In all the cases, desired products were obtained in
good to excellent yields. Remarkably, this chitosan supported catalyst could be recovered and reused for five
times without any significant decrease of reactivity.
© 2016 Published by Elsevier B.V.
Keywords:
Chitosan supported copper
Heterogeneous catalyst
C\\B bond formation
Recovery and reuse
1. Introduction
Immobilization of metal catalysts on heterogeneous support is one
of the best solutions to overcome the problems associated with recovery
Organoboron derivatives are one of the most important intermedi-
ates because the C\\B linkages can be transferred to C\\O, C\\N and
C\\C bonds [1]. In contrast to the classical methods, transition metal-
catalyzed boron conjugate additions to α,β-unsaturated carbonyl and
related compounds provide most efficient and straightforward pathway
to form functionalized organoboron reagents [2]. After Hosomi [3] and
Miyaura's [4] exploration of this area, borylation of α,β-unsaturated ac-
ceptors have been successfully achieved by using various transition
metal catalysts, such as Rh [5], Ni [6], Pt [7], Pd [8], Zn [9] etc. Among
the other reported catalysts, Cu-based catalyst has drawn substantial at-
tention for this transformation because of its inexpensiveness, lower
toxicity and abundance. Although CuI-based catalyst [10] effectively cat-
alyzes boron conjugate addition reaction still, it particularly involves
complicated experimental procedures, requires strong base and is
mainly limited to homogeneous systems. On the other hand, CuII-
catalyzed borylation [11,12] which was recently developed, exhibited
a more green and convenient pathway by using only water as solvent.
In order to further fulfill the requirements of sustainability, develop-
ment of alternative highly active and low cost heterogeneous CuII
based catalyst for recycle and reuse is still highly desirable.
and reuse of expensive homogeneous catalysts and to avoid product
contamination [13]. Also, recovery and recyclability of immobilized
metal catalysts makes the whole protocol green and sustainable both
economically and environmentally. Magnetic materials [14], silica [15],
zeolites [16] and polymers [17] were successfully employed as the sup-
port in the past. Recently, chitosan (CS) has attracted significant interest
due to its green property, stability and ability of chelation [18]. Several
examples have been reported by using chitosan-supported metal cata-
lyst for C\\C [19], C\\N [20], C\\S [21] and C\\O [22] bond formation.
However, to best of our knowledge, no such example of C\\B bond for-
mation has yet been reported before and it still remains a challenge. In
2013, Časar et al. [23] disclosed that D-glucosamine could be used as
ligand for the borylation in the presence of basic CuCO3. This result indi-
cated the possibility of applying chitosan as support towards similar
transformation. In this work, we describe a new strategy towards the
borylation of α,β-unsaturated acceptors by utilizing a heterogeneous,
green and recyclable chitosan supported copper catalyst.
2. Methods
2.1. Materials
⁎
Corresponding authors.
E-mail addresses: wangls@hbeu.edu.cn (L. Wang), Lei.zhu@hbeu.edu.cn (L. Zhu).
These authors contributed equally.
Reagents were commercially supplied and used as received. Chito-
san was purchased from Aldrich (medium molecular weight, CAS
1
http://dx.doi.org/10.1016/j.catcom.2016.08.002
1566-7367/© 2016 Published by Elsevier B.V.

24
P. Xu et al. / Catalysis Communications 86 (2016) 23–26
9012-76-4). CS@Cu(OH)2, CS@CuO [24] and other CS@Cu catalysts [21]
were prepared following the procedures reported.
intervals until the resulting solution was clear. After the solution was
cooled to room temperature, 1 mL of aqua regia was added carefully.
Effervescence of gas was observed and the solution becomes clearer.
The solution was then transferred to a volumetric flask and made up
to 50 mL with water which was submitted for ICP analysis.
2.2. Analytical methods
1H NMR and 13C NMR spectra were recorded on a Bruker Avance 500
or 600 MHz spectrometers at ambient temperature. Data for 1H NMR
are reported as follows: chemical shift (ppm, scale), multiplicity (s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet and/or mul-
tiplet resonances, br = broad), coupling constant (Hz), and integration.
Data for 13C NMR are reported in terms of chemical shift (ppm, scale),
multiplicity, and coupling constant (Hz). Flash column chromatographic
purification of products was accomplished using forced-flow chroma-
tography on Silica Gel (200–300 mesh). High Resolution Mass Spectra
(HRMS) were recorded using a JEOL JMS-700 spectrometer. The weight
percentage and metal leaching of copper were determined by induc-
tively coupled plasma-atomic emission spectroscopy (ICP-AES) analy-
sis. The copper loading of CS@Cu(OH)2 was found to be 1.50 mmol/g.
Procedure for the preparation of sample for ICP analysis to deter-
mine the catalyst loading: Catalyst (~20 mg) was placed in a clean test
tube and heated with H2SO4 (1 mL) at 200 °C. After 30 min, several
drops of concentrated HNO3 were added carefully and the tube was
shaken occasionally. HNO3 was continuously added until a clear
solution was obtained and excess amount of HNO3 was allowed to evap-
orate under heating. After the solution was cooled to room temperature,
1 mL of aqua regia was added carefully. Effervescence of gas was ob-
served and the solution becomes clearer. The solution was then trans-
ferred to a volumetric flask and made up to 50 mL with water which
was submitted for ICP analysis.
2.3. General procedure for CS@Cu catalyzed borylation of α,β-unsaturated
acceptors in water
CS@Cu(OH)2 (6.7 mg, 5 mol% Cu loading) and L4 (2.4 mg, 6 mol%)
were mixed in water (2 mL). The mixture was stirred for 1 h at room
temperature, followed by successive addition of chalcone 1a (41.0 mg,
0.2 mmol) and B2(pin)2 (60.9 mg, 0.24 mmol). After stirring for 12 h
at room temperature, the reaction mixture was filtered and washed
with THF (3 mL). Excess amount of NaBO3·4H2O (244 mg) was then
added to filtrate and the mixture was stirred at room temperature for
4 h. The aqueous layer was extracted with EtOAc (20 mL) three times,
and the combined organic layers were dried over anhydrous Na2SO4.
After concentrated under reduced pressure, the crude mixture was pu-
rified by preparative TLC (nhexane/EtOAc = 4/1) to afford the desired
product 3a (45.3 mg, quant.).
3. Results and discussion
We conducted the reaction of bis(pinacolato)diboron (B2(pin)2)
with chalcone 1a in water by using Cu(OH)2 as catalyst, 2,2′-bipyridine
ligand L1 as additive. The desired 1,4-addition product 2a was obtained
in 93% yield with 1.6% leaching (Table 1, entry 1). Since Cu(OH)2 is al-
most insoluble in water, metal leaching could probably be ascribed to
the generation of pinBOH by-product [12] during the reaction process.
Based on the catalytic cycle [11], stoichiometric amount of pinBOH
was generated as by-product which presumably might be absorbed on
the surface of CS@Cu catalyst and caused the metal leaching. When
we tried to immobilize Cu(OH)2 on series of different supports, the
yield decreased in all cases (Table 1, entries 2–4). However, chitosan
(CS) could give comparable results with non-immobilized Cu(OH)2
(Table 1, entry 5). To be mentioned, this reaction could also proceed
Procedure for the preparation of ICP analysis to determine the metal
leaching: After the reaction was finished, the reaction mixture was fil-
tered. The filtrate obtained was concentrated and diluted with 10 mL
of THF. Then 50% v/v of the crude THF solution (5 mL) was then passed
through a membrane filter (0.25 or 0.45 μm) into a clean test tube. After
evaporation of solvent, the solid obtained in the test tube was heated to
200 °C and 1.0 mL of concentrated H2SO4 was added. Following similar
procedure described above, concentrated HNO3 were added at regular
Table 1
Optimization of reaction conditionsa.
Entry
Metal salts
Support
Ligand
Yield (%)b
Leaching (%)c
1
2
3
4
5
6
7
8
Cu(OH)2
Cu(OH)2
Cu(OH)2
Cu(OH)2
Cu(OH)2
Cu(OH)2
CuCN
CuSO4
CuCl2
CuF2
CuBr2
–
TiO2
Fe3O4
SiO2
L1
L1
L1
L1
L1
–
L1
L1
L1
L1
L1
L1
L2
L3
L4
93
64
52
69
90
47
NR
46
37
71
39
17
83
91
Quant.
1.6
0.7
16.5
1.2
0.7
1.1
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
Chitosan
No leachingd
65.7
59.0
25.6
72.1
3.9
9
10
11
12
13
14
15
CuO
Cu(OH)2
Cu(OH)2
Cu(OH)2
1.4
No leachingd
No leachingd
a
Reaction conditions: substrate 1 (0.2 mmol), B2(pin)2 (1.2 equiv), Support@Cu (5 mol% Cu loading), L (6 mol%), H2O (2 mL), rt, 12 h.
Isolated yield.
Determined by ICP analysis.
Under detection limit.
b
c
d

P. Xu et al. / Catalysis Communications 86 (2016) 23–26
25
Table 2
Substrate scopea,b
.
a
Reaction conditions: substrate 1 (0.2 mmol), B2(pin)2 (1.2 equiv), CS@Cu(OH)2 (5 mol% Cu loading),
L4 (6 mol%), H2O (2 mL), rt, 12 h.
b
Isolated yields were shown.
without any external ligand, but significant decrease of reactivity ac-
company with metal leaching was observed (Table 1, entry 6). Ligand
was known to be crucial for the improved reactivity of borylation by
coordination to metal center [10,23].
pharmaceuticals [28] was obtained in 82% yield. The nitrile group can
be further transformed to other functional groups such as amides [29],
carboxylic acids [30], and so on.
In order to illustrate the practical applications of such heterogeneous
catalytic system, the recovery and reuse of catalyst are very important
factors. Actually, the CS@Cu(OH)2 catalyst remained insoluble during
the reaction process and after the completion of reaction. It could
be easily removed and recovered from the reaction mixture by simple
filtration process when reaction was finished. The recovered catalyst
was washed slightly by diethyl ether, dried and then directly used
for next cycle. Up to five reused cycles, the desired product was all
obtained in excellent yields (Fig. 1) and no metal leaching was observed.
Although slightly decrease of yield was found in the fifth run, the
Next, we examined various chitosan supported copper (CS@Cu) cat-
alysts obtained from different copper salts. It was found CS@CuCN gave
no desired products although no metal leaching was observed (Table 1,
entry 7). Catalyst derived from soluble copper salts caused serious metal
leaching problems (Table 1, entries 8–11). CS@CuO prepared from in-
soluble CuO resulted in very poor reactivity (Table 1, entry 12). Follow-
ing, other types of pyridine ligands [25] were tested to avoid the
leaching of copper from chitosan support. Silane-type pyridine ligand
is known to possess a good affinity towards chitosan. However, the
leaching level could not be suppressed and yield decreased when L2
was used (Table 1, entry 13). To our delight, pyridine ligand substituted
with aliphatic secondary amine L3 could give the desired product in
91% yield without any metal leaching. Finally, in the presence of
amide-type pyridine ligand L4, the desired product 2a was obtained in
quantitative yield and no metal leaching was observed (Table 1, entry
15).
Under the optimized conditions, the substrate scope of α,β-unsatu-
rated acceptors was surveyed and the results are summarized in Table 2.
Chalcone derivatives bearing mono- or di-substituted functional groups
are suitable substrates and the corresponding 1,4-adducts were obtain-
ed in good to excellent yields. By successive oxidation of achieved
organoboron compounds [26], the β-hydroxy products could be directly
synthesized (3a–3h). Both electron-withdrawing and electron-
donating groups could be applied. Borylation of α,β-unsaturated ke-
tones having aromatic-aliphatic structures all proceeded well under
the optimized conditions (3i, 3j, 3l), even where steric hindrance
existed (3k). Methyl- and Ethyl- esters were also well tolerated (3m,
3n). It is worth mentioning that β-hydroxy nitriles (3o) which have
emerged as versatile synthons for several natural products [27] and
Fig. 1. Recovery and reuse of catalyst. Reaction conditions: see Table 1, entry 15. The
catalyst was washed with NaHCO3(aq) during the sixth run.

26
P. Xu et al. / Catalysis Communications 86 (2016) 23–26
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.08.002.
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