Ligand-, base-, co-catalyst-free copper fluorapatite (CuFAP) as a versatile, ecofriendly, heterogeneous and reusable catalyst for an efficient homocoupling of arylboronic acid at ambient reaction conditions

Article abstract of DOI:10.1039/c4ra16760k

This paper describes the first report in which copper species containing copper fluorapatite (CuFAP) acts as a versatile, eco-friendly, recyclable, heterogeneous catalyst for an efficient synthesis of symmetric biaryls from the homo-coupling of arylboronic acids in methanol solvent at ambient reaction conditions. The developed protocol is ligand-, base-, and co-catalyst-free, sustainable, mild, inexpensive, and compatible with a wide range of aromatic/heterocyclic boronic acids and provides the corresponding products in excellent yields without purification. The catalyst was easily recovered from the reaction mixture and reused several times without loss of activity.

Full text of DOI:10.1039/c4ra16760k

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PAPER
Ligand-, base-, co-catalyst-free copper fluorapatite
(CuFAP) as a versatile, ecofriendly, heterogeneous
and reusable catalyst for an efficient homocoupling
of arylboronic acid at ambient reaction conditions†
Cite this: RSC Adv., 2015, 5, 24675
*
Shafeek A. R. Mulla, Santosh S. Chavan, Mohsinkhan Y. Pathan, Suleman M. Inamdar
and Taufeekaslam M. Y. Shaikh
This paper describes the first report in which copper species containing copper fluorapatite (CuFAP) acts as
a versatile, eco-friendly, recyclable, heterogeneous catalyst for an efficient synthesis of symmetric biaryls
from the homo-coupling of arylboronic acids in methanol solvent at ambient reaction conditions. The
developed protocol is ligand-, base-, and co-catalyst-free, sustainable, mild, inexpensive, and
compatible with a wide range of aromatic/heterocyclic boronic acids and provides the corresponding
products in excellent yields without purification. The catalyst was easily recovered from the reaction
mixture and reused several times without loss of activity.
Received 20th December 2014
Accepted 18th February 2015
DOI: 10.1039/c4ra16760k
www.rsc.org/advances
complexes,10d AuNP,10e Pd(OAc)2/N,O-ligands,10f [{(1,10-phenan-
Introduction
throline)Cu(u-OH)}2Cl2]$3H2O,10g
CuCl,10h
Ag2O/CrCl2,10i
Pd(PPh3)2Cl2,10j PdCl2,10k Pd(OAc)2,10l Pd(OAc)2/AgNO3,10m
NaAuCl4$2H2O,10n AuNP/In2O3,10o I2,10p have been well reported
in the literature. Nonetheless, so far only four papers,10a,b,g,h have
been reported in which copper species act as catalysts for the
synthesis of biaryls from the homo-coupling of arylboronic
acids. Moreover, all these approaches reported so far have used
either costly transition metals or moisture-sensitive, toxic,
corrosive homogeneous catalysts in excess or stoichiometric
quantity with the requirements of oxidants, high concentra-
tions of base, highly expensive ligands/co-catalysts in the
presence/absence of hazardous/malodorous solvents, which
leads to problems of toxic waste disposal besides long reaction
times, high temperatures and low yields of the desired product.
Hence, all these approaches lack generalization to implement
the commercial scale production of biaryls from homo-coupling
of arylboronic acids.
Diaryl motifs being a key constituent of the structural
backbone of many bioactive natural/pharmaceutical
compounds, the construction of their structural units by
developing more general, ligand-, base-, co-catalyst-free,
straightforward, environment friendly, and cost effective
protocols using non-toxic, environmentally benign solvents,
cost effective recyclable catalysts at milder reaction conditions
is still challenging and the subject of intense investigation. To
the best of our knowledge, so far no report on the biaryls
synthesis from homocoupling of arylboronic acids using copper
species as a heterogeneous, reusable catalyst has been pub-
lished in the literature. As we were stimulated from our research
work on the organic transformations using recyclable,
Biaryl linkages act as building blocks not only in polymer3 and
ligand syntheses4 but also in variety of synthetic/naturally
occurring biologically active molecules, which possess a wide
range of bioactivities, such as antiviral,1 antibacterial (crisami-
cin A and biphenomycin B),2 and so on. Owing to the potent
importance of biaryl moieties in life sciences, pharmaceuticals,
polymers and catalysis, the synthesis of their structural units
have attracted immense attention among researchers who have
developed various methods, such as metal catalyzed coupling of
aryl halides,5 Grignard reagents, arenediazonium salts,
1,2-diarylditellanes,6 and name reactions for their synthesis.7
However, a copper catalyzed Ullmann coupling reaction of
arylhalides is a well known conventional method for the
synthesis of biaryls.8
Arylboronic acids are easily available, more stable, non-toxic,
and compatible with different functional groups compared to
arylhalide/organometallic reagents, hence their utilization has
become a centre of interest among chemists/biologists for the
introduction of phenyl ring in various biologically active mole-
cules9 and/or for the synthesis of biaryl motifs by homo-
coupling of arylboronic acids. Because of these novel features
of arylboronic acids, homo-coupling of arylboronic acids for the
synthesis of symmetrical biaryl using various catalysts, such as
CuSO4,10a Cu(OAc)2,10b RhCl (PPh3)3,10c Au(III) Schiff base-
Chemical Engineering & Process Development Division, CSIR-National Chemical
Laboratory, Dr. Homi Bhaba Road, Pune-411008, Maharashtra, India. E-mail: sa.
mulla@ncl.res.in; Fax: +91-20-25902676; Tel: +91-20-25902316
† Electronic supplementary information (ESI) available: Experimental details and
spectral data of all the new compounds. See DOI: 10.1039/c4ra1670k
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heterogeneous copper uorapatite (CuFAP) catalysts11a–c as well
as the use of copper in the classical Ullmann coupling reaction
of arylhalide for the synthesis of biaryls, fascinated us to
investigate the application of copper uorapatite (CuFAP)
catalysts for the homo-coupling of arylboronic acids. Therefore
this paper describes the rst report in which copper species
containing CuFAP act as a versatile, recyclable, heterogeneous
catalyst for the homo coupling reaction of arylboronic acids.
As part of our ongoing program to develop general, green,
ecofriendly, and cost effective approaches for organic trans-
formations using recyclable catalysts,11a–h we herein report a
ligand-, base-, co-catalyst-free, environment friendly, cost
effective, and mild protocol for an efficient synthesis of biaryls
from the homo-coupling of arylboronic acids over an eco-
friendly, heterogeneous reusable CuFAP catalyst in methanol
solvent at ambient temperature (Scheme 1).
Table 1 Optimisation of reaction conditions for biaryl synthesis from
phenyl boronic acida
Entry Catalyst
Solvent Temp (ꢀC) Time (h) Yieldb (%)
Effect of solvent
1
2
3
4
5
6
7
8
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
CuFAP
MeOH
EtOH
n-PrOH rt
iPrOH
THF
Dioxane rt
PhMe
DMSO
DMF
CHCl3
DCM
rt
rt
2
8
8
8
8
92
91
80
84
71
74
rt
rt
8
rt
rt
rt
rt
rt
rt
12
12
12
12
12
12
N.R.c
N.R.c
N.R.c
N.R.c
N.R.c
N.R.c
9
10
11
12
Results and discussion
MeCN
To develop a protocol for the synthesis of biaryls from the homo
coupling of arylboronic acids, phenyl boronic acid (1 mmol),
catalyzed by a 100 mg of CuFAP catalyst in a 5 mL solvent, was
selected as the model reaction to optimize reaction conditions.
Initially, the screening of different solvents, such as MeOH,
n-PrOH, THF, i-PrOH, EtOH, 1,4-dioxane, PhMe, DMSO, DMF,
CHCl3, DCM, and MeCN were carried out at room temperature
(Table 1, entries 1–12). However, MeOH as a solvent gives a 92%
yield of the desired product in 2 h, whereas EtOH, n-PrOH,
i-PrOH, THF, and 1,4-dioxane solvents give 91%, 80%, 84%,
71%, and 74% yields, respectively in 8 h (Table 1, entries 2–6).
The formation of desired product was not observed in PhMe,
DMSO, DMF, CHCl3, DCM, and MeCN solvents even aer 12 h
(Table 1, entries 7–12).
The excited results using MeOH as a solvent over ligand-,
base-, and co-catalyst-free CuFAP catalyst made us keen to
design, develop, and explore the scope of various catalysts for
the homo-coupling of arylboronic acids. The various catalysts,
such as CuFAP, rhodium uorapatite (RhFAP), palladium u-
orapatite (PdFAP), and silica supported dodecatungstophos-
phoric acid (DTP/SiO2), were screened for the synthesis of
biaryls from the homo-coupling of phenylboronic acids in 5 mL
methanol solvent at room temperature. However, the CuFAP
catalyst provided the desired product in excellent yield in a
short reaction time (Table 1, entry 1) as compared to the PdFAP
catalyst (Table 1, entry 13). The RhFAP, and DTP/SiO2 catalysts
do not show any sign of desired product formation even though
the reaction was heated at 70 ꢀC for 24 h (Table 1, entry 14
and 15).
Catalyst screening
13
14
15
PdFAP
RhFAP
DTP/SiO2
MeOH
MeOH
MeOH
rt
70
70
5
24
24
88
N.R.c
N.R.c
Effect of catalyst loading
16
17
18
CuFAP (50 mg)
CuFAP (75 mg)
CuFAP (125 mg) MeOH
MeOH
MeOH
rt
rt
rt
2
2
2
57
80
93
a
Reaction conditions: phenyl boronic acid (1 mmol), solvent (5 mL),
b
catalyst (100 mg). Isolated yield aer chromatographic purication.
c
No reaction.
The excellent results of the CuFAP catalyst made us further
optimize the catalyst loading. Interestingly, increasing the
catalyst loading from 50 mg to 100 mg resulted in a dramatic
enhancement in the yield from 57% to 92% (Table 1, entry 16,
17 and 1). However, no progress in the yield was observed by
further increasing to 125 mg catalyst loading (Table 1, entry 18).
Fig. 1 Effect of reaction time: phenylboronic acid (1 mmol), CuFAP
Scheme 1 CuFAP catalyzed homo coupling of arylboronic acid.
24676 | RSC Adv., 2015, 5, 24675–24680
(100 mg) in MeOH (5 mL) at room temperature.
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Table 2 Substrate scope of CuFAP catalyzed homocoupling of aryl-
We optimized the reaction time as shown in the Fig. 1. The
yield of the desired product increased with increasing reaction
time from 0.5 h to 2 h and remains unchanged by further
increasing the reaction time to 2.5 h. These optimization results
reveal that the CuFAP catalyst with 100 mg loading in methanol
shows excellent catalytic activity at room temperature in 2 h
(Table 1, entries 1–18 and Fig. 1). The formation of biaryl was
not observed in the absence of CuFAP catalyst.
boronic acidab
Entry
Product (2a–2q)
Yieldb (%)
The promising performance of ligand-, base-, co-catalyst-free
CuFAP catalyst at optimized reaction conditions, encouraged us
to ascertain general compatibility of the CuFAP catalyst with
various arylboronic acids, hence, homo-coupling reactions of
various arylboronic acids have been examined and results are
shown in Table 2. Surprisingly, the homo-coupling reactions of
various substituted arylboronic acids in the presence of the
ligand-, base-, co-catalyst-free CuFAP catalyst under optimized
reaction conditions provided the corresponding biaryl products
in very good to excellent yields (Table 2, entries 1–21). Initially,
the homo-coupling reaction of various para substituted phe-
nylboronic acids were carried out under the optimized reaction
conditions and the electron donating/withdrawing groups, such
as Me, OMe, C(Me)3, F, Cl at the para position on the aryl ring
provided excellent yields (Table 2, entry 2–6). Moreover, phe-
nylboronic acids bearing a sensitive functional groups such as
–CHO and –CH]CH2 were successfully reacted to afford the
desired homo-coupling compounds in 85% and 92% yield,
respectively (Table 2, entries 7 and 8), whereas phenyl boronic
acids bearing –OMe, and –Cl group at ortho and meta positions,
respectively, afforded the corresponding homo-coupling
product in excellent yields (Table 1, entry 9 and 10).
1
2
3
4
5
6
7
8
2a, R ¼ H
92
94
93
96
90
93
85
92
2b, R ¼ Me
2c, R ¼ OMe
2d, R ¼ C(Me)3
2e, R ¼ F
2f, R ¼ Cl
2g, R ¼ CHO
2h, R ¼ CH]CH2
9
95
95
98
97
97
10
11
12
13
Encouraging results on various mono-substituted phenyl-
boronic acids create a keen interest to further elaborate the
scope to various di and tri-substituted phenyl boronic acids.
Interestingly, 2,3,4-trimethoxy phenylboronic acid, (4-methoxy-
3,5-dimethylphenyl)boronic
acid,
(3,5-bis(triuoromethyl)-
phenyl)-boronic acid, 3,4-diuoro phenyboronic acid, 4-acetyl-
phenyboronic acid, and 2-naphthylboronic acid were reacted
without any problem under the optimized reaction conditions
and afforded the corresponding biaryl products in good to excel-
lent (88–98%) yields (Table 2, entries 11–16). Amazing results on
homo-coupling of mono-, di-, tri- arylboronic acids as well as 2-
naphthylboronic acid, encouraged us to investigate the homo-
coupling reaction of substituted trans-2-phenylvinylboronic
acids, and to our surprise, the homo-coupling reactions of various
electronically rich and poor trans-2-phenylvinylboronic acids were
amenable to produce the anticipated product in excellent yields
(Table 2, entries 17–21).
14
15
16
93
88
90
The incredible performance of ligand-, base-, co-catalyst-free
CuFAP catalyst on various substituted arylboronic acid/
2-napthyleneboronic acid/phenylvinylboronic acids (Table 2,
entries 1–21), made us eager to check the feasibility of this
protocol for hetrocyclicboronic acids. Hence, various hetrocy-
clicboronic acids such as 2-thiopheneboronic acid, 3-thio-
pheneboronic acid, 2-benzothiopheneboronic acid, 3-benzo-
thiopheneboronic acid, and 2-furanylboronic acid were sub-
jected to the homo-coupling reaction under the optimized
reaction conditions. Miraculously, all hetrocyclicboronic acids
17
18
19
20
21
2q, R ¼ H
2r, R ¼ F
96
93
90
94
96
2s, R ¼ CF3
2t, R ¼ Cl
2u, R ¼ Ph
a
Reaction conditions: arylboronic acid/(1 mmol), CuFAP (100 mg) in
b
MeOH (5 mL) at room temperature for 2 h. Isolated yield without
chromatographic purication.
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Table 3 CuFAP catalyzed homo coupling of hetrocyclicboronic acidab
Entry
Product (3a–e)
Yieldb (%)
1
2
95
96
3
4
93
95
Scheme 3 Plausible mechanism for homo coupling over CuFAP.
Therefore, phenylboronic acid and 2-methoxy phenylboronic
acid were coupled under the optimized reaction conditions, and
provided the homo-coupling and cross-coupling products in
31% (4a), 43% (5a) and 26% (3a) yields, respectively, as pre-
dicted in Scheme 2. The yield is based on GC analysis.
As per our,11a–c and previous12a,b research work reported in
the literature, the probable mechanism proposed for the
synthesis of biaryls from the homo coupling reaction of aryl-
boronic acids over ligand-, base-, co-catalyst-free CuFAP catalyst
is shown in Scheme 3. Initially, phenylboronic acid reacts with
the CuFAP catalyst to generate the intermediate complex
I. Subsequently, complex I reacts with another mole of phe-
nylboronic acid to form intermediate complex II, which
instantaneously undergoes the formation of C–C coupling to
eliminate the biaryl product as well as the CuFAP catalyst for
reuse.
5
87
a
Reaction conditions: heterocyclic boronic acid (1 mmol), CuFAP (100
b
mg) in MeOH (5 mL) at room temperature for 2 h. Isolated yield
without chromatographic purication.
provided desired coupling products in excellent yields (Table 3,
entries 1–4) except 2-furanylboronic acid, which afforded an
87% yield (Table 3, entry 5). The results in the Tables 2 and 3
reveal that a novel ligand-, base-, co-catalyst-free protocol over a
versatile, eco-friendly, recyclable, heterogeneous copper uo-
rapatite catalyst is tolerant to homo-coupling reactions of
various aryl/hetrocyclicboronic acids having different func-
tional groups. However, the yield obtained does not depend on
the nature of substituents on aryl/hetrocyclicboronic acids and
provided excellent yields except Table 2 entry 7 (85% yield),
15 (88% yield), Table 3 entry 5 (87% yield).
Delighted with the performance of the ligand-, base- and
co-catalyst-free CuFAP catalyst for homo-coupling reactions of
the aromatic, hetrocyclicboronic acids, we were curious to
investigate the coupling of two different arylboronic acids
(Scheme 2).
Recyclability of CuFAP catalyst
Today's environmental concerns demand the development of
“Green Chemical Processes” hence, recovery, and recyclability of
catalyst is the choice to achieve environmental sustainable and
economical viable processes. Therefore, the recyclability and
recovery of the ligand-, base-, co-catalyst-free CuFAP catalyst was
investigated for the synthesis of symmetrical biaryls by the
homo-coupling reaction of phenylboronic acids at optimized
reaction conditions. The results are shown in Table 4. The
Table 4 Recoverability and reusability of CuFAP catalysta
Entry
Run
Yieldb (%)
1
2
3
4
5
Fresh
Run-1
Run-2
Run-3
Run-4
92
92
90
92
91
a
Reaction conditions: phenylboronic acid (1 mmol), CuFAP (100 mg) in
b
MeOH (5 mL) at RT for 2 h. Isolated yield without chromatographic
Scheme 2 CuFAP catalyzed the coupling reaction of phenylboronic
purication.
acid and 2-methoxy phenylboronic acid.
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catalyst was recovered and recycled several times without loss of
catalytic activity (Table 4, entries 2–5). The isolated yield
obtained at the end of 4th recycle (Table 4, entry 5) is very much
consistent with the fresh catalyst (Table 4, entry 1). The consis-
tent performance of reused catalyst clearly indicates that there
was no loss or leaching of copper species during the course of
the reaction, which was conrmed by ICP analysis as well.
Notes and references
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Conclusions
In conclusion, a novel, general, ligand-, base-, co-catalyst-free,
sustainable, mild, and inexpensive protocol has been devel-
oped over a versatile, eco-friendly, recyclable, heterogeneous
copper uorapatite catalyst for an efficient synthesis of
symmetric biaryls in good to excellent yield from the homo-
coupling of aryl/heterocylicboronic acids in methanol solvent
at ambient reaction conditions in a short reaction time. The
CuFAP catalyst was recovered by simple ltration from the
reaction mixture and reused several times without the loss of
catalytic activity. The described protocol is very much compat-
ible with wide range of sensitive functional groups and may be
applicable for the synthesis of various bioactive and natural
products containing biaryls; hence further studies are in prog-
ress in this direction.
Experimental section
All chemicals and reagents were procured from Sigma Aldrich,
S.D. Fine chemical and commercial suppliers, and used without
further purication. The products were characterized using 1H
NMR, and 13C NMR spectroscopy. NMR spectra of the products
were obtained using a Bruker AC-200 MHz spectrometer with TMS
as the internal standard. Column chromatography was performed
on silica gel, Merck grade 60–120 mesh size. TLC was performed
on 0.25 mm E. Merck precoated silica gel plates (60 F254).
General experimental procedure for the synthesis of biaryls
over CuFAP
Arylboronic acid (1 mmol) in 5 mL methanol solvent was stirred
at room temperature for 2 h in the presence of 100 mg of CuFAP
catalyst. The completion of the reaction was monitored by TLC.
Aer the completion of the reaction, the reaction mixture was
diluted with 10 mL methanol followed by ltration to recover
the catalyst. The ltrate was dried under vacuum, thereaer
10 mL water and 10 mL ethyl acetate were added to separate out
the organic layer. The organic layer was dried over anhydrous
Na2SO4 and concentrated under vacuum to give the pure
product. All the isolated reaction products were characterized
and conrmed by NMR.
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Acknowledgements
SSC and TMYS thank CSIR and UGC, New Delhi, India, for SRF
and JRF, respectively. The authors are also thankful to Dr V. V.
Ranade, Dy. Director for his constant encouragement.
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