A green protocol for the Pd catalyzed ligand free homocoupling reaction of arylboronic acids under ambient conditions

Article abstract of DOI:10.1039/c4ra05230g

A simple, competent, green pathway has been developed for the Pd catalyzed ligand free homocoupling reaction of arylboronic acids in water under ambient conditions. An efficient reaction environment is generated using a combination of Pd(OAc)₂and 'green additives', which exhibited excellent activity and results in high yields of the desired coupled products within 15 minutes. This journal is

Full text of DOI:10.1039/c4ra05230g

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PAPER
A green protocol for the Pd catalyzed ligand free
homocoupling reaction of arylboronic acids under
ambient conditions†
Cite this: RSC Adv., 2014, 4, 41045
Seema Dwivedi, Soumik Bardhan, Prasanjit Ghosh and Sajal Das*
Received 2nd June 2014
Accepted 8th August 2014
A simple, competent, green pathway has been developed for the Pd catalyzed ligand free homocoupling
reaction of arylboronic acids in water under ambient conditions. An efficient reaction environment is
DOI: 10.1039/c4ra05230g
generated using a combination of Pd(OAc)
2
and ‘green additives’, which exhibited excellent activity and
www.rsc.org/advances
results in high yields of the desired coupled products within 15 minutes.
reported to catalyze C–C coupling reactions. Attempts to conduct
Introduction
10
11
this reaction in water report the use of PTC, an added oxidant,
12
13
Owing to accelerating environmental concerns, the present era sulphonyl chloride additives, and ligands. Recently, Wu et al.
is inducing the search of greener alternatives to chemical reported the reaction using 6 mol% Pd(OAc) simply in presence
processes both for academic and industrial purposes. The idea of K CO (base) and water (solvent). However, the reaction
of green technology is hence, receiving momentous thought. required a long time span of 24 h. In contrast, our newly
Minimization in the use of organic solvents and subsequent developed protocol requires only 1 mol% of Pd(OAc) for the
2
2
3
14
2
exploration of aqueous biphase catalysis, supercritical uids as effective transformation of arylboronic acids to their corre-
well as ionic liquids as reaction media are emerging areas of sponding biaryls within 15 minutes under ambient conditions.
1
research. However, the selection of water as a reaction medium
is undoubtedly the best option as it is readily available, cost
effective and most importantly environmentally benign. There
Results and discussion
are a few major drawbacks to this approach, which include poor
We started our investigation to know the effect of phase transfer
substrate solubility, poor yield, side products and prolonged
catalysts on the homocoupling of phenylboronic acid in water.
reaction times. These predicaments are oen solved either by
In a typical reaction, an equivalent mixture of phenylboronic
using water soluble reagents, additives and/or phase transfer
acid and PTC was treated with 2 equiv. of potassium carbonate
2
catalysts (PTC), which are themselves not green.
(
K CO ) and 3 mol% of Pd(OAc) in water for 5 h under ambient
2 3 2
Our main effort was directed towards C–C bond forming
reactions. Without deviating from the main objective of a green
conditions (Scheme 1).
3
Both TBAB (tetrabutylammonium bromide) and TBAHS
tetrabutylammonium hydrogen sulphate) gave disappointing
protocol, herein, we disclose a new class of green additives that
effectively enhance the yield and reaction rate of the homocou-
pling reaction of arylboronic acids in water. Biaryl compounds
serve as imperative motifs in natural, biologically active, phar-
maceutically and agrochemically efficient moieties. In addition,
the widespread application of biaryls as ligands and components
in new resources make these compounds attractive man-made
targets. Several synthetic methods have been developed so far
for the construction of biaryl skeletons using Csp2–Csp2 bond
forming reactions. For the synthesis of symmetrical biaryls, the
Pd catalyzed Suzuki type homocoupling of arylboronic acids is
known to be an indispensible tool. In addition to palladium
catalysis, Au, Rh, Cu and Mn. Catalysts have also been
(
results as only a 31% and 35% yield of biphenyl were obtained,
respectively. Moreover, an equivalent amount of phenol, a
15
common side product was isolated aer the reaction. Then, we
moved to improve the reaction conditions and explore the
possibility of substituting the PTC with long chain esters. A
variety of long chain esters are commonly used as cosolvents to
enhance the solubility of many drugs in dermal and trans-
16
dermal drug delivery processes. We have brought the concept
here for the fabrication biaryls motifs.
4
5
6
7
8
9
Department of Chemistry, University of North Bengal, Darjeeling 734 013, India.
E-mail: sajal.das@hotmail.com; Fax: +91 353 269 9001; Tel: +91 353 277 6381
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra05230g
Scheme 1 Homocoupling of phenylboronic acid in the presence of
PTC.
1
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We tried the model reaction with an equivalent amount
With this development in hand, we proceeded to navigate
(
160 mL) of IPM (isopropyl myristate) in place of the phase the standard conditions for this reaction by varying reaction
transfer catalyst (Table 1). The reaction was completed within parameters like catalyst and base (Table 1). Among the various
5 minutes and resulted in an 83% yield of biphenyl (Table 1, catalysts used for the reaction, Pd(OAc) provided best results
1
2
entry 1). We then varied the additive loading and observed that with complete conversion within 15 minutes (Table 1, entry 2).
a catalytic amount of IPM (10 mL ¼ 0.06 mol%) was sufficient for Copper and iron based catalysts were also found active,
an effective transformation (Table 1, entry 5). However, 40 mL of however, they provided lower yields (Table 1, entry 14–17).
IPM (0.25 equiv.) showed the highest yield 100% (Table 1, entry Moreover, an optimization of base showed that the reaction was
2). Consequently, we checked some similar naturally occurring not dependent on the nature of the base to any great extent.
long chain green esters viz.; ethyl palmitate (EP), ethyl myristate Since, both Cs CO and K CO were equally effective for the
2
3
2
3
(EM) and ethyl oleate (EO) in place of IPM under the same reaction (Table 1, entry 18–19). Only one equivalent of base was
reaction conditions. All these esters showed excellent activity sufficient for an efficient transformation. The reaction also
and provided rather synergistic results with slight variations of occurred in the absence of base however, a large amount of
yields (Table 1, entry 6–8). This indicates that long chain ester phenylboronic acid was le unreacted providing a low yield
molecules in some way stabilize the system and facilitate the (53%) in 3 h (Table 1, entry 20). Gradual addition of IPM
reaction. We then performed the reaction in neat IPM and increased the substrate to product ratio. With an equivalent
observed that no reaction occurred in 15 minutes. The higher amount of IPM, 81% yield of biphenyl product was obtained in
yield (100%) with 40 mL of IPM when compared to 160 mL of IPM 3 h in the absence of a base. It was observed that decreasing the
(83%), suggested the optimum amount of the additive required amount of Pd catalyst to 1 mol% did not record any effect on the
for effective transformation of arylboronic acid to its corre- reaction performance (Table 2). Pd(OAc)2 recorded excellent
sponding biphenyl. Subsequently, we performed the reaction in activity reporting a 85% yield with just 0.1 mol% catalytic
the presence of long chain acid, stearic acid and obtained an loading (Table 2, entry 5).
88% yield in 1 hour (Table 1, entry 9). The sodium salt of
Subsequently, we performed the homocoupling reaction of
myristic acid was found to be less effective and furnished a 72% various arylboronic acids using the optimized reaction condi-
yield in 1 hour (Table 1, entry 10). To assure the requirement of tions (Table 3). All the arylboronic acids smoothly reacted to
a long hydrocarbon chain containing moiety as an additive we provide the desired homocoupling products. Arylboronic acids
performed the reaction with ethyl acetate as an additive. A poor with electron donating groups gave considerably good results.
conversion of 10% was observed in 1 hour (Table 1, entry 11).
4-Methoxyphenylboronic acid and 3-methylphenylboronic acid
a
Table 1 Optimization of homocoupling reaction
b
Entry
Catalyst
Base
Additive
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
2
2
2
2
2
2
2
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
IPM (160 mL)
IPM (40 mL)
IPM (30 mL)
IPM (20 mL)
IPM (10 mL)
EM (40 mL)
EO (40 mL)
EP (40 mL)
Stearic acid (0.25 equiv.)
Sodium myristate (0.25 equiv.)
Ethyl acetate (0.25 equiv.)
IPM (40 mL)
IPM (40 mL)
IPM (40 mL)
IPM (40 mL)
IPM (40 mL)
IPM (40 mL)
IPM (40 mL)
15
15
15
15
15
15
15
15
60
60
60
60
60
60
60
60
60
15
15
180
180
83
100
99
97
98
99
98
99
88
72
10
94
58
14
—
10
11
12
13
14
15
16
17
18
19
20
21
PdCl
Pd/C
2
c
CuSO
4
$5H
2
O
Ni(OAc)
Fe(III)acetylacetonate
Fe(II)triuoromethanesulphonate
2
$4H
2
O
c
—
51
100
99
53
81
d
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
2
2
2
d
Cs
Nil
Nil
2
CO
3
IPM (40 mL)
IPM (40 mL)
IPM (160 mL)
a
b
c
Reaction conditions: phenylboronic acid (0.5 mmol), catalyst (3 mol%), base (2 equiv.), water (3 mL), room temperature. HPLC yields. 10 mol%
d
of catalyst was used. 1 equivalent of base was used.
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a
a
Table 2 Optimization of catalyst loading
Table 3 Homocoupling of arylboronic acids
b
Entry
1
Arylboronic acid
Yield (%)
b
Entry
Pd(OAc)
2
Base (1 equiv.)
Time (min)
Yield (%)
1
2
3
4
5
3 mol%
2 mol%
1 mol%
0.5 mol%
0.1 mol%
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
3
3
3
3
3
15
15
15
15
15
100
100
99
87
85
94
95
2
a
Reaction conditions: phenylboronic acid (0.5 mmol), Pd(OAc)
2
, base
b
(1 equiv.), IPM (40 mL), room temperature. HPLC yields.
3
4
5
93
47
77
provided the corresponding coupling products in 93% and 95%
yield, respectively (Table 3, entry 2–3). Unsubstituted phenyl-
boronic acid and napthylboronic acid also gave high yields of
94% and 97%, respectively, upon isolation (Table 3, entry 1, 6).
Again, electron decient substrates like 4-uorophenylboronic
acid, 2-chlorophenylboronic acid and 3-nitrophenylboronic
acid also performed very well and accomplished 77%, 74% and
6
7
97
73% yield, respectively (Table 3, entry 5, 7, 8). However, 4-cya-
nophenylboronic acid and 4-formylphenylboronic acid
0
provided the corresponding products 4,4 -dicyanobiphenyl and
74
73
0
0
1
,1 -biphenyl-4,4 -dicarbaldehyde in only 47% and 49% yield,
respectively (Table 3, entry 4 and 9).
0
0
The very low yield in the case of 4,4 -dicyanobiphenyl and 1,1 -
0
8
9
biphenyl-4,4 -dicarbaldehyde may be attributed to the para
positioning of the cyano and formyl groups, respectively, which
have strong negative mesomeric effects. To test the application of
our reaction system for heteroaryl substrates we conducted the
homocoupling reaction of 2-thienylboronic acid and 5-methyl-2-
thienylboronic acid. They also underwent smooth coupling to
accomplish a 57% and 59% yield, respectively, of the desired
products (Table 3, entry 10, 11). Hence, our developed protocol is
equally effective for heteroaryl systems as well.
49
57
1
0
1
1
59
A recycling experiment was performed by taking the model
homocoupling reaction of phenylboronic acid to ensure the
reusability of reaction medium. The reaction was performed
using the optimized reactions conditions. Aer the rst run, the
product was isolated by solvent extraction methods and the
reaction media used directly for the next run. No other reagents
except phenylboronic acid were added for the subsequent runs.
a
Phenylboronic acid (1 mmol), Pd(OAc)
2
(1 mol%), K
Isolated yields (column
2
CO
3
(1 equiv.),
b
IPM (0.25 equiv.), water (3 mL).
chromatography).
Five consecutive runs were tested and the corresponding results system via several interactions and thus remains in the medium
given in Fig. 1. The reaction system was found to be efficient aer solvent extraction.
with a slight decrease in yield observed from the second run
In brief, an in situ generated stimulant combining Pd(OAc)2
onwards. IPM is immiscible in water but remains in the reac- and green additives signicantly promotes the homocoupling
tion medium aer extraction of the product. The presence of reactions of arylboronic acids in water. Further, these additives are
IPM in the reaction media was detected by spectroscopic tech- capable of performing the reaction in the absence of a base and
niques. FTIR analysis of the reaction media aer the rst run are very effective compared to traditional PTC. This scenario
ꢀ1
showed the presence of C]O at 1729 cm . This fact was prompted us to investigate the role and signicance of green
further supported by NMR analysis. Most plausibly, the IPM additives in this homocoupling reaction. The homocoupling
molecules are attached with the other reagents present in the reaction of arylboronic acids performs well in water in the absence
of additives aer producing their water soluble ‘trihydroxyborate
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Fig. 2 (A) Shifting of the C]O stretching band of the ester group in
IPM (specification: a; neat IPM, b; IPM in the presence of PhB(OH) )
2
(1 : 1). (B) Gaussian deconvolution of the C]O stretching band of IPM
2
in the presence of PhB(OH) ) (1 : 1) (specification: experimental
spectra, overall fitted point (open circle) and deconvoluted curves, 1:
free C]O stretching band; 2: bound C]O stretching band.
program, and the vibrational characteristics further analyzed.
Upon deconvolution, the broad peak was found to be a combi-
nation of two different peaks (Fig. 2B). The higher frequency
Fig. 1 Recycling experiment of the reaction medium.
ꢀ1
region peak (Fig. 2B-1) at 1734 cm is similar to that of the C]O
stretching frequency of neat IPM. Whereas the new peak at 1725
salts’. However, a relatively low yield (62%) and a comparatively
longer time span of 1 hour was required in that case.
ꢀ1
cm (Fig. 2B-2) corresponds to the H-bonded C]O of IPM with
phenylboronic acid. Nevertheless, a high population (86%) of
H-bonded C]O stretching band has been observed from the
variation of Gaussian proles (area fraction) of the normalized
spectra. Hence, it may be concluded that most of the IPM
molecules are forming H-bonds with the complementary
modules of phenylboronic acid and provide a unique platform
for the homocoupling reaction of arylboronic acid.
We surmise that the presence of a small amount of long
chain ester not only resolved the solubility problem of phenyl-
boronic acid in water, it encourages substrate–catalyst
communication. The reaction is considered to be homoge-
neously proceeding around the long chain ester molecules of
IPM (Scheme 2). Plausibly, the long chain ester molecules grasp
the substrates and bring it to the proximity of the catalyst. The
17
hydrophobic and multiple hydrogen bonding interactions
between the hydrophobic and hydrophilic branch of the esters
and complementary modules of substrates might be respon-
sible for such type of special arrangement.
Experimental
General comments
The presence of H-bonding interactions has been further Unless stated otherwise, all reagents such as palladium acetate,
supported by FT-IR spectroscopic analysis. IPM exhibits a sharp arylboronic acids and solvents were used as received from
ꢀ1
peak for the C]O stretching frequency at 1738 cm (Fig. 2A and commercial suppliers. NMR spectra were recorded on 300 MHz
ESI†). However, in the presence of phenylboronic acid the peak spectrometer at 298 K with calibration done on the basis of the
ꢀ1
ꢀ1
becomes broader appearing at 1729 cm . This downward shi residual solvent peak. FTIR spectra in the 400–4000 cm
and broadening nature indicates the formation of intermolecular window were recorded on a Perkin Elmer Spectrum RXI spec-
H-bonding between the C]O and the complementary module of trometer (USA) (Absorbance mode) with 100 scans and spectral
18
ꢀ1
phenylboronic acid. Then, the observed broad band was tted resolution of 4 cm at 303 K. Deconvolution of the spectra has
as a sum of the Gaussian with the help of a Gaussian curve tting been performed with the help of a Gaussian curve tting
program (Origin soware). The products were puried using
column chromatography on silica gel (60–120 mesh). Ethyl
ꢁ
acetate and petroleum ether (60–80 C) were used as eluents.
The progress of reaction was monitored using silica gel TLC.
General procedure for the synthesis of biphenyls
In a 25 mL round bottom ask 1 mmol of aryl boronic acid was
added. 1 mmol of K CO , 0.25 mmol of IPM and 3 mL of water
2 3
were added to the reaction ask. The reaction mixture was
stirred at room temperature for 15 minutes. The reaction
mixture was poured into water and the organic layer extracted
with DCM. The organic layer was dried by passing over
anhydrous Na SO and concentrated under reduced pressure.
2
4
The crude material was further puried by column chroma-
Scheme 2 Schematic representation of the homocoupling reaction in
the presence of IPM.
tography using petroleum ether as eluent.
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Recycling of the reaction medium
can effectively support a smooth homocoupling of a range of
arylboronic acids in water and aerobic conditions at room
temperature. The reaction uses a low catalyst loading and is
completed within a short reaction time of 15 minutes.
Substrates bearing electron rich and electron withdrawing
substituents are well tolerated in the reaction. Hetero-
arylboronic acids also performed well under the reaction
conditions. Interestingly, a class of new green additives stand as
To the water portion le aer solvent extraction, phenylboronic
acid (1 equiv.) was added and the reaction performed at room
temperature for 15 minutes. No added base or catalyst was
required for the four subsequent runs.
Analytical data for biaryl products
19
ꢁ
1
(
1) Biphenyl. White solid; mp 68–71 C; H NMR (300 MHz, a relevant parameter for the design and development of cata-
): d 7.30–7.35 (m, 2H), 7.39–7.45 (m, 4H), 7.56–7.59 (m, 4H); lysts for homocoupling reactions in water. We are presently
C NMR (75 MHz, CDCl ): d 127.26, 127.34, 128.85, 141.31.
investigating the possibility of broadening this concept to
2) 3,3 -Dimethylbiphenyl. Colourless liquid; bp 286 C; advanced catalytic process as a future outlook.
H NMR (300 MHz, CDCl
): d 2.406 (s, 6H), 7.14 (d, J ¼ 7.5 Hz,
H), 7.28–7.33 (m, 2H), 7.37–7.37 (m, 4H); C NMR (75 MHz,
CDCl
3
1
3
3
0
19
ꢁ
(
1
3
1
3
2
Acknowledgements
CDCl ): d 21.61, 124.34, 127.97, 128.03, 128.66, 138.30, 141.40.
3
0
19
ꢁ
(
3) 4,4 -Dimethoxybiphenyl. White solid; mp 177–179 C; We thank the Council of Scientic & Industrial Research, New
H NMR (300 MHz, CDCl ): d 1.537 (s, 6H), 6.93–6.98 (m, 4H), Delhi for nancial support (02(0022)/11/EMR-II).
1
3
1
3
7
1
3
.45–7.50 (m, 4H); C NMR (75 MHz, CDCl ): d 55.36, 114.18,
27.76, 133.48, 158.70.
References
0
8
ꢁ
1
(
4) 4,4 -Dicyanobiphenyl. White solid; mp 233–234 C; H
NMR (300 MHz, CDCl ): d 7.68–7.72 (m, 4H), 7.77–7.81 (m, 4H);
C NMR (75 MHz, CDCl ): d 112.44, 118.44, 127.95, 132.91,
3
3
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1
3
1
43.53.
0
8
ꢁ
(
5) 4,4 -Diuorobiphenyl. Pale yellow solid; mp 88–90 C;
1
H NMR (300 MHz, CDCl ): d 7.08–7.15 (m, 4H), 7.45–7.52 (m,
3
1
3
4
H); C NMR (75 MHz, CDCl
3
): d 115.70 (d, J ¼ 21 Hz), 128.58
(
d, J ¼ 8 Hz), 136.39 (d, J ¼ 3 Hz), 162.42 (d, J ¼ 245 Hz).
0
20
ꢁ
1
(
6) 1,1 -Binapthyl. Colourless solid; mp 141–143 C; H
NMR (300 MHz, CDCl ): d 7.16–7.23 (2H, m), 7.30–7.37 (2H, m),
.39–7.43 (4H, m), 7.46–7.53 (2H, m), 7.85–7.88 (4H, m);
NMR (75 MHz, CDCl ): 124.34, 124.76, 124.93, 125.51, 126.78,
3
1
3
7
C
3
1
26.84, 127.10, 131.79, 132.46, 137.40.
0
20
ꢁ
1
(
7) 2,2 -Dichlorobiphenyl. White solid; mp 59–62 C; H
NMR (300 MHz, CDCl ): d 7.27–7.29 (m, 2H), 7.32–7.35 (m, 4H),
3
1
3
3
7.47–7.50 (m, 2H); C NMR (75 MHz, CDCl ): d 126.50, 129.23,
1
29.44, 131.18, 133.52, 138.36.
(
0
21
ꢁ
1
8) 3,3 -Dinitrobiphenyl. Yellow solid; mp 201–202 C; H
NMR (300 MHz, CDCl
): d 7.71 (t, J ¼ 8.0 Hz, 2H), 7.96–8.0 (m,
H), 8.29–8.33 (m, 2H), 8.51 (t, J ¼ 2.1 Hz, 2H); C NMR (75
MHz, CDCl ): d 122.13, 123.32, 130.32, 133.09, 140.34, 148.88.
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3
1
3
2
3
0
0
20
(
9) 1,1 -Biphenyl-4,4 -dicarbaldehyde. White solid; mp
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ꢁ
1
1
7
1
46–148 C; H NMR (300 MHz, CDCl ): d 7.79–7.82 (m, 4H),
3
1
3
3
.98–8.02 (m, 4H), 10.09 (s, 2H); C NMR (75 MHz, CDCl ): d
28.03, 130.36, 136.00, 145.56, 191.73.
0
19
ꢁ
1
(
10) 2,2 -Dithiophene. White solid; mp 33–34 C; H NMR
300 MHz, CDCl
): d 7.00–7.03 (m, 2H), 7.18 (dd, J ¼ 3.6 Hz, J ¼
.2 Hz, 2H), 7.22 (dd, J ¼ 5.1 Hz, J ¼ 0.9 Hz, 2H); C NMR (75
MHz, CDCl ): d 123.77, 124.36, 127.78, 137.40.
(
1
3
1
3
3
0
0
22
(
ꢁ
11) 5,5 -Dimethyl-2,2 -dithiophene. Yellow solid; mp 65–
1
6
2
1
8 C; H NMR (300 MHz, CDCl
H), 6.86–6.87 (m, 2H); C NMR (75 MHz, CDCl ): d 15.31,
3
3
): d 2.46 (m, 6H), 6.62–6.63 (m,
1
3
22.87, 125.74, 135.51, 138.48.
7
T. Volgler and A. Studer, Adv. Synth. Catal., 2008, 350, 1963–
967.
G. Cheng and M. Luo, Eur. J. Org. Chem., 2011, 2519.
1
Conclusions
8
¨
In summary, we have reported an efficient reaction environ-
ment using a combination of Pd(OAc) and green additives that
9 A. S. Demir, O. Reis and M. Emrullahoglu, J. Org. Chem.,
2003, 68, 578.
2
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41050 | RSC Adv., 2014, 4, 41045–41050
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