Highly effective alternative aryl trihydroxyborate salts for a ligand-free, on-water Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1039/c0gc00122h

Aryl trihydroxyborate salts of sodium, an easily accessible and stable alternative source of organoboron species, can efficiently promote Pd-catalyzed ligand-free, on-water Suzuki-Miyaura (SM) coupling reactions at ambient temperature.

Full text of DOI:10.1039/c0gc00122h

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www.rsc.org/greenchem | Green Chemistry
Highly effective alternative aryl trihydroxyborate salts for a ligand-free,
on-water Suzuki–Miyaura coupling reaction
Basudeb Basu,* Kinkar Biswas, Sekhar Kundu and Sujit Ghosh
Received 18th May 2010, Accepted 9th July 2010
DOI: 10.1039/c0gc00122h
Aryl trihydroxyborate salts of sodium, an easily accessible and stable alternative source of
organoboron species, can efficiently promote Pd-catalyzed ligand-free, on-water Suzuki–Miyaura
(SM) coupling reactions at ambient temperature.
Efforts have been made to overcome the problem by introducing
Introduction
phase transfer catalysts,15 water soluble salts of reagents16 or
The seminal paper of Miyaura, Yamada and Suzuki1 laid
the foundation of one of the most important and useful
catalysts17 or carrying out the reaction in aqueous buffer.18
Two types of water-soluble organoborate salts viz. potassium
methods for the construction of carbon–carbon bonds, in
aryl trifluoroborates16a–d and sodium aryl trihydroxyborates,16e,f
particular for the formation of unsymmetrical biaryls. Despite
which are easy to prepare, store and handle, have been employed
other alternative approaches for C–C bond formation such
in Pd-catalyzed cross-couplings with aryl halides. Yet, despite
as Kharash coupling,2 Negishi coupling,3 Stille coupling,4
some positive features of using aryl trihydroxyborate salts, aque-
Hiyama coupling,5 and Kumuda coupling,6 the Suzuki–Miyaura
ous SM coupling usually requires elevated temperatures, organic
(SM) coupling reaction has arguably received much more
co-solvents, ligand-based Pd-catalysts, high catalyst loadings
popularity due to stability, commercial availability and ease of
and/or tedious work-up. In this paper we present an ambient
handling of the organoboron compounds. The Suzuki–Miyaura
on-water protocol for the SM coupling reaction of a wide
coupling has found widespread applications in academic lab-
range of aryl halides (I, Br or Cl) including heteroaryl halides
oratories, fine chemical industries, synthesis of biologically
with different sodium aryl trihydroxyborates. Our observations
active pharmaceuticals, as well as in the burgeoning area of
practically constitute an efficient, mild, ligand-free method for
nanotechnology, as reflected from contributions from myriad
the SM coupling reactions in water at ambient temperature
research groups.7 For example, Losartan, an antihypertensive
by using aryl trihydroxyborate salt as one of the coupling
drug,8a CI-1034, a potent endothelian receptor antagonist,8b CE-
partners (Scheme 1). This paper also reports successful extension
178,253 benzenesulfonate, a CB1 antagonist for the treatment of
of the procedure through the use of polymer-supported Pd-
obesity8c or apoptolidin A, a potent antitumor agent8d have been
catalyst (ARF–Pd), a heterogeneous Pd-catalyst developed by
synthesised on a large scale employing the SM coupling as a key
our group,19 covering the essential aspects of green chemistry.
step. Similarly, benzimidazole derivatives bearing substituted
Furthermore, we have demonstrated modular synthesis of
biphenyl moieties, potential inhibitors of hepatitis C virus, have
pharmaceutically important benzimidazole- and benzotriazole-
been prepared using the SM coupling reaction.9 Review articles
based biphenyl scaffolds using an alternative water-soluble
by Danishefsky et al.10 and Nicolaou et al.11 amply demonstrate
various applications of the SM coupling reaction in the synthesis
of natural products.
sodium organoborate salt.
Results and discussion
In recent years, amelioration of the SM coupling reaction has
been directed towards the more efficient, economic and greener
techniques, especially in respect of Pd-catalyst, requirement
of base and carrying out the reaction in water or in the
absence of any solvent.12 Recent trends in organic synthesis
involve reactions under solvent-free or on-water conditions to
obtain the target molecule in a cleaner and environmentally
benign way.13 Although many organic reactions are facilitated
in aqueous media, some reactions proceed very slowly because
of poor solubility of the substrate/reagents in water. In the
case of SM couplings, hydrophobic aryl boronic acids often
show very slow and/or incomplete conversions along with the
difficulty to isolate the products from the reaction mixture.14
Preliminary optimization of the SM coupling reactions was car-
ried out using 3-iodoanisole and phenyltrihydroxyborate with
the aid of 0.5 mol% Pd(OAc)2 (Table 1). The phenyl trihydroxyb-
orate salt was prepared following the reported procedure,16e and
used directly without further purification. Investigations using
different solvents revealed that the coupling is unsuccessful in
toluene (Table 1 entry 1), partly successful in dioxane (Table 1,
entry 2) but worked efficiently in DMF (Table 1, entry 3). On
switching over to aqueous media, it was found that a mixture
of acetone–water also worked efficiently within 8 h under mild
conditions (Table 1, entry 4). However, carrying out the reaction
in only water resulted in the formation of the biphenyl derivative
in 38% yield (Table 1, entry 5), which may be attributed to
the poor solubility of aryl iodide in water. To overcome this
shortcoming, we decided to use tetrabutylammonium bromide
(TBAB), a phase transfer reagent, in an equimolar amount.
Department of Chemistry, North Bengal University, Darjeeling, 734 013,
India. E-mail: basu_nbu@hotmail.com; Fax: 91 353 2699001; Tel: 91
353 2776381
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Scheme 1
◦
Table 1 Optimization of reaction conditions for the SM coupling using
3-iodoanisole and phenyltrihydroxyborate
aryl chlorides at 150–175 C in aqueous media indicating that
aryl chlorides are very sluggish towards the SM coupling reac-
tion and require relatively higher temperature, longer reaction
time and/or the presence of electron-withdrawing groups. We
examined aryl chlorides bearing nitro or acetyl groups, which
however afforded the desired coupled products in excellent
yields at refluxing temperatures (100 ◦C) (Table 2, entries 16–
17). Changing the coupling partner phenyltrihydroxyborate with
m-tolyltrihydroxyborate and p-anisyltrihydroxyborate did work
efficiently with bromo and iodoarenes (Table 2, entries 18–22
and 24). The SM coupling reaction with heteroaryl halides
was also successful. For example, 3-bromoquinoline or 2,6-
dibromopyridine gave the desired coupled products in 66% and
83% yields respectively (entries 22–23), while similar coupling
of 2-iodothiophene with p-anisyltrihydroxyborate afforded the
corresponding unsymmetrical biphenyl in 92% yield within 3 h
(Table 2, entry 24).
Entry
Solvent
Temperature
Time
% of Yielda
1
2
3
4
5
6
7
Toluene
Dioxane
DMF
100 ◦
RT
RT
RT
RT
RT
RT
C
8 h
24 h
4 h
8 h
4 h
4 h
8 h
00
45
96
93
38
92
50
Acetone : water
Water
Waterb
Waterc
a Isolated yields after purification by column chromatography on silica.
b 1 equiv. of TBAB was added. c 0.5 equiv. of TBAB was added. All
reactions were carried out using 0.5 mol% Pd(OAc)2.
Recently, we developed a new Pd-catalyst (where Pd was
immobilized onto ion-exchange resins), designated as ARF–
Pd, which was successfully applied to Heck, Suzuki–Miyaura
This led to the formation of the desired unsymmetrical biphenyl
within 4 h at room temperature in 92% yield (Table 1, entry 6). An
experiment with 0.5 equivalents of TBAB, however, afforded the
desired product only in 50% yield, even after 8 h (Table 1, entry
7). It was revealed that polar protic or aprotic solvents are good
enough to effect the SM coupling at room temperature. Thus,
the optimized reaction conditions are: 0.5 mol% of Pd(OAc)2
and 1 equivalent of TBAB in water at room temperature.
† Spectral data of selected biphenyls: 3-Methoxy biphenyl (liquid);
Table-2, Entry-1: IR (film): nmax 1574, 1610 cm-1. 1H NMR (CDCl3,
d ppm-1 relative to TMS): 3.75 (3H, s, –OCH3); 6.77–6.81 (1H, m,
aromatic proton); 7.03–7.10 (2H, m, 2 aromatic protons); 7.21–7.36
(4H, m, all aromatic protons); 7.47–7.51 (2H, m, 2 aromatic protons).
13C NMR (CDCl3, d ppm-1): 55.2 (OCH3); 112.6; 112.8; 119.6; 127.1;
127.4; 128.7; 129.7; 141.0; 142.7; 159.9 (aromatic carbons). 2-Methoxy
biphenyl (liquid); Table-2, Entry-3: IR (film): nmax 1504, 1597 cm-1. 1H
NMR (CDCl3, d ppm-1 relative to TMS): 3.79 (3H, s, –OCH3); 6.96–7.05
(2H, m, 2 aromatic protons); 7.29–7.42 (5H, m, all aromatic protons);
7.51–7.54 (2H, m, 2 aromatic protons). 13C NMR (CDCl3, dPpm-1):
55.54 (OCH3); 111.2; 120.8; 126.9; 127.9; 128.6; 129.5; 130.7; 130.8;
138.5; 156.5 (aromatic carbons). 3,4¢-Dimethyl biphenyl (liquid); Table-
2, Entry-19: IR (film): nmax 1588, 1606 cm-1. 1H NMR (CDCl3, d ppm-1
relative to TMS) 2.390 (6H, s, CH3); 7.13–7.50 (8H, m, 8, all aromatic
protons). 13C NMR (CDCl3, d ppm-1): 21.3 (CH3); 124.1; 127.0; 127.7;
127.8; 128.6; 129.4; 136.9; 138.2; 138.5; 141.1 (aromatic carbons). 3-
Methoxy 3¢-methyl biphenyl (liquid); Table-2, Entry-20: IR (neat): nmax
1593 cm-1. 1H NMR (CDCl3, d ppm-1 relative to TMS): 2.41 (3H, s,
CH3); 3.86 (3H, s, –OCH3); 7.11–7.39 (8H, m, all aromatic protons).13C
NMR (CDCl3, d ppm-1): 21.5 (CH3); 55.3 (OCH3); 112.6; 112.9; 119.7;
124.3; 128.0; 128.1; 128.6; 129.6; 138.3; 141.1; 142.9; 159.9 (aromatic
carbons). 3-(3-Methyl phenyl) quinoline (liquid); Table-2, Entry-22: IR
(film): nmax 1580, 1606 cm-1. 1H NMR (CDCl3, d ppm-1 relative to TMS):
1.59 (3H, s, CH3); 6.36–6.87 (6H, m, 6 aromatic protons); 7.00 (1H, d,
J = 8.1 Hz, aromatic proton); 7.28 (1H, d, J = 8.4 Hz, aromatic proton);
7.43 (1H, s); 8.3 (1H, s). 13C NMR (CDCl3, d ppm-1): 21.6 (CH3); 124.5;
127.1; 128.0; 128.1; 128.2; 128.9; 129.0; 129.1; 129.4; 133.4; 134.0; 137.7;
138.9; 147.1; 149.8 (aromatic carbons). 2-(4-Methoxy phenyl) thiophene;
Table-2, Entry-24: mp 106 ◦C; IR (KBr): nmax 1500, 1533, 1606 cm-1. 1H
NMR (CDCl3, d ppm-1 relative to TMS): 3.81 (3H, s, –OCH3); 6.91
(2H, d, J = 9 Hz, 2 aromatic protons); 7.03–7.25 (3H, m, all aromatic
protons); 7.53 (2H, d, J = 8.7 Hz, 2 aromatic protons). 13C NMR (CDCl3,
d ppm-1): 55.3 (OCH3); 114.3; 122.1; 123.8; 127.2; 127.3; 127.9; 144.3;
159.2 (aromatic carbons).
After identification of the optimal conditions, the scope and
limitations of the reaction were examined. Initially, we applied
these reaction conditions to the coupling of various functional-
ized aryl iodides with the sodium salt of phenyltrihydroxyborate
in water. The results are presented in Table 1. Aryl iodides
bearing different substituents such as OMe, Me, NH2, F and
I underwent smooth SM coupling affording the corresponding
unsymmetrical biphenyls in 84–94% yields (Table 2, entries 1–
7). Mechanistically, the oxidative addition of aryl halides to
palladium(0) depends on the nature of halogens and occurs in
the descending order of I > Br > Cl.20 We therefore examined
the couplings of other aryl electrophiles bearing bromide and
chloride. Several aryl bromides including di- and tribromoarenes
were found to give the corresponding unsymmetrical biaryls
in good to excellent yields (Table 2, entries 8–13). While p-
bromoacetophenone showed a faster rate of reaction (2 h)
(Table 2, entry 9), 2,4,6-tribromophenol required a longer time
(24 h) (Table 2, entry 13) for the coupling reaction, which may be
due to the presence of the electron-withdrawing acetyl group in
the former example. Thus, aryl iodides and bromides underwent
easy coupling with phenyl trihydroxyborate. A similar reaction
with aryl chloride was not successful even after heating the
reaction mixture at 100 ◦C for 24 h (Table 2, entries 14–15). Lead-
beater et al.18a reported the microwave-assisted SM coupling of
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and Sonogashira coupling reactions.19 To extend further, we
employed the heterogeneous Pd-catalyst (ARF–Pd) replacing
Pd(OAc)2. Indeed, trihydroxyborate salts were found to be
equally active in SM coupling reactions in the presence of
a catalytic amount of ARF–Pd. The results are presented in
Table 3. In all the cases, the ARF–Pd was separated by filtration
and the desired products were obtained after chromatographic
purification in excellent yields (85–93%) (Table 3, entries 1–5).
As shown above, water-soluble sodium salts of aryl trihy-
droxyborates have proven to be highly effective in SM coupling
reactions in water at ambient temperatures. Low loading of
the Pd-catalyst (direct use of Pd(OAc)2 or polymer-bound Pd)
and absence of any phosphine ligands are notable features to
mention. Having established a general, mild, aerobic and on-
water protocol for the SM coupling reactions using aryl trihy-
droxyborate salts, we probed the utility of this protocol in mod-
ular synthesis of pharmaceutically important benzimidazole-
and benzotriazole-based biphenyl scaffolds. Thus, compounds 2
and 3 were synthesized from compounds 1a and 1b respectively
(Scheme 2), where the SM couplings were efficiently performed
using sodium phenyltrihydroxyborate in a mixture of DMF–
H2O (2 : 1).
Table 2 On-water SM coupling reactions with sodium aryl trihydrox-
yborates using 0.5 mol% of Pd(OAc)2
Conclusions
In summary, our studies have established that easily accessible
and air-stable sodium aryl trihydroxyborates can be effectively
used as an alternative source of organoboron species in ligand-
free Pd-catalyzed SM cross-coupling reactions in water under
an aerobic atmosphere and at room temperature. The protocol
has been found to be broadly applicable to a variety of aryl
halides (X = Br, I) and also to aryl chlorides bearing electron-
withdrawing groups. It is further shown to be effective with
heterogeneous Pd-catalysts and also extended to the modular
synthesis of some pharmaceutically important benzimidazole-
and benzotriazole-based biphenyl scaffolds.
Experimental
General procedure for Suzuki–Miyaura coupling
A mixture of 3-iodoanisole (468 mg, 2 mmol), sodium phenyltri-
hydroxyborate (354 mg, 2.2 mmol), Pd(OAc)2 (2.2 mg, 0.5 mol%)
and TBAB (644 mg, 2 mmol; 1 equiv) was taken in water (5 mL).
The mixture was magnetically stirred at room temperature for
several hours (see Table 2). After the reaction was complete
(monitored by tlc), the mixture was extracted with ether (3
¥ 20 mL). The combined organic layer was then washed with
brine (10 mL), dried (anhydrous Na2SO4), and evaporated. The
residue was purified on a short column of silica using light
petroleum as the eluent to afford the desired unsymmetrical
biphenyl (338 mg, 92%); liquid.
Synthesis of compounds 2 and 3
A mixture of 1-(4-iodobenzyl)-1H-benzo[d]imidazole (334 mg,
a
1
mmol) or 1-(4-iodobenzyl)-1H-benzo[d][1,2,3]triazole
Aryl halide and arylboronic acid salt used in 1 : 1.1 ratios for
mono-coupling. b Isolated yields after purification by column
chromatography on silica.†
(335 mg, 1 mmol) and sodium salt of phenyltrihydroxyborate
(177 mg, 1.1 mmol), ARF–Pd (300 mg, 0.94 mol% of Pd)
and TBAB (322 mg, 1 mmol) was taken in a DMF–water
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Table 3 SM coupling reactions with aryl trihydroxyborates in water using heterogeneous Pd-catalyst (ARF–Pd)
Entry
1
Aryl halidesa
Sodium trihydroxyborate
Temp.
RT
Time/h
5
Product
Yieldb (%)
85
2
3
RT
5
4
88
92
100 ◦
C
4
5
100 ◦
100 ◦
C
C
3
5
93
87
a 300 mg ARF-Pd (0.94 mol% Pd) was used. b Isolated yields after purification by column chromatography on silica.
Scheme 2 Conditions: a1a or 1b (1 mmol), PhB(OH)3Na (1.1 mmol) in DMF–H2O (2 : 1; 3 mL), Pd(OAc)2 (1.1 mg, 0.5 mol%), 100 ◦C for 24 h.
mixture (2 : 1; 3 mL). The mixture was heated at 100 ◦C for
24 h. After completion of the reaction (monitored by tlc), the
mixture was extracted with ethyl acetate (2 ¥ 20 mL). The
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Acknowledgements
Weare grateful totheDepartment of Science &Technology, New
Delhi for financial support (Grant No. SR/S1/OC–49/2006).
KB and SG thank CSIR, New Delhi for awarding junior
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