An efficient and new protocol for phosphine-free Suzuki coupling reaction using palladium-encapsulated and air-stable MIDA boronates in an aqueous medium

Article abstract of DOI:10.1039/c4ra03586k

A simple methodology that uses a system based on polyurea microencapsulated palladium (PdEnCat 30) and aryl or (2-pyridyl) MIDA boronates for Suzuki-Miyaura cross-coupling reactions of (hetero)aryl halides in water-alcohol under phosphine-free conditions was developed. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03586k

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An efficient and new protocol for phosphine-free
Suzuki coupling reaction using palladium-
encapsulated and air-stable MIDA boronates in an
aqueous medium
Cite this: RSC Adv., 2014, 4, 28148
Joaquim Fernando Mendes da Silva, a Andres Felipe Yepes Perezab and Natalia Pinto
*
´
de Almeidaa
Received 20th April 2014
Accepted 13th June 2014
A simple methodology that uses a system based on polyurea microencapsulated palladium (PdEnCat 30™)
and aryl or (2-pyridyl) MIDA boronates for Suzuki–Miyaura cross-coupling reactions of (hetero)aryl halides
in water–alcohol under phosphine-free conditions was developed.
DOI: 10.1039/c4ra03586k
www.rsc.org/advances
Suzuki–Miyaura cross-coupling reactions of highly challenging
aryl and heteroarylchlorides, bromides or iodides.
Introduction
Also, as part of our ongoing interest in palladium-catalyzed
reactions, we describe the Suzuki–Miyaura cross-coupling
reaction of aryl(heteroaryl)halides with the corresponding
MIDA boronates catalyzed by an air-stable and available palla-
dium-encapsulated catalyst (Pd EnCat30™). PdEnCat catalyst
signicantly facilitates these reactions, beneting from a much
simplied workup procedure: the heterogeneous catalyst is
readily removed by ltration.
Therefore, these transformations are oen mediated by Pd
species in the presence of phosphine ligands, and this may
result in difficulties associated with the removal of homoge-
neous material from reaction mixtures. We have developed an
effective methodology that use phosphine-free supported Pd
catalyst entrapped in a polyurea matrix, with potential for both
economic and environmental advantages. PdEnCat 30™ have
proven to be effective as recyclable catalysts for use in Suzuki
type cross-coupling reactions and do not require expensive and
toxic phosphines as co-ligands.11–13
On the other hand, due to their signicant and varied bio-
logical activities, the design and development of novel methods
for the construction of 2-thienyl, 2-pyridil, 3-pyridil, 4-pyridil and
2-phenyl functionalized pyridines have drawn remarkable
interest from the synthetic, organic as well as medicinal chem-
ists.14 2-(20-Thienyl)pyridine derivatives have been used
frequently to obtain complexes with several transition metals
with a wide range of applications in solar energy conversion
devices as photocatalysts, photosensitizers and electrolumines-
cent emitters.15 Also, derivatives of 2-thienylpyridine skeleton
exhibited considerable topoisomerase I and II inhibitory activity16
and are being used in the design of various anticancer agents.17
Secondly, phenyl pyridines have emerged as the integral back-
bone of several potent azapeptide HIV protease inhibitors with
anti HIV activity such as Atazanavir (BMS-232632), an orally
The palladium catalyzed Suzuki–Miyaura cross-coupling reac-
tion is one of the most powerful and convenient approaches for
the synthesis of biaryl compounds.1 Palladium-catalyzed cross-
coupling processes for carbon–carbon bond formation of (het)
aryl halides have become an indispensable tool for synthetic
and medicinal chemists in their search for new derivatives with
wide-ranging therapeutic potential.2 During the last two
decades, there has been an increasing interest in the use of
water as solvent for many homogeneously catalyzed reactions.3
Cost, environmental benets and safety are among the reasons
most oen argued to justify the replacement of organic solvents
by water in organic transformations. The use of water in Pd-
catalyzed cross-coupling reactions goes back to the early
development of Suzuki–Miyaura cross-coupling reactions.4
On another hand, although boronic acids can serve as
excellent building blocks for the synthesis of a wide range of
natural products, pharmaceuticals, and materials,5 some of the
potentially most useful boronic acids, including 2-heterocy-
clic6–8 derivatives, are inherently unstable, which can signi-
cantly limit their benchtop storage and/or efficient cross-
coupling. N-Methyliminodiacetic acid (MIDA) boronates9,10
represent the rst general solution to this problem by virtue of
their stability and remarkable capacity for in situ slow release of
unstable boronic acids. We herein report the use of (aryl) or
(2-pyridyl) N-methyliminodiacetic acid (MIDA) boronate for
a
´
´
Instituto de Quımica, Universidade Federal do Rio de Janeiro, Rua Helio de Almeida
´
´
40, Polo de Xistoquımica Prof. Claudio Costa Neto, PXQmed
A 107, Cidade
´
Universitaria, Rio de Janeiro, CEP 21941-614, Brazil. E-mail: joaquim@iq.ufrj.br;
andre_yepes@hotmail.com; Fax: +55 21 2560 2069
b
´
Facultad de Ciencias Fısicas Exactas y Naturales Universidad de Santander (UDES).
´
´
´
Centro Investigacion en Ciencias Quımicas y Tecnologıas Sostenibles-QUIMISOST
´
Cidade Universitaria, Campus Lagos del Cacique Bucaramanga, Colombia. E-mail:
andre_yepes@hotmail.com; Fax: +57 7 6516492
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occurrence of the debrominated or homocoupling side prod-
ucts, while the formation of black palladium was not observed.
In the other ratios tested, the coupling reactions were accom-
panied by different extents of hydrodehalogenation or homo-
coupling, which were isolated always as major products in these
cases where formation of black palladium was observed.
Heterocyclic compounds are of particular interest to the
pharmaceutical industry,20 but the application of heterocyclic
compounds in cross-coupling reactions remains a challenge.21
To the best of our knowledge, only a few successful examples of
aqueous-phase Suzuki–Miyaura cross-coupling reactions of
heteroaryl chlorides are known.22
Thus, there is a need for the development of efficient
procedures for the cross-coupling reactions of heteroaryl chlo-
rides with arylboronic acids in water. To carry out an econom-
ical large-scale synthesis of pharmaceutical intermediates or
other bulk ne chemicals, a low loading catalyst is needed. So
then, we nally investigated the minimum quantity of catalyst
required to achieve a fast and complete conversion. We
Fig. 1 Structure of 2-thienylpyridines, a-terpyridine and 4,6-diaryl-
2,40-bipyridine derivatives.
available azapeptide.18 Bipyridine compounds as a-terpyridine or
4,6-diaryl-2,40-bipyridine molecule can act as tridentate ligands
and form stable complexes by chelating a broad variety of tran-
sition metal ions. The numerous reports on DNA binding prop-
erty and antitumor activity of terpyridine complexes have
attracted multiple researchers.19 (Fig. 1)
Table
1 Suzuki–Miyaura reaction of 4-bromoacetophenone with
2-formylphenylboronic acid MIDA ester as a model to optimize the
conditions
One of the main objectives of eco-friendly methods in
organic chemistry is to develop cost-effective and environmen-
tally benign synthetic protocols which have become one of the
main themes of contemporary synthetic chemistry. Thus, we
developed a mild, phosphine-free single-step procedure for the
Suzuki coupling of readily available hetaryl(aryl)halides in water
medium to the corresponding biaryl, bipyridyl and thienylpyr-
idine as a part of our ongoing research in the development of
new ecofriendlyprotocols.
Volumetric
ratios
EtOH : H2O
Catalyst
loading
(mol%)
Suzuki-coupling
product (1a)
yieldb (%)
Entrya
1
2
3
4
5
6
7
8
9
10
1 : 1
2 : 1
3 : 1
4 : 1
4 : 1
4 : 1
4 : 1
4 : 1
6 : 1
8 : 1
10% mol
10% mol
10% mol
10% mol
8% mol
6% mol
2.5% mol
1% mol
(51%)
(56%)
(60%)
(76%)
(77%)
(80%)
(70%)
(67%)
(61%)
(63%)
Results and discussion
In order to optimize the reaction conditions for the Suzuki–
Miyaura synthesis we studied the coupling reaction of 4-bro-
moacetophenone with 2-formylphenylboronic acid MIDA or
6-methoxypiridylboronic acid MIDA ester (1.5 equiv.) in the
presence of K2CO3 (3.0 equiv.) as a model.
Two parameters were investigated: (1) optimal ratio of
aqueous solvent used and (2) palladium catalyst concentration
(Table 1).
10% mol
10% mol
3.0 equiv. of K2CO were used. b Isolated yield.
a
Initially, the reaction was carried out using only ethanol as
solvent, Pd EnCat30 (10 mol%) as the catalyst and K2CO3
(3 equiv.) as base, heating under reux for 3–9 h, when we
isolated the Suzuki product in very low yields, the majors
products being the homocoupling and the dehalogenation
products, alongside with the formation of black palladium.
However, yields increased substantially when signicant
volumes of water were added to the reaction. Then, we tested
Table 2 Reuse of Pd-catalyst in the Suzuki–Miyaura reaction of 4-
bromoacetophenone with 2-formylphenylboronic acid MIDA estera
several volumetric ratios of ethanol : H2O (1 : 1, 2 : 1, 3 : 1, 4 : 1, % Yield 1a
6 : 1, 8 : 1) and we found that in the reaction conditions used,
1st recycle
78
2nd recycle
77
3rd recycle
74
the highest conversion to 1 was observed when the volumetric
ratio of ethanol : H2O was 4 : 1, together with the lowest
a
Conditions: K2CO3 (3.0 mmol), EtOH : H2O (4 : 1), reux, 8 h.
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Table 3 Suzuki–Miyaura reaction of aryl (hetaryl) halides with 2-for- Table 3 (Contd.)
mylphenylboronic acid MIDA ester and two 2-pyridyl boronic acid
MIDA estera
Reaction Suzuki-coupling
time (h)
Entryc R–X
R1
product (1) yieldb (%)
Reaction Suzuki-coupling
time (h)
Entryc R–X
R1
product (1) yieldb (%)
20
(8)
1
2
3
4
5
6d
7
8
(8)
(5)
(3)
(9)
(8)
(8)
(9)
(8)
a
Reaction conditions: 1 equiv. of aryl halide (1 mmol), 1.5 equiv. of
MIDA boronate, 6 mol% Pd EnCat30, 3 equiv. of K2CO3, 0.1 M in 4 : 1
b
c
ethanol–H2O. Isolated yield. Cross-couplings were run at reux.
d
4-Bromopyridine hydrochloride was used.
compared three conditions with decreasing amounts of Pd
EnCat30 (without phosphine) and using K2CO3 as base and
ethanol : H2O (4 : 1) as solvent. Then, carrying out the coupling
reaction at palladium loadings of 10, 8, 6, 2.5 or 1 mol% always
led to the desired coupling product in similar yields aer 8 h at
reux. The best results were obtained in the presence of a 6
mol% catalyst concentration.
Additionally, we performed a study of the catalytic activity of
Pd EnCat30 for our reaction under the best conditions found
using the reaction of 4-bromoacetophenone with 2-for-
mylphenylboronic acid MIDA ester as model. The results
showed that when the catalyst was recycled 3 times at the
reaction conditions established no signicant loss of activity or
yield occurred. During these reactions, the formation of black
palladium was not observed (Table 2).
9
(5)
(9)
10
Under the optimized reaction conditions, we investigated
the reaction of some aryl halides with 2-formylphenylboronic
and 6-methoxy-2-pyridylboronic acids MIDA esters. This inves-
tigation showed that the reaction successfully occurred without
any phosphine ligand to give the corresponding Suzuki-
coupling products 1a–t in good to excellent yields (Table 3).
Then, a wide array of aryl halides reacted efficiently with
2-formylphenylboronic acid MIDA ester and 6-methoxy-2-pyr-
idylboronic acid MIDA ester respectively to provide the corre-
sponding products 1a–t. For example, 4-bromobenzaldehyde
and 4-iodonitrobenzene reacted with 2-formylphenylboronic
acid MIDA ester to yield in 90% and 98% respectively, the
coupling products 1b and 1c.
The coupling reactions of representative heteroaromatic
halides were also effective with this protocol. Under the same
conditions, 3-bromopyridine reacted efficiently with 6-methox-
ypyridylboronic acid MIDA ester within 8 h, providing 85% yield
of the desired product 1q, while, the quinoline 1s was prepared
by the reaction of 2-bromoquinoline with 6-methoxypyr-
idylboronic acid MIDA ester to give the desired product in good
yield (80%). The 2,20-bipyridine unit is a key structural feature
found in some important natural products that show antibiotic
and cytotoxic activities.23
11
12
13
14
15
16
17
18
19
(3)
(8)
(8)
(9)
(9)
(3)
(8)
(8)
(3)
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The feasibility of our strategy was demonstrated in the chromatography (hexane/ethyl acetate as eluent) to afford the
synthesis of thio-derivatives, when 2-bromothiophene was desired product 1a–t. All the compounds prepared are known
treated with 2-formylphenylboronic acid MIDA ester or compounds.24–42 The products were conrmed by comparing
6-methoxypyridylboronic acid MIDA ester (1.5 equiv.) under the the 1H NMR and mass spectral data with authentic samples
conditions already established. The reaction proceeded well to reported in the literature.
give 2-(thiophen-2-yl)benzaldehyde (1h) and 2-methoxy-6-(thio-
40-Acetyl-[1,10-biphenyl]-2-carbaldehyde (1a).24 Following the
phen-2-yl)pyridine (1t) respectively, in good yields via C–C bond general procedure for Table 3, using 4-bromoacetophenone
formation. Since most of the strategies for the synthesis of these (70 mg, 0.351 mmol), 2-formylphenylboronic acid MIDA ester
phenyl-pyridines or thienylpyridines by condensation and (110 mg, 0.422 mmol), potassium carbonate (145 mg, 1.054
substitution reactions frequently afford the desired products in mmol), Pd EnCat30 (53 mg, 0.021 mmol), 4 : 1 aq. ethanol (10
low yields and with low regioselectivity towards the 2-position, mL) were heated under reux for 8 h. Aer work-up the residue
our synthetic strategy allows the effective construction of large was puried by ash column chromatography (SiO2, 10 : 1
libraries of 2-thienyl and 2-phenylpyridinederivatives in an hexane : ethyl acetate) to provide 1a as a colorless oil (63 mg,
efficient and eco-friendly method.
80% yield). 1H NMR (400 MHz, CDCl3) d 2.67 (s, 3H), 7.43–7.61
(m, 3H), 7.63–7.72 (m, 2H), 8.02–8.09 (m, 3H), 9.97 (s, 1H); 13C
NMR (100 MHz, CDCl3) d 26.9, 128.3, 128.6, 128.7, 129.7, 130.0,
130.3, 130.5, 130.8, 133.9, 136.8, 142.9, 144.7, 191.9, 197.7.
40-Nitro-[1,10-biphenyl]-2-carbaldehyde (1b).25 Following the
general procedure for Table 3, using 4-iodonitrobenzene
(70 mg, 0.281 mmol), 2-formylphenylboronic acid MIDA ester
(88 mg, 0.337 mmol), potassium carbonate (117 mg, 0.843
mmol), Pd EnCat30 (43 mg, 0.017 mmol), 4 : 1 aq. ethanol
(10 mL) were heated under reux for 5 h. Aer work-up the
residue was puried by ash column chromatography (SiO2,
10 : 1 hexane : ethyl acetate) to provide 1b as a light yellow solid
(54 mg, 90% yield), mp 118–120 ꢁC. 1H NMR (400 MHz, CDCl3) d
7.43 (d, J ¼ 7.6 Hz, 1H), 7.57–7.67 (m, 4H), 7.72 (t, J ¼ 7.5 Hz,
1H), 8.04 (d, J ¼ 7.6 Hz, 1H), 8.34 (d, J ¼ 8.3 Hz, 1H), 9.97 (s, 1H);
13C NMR (100 MHz, CDCl3) d 123.8, 129.1, 129.3, 130.8, 131.0,
133.8, 134.1, 143.2, 145.0, 147.9, 191.2.
[1,10-Biphenyl]-2,40-dicarbaldehyde (1c).26 Following the
general procedure for Table 3, using 4-bromobenzaldehyde
(80 mg, 0.432 mmol), 2-formylphenylboronic acid MIDA ester
(170 mg, 0.648 mmol), potassium carbonate (180 mg, 1.30
mmol), Pd EnCat30 (64.8 mg, 0.026 mmol), 4 : 1 aq. ethanol
(10 mL) were heated under reux for 3 h. Aer work-up the
residue was puried by ash column chromatography (SiO2,
7 : 1 hexane : ethyl acetate) to provide 1c as a white solid (87 mg,
98% yield), mp 94–96 ꢁC. 1H NMR (400 MHz, CDCl3) d 7.27–7.74
(m, 5H), 7.98–8.08 (m, 3H), 9.98 (s, 1H), 10.11 (s, 1H); 13C NMR
(100 MHz, CDCl3) d 128.4, 128.8, 129.8, 130.7, 130.8, 133.8,
133.9, 136.0, 144.3, 144.4, 191.6, 191.8.
Conclusions
In conclusion, a convenient methodology was developed for the
palladium catalyzed Suzuki–Miyaura reaction in water. The
reaction is catalyzed by a system based on Pd- micro-
encapsulated in a polyurea matrix (Pd EnCat30), through a
simple and efficient protocol for the phosphine-free Suzuki
coupling of aryl (hetaryl) halides with arylboronic acids MIDA
esters. The advantages of this procedure are simple operation,
good to excellent yields, short reaction time, absence of phos-
phine-ligands and mild and environmentally friendly condi-
tions. A great diversity of substrates was tolerated for this
method.
Experimental procedure
General information
All reagents were purchased from Sigma Aldrich and used
without further purication. Ethanol were purchased from local
suppliers. The analyses by Gas Chromatography-Mass Spec-
trometry (CG-MS) were performed on a Shimadzu GCMS-
QP2010S with a RTX-5MS capillary column (30 m ꢀ 0.25 mm ꢀ
0.25 mm, with the stationary phase 5% diphenyl and 95%
dimethylpolysiloxane), mass detector in mode electron
ionization (EI) standard (70 eV), with helium as a carrier gas.
The 1H- and 13C NMR spectra were recorded on a Bruker AMX-
200 spectrometer operating at 200 MHz (1H) and 50 MHz (13C)
in CDCl3 with TMS as an internal standard. The 1H- and 13C
NMR spectra were processed using MESTREC/NMR.
40-Methoxy-[1,10-biphenyl]-2-carbaldehyde (1d).27 Following
the general procedure for Table 3, using 4-iodoanisol (85 mg,
0.363 mmol), 2-formylphenylboronic acid MIDA ester (142 mg,
0.540 mmol), potassium carbonate (151 mg, 1.09 mmol), Pd
EnCat30 (54 mg, 0.022 mmol), 4 : 1 aq. ethanol (11 mL) were
General method for carrying out the Suzuki-coupling (Table 3)
Under ambient atmosphere, to a 15 mL vial equipped with a stir heated under reux for 9 h. Aer work-up the residue was
bar was added the halide (1.0 mmol), 2-formylphenyl or 2-pyr- puried by ash column chromatography (SiO2, 15 : 1 hex-
idyl MIDA boronate (1.5 mmol), K2CO3 (3.0 mmol), Pd enCat30 ane : ethyl acetate) to provide 1d as a colorless oil (60 mg, 78%
(6 mol%) and ethanol : H2O (4 : 1 v/v). The reaction mixture was yield). 1H NMR (400 MHz, CDCl3) d 3.88 (s, 3H), 7.01 (d, J ¼ 8.8
stirred at reux for 3–9 h. The mixture was cooled to room Hz, 2H), 7.31 (d, J ¼ 8.8 Hz, 2H), 7.42–7.46 (m, 2H), 7.62 (t, J ¼
temperature and then was transferred to a 60 mL separatory 7.6 Hz, 1H), 8.01 (d, J ¼ 7.6 Hz, 1H), 10.0 (s, 1H); 13C NMR
funnel and was added brine (10 mL). The mixture was extracted (100 MHz, CDCl3) d 55.6, 114.2, 127.6, 127.8, 130.3, 131.0, 131.5,
with AcOEt (3 ꢀ 10 mL). The combined organics were dried over 133.7, 134.0, 145.9, 159.9, 192.9.
Na2SO4, ltered and concentrated in vacuo. The organic residue
was adsorbed onto silica gel, and then puried by column ash the general procedure for Table 3, using 4-bromotoluene
40-Methyl-[1,10-biphenyl]-2-carbaldehyde (1e).28 Following
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(75 mg, 0.44 mmol), 2-formylphenylboronic acid MIDA ester 84% yield). 1H NMR (400 MHz, CDCl3) d 7.34 (dd, J ¼ 1.0, 7.8 Hz,
(171 mg, 0.657 mmol), potassium carbonate (181 mg, 1.31 2H), 7.58 (t, J ¼ 7.6 Hz, 2H), 7.66 (dt, J ¼ 1.2, 7.6 Hz, 2H), 8.06
mmol), Pd EnCat30 (66 mg, 0.026 mmol), 4 : 1 aq. ethanol (10 (dd, J ¼ 1.0, 7.8 Hz, 2H), 9.83 (s, 2H);13C NMR (100 MHz, CDCl3)
mL) were heated under reux for 8 h. Aer work-up the residue d 191.1, 141.3, 134.7, 133.5, 131.8, 128.9, 128.7.
was puried by ash column chromatography (SiO2, 20 : 1
40-Hydroxy-20-methyl-[1,10-biphenyl]-2-carbaldehyde
(1j).32
hexane : ethyl acetate) to provide 1d as a light yellow oil (68 mg, Following the general procedure for Table 3, using 4-chloro-3-
80% yield). 1H NMR (400 MHz, CDCl3) d 2.40 (s, 3H), 7.22–7.30 methylphenol (70 mg, 0.491 mmol), 2-formylphenylboronic
(m, 4H), 7.39 (d, J ¼ 8.2 Hz, 1H), 7.48 (d, J ¼ 7.5 Hz, 1H), 7.60 (td, acid MIDA ester (192 mg, 0.73 mmol), potassium carbonate (203
J ¼ 7.5, 1.2 Hz, 1H), 8.00 (dd, J ¼ 8.2, 1.2 Hz, 1H), 9.97 (s, 1H);13
C
mg, 1.47 mmol), Pd EnCat30 (73 mg, 0.0294 mmol), 4 : 1 aq.
NMR (100 MHz, CDCl3) d 55.5, 114.4, 127.3, 127.6, 130.6, 130.9, ethanol (10 mL) were heated under reux for 8 h. Aer work-up
131.5, 133.9, 134.1, 146.0, 159.8, 192.8.
the residue was puried by ash column chromatography (SiO2,
2-(Pyridin-4-yl)benzaldehyde (1f).29 Following the general 1 : 1 hexane : ethyl acetate) to provide 1j as a white solid (81 mg,
procedure for Table 3, using 4-bromopyridine hydrochloride (95 81% yield), mp 148–150 ꢁC. 1H NMR (400 MHz, CDCl3) d 2.51 (s,
mg, 0.49 mmol), 2-formylphenylboronic acid MIDA ester 3H), 7.27–7.17 (m, 2H), 7.35 (d, J ¼ 7.4 Hz, 1H), 7.60–7.55 (m,
(192 mg, 0.734 mmol), potassium carbonate (203 mg, 1.47 2H), 7.87–7.90 (m, 2H), 9.86 (s, 1H); 13C NMR (100 MHz, CDCl3)
mmol), Pd EnCat30 (74 mg, 0.029 mmol), 4 : 1 aq. ethanol d 55.5, 114.1, 121.4, 128.6, 128.7, 132.3, 133.3, 133.6, 133.7,
(12 mL) were heated under reux for 8 h. Aer work-up the 134.8, 135.4, 139.7, 155.6, 195.9.
residue was puried by ash column chromatography (SiO2, 3 : 2
2-Methoxy-6-(4-nitrophenyl)pyridine (1k).33 Following the
hexane : ethyl acetate) to provide 1f as a light orange solid (80 mg, general procedure for Table 3, using 4-iodonitrobenzene (85 mg,
91% yield); mp 64–66 ꢁC. 1H NMR (400 MHz, CDCl3) d 7.32–7.35 0.34 mmol), 6-methoxypyridylboronic acid MIDA ester (135 mg,
(m, 2H), 7.43 (d, J ¼ 7.5 Hz, 1H), 7.60 (t, J ¼ 7.5 Hz, 1H), 7.73 (dt, J 0.51 mmol), potassium carbonate (142 mg, 1.02 mmol), Pd
¼ 7.5, 1.5 Hz, 1H), 8.07 (d, J ¼ 7.5 Hz, 1H), 8.72 (dd, J ¼ 6.2, 1.1 EnCat30 (51 mg, 0.020 mmol), 4 : 1 aq. ethanol (10 mL) were
Hz, 2H), 9.98 (s, 1H); 13C NMR (100 MHz, CDCl3) d 124.9, 128.6, heated under reux for 3 h. Aer work-up the residue was puri-
129.2, 130.5, 133.6, 134.0, 142.9, 146.1, 150.0, 191.3.
ed by ash column chromatography (SiO2, 10 : 1 hexane : ethyl
2-(Pyridin-3-yl)benzaldehyde (1g).30 Following the general acetate) to provide 1k as a light yellow solid (64 mg, 82% yield);
procedure for Table 3, using 3-bromopyridine (70 mg, 0.44 mp 104–106 ꢁC. 1H NMR (400 MHz, CDCl3) d 4.05 (s, 3H), 6.80 (d,
mmol), 2-formylphenylboronic acid MIDA ester (139 mg, 0.531 J ¼ 8.0 Hz, 1H), 7.42 (d, J ¼ 8.0 Hz, 1H), 7.69 (t, J ¼ 8.0 Hz, 1H),
mmol), potassium carbonate (184 mg, 1.33 mmol), Pd EnCat30 8.18–8.32 (m, 4H). 13C NMR (100 MHz, CDCl3) d 53.6, 111.5,
(66 mg, 0.026 mmol), 4 : 1 aq. ethanol (10 mL) were heated 114.0, 124.1, 127.5, 139.6, 145.2, 148.2, 152.2, 164.2.
under reux for 9 h. Aer work-up the residue was puried by
4-(6-Methoxypyridin-2-yl)acetophenone (1l).34 Following the
ash column chromatography (SiO2, 5 : 1 hexane : ethyl general procedure for Table 3, using 4-bromoacetophenone
1
acetate) to provide 1g as a colorless oil (80 mg, 91% yield). H (50 mg, 0.25 mmol), 6-methoxypyridylboronic acid MIDA ester
NMR (400 MHz, CDCl3) d 7.40–7.46 (m, 2H), 7.58 (d, J ¼ 7.5 Hz, (99 mg, 0.38 mmol), potassium carbonate (104 mg, 0.75 mmol),
1H), 7.60 (t, J ¼ 7.5 Hz, 1H), 7.73 (d, J ¼ 7.5 Hz, 1H), 8.06 (d, J ¼ Pd EnCat30 (38 mg, 0.015 mmol), 4 : 1 aq. ethanol (8 mL) were
7.5 Hz, 1H), 8.70 (dd, J ¼ 6.4, 1.5 Hz, 2H), 9.99 (s, 1H); 13C NMR heated under reux for 8 h. Aer work-up the residue was
(100 MHz, CDCl3) d 123.3, 128.7, 128.9, 131.2, 133.8, 133.9, puried by ash column chromatography (SiO2, 10 : 1 hex-
134.1, 137.4, 142.0, 149.5, 150.3, 191.5.
ane : ethyl acetate) to provide 1l as a white solid (48 mg, 84%
1
2-(Thiophen-2-yl)benzaldehyde (1h).30 Following the general yield); mp 73–75 C. H NMR (400 MHz, CDCl3) d 2.64 (s, 3H),
procedure for Table 3, using 2-bromothiophene (100 mg, 0.61 4.05 (s, 3H), 6.75 (d, J ¼ 8.0 Hz, 1H), 7.38 (d, J ¼ 8.0 Hz, 1H), 7.66
mmol), 2-formylphenylboronic acid MIDA ester (240 mg, 0.92 (t, J ¼ 8.0 Hz, 1H), 8.01–8.16 (m, 4H). 13C NMR (100 MHz, CDCl3)
mmol), potassium carbonate (253 mg, 1.83 mmol), Pd EnCat30 d 26.9, 53.5, 110.6, 113.7, 126.9, 128.9, 137.2, 139.5, 143.5, 153.4,
(92 mg, 0.036 mmol), 4 : 1 aq. ethanol (13 mL) were heated 164.1, 198.0.
ꢁ
under reux for 8 h. Aer work-up the residue was puried by
4-(6-Methoxypyridin-2-yl)benzaldehyde (1m).35 Following the
ash column chromatography (SiO2, 30 : 1 hexane : ethyl general procedure for Table 3, using 4-bromobenzaldehyde
acetate) to provide 1h as a light yellow oil (98 mg, 85% yield). 1H (60 mg, 0.32 mmol), 6-methoxypyridylboronic acid MIDA ester
NMR (400 MHz, CDCl3) d 7.07–7.14 (m, 1H), 7.15–7.18 (m, 1H), (128 mg, 0.48 mmol), potassium carbonate (134 mg, 0.96
7.46–7.49 (m, 3H), 7.60 (d, J ¼ 8.2 Hz, 1H), 8.01 (d, J ¼ 7.5 Hz, mmol), Pd EnCat30 (49 mg, 0.019 mmol), 4 : 1 aq. ethanol
1H), 10.20 (s, 1H); 13C NMR (100 MHz, CDCl3) d 127.5, 127.9, (8 mL) were heated under reux for 8 h. Aer work-up the
128.0, 128.4, 129.7, 131.5, 133.7, 134.4, 138.2, 138.9, 192.2.
residue was puried by ash column chromatography (SiO2,
[1,10-Biphenyl]-2,20-dicarbaldehyde (1i).31 Following the 10 : 1 hexane : ethyl acetate) to provide 1m as a white solid (66
general procedure for Table 3, using 2-bromobenzaldehyde mg, 96% yield); mp 74–76 ꢁC. 1H NMR (400 MHz, CDCl3) d 4.05
(60 mg, 0.324 mmol), 2-formylphenylboronic acid MIDA ester (s, 3H), 6.77 (d, J ¼ 7.6 Hz, 1H), 7.42 (d, J ¼ 7.6 Hz, 1H), 7.67 (t, J
(127 mg, 0.49 mmol), potassium carbonate (134 mg, 0.972 ¼ 7.6 Hz, 1H), 7.96 (d, J ¼ 7.6 Hz, 2H), 8.22 (d, J ¼ 7.6 Hz, 2H),
mmol), Pd EnCat30 (48 mg, 0.020 mmol), 4 : 1 aq. ethanol (10 10.07 (s, 1H). 13C NMR (100 MHz, CDCl3) d 53.5, 110.9, 113.9,
mL) were heated under reux for 5 h. Aer work-up the residue 127.3, 130.2, 136.5, 139.5, 144.8, 153.2, 164.1, 192.1.
was puried by ash column chromatography (SiO2, 10 : 1
hexane : ethyl acetate) to provide 1i as a light yellow oil (56 mg, the general procedure for Table 3, using 4-iodoanisol (50 mg,
2-Methoxy-6-(4-methoxyphenyl)pyridine (1n).36 Following
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0.21 mmol), 6-methoxypyridylboronic acid MIDA ester (84 mg, 2 : 1 hexane : ethyl acetate) to provide 1r as a colorless oil (58
1
0.32 mmol), potassium carbonate (89 mg, 0.63 mmol), Pd mg, 76% yield). H NMR (400 MHz, CDCl3) d 3.97 (s, 3H), 6.72
EnCat30 (32 mg, 0.013 mmol), 4 : 1 aq. ethanol (8 mL) were (dd, J ¼ 8.3, 0.6 Hz, 1H), 7.34 (dd, J ¼ 7.4, 0.6 Hz, 1H), 7.60 (t, J ¼
heated under reux for 9 h. Aer work-up the residue was 7.4 Hz, 1H), 7.81–7.87 (m, 2H), 8.59–8.66 (m, 2H). 13C NMR (100
puried by ash column chromatography (SiO2, 10 : 1 hexa- MHz, CDCl3) d 53.5, 111.7, 113.6, 121.0, 139.5, 146.3, 140.4,
ne : ethyl acetate) to provide 1n as a colorless oil (42 mg, 91% 151.9, 164.2.
yield). 1H NMR (400 MHz, CDCl3) d 3.85 (s, 3H), 4.02 (s, 3H), 6.63
2-(6-Methoxypyridin-2-yl)quinoline (1s).41 Following the
(d, J ¼ 7.8 Hz, 1H), 6.97 (d, J ¼ 8.0 Hz, 1H), 7.26 (d, J ¼ 8.0 Hz, general procedure for Table 3, using 2-bromoquinoline (70 mg,
1H), 7.55 (t, J ¼ 8.0 Hz, 1H),7.98–8.02 (m, 3H). 13C NMR (100 0.33 mmol), 6-methoxypyridylboronic acid MIDA ester (133 mg,
MHz, CDCl3) d 53.3, 55.5, 108.5, 112.1, 114.2, 114.4, 127.9, 0.50 mmol), potassium carbonate (139 mg, 1.00 mmol), Pd
128.2, 132.0, 139.3, 154.6, 160.6, 163.9.
EnCat30 (50 mg, 0.020 mmol), 4 : 1 aq. ethanol (10 mL) were
6-Methoxy-2,20-bipyridine (1o).37 Following the general heated under reux for 3 h. Aer work-up the residue was
procedure for Table 3, using 2-bromopyridine (60 mg, 0.38 puried by ash column chromatography (SiO2, 20 : 1 hexa-
mmol), 6-methoxypyridylboronic acid MIDA ester (150 mg, 0.57 ne : ethyl acetate) to provide 1s as a white solid (63 mg, 80%
1
ꢁ
mmol), potassium carbonate (157 mg, 1.14 mmol), Pd EnCat30 yield); mp 79–81 C. H NMR (400 MHz, CDCl3) d 4.02 (s, 3H),
(57 mg, 0.023 mmol), 4 : 1 aq. ethanol (8 mL) were heated under 6.83 (d, J ¼ 8.1 Hz, 1H), 7.54 (t, J ¼ 7.4 Hz, 1H), 7.68–7.80 (m,
reux for 9 h. Aer work-up the residue was puried by ash 2H), 7.84 (d, J ¼ 8.1 Hz, 1H), 8.17 (d, J ¼ 8.5 Hz, 1H), 8.17–7.30
column chromatography (SiO2, 15 : 1 hexane : ethyl acetate) to (m, 2H), 8.57 (d, J ¼ 8.5 Hz, 1H). 13C NMR (100 MHz, CDCl3) d
1
provide 1o as a colorless oil (51 mg, 72% yield). H NMR (400 53.5, 111.5, 114.7, 119.2, 126.8, 127.7, 128.4, 129.6, 130.0, 136.7,
MHz, CDCl3) d 4.05 (s, 3H), 6.76 (d, J ¼ 8.0 Hz, 1H), 7.28–7.33 139.6, 148.1, 153.9, 156.3, 163.8.
(m, 1H), 7.74 (t, J ¼ 8.0 Hz, 1H), 7.80 (td, J ¼ 7.4, 1.0 Hz, 1H),
2-Methoxy-6-(thiophen-2-yl)pyridine (1t).42 Following the
8.00 (d, J ¼ 7.4 Hz, 1H), 8.41 (d, J ¼ 7.5 Hz, 1H), 8.65 (dd, J ¼ 7.4, general procedure for Table 3, using 2-bromothiophene (100 mg,
1.0 Hz, 1H). 13C NMR (100 MHz, CDCl3) d 53.4, 111.2, 113.9, 0.61 mmol), 6-methoxypyridylboronic acid MIDA ester (240 mg,
121.2, 123.7, 136.9, 139.6, 149.3, 153.7, 156.3, 163.7.
0.92 mmol), potassium carbonate (253 mg, 1.83 mmol), Pd
60-Methoxy-5-nitro-2,20-bipyridine (1p).38 Following the EnCat30 (91 mg, 0.036 mmol), 4 : 1 aq. ethanol (13 mL) were
general procedure for Table 3, using 2-chloro-4-nitropyridine heated under reux for 8 h. Aer work-up the residue was puri-
(65 mg, 0.41 mmol), 6-methoxypyridylboronic acid MIDA ester ed by ash column chromatography (SiO2, 20 : 1 hexane : ethyl
(163 mg, 0.62 mmol), potassium carbonate (170 mg, 1.23 acetate) to provide 1t as a light yellow oil (88 mg, 75% yield). 1H
mmol), Pd EnCat30 (62 mg, 0.025 mmol), 4 : 1 aq. ethanol NMR (400 MHz, CDCl3) d 4.05 (s, 3H), 7.23 (dd, J ¼ 3.6, 0.9 Hz,
(10 mL) were heated under reux for 3 h. Aer work-up the 1H), 7.69 (dd, J ¼ 5.1, 0.9 Hz, 1H), 7.92 (dd, J ¼ 5.1, 0.6 Hz, 1H),
residue was puried by ash column chromatography (SiO2, 8.02 (dd, J ¼ 8.0, 1.2 Hz, 1H), 8.04 (t, J ¼ 8.0, 1.2 Hz, 1H), 8.49 (dd,
10 : 1 hexane : ethyl acetate) to provide 1p as a white solid J ¼ 7.8, 1.2 Hz, 1H). 13C NMR (100 MHz, CDCl3) d 53.4, 108.9,
(72 mg, 76% yield); mp 114–116 ꢁC. 1H NMR (400 MHz, CDCl3) d 111.3, 124.3, 127.1, 127.9, 139.1, 144.9, 150.0, 163.5.
4.06 (s, 3H), 6.88 (d, J ¼ 7.8 Hz, 1H), 7.76 (t, J ¼ 7.6 Hz, 1H), 8.13
(d, J ¼ 7.4 Hz, 1H), 8.57–8.64 (m, 2H), 9.45 (s, 1H).13C NMR (100
Acknowledgements
MHz, CDCl3) d 53.6, 113.4, 115.7, 121.0, 132.0, 139.8, 144.0,
144.9, 151.3, 161.1, 164.0.
The authors acknowledge CAPES and FAPERJ for nancial
6-Methoxy-2,30-bipyridine (1q).39 Following the general
support for this work.
procedure for Table 3, using 3-bromopyridine (60 mg, 0.38
mmol), 6-methoxypyridylboronic acid MIDA ester (150 mg, 0.57
mmol), potassium carbonate (157 mg, 1.14 mmol), Pd EnCat30
(57 mg, 0.023 mmol), 4 : 1 aq. ethanol (8 mL) were heated under
Notes and references
reux for 8 h. Aer work-up the residue was puried by ash
column chromatography (SiO2, 5 : 1 hexane : ethyl acetate) to
provide 1q as a colorless oil (60 mg, 85% yield). 1H NMR
(400 MHz, CDCl3) d 4.03 (s, 3H), 6.74 (d, J ¼ 8.0 Hz, 1H), 7.35–
7.41 (m, 2H), 7.66 (t, J ¼ 8.0 Hz, 1H), 7.80 (dt, J ¼ 8.0, 1.0 Hz,
1H), 8.62 (dd, J ¼ 8.0, 1.0 Hz, 1H), 9.25 (s, 1H). 13C NMR (100
MHz, CDCl3) d 53.5, 110.4, 114.0, 123.6, 134.1, 134.7, 139.5,
148.4, 149.8, 152.1, 164.2.
6-Methoxy-2,40-bipyridine (1r).40 Following the general
procedure for Table 3, using 4-bromopyridine hydrochloride
(80 mg, 0.41 mmol), 6-methoxypyridylboronic acid MIDA ester
(163 mg, 0.61 mmol), potassium carbonate (170 mg, 1.23
mmol), Pd EnCat30 (62 mg, 0.025 mmol), 4 : 1 aq. ethanol
(10 mL) were heated under reux for 8 h. Aer work-up the
residue was puried by ash column chromatography (SiO2,
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