Acetanilide palladacycle: An efficient catalyst for room-temperature Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1002/aoc.3113

The catalytic activity of three acetanilide palladacycles derived from easily accessible and commercially available acetanilide derivatives, viz. N-phenylacetamide (L1), N-(4-chlorophenyl)acetamide (L2) and N-(4-methylphenyl)acetamide (L3) has been examined in Pd-catalyzed Suzuki-Miyaura reaction of arylboronic acid with aryl bromides at room temperature. The complex 1L3 exhibited efficient activity in the Suzuki-Miyaura reaction (up to 99% isolated yield) under mild reaction conditions. Copyright

Full text of DOI:10.1002/aoc.3113

Full Paper
Received: 28 August 2013
Revised: 7 November 2013
Accepted: 11 November 2013
Published online in Wiley Online Library: 30 January 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3113
Acetanilide palladacycle: an efficient catalyst
for room-temperature Suzuki–Miyaura cross-
coupling reaction
Anindita Dewana, Zenith Buragohaina, Manoj Mondala, Gayatri Sarmaha,
Geetika Boraha and Utpal Boraa,b*
The catalytic activity of three acetanilide palladacycles derived from easily accessible and commercially available acetanilide
derivatives, viz. N-phenylacetamide (L1), N-(4-chlorophenyl)acetamide (L2) and N-(4-methylphenyl)acetamide (L3) has been
examined in Pd-catalyzed Suzuki–Miyaura reaction of arylboronic acid with aryl bromides at room temperature. The complex
1L3 exhibited efficient activity in the Suzuki–Miyaura reaction (up to 99% isolated yield) under mild reaction conditions.
Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: palladacycle; acetanilide; Suzuki–Miyaura; isopropanol; biaryl
achieved using easily accessible acetanilide (N-phenylacetamide)
derivatives as a ligand. Acetanilide is a routinely used cheap
Introduction
Biaryl compounds served as building block materials for the
synthesis of a wide range of vital compounds, such as pharmaceu-
ticals, fine chemicals, agrochemicals, natural products and smart
engineering materials, including conducting polymers and molecu-
lar wires.[1] Palladium-catalyzed Suzuki–Miyaura cross-coupling
reaction is one of the most widely used and versatile methodolo-
gies for the synthesis of unsymmetrical biaryls under mild reaction
conditions.[2] The Suzuki–Miyaura cross-coupling reaction is effi-
ciently catalyzed by wide variety of Pd-based catalysts, and efficacy
of the catalytic system has been achieved by changing the ligand
environment around palladium.[3] Consequently, significant efforts
have been made in designing novel catalyst systems of Pd(0) or Pd
(II) complexes with phosphine-based ligands such as tertiary phos-
phines, hemilabile-type phosphines, sterically crowded biphenyl-
type phosphines, and imidazole- and imidazolium-functionalized
phosphines.[4] Although complexes containing such ligands often
show excellent activity, moisture sensitivity,[5] and the requirement
of high temperature and undesirable solvents such as DMF, N-
methylpyrrolidine, dimethoxyethane,[6] are some of the major
drawbacks in majority of cases. Moreover, in most of cases the
laboratory chemical with immense potential as a ligand. Moreover,
acetanilide derivatives are well-known precursors for palladacycles,
which have wide applicability in various organic transformations
such as orthoalkylation[16] and Fujiwara–Moritani reaction.[17] The
cyclopalladated acetanilide with bulky N-heterocyclic carbene
(NHC), 1,3-bis(2,6 diisopropylphenyl) imidazol-2-ylidene (IPr), has
been found an effective palladium pre-catalyst for the Suzuki–-
Miyaura cross-coupling reaction.[18] However, to the best of our
knowledge, the potential of simple acetanilide-derived palladacycles
has not been utilized in the Suzuki–Miyaura cross-coupling reaction.
Herein, we wish to report a facile and efficient room-temperature
Suzuki–Miyaura reaction strategy using acetanilide-based
palladacycles prepared by well-precedented orthometalation of
acetanilide derivatives. For this we have used cheap, easily accessible
and commercially available acetanilide derivatives, viz.
N-phenylacetamide (L1), N-(4-chlorophenyl)acetamide (L2) and N-
(4-methylphenyl)acetamide (L3) as palladacycle precursors (Fig. 1).
This type of amide-based palladacycle can easily be synthesized
within short reaction times under mild reaction conditions (Fig. 1).[16b]
ligands are either very expensive or difficult to synthesize. Experimental
Recently, nitrogen-containing ligands such as N-heterocyclic
carbenes,[7] amines[8] and oxime[9] have emerged as efficient
General Information
ligands for Suzuki–Miyaura reaction with the potential to overcome
most of the drawbacks of traditional phosphine ligands. Addition-
ally, some common laboratory chemicals and reagents such as
sodium sulfate,[10], PEG[11] and EDTA[12] have been successfully
utilized as promoters for the reaction. Moreover, some easily accessi-
ble Schiff base type[13] and thiosemicarbazone[14] ligands have been
used effectively in Pd-catalyzed Suzuki–Miyaura reaction. Very
recently, Costa and Nobre utilized bisamides as ligands in Suzuki
coupling of aryl bromides at a catalyst loading of 2 mol% PdCl2.[15]
This study ascertained that a similar catalytic tendency can be
Melting points were determined using a Buchi B450 melting
point apparatus. Elemental analyses were performed using an
Elementar Vario EL III Carlo Erba 1108 instrument. IR spectra
*
Correspondence to: Utpal Bora, Department of Chemistry, Dibrugarh Univer-
sity, Dibrugarh 786004, Assam, India. Email: utbora@yahoo.co.in
a
b
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
Department of Chemical Sciences, Tezpur University, Tezpur 784028, Assam, India
Appl. Organometal. Chem. 2014, 28, 230–233
Copyright © 2014 John Wiley & Sons, Ltd.

Palladacycle-catalyzed Suzuki–Miyaura coupling
Table 1. Effect of acetanilide derivatives in Suzuki–Miyaura coupling
Figure 1. Structure of acetanilide derivatives L1, L2 and L3, and their
corresponding palladacycles.
Entry
X
Catalyst (0.5 mol%)
Yield (%)a
1
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Br
Pd(OAc)2
55
78
70
85
90
85
99
25
28
40
95b
(4000–250 cmÀ1) were recorded in KBr on a Shimadzu Prestige-
21 FT-IR spectrophotometer. 1H and 13C NMR spectra were
recorded in CDCl3 solutions operating at 400.13, and 100.62
MHz, respectively, on a Bruker Advance II 400 NMR spectrometer,
and chemical shifts were reported relative to tetramethylsilane.
Electrospray ionization (ESI) (+) mass spectra were recorded on
a Water ZQ-4000 mass spectrometer.
2
Pd(OAc)2/L1
3
Pd(OAc)2/L2
4
Pd(OAc)2/L3
5
[Pd(OAc)2/L1](1L1)
[Pd(OAc)2/L2](1L2)
[Pd(OAc)2/L3](1L3)
[Pd(OAc)2/L1](1L1)
[Pd(OAc)2/L2](1L2)
[Pd(OAc)2/L3](1L3)
[Pd(OAc)2/L3](1L3)
6
7
8
9
10
11
General Information about Catalytic Experiments
Suzuki–Miyaura cross-coupling reactions were carried out under
aerobic condition at room temperature. Progress of the reactions
was monitored by TLC using aluminum-coated TLC plates (Merck
silica gel 60F254) under UV light. The products were purified by
column chromatographic technique using silica gel (60–120
mesh). The various products separated were characterized by
melting point, 1H and 13C NMR and mass spectral data and com-
pared with the authentic samples.
Reaction conditions: 4-bromonitrobenzene (0.50 mmol), PhB(OH)2
(0.55 mmol), K2CO3 (1.5 mmol), i-PrOH (3 ml), in situ prepared
catalyst was used (entries 2–4): Pd(OAc)2 (0.5 mol%),
acetanilide L1–L3 (0.5 mol%), pre-formed palladacycle 1L1–
1L3 was used (entries 5–10).
aIsolated yield.
bPd complex 1L3 (0.25 mol%) was used.
improved yield (Table 1, entry 2). Moreover, the use of ligand
L1 significantly reduced the formation of homodiaryl. Similarly,
use of ligand N-(4-chlorophenyl)acetamide (L2) and N-(4-
methylphenyl)acetamide (L3) gave very good yields of isolated
cross-coupling product (Table 1, entries 3 and 4). Among the
three ligands used, N-(4-methylphenyl)acetamide (L3) demon-
strated the highest catalytic activity (Table 1, entry 4), whereas
4-chloroacetanilide (L2) showed the lowest activity (Table 1,
entry 3). In the Suzuki–Miyaura reaction, the in situ generated
catalytic species and the pre-formed catalyst often exhibit differ-
ent catalytic activities.[20] Therefore, to compare the catalytic
activities of these in situ formed catalysts with that of pre-formed
catalyst, we synthesized acetanilide-based palladacycles using
acetanilide derivatives (L1–L3) as ligand.[16b] Acetanilide deriva-
tives are well-known precursors for CN-palladacycle, and it is
worth mentioning here that the acetanilide-derived palladacycle
complexes 1L1–1L3, which can be prepared vary easily, were
previously reported in the literature.[16b] It is observed that the
use of 0.5 mol% pre-formed palladacycle complexes 1L1–1L3
as catalyst under the above-mentioned conditions provides
excellent yields compared to that of the in situ formed catalyst
(Table 1, entries 2 vs. 5, 3 vs. 6, 4 vs. 7). Although the reason for
different catalytic activities is not clear, one possible reason could
be the slow rate of formation of the in situ complex at room tem-
perature, as synthesis of the complexes (1L1–1L3) requires
refluxing conditions. The best result was obtained with the
palladacycle derived from N-(4-methylphenyl)acetamide, 1L3
(Table 1, entry 7). We have also examined the effectiveness of
the current catalytic system in the cross-coupling chemistry of
aryl chlorides using 4-chloronitrobenzene as coupling partner.
Under the given set of reaction conditions, 4-chloronitrobenzene
General Procedure for Suzuki–Miyaura Reactions of Aryl Ha-
lides Using Palladium Complex
A 50 ml round-bottom flask was charged with a mixture of aryl
halide (0.5 mmol), aryl boronic acid (0.55 mmol), base (1.5 mmol)
and palladium complex (0.5 mol%), and the mixture was stirred at
room temperature for the required time. After completion, the re-
action mixture was diluted with water (20 ml) and extracted with
ether (3 × 20 ml). The combined extract was washed with brine
(2 × 20 ml) and dried over Na2SO4. After evaporation of the sol-
vent under reduced pressure, the residue was subjected to chro-
matography (silica gel, ethyl acetate–hexane, 1:9) to obtain the
desired products. The products were confirmed by comparing
1
the melting point, H and 13C NMR and mass spectral data with
authentic samples.
Results and Discussion
To examine the efficiency of the different acetanilide derivatives
(L1–L3) in Suzuki–Miyaura reaction, the reaction of 4-
bromonitrobenzene (0.50 mmol) and phenylboronic acid (0.55
mmol) was chosen as a model reaction using isopropanol as sol-
vent and K2CO3 as base, and the reaction was performed under
aerobic conditions at room temperature in the presence of Pd
(OAc)2 (0.5 mol%) and ligands L1–L3 (0.5 mol%).[19] The results
are summarized in Table 1. It may be observed from the table
that, even in the absence of any ligand, the coupling reaction
of 4-bromonitrobenzene and phenylboronic acid proceeded with
55% isolated yield (Table 1, entry 1), accompanied by a substan-
tial amount of homodiaryl as by-product. However, in the pres-
ence of the ligand L1 the desired product was obtained with
Appl. Organometal. Chem. 2014, 28, 230–233
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A. Dewan et al.
underwent the cross-coupling reaction with lower yields com-
pared to the aryl bromide counterpart (Table 1, entries 8–10).
The cross-coupling reaction also proceeded with 0.25 mol% Pd
complex 1L3 (Table 1, entry11).
It is a well-known fact that the choice of solvent and base
significantly influences the Suzuki–Miyaura reaction. Thus, to
study the effect of different solvents in our system, we performed
evaluated by investigating the reactions of a wide array of elec-
tronically diverse aryl bromides with arylboronic acids. The
coupling reactions were conducted at room temperature in
isopropanol with 0.5 mol% 1L3 as catalyst and 3 equiv. K2CO3
(Table 3). It was observed that the aryl bromides with electron-
withdrawing and electron-donating substituent underwent
coupling reactions with phenylboronic acid, effectively affording
the desired biaryls in excellent yields (90–100%). The catalytic
system is equally effective for electronically diverse arylboronic
acids under similar reaction conditions. Moreover, it was found
that the deactivated aryl bromides possessing MeO and Me
groups gave almost the same or higher yields compared to the
activated bromides possessing NO2 and CHO groups. We have
also observed a similar trend in reactivity of activated and
deactivated aryl halides with phosphinoamine–Pd(II)–imidazole
complexes.[4g] However, the actual reason for the equal effi-
ciency of the catalyst for both activating and deactivating sub-
strates is not clear. It is a well-known fact that dissociation of
ligand in the reaction medium produces palladium(0) colloids
or nanoparticles and the palladium(0) nanoparticle is the active
catalytic species for the Suzuki–Miyaura reaction. With these
aspects in mind, we investigated the nature of the catalytic
species after completion of the reaction. Transmission electron
microscopic images of the catalyst after reaction clearly show
the formation of Pd(0) nanoparticles (Fig. S1, supporting
a
series of reactions between 4-bromonitrobenzene and
phenylboronic acid in the presence of various solvents using
K2CO3 as base, and acetanilide-derived palladacycle 1L3 as cata-
lyst. The results are summarized in Table 2. It is seen from the
table that the reaction proceeds in both protic (Table 2, entries
1–5) and aprotic (Table 2, entries 6–8) solvents, although signifi-
cant variations in yields were noted. The reaction seemed to be
more facile in polar alcoholic solvent (Table 2, entries 1 and 3)
than in other polar solvents like THF, DMF or CH3CN. However,
the reaction did not proceed well in water and gave only 60% iso-
lated yield (Table 2, entry 9). The best result was obtained in
isopropanol, which is considered an eco-friendly and desirable
solvent for organic synthesis.[21] To study the effect of bases in
our system, we carried out the reaction in the presence of various
bases using isopropanol as solvent. The study showed that the
reaction proceeds smoothly in the presence of K2CO3, Na2CO3
and Na3PO4 with equal ease (Table 2, entries 1, 10 and 14). How-
ever, the yield of cross-coupling product significantly diminished
in the presence of NaOH and KOH (Table 2, entries 11 and 12).
Moreover, an organic base such as Et3N was not suitable for
the current catalyst system (Table 2, entry 13). The coupling reac-
tion did not proceed in the absence of base (Table 2, entry 15).
The scope of the described Suzuki–Miyaura coupling was
Table 3. Suzuki–Miyaura reaction of various substrates
Table 2. Effect of solvent and base in Suzuki–Miyaura coupling
Entry
R1
4-NO2
R2
Time (h)
Yield (%)a
1
H
20
20
22
18
20
15
15
13
15
10
5
99
90
95
90
97
98
93
90
97
95
94
96
97
98
90
88
90
98
95
80
2
4-NO2
4-NO2
4-NO2
4-NO2
4-OCH3
4-OCH3
4-OCH3
4-OCH3
4-OCH3
2-CH3
4-CH3
4-CH3
4-CH3
4-CH3
4-CHO
4-COCH3
H
4-OCH3
4-C(CH3)3
4-Cl
3
4
Entry
Solvent
i-PrOH
Base
K2CO3
Yield
5
3-OCH3
H
1
99
90
95
90
65
70
80
85
60
98
30
30
45
99
Nil
6
2
i-PrOH–H2O
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaOH
KOH
7
4-OCH3
4-C(CH3)3
4-Cl
3
8
4
EtOH–H2O
DMF–H2O
DMF
9
5
10
11
12
13
14
15
16
17
18
19
20
3-OCH3
H
6
7
THF
H
7
8
CH3CN
H2O
4-OCH3
4-C(CH3)3
4-Cl
12
17
20
22
10
8
9
10
11
12
13
14
15
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
i-PrOH
H
H
Et3N
H
Na3PO4
—
H
4-OCH3
2-OCH3
14
20
2-CH3
Reaction conditions: 4-bromonitrobenzene (0.50 mmol), PhB(OH)2
(0.55 mmol), base (1.5 mmol), solvent (3 ml), 1L3 (0.5 mol%).
aIsolated yield.
Reaction conditions: arylbromide (0.50 mmol), arylboronic acid
(0.55 mmol), K2CO3 (1.5 mmol), i-PrOH (3 ml), 1L3 (0.5 mol%).
aIsolated yield.
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Appl. Organometal. Chem. 2014, 28, 230–233

Palladacycle-catalyzed Suzuki–Miyaura coupling
Nekoueishahraki, J.-M. Basset, V. Polshettiwar, Chem. Soc. Rev.
2011, 40, 5181. j)A. Balanta, C. Godard, C. Claver, Chem. Soc. Rev.
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information). To investigate this issue further, we next examined
the role of possible active palladium(0) nano species by
performing the Hg(0) poisoning test,[22,23] in which a reaction
catalyzed by metal(0) or nanoparticle species become retarded
owing to amalgamation with mercury(0).[24,25] However, metal–
ligand complexes bearing metal in higher oxidation states
usually remain unchanged in the presence of mercury(0).
Therefore an experiment with Hg(0) was performed using
4-bromonitrobenzene (0.5 mmol) and phenylboronic acid
(0.55 mmol) as substrate and 1L3 (0.5 mol%) as the palladium
source. The palladium complex 1L3 was stirred in isopropanol
containing K2CO3 and a stoichiometric amount of Hg(0) for
30 min prior to addition of the reacting substrates. Progress
was monitored by TLC. Initially, the reaction proceeded
smoothly, but subsequently it became retarded, and after 24 h
we were able to isolate only 35% of the cross-coupled product.
The retarding effect of Hg(0) became prominent as the reaction
proceeded, possibly due to the formation of Pd(0) nanoparticles
from complex 1L3 during the course of the reaction.
Thus our present result is quite significant as the desired biaryls
could be achieved at room temperature using isopropanol as a
solvent and with relatively low catalyst loading (0.5 mol% of the Pd
complex 1L3). In comparison with our previous results on the
Suzuki–Miyaura cross-coupling reaction involving triphenylphosphine
chalcogenides,[4f] triphenylmethylamine[8a] and phosphinoamine–Pd
(II)–imidazole complexes,[4g] this method offers a mild and efficient
methodology that can promote Suzuki–Miyaura reactions with an
easily accessible and cheap ligand system.
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Conclusion
We have found that the Suzuki–Miyaura coupling can be carried
out in isopropanol with aryl bromides and arylboronic acid in the
presence of stable and easily accessible acetanilide-based
palladacycles. The reaction advances efficiently at low catalyst load-
ing (0.5 mol% Pd complex) in isopropanol, which is a desirable sol-
vent for organic synthesis. Synthesis of this catalytic complex is very
simple and the starting materials used are commercially available
and cheap. The formation of Pd(0) nanoparticles was observed dur-
ing the reaction, which probably acted as the real catalyst. This new
protocol is inexpensive and effective for many electronically di-
verse aryl bromides and arylboronic acids, providing biphenyl de-
rivatives in good to excellent yields (88–99%).
[19] General procedure for the Suzuki–Miyaura reaction with in situ gen-
erated palladium catalyst. A 50 ml round-bottom flask was charged
with a mixture of aryl halide (0.5 mmol), aryl boronic acid (0.55
mmol), base (1.5 mmol), Pd(OAc)2 (0.5 mol%) and acetanilide L1–
L3 (0.5 mol%), and the mixture was stirred at room temperature
for the required time. After completion, the reaction mixture was di-
luted with water (20 ml) and extracted with ether (3 × 20 ml). The
combined extract was washed with brine (2 × 20 ml) and dried over
Na2SO4. After evaporation of the solvent under reduced pressure,
the residue was subjected to chromatography (silica gel, ethyl ace-
tate–hexane, 1:9) to obtain the desired products. The products were
confirmed by comparing the melting point, 1H and 13C NMR and
mass spectral data with authentic samples.
Acknowledgments
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The authors acknowledge the Department of Science and Tech-
nology, New Delhi, for financial support for this work under the
DST Woman Scientist Scheme (No. SR/WOS-A/CS-78/2011(G).
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