Cooperative catalysis by palladium-nickel binary nanocluster for suzuki-miyaura reaction of ortho-heterocycle-tethered sterically hindered aryl bromides

Article abstract of DOI:10.1021/ol500587m

The palladium-nickel binary nanocluster is reported as a new catalyst system for Suzuki-Miyaura cross-coupling of ortho-heterocycle-tethered sterically hindered aryl bromides. The inferior results obtained with the reported Pd/Ni salts/complexes or individual Pd/Ni nanoparticles as catalyst reveal the cooperative catalytic effect of the Pd and Ni nanoparticles in the Pd-Ni nanocluster. The broad substrate scope with respect to variation of the 2-arylbenzoxazole moiety and boronic acids, which offers a means for diversity generation and catalyst recyclability, marks a distinct advantage.

Full text of DOI:10.1021/ol500587m

Letter
pubs.acs.org/OrgLett
Cooperative Catalysis by Palladium−Nickel Binary Nanocluster for
Suzuki−Miyaura Reaction of Ortho-Heterocycle-Tethered Sterically
Hindered Aryl Bromides
Kapileswar Seth, Priyank Purohit, and Asit K. Chakraborti*
†Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), Sector 67, S. A. S.
Nagar 160 062, Punjab, India
S
* Supporting Information
ABSTRACT: The palladium−nickel binary nanocluster is reported
as a new catalyst system for Suzuki−Miyaura cross-coupling of ortho-
heterocycle-tethered sterically hindered aryl bromides. The inferior
results obtained with the reported Pd/Ni salts/complexes or
individual Pd/Ni nanoparticles as catalyst reveal the cooperative
catalytic effect of the Pd and Ni nanoparticles in the Pd−Ni
nanocluster. The broad substrate scope with respect to variation of
the 2-arylbenzoxazole moiety and boronic acids, which offers a means for diversity generation and catalyst recyclability, marks a
distinct advantage.
he 1,2-diaryl/arylheteroaryl-substituted heterocyclic frame-
Twork has been recognized as the essential pharmacophoric
feature of cyclooxygenase-2 (COX-2) selective inhibitors,
typically known as coxibs, that have emerged as nonsteroidal
anti-inflammatory drugs (NSAIDs) for the treatment of
rheumatoid arthritis.1 However, the cardiovascular side effects
associated with chronic administration of some of the coxibs led
to their withdrawal from the market2 and fueled a debate on the
safe use of the existing COX-2 inhibitors3 pressing the need for a
novel anti-inflammatory scaffold. Toward this initiative, we
recently reported 2-(2-arylphenyl)benzoxazoles as new anti-
inflammatory chemotypes4 affording new leads (Figure 1).
lead optimization it is necessary to generate diversity with the
newly identified scaffold A (Figure 1) through modification on
the (i) central aromatic ring incorporating substitution, (ii)
benzoxazole moiety, introducing substitution in the phenyl ring
as well as replacing it with naphthoxazole, and (iii) the 2-aryl
group by replacing it with heteroaryl moiety. In view of the longer
reaction time required in the reported procedure,4 it becomes
necessary to develop a more effective Suzuki−Miyaura cross-
coupling protocol that would enrich the medicinal chemists’
toolbox.5
It was observed that the Suzuki−Miyaura cross-coupling
involving ortho-heterocycle-tethered sterically hindered sub-
strates as the electrophilic coupling partner remained elusive.6
Herein we report a binary palladium−nickel nanocluster (Pd−Ni
NC) as a new and efficient catalyst system for Suzuki−Miyaura
cross-coupling of ortho-heterocycle-tethered sterically hindered
aryl bromides represented by the generalized structure 6.
In search of a more effective catalytic system, the Suzuki cross-
coupling of 6a7 with 7a was performed separately in the presence
of Pd(OAc)2/(biphenyl)PCy2 and Pd(OAc)2/PCy3 following
the reported conditions8 as well as under different variations by
changing the solvent, but no significant improvement was
observed (16−27 and 12−14% yields, respectively) (Supporting
Information: entries 1−4, Table B).9 Anticipating better catalytic
potential of palladium nanoparticles (PdNPs),10 the effect of the
PdNPs was assessed using various Pd compounds (2.5 mol %)
such as PdCl2, Pd(OAc)2, Pd(TFA)2, Pd(acac)2, Na2PdCl4, and
PdCl2(PPh3)2 as the precatalysts11 for the reaction of 6a with 7a
but 8a was obtained in poor (21−47%) yields (Supporting
Information: entries 1−12 and 14−18, Table C).9
Figure 1. Representative COX-2 inhibitors and new leads.
The new anti-inflammatory leads (5a−c) were synthesized
following the Suzuki−Miyaura cross-coupling (Scheme 1). For
Scheme 1. Suzuki−Miyaura Reaction of 6a with 7 To Form 8
Received: February 24, 2014
Published: April 10, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
Being attracted by the recent trend toward nickel-catalyzed12
Suzuki−Miyaura cross-coupling, various Ni compounds such as
NiCl2·6H2O, [NiCl2(PPh3)2], and NiSO4 were used (3 mol %)
for the Suzuki cross-coupling of 6a with 7a in DMF at 130 °C or
under reflux for 1 h that resulted 8a in insignificant (0−18%)
yields (Supporting Information: entries 1−6, Table D).9 The use
of NiCl2−PCy3 [so as to generate NiCl2(PCy3)2 in situ] or
NiCl2(PPh3)2-PCy3 under reported13 as well as modified
conditions with different solvents gave poor yields (19−28%)
(Supporting Information: entries 5 and 6, Table B).9 As NiNPs
are also reported to catalyze the Suzuki−Miyaura coupling,14
NiCl2·6H2O, [NiCl2(PPh3)2], and NiSO4 were used as
precatalysts for in situ formation of the NiNPs for the reaction
of 6a with 7a but 8a was obtained in poor yields (12−46%)
(Supporting Information: entries 1−12, Table E).9
The ineffectiveness of the classical Suzuki−Miyaura cross-
coupling reaction conditions using the Pd or Ni compounds and
the lack of the desirable efficiency of the individual Pd and Ni
NPs to promote the reaction of 6a with 7a to form 8a in shorter
reaction time led us to explore cooperative effects of the binary
Pd−Ni NC. The high reactivity of isolated metal atoms makes
them prone to form dinuclear species in contact with other metal
centers involving their valence orbitals.15 As Pd and Ni are
elements of the same group with 4d105s0 and 3d84s2 outer
electronic configuration, respectively,16 they are likely to get
involved in Pd−Ni bonding during the nanocluster formation,
with different electronic properties than that of the individual
metal (Pd or Ni) NPs, and such a Pd−Ni binary NC may be a
more effective catalyst.
Initial experiments on the use of 10 mol % each of
NiCl2(PPh3)2 and PdCl2(PPh3)2 in the presence of K2CO3
(1.2 equiv) and TBAB (1 equiv) afforded 8a in 74% yield in
DMF at 130 °C for 40 min (Table 1, entry 1). The poor yields
using NiCl2(PPh3)2 or PdCl2(PPh3)2 alone (entries 2 and 3,
respectively) revealed the distinct cooperative catalytic effect of
the Pd and Ni NPs in the binary Pd−Ni NC. The use of other Ni
compounds such as NiSO4 and NiCl2·6H2O in place of
NiCl2(PPh3)2 but in combination with PdCl2(PPh3)2 afforded
8a in 26 and 62% yields, respectively (Table 1, entries 4 and 5).
The replacement of PdCl2(PPh3)2 by other Pd salts/complexes
such as [Pd(PPh3)4], PdCl2, Na2PdCl4, Pd(OAc)2, Pd(TFA)2,
Pd(acac)2, Pd(dba)2, and Pd2(dba)3 afforded 8a in moderate
(60−64%) yields (entries 6−12). The optimal amounts of the
NiCl2(PPh3)2 and PdCl2(PPh3)2 could be reduced to 2.5 mol %
each under the working temperature of 100 °C (entry 18). A
decrease of the amount of NiCl2(PPh3)2 and PdCl2(PPh3)2 less
than 2.5 mol % (entries 15−17) or the reaction temperature
(entries 19−21) resulted in decreased yields. Further studies on
optimization of various reaction parameters such as the use of
various tetraalkylammonium salts as the stabilizer17 (Supporting
Information: Table H), different bases (Supporting Information:
Table G), and solvents (Supporting Information: Table I)
revealed that the best operative reaction condition is to use
NiCl2·6H2O (2.5 mol %) and PdCl2 (2.5 mol %) in DMF in the
presence of K2CO3 (1.2 equiv) and TBAF (10 mol %) at 100 °C
for 40 min, which afforded 8a in 80% yield (Supporting
Information: entry 8, Table I).9 The optimal Pd/Ni ratio was
found to be 1:1, as a decrease in the product yield was observed in
using higher amounts of Ni (Pd/Ni ratio increased to 1:2 and
1:4), while the increase of the Pd content (Pd/Ni ratio increased
to 2:1 and 4:1) did not have any significant change in the product
yield (Supporting Information: entries 48−51, Table F). The
addition of external ligands such as PPh3 or PCy3 did not offer
Table 1. Suzuki Cross-Coupling of 6a with 7a To Form 8a in
the Presence of Binary Pd−Ni NC Formed in Situ under
a
Various Conditions
b
temp
(°C)
yield
(%)
entry precatalyst A (mol %) precatalyst B (mol %)
1
[NiCl2(PPh3)2](10) [PdCl2(PPh3)2](10)
[NiCl2(PPh3)2](10) none
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
100
130
100
80
74
27
30
26
62
62
64
60
64
60
61
42
80
80
60
42
trace
76
63
32
trace
0
2
3
none
[PdCl2(PPh3)2](10)
[PdCl2(PPh3)2](10)
[PdCl2(PPh3)2](10)
4
NiSO4 (10)
NiCl2·6H2O (10)
5
6
[NiCl2(PPh3)2](10) [Pd(PPh3)4](10)
[NiCl2(PPh3)2](10) PdCl2(10)
[NiCl2(PPh3)2](10) Na2PdCl4(10)
[NiCl2(PPh3)2](10) Pd(OAc)2(10)
[NiCl2(PPh3)2](10) Pd(TFA)2(10)
[NiCl2(PPh3)2](10) Pd(acac)2(10)
[NiCl2(PPh3)2](10) Pd/C(10)
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
[NiCl2(PPh3)2](5)
[PdCl2(PPh3)2](5)
[NiCl2(PPh3)2](2.5) [PdCl2(PPh3)2](2.5)
[NiCl2(PPh3)2](1)
[NiCl2(PPh3)2](1)
[PdCl2(PPh3)2](1)
[PdCl2(PPh3)2](1)
[NiCl2(PPh3)2](0.5) [PdCl2(PPh3)2](0.5)
[NiCl2(PPh3)2](2.5) [PdCl2(PPh3)2](2.5)
[NiCl2(PPh3)2](2.5) [PdCl2(PPh3)2](2.5)
[NiCl2(PPh3)2](2.5) [PdCl2(PPh3)2](2.5)
[NiCl2(PPh3)2](2.5) [PdCl2(PPh3)2](2.5)
[NiCl2(PPh3)2](2.5) [PdCl2(PPh3)2](2.5)
60
40
c
100
100
100
100
100
100
100
100
NiCl2·6H2O (2.5)
NiCl2·6H2O 2.5)
NiCl2·6H2O (2.5)
[PdCl2(PPh3)2](2.5)
Pd(dba)2(2.5)
70
48
43
67
74
0
Pd2(dba)3(2.5)
[NiCl2(PPh3)2](2.5) PdCl2(2.5)
NiCl2·6H2O (2.5)
NiCl2·6H2O (2.5)
none
PdCl2(2.5)
PdCl2(2.5)
none
c
0
a
6a (1.0 mmol) was treated with 7a (1.2 mmol, 1.2 equiv) under the
b
various conditions in DMF (2 mL) for 40 min. Isolated yield of 8a.
c
No K2CO3 was used.
any improvement of the product yield (Supporting Information:
footnotes c−h, Table F).9
The formation of the NPs was indicated by the color transition
of the reaction medium from blue to dark brown to green to
yellow in the initial stage of the reaction after the addition of all of
the components. The formation of the Pd−Ni binary NC was
confirmed by a high-resolution transmission electron microscopy
(HRTEM) image of the reaction mixture after 10 min of mixing
of all the components that showed the presence of 2.3−2.5 nm
particles that have higher surface areas with highly accessible
active sites and obviously will show enhanced properties due to
the quantum size effects.18 The identity of the particles was
confirmed by energy dispersive acquired X-ray (EDAX).9 The
size of the Pd and Ni NPs formed separately/individually were
on the order of 8−10 nm and 10−12 nm, respectively. Thus, the
use of the mixed metal salts makes the reduction more feasible to
form smaller MNPs. As Pd(II) has higher reduction potential
(0.83 V) than that of Ni(II) (−0.23 V),19 it acts as electron
carrier and helps to reduce Ni(II).20
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The cooperative catalytic effect of the Pd−Ni binary NC was
extended to the Suzuki−Miyaura cross-coupling of 6a with
various substituted arylboronic acids (Table 2). The reaction
4-methoxyphenylboronic acid 7b and 2-naphthylboronic acid 7c,
and the results (Table 3) demonstrated the broad substrate scope
with respect to the variation of the heterocyclic scaffold as well as
the aryl moiety bearing the electrophilic center.
Table 2. Suzuki−Miyaura Cross-Coupling of 6a with Various
a
Arylboronic Acids Catalysed by Pd−Ni Binary NC
Table 3. Pd−Ni NC-Catalyzed Suzuki−Miyaura Cross-
Coupling of Different Variations of the Ortho-Heterocycle-
a
Tethered Aryl Bromide 6 with 7b/7c
a
6 (1.0 mmol) in DMF (2 mL) was treated with 7b/7c (1.2 mmol, 1.2
equiv) in the presence of NiCl2·6H2O (2.5 mol %), PdCl2 (2.5 mol
b
%), K2CO3 (1.2 equiv), and TBAF (10 mol %) at 100 °C. Isolated
c
yield of the corresponding biaryl 8. The number in parentheses is the
yield of the homocoupling product of the boronic acid.
The tolerability of the reaction conditions to the presence of
Cl and OH groups signifies the chemoselectivity. The
benzoxazole moiety can be replaced by a more sterically
demanding naphthoxazole (entry 6, Table 3). The reaction of
2-(2-bromophenyl)naphthoxazole 6f with 7c (entry 7, Table 3)
exemplify the reaction involving sterically hindered coupled
partners. No significant amount of the Suzuki−Miyaura cross-
coupling product was obtained during the reaction of 6f with 7c
using Pd(OAc)2 (1 mol %), PCy3 (1.2 mol %), KF (3.3 equiv) in
THF at rt for 4 h or in DMF at 100 °C for 4 h, akin to the
reported catalyst system.8b,9
The Pd−Ni NC was recycled five times without a significant
decrease in the catalytic activity. However, after the fifth cycle the
recovered catalyst showed slightly lower activity (Supporting
Information, Table O). The transmission electron microscopy
(TEM) image of the catalyst system at this stage indicated the
presence of slightly larger particles (4−6 nm).9
The possible role of the Pd−Ni NC’s to catalyze the reaction is
depicted in Scheme 2. Coordination with the N atom of the
oxazole moiety of 6 with the hard Ni site (smaller in size than the
larger and softer Pd atom) forms Ia and makes the Ni site
electron rich. Transfer of the electron density from Ni to the
larger and softer Pd site makes it more nucleophilic (electron
rich), and thus, Pd oxidatively inserts into the C−Br bond to
form the metallacycle I. The formation of the six-membered
metallacycle with the Pd−Ni binary NC compared to the five-
membered metallacycles in the case of either the PdNPs or the
NiNPs also adds up to the cause of better catalytic potential of
the binary NC apart from the cooperative effect of the Pd and Ni
NPs in the binary Pd−Ni NC system. Subsequent ligand (Br−
anion) dissociation and base-induced transmetalation with the
aryl moiety of the arylboronic acid 7 (through its anion) with I
forms II. Reductive elimination from II gives rise to the product 8
a
6a (1.0 mmol) in DMF (2 mL) was treated with the various
arylboronic acids (1.2 mmol, 1.2 equiv) in the presence of NiCl2·6H2O
(2.5 mol %), PdCl2 (2.5 mol %), K2CO3 (1.2 equiv), and TBAF (10
mol %) at 100 °C. Isolated yield of the corresponding biaryl 8.
b
condition was compatible with various functional groups, e.g., Cl,
OCH3, SCH3, OCH2Ph, CHO, NO2, COMe, CO2H, and
CH2OH. Sterically hindered 2-naphthylboronic acid underwent
the cross-coupling efficiently. The successful reaction with the
heteroaryl (entries 21 and 22), styryl (entry 23), and alkyl (entry
24) boronic acids demonstrate the broad substrate scope. In
particular, the reaction of 6a with 2-benzofuranboronic acid
(entry 21) demonstrates the feasibility of the Pd−Ni NC to
catalyze the Suzuki−Miyaura cross-coupling involving ortho-
heterocycle-tethered sterically hindered aryl bromide with
sterically hindered heteroaryl boronic acid. The use of Pd(OAc)2
(1 mol %), PCy3 (1.2 mol %), KF (3.3 equiv) in THF at rt for 4 h,
akin to the reported catalyst system,8b failed to produce the
Suzuki product for the reaction of 6a with 2-benzofuranboronic
acid, and only 19% yield was obtained in DMF at 100 °C for 4 h.9
The scope of the reaction was further evaluated for different
variation of the ortho-heterocycle-tethered aryl bromides 6 with
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Organic Letters
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Scheme 2. Plausible Role of the Pd−Ni NC To Catalyze the
Suzuki Cross-Coupling Reaction of 6 with 7 To Form 8
ACKNOWLEDGMENTS
K.S. thanks CSIR and DST (SR/S1/OC-33/2008), New Delhi,
India, for a senior research fellowship.
■
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ASSOCIATED CONTENT
■
S
* Supporting Information
(23) The role of the base has been ascribed to form the quadrivalent
aryl boron species through nucleophilic attack by the F−/OH−
[
Additional data, spectral data of all compounds, and scanned
spectra of new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
(a) Amatore, C.; Jutand, A.; Duc, G. L. Angew. Chem., Int. Ed. 2012, 51,
1379. (b) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 2116
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Soc. Chem. Commun. 2013, 49, 5886 and references cited therein.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: 91-(0)-172 2214683. Fax: 91-(0)-172 2214692. E-mail:
akchakraborti@niper.ac.in; akchakraborti@rediffmail.com.
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/ol500587m | Org. Lett. 2014, 16, 2334−2337