P,N-Type benzimidazolyl phosphine ligands for the palladium-catalyzed Suzuki coupling of potassium aryltrifluoroborates and aryl chlorides

Article abstract of DOI:10.1016/j.tetlet.2012.05.017

This study describes the efficacy of P,N-type benzimidazolyl phosphine ligands, which can be easily synthesized from commercially available and inexpensive starting materials. The application of this ligand array in palladium-catalyzed Suzuki-Miyaura coupling reaction of aryl chlorides with potassium aryltrifluoroborates is described. The air stable benzimidazolyl phosphines in combination with a palladium metal precursor provides highly effective catalysts for the Suzuki-Miyaura coupling of unactivated aryl chlorides and can achieve a catalyst loading of only 0.05 mol %.

Full text of DOI:10.1016/j.tetlet.2012.05.017

Tetrahedron Letters 53 (2012) 3754–3757
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
P,N-Type benzimidazolyl phosphine ligands for the palladium-catalyzed
Suzuki coupling of potassium aryltrifluoroborates and aryl chlorides
⇑
⇑
Shun Man Wong, Chau Ming So , Kin Ho Chung, Chi Him Luk, Chak Po Lau, Fuk Yee Kwong
State Key Laboratory of Chirosciences and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
This study describes the efficacy of P,N-type benzimidazolyl phosphine ligands, which can be easily syn-
thesized from commercially available and inexpensive starting materials. The application of this ligand
array in palladium-catalyzed Suzuki–Miyaura coupling reaction of aryl chlorides with potassium aryltri-
fluoroborates is described. The air stable benzimidazolyl phosphines in combination with a palladium
metal precursor provides highly effective catalysts for the Suzuki–Miyaura coupling of unactivated aryl
chlorides and can achieve a catalyst loading of only 0.05 mol %.
Received 30 January 2012
Revised 27 April 2012
Accepted 4 May 2012
Available online 11 May 2012
Keywords:
Ó 2012 Elsevier Ltd. All rights reserved.
Suzuki coupling
P,N-Type ligand
Phosphine
Trifluoroborate
Palladium
Palladium-catalyzed Suzuki–Miyaura coupling reactions of
arylboronic acids with aryl halides represent a versatile method
for the preparation of diversified biaryls, which have a myriad of
applications in pharmaceutical, material, and agricultural chemis-
try.1 Advantages of Suzuki–Miyaura coupling include high func-
tional group tolerance, easily separable, and nontoxic byproducts,
and the commercial availability of boron coupling partners. These
advantages have made Suzuki–Miyaura coupling both a popular
and effective tool.2
In general, arylboronic acids are good nucleophilic organoboron
reagents in Suzuki–Miyaura coupling reaction.3 However, boronic
acids are noted to never be ideal because they exhibit several draw-
backs such as the partial formation of dimeric and cyclic trimeric
boroxines (which depend on storage water content). This structural
ambiguity affects the stoichiometry of boronic acids added to the
intended reaction. Organotrifluoroborate salts are attractive alter-
natives to aryl and heteroarylboronic acids because they are usually
sold in high purity from commercial sources and are readily avail-
able from other organoboron compounds.4,5 Recently, Molander,4,6
Buchwald,7 and many other respected researchers8 made superb
efforts to make advancements toward transition metal-catalyzed
Suzuki–Miyaura coupling using organotrifluoroborate salts as
coupling partners.
strategic ligand modification with appropriate steric/electronic nat-
ures toward great diversity is crucial in dealing with problematic
and specific substrates in this area. The advancement of some nota-
ble ligands such as t-Bu3P,9 Bellers’s PAP,10 Buchwald’s biaryl-phos-
phines,11 Hartwig’s Q-Phos,12 and Verkade’s amino-phosphine13
provide excellent catalytic activity in various cross-coupling of aryl
halides (especially aryl chlorides)14 (Fig. 1).15
Phosphine ligands that possess a potential hemilabile coordi-
nating group have been studied in the past decade.16 In 2004,
Hor co-workers reported a study on active ferroceneyl P,N-type
ligands with imino group for Suzuki–Miyaura coupling reaction.17
The X-ray crystal structures from the study showed that the
ferrocenyl hemilabile ligands provided both P and N coordination
to the palladium center. In 2005, a similar ligand using an aryl scaf-
fold was also found to be effective in coupling processes.18 Addi-
tionally, Liang et al. reported an amido phosphino complex as an
effective catalyst for the Suzuki coupling of aryl chloride19 and
He et al. reported the use of bis(aminephosphine) palladium che-
lated complexes for the Suzuki coupling of aryl bromide (Fig. 2).20
Hartwig group
Pt-Bu2
Ph
Fu group Buchwald group
R'
Beller group
PCy2
Verkade
group
R
Cross-coupling process efficiency has been widely recognized to
be significantly affected by the structure of the ligand. Therefore, a
R
N
Fe
Ph
N
PCy2
N
Ph
Ph
P
Pt-Bu3
R
N
Ph
R
R
N
⇑
Corresponding authors. Tel.: +852 3400 8682; fax: +852 2364 9932.
E-mail addresses: bccmso@inet.polyu.edu.hk (C.M. So), bcfyk@inet.polyu.edu.hk
PAP
Q-Phos
(F.Y. Kwong).
Figure 1. Recent development on effective phosphine ligands.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.05.017

S. M. Wong et al. / Tetrahedron Letters 53 (2012) 3754–3757
3755
Ar
N
Although a variety of ligands have been introduced, a rapid
assembly of structurally diverse ligand systems via simple syn-
thetic methods is still important for the development of versatile
catalysts for widespread applications of coupling reactions. In
continuing our interest in developing heterocyclic phosphine
ligands (Fig. 3),21 we herein report our exploration of a benzimid-
azole-derived phosphine. This class of ligands contains a tunable
main 2-phenylbenzimidazole skeleton. The N-chelating group on
the benzimidazole ring provides weak coordinating properties for
potential dynamic interaction to possibly lengthen catalyst longev-
ity. The attached tunable phosphino group at the bottom ring can
provide electron-richness and potential electronic fine-tuning.
The ligand precursor can be synthesized through simple con-
densation of the inexpensive o-phenylenediamine and 2-bromo-
benzoic acid in the presence of polyphosphoric acid.22 Moreover,
the phosphination of the benzimidazolyl precursor can give the
corresponding benzimidazolyl phosphines in good to excellence
yields (Scheme 1). It is noteworthy that this class of ligand exhibits
high air stability in both solid and solution states.23
To test the effectiveness of the benzimidazolyl phosphine
ligands, 2-chlorotoluene coupled with potassium phenyltriflourob-
orate was used as the benchmark entry (Table 1). Commonly used
organic solvents were firstly examined. Alcoholic solvents such as
t-BuOH and t-amyl alcohol showed superior result when compared
to the other polar aprotic solvents (Table 1, entries 1–5). These bet-
ter results of the alcoholic solvents may be due to the increasing
solubility of the triflouroborate salt. Mixing t-BuOH with toluene
gave the best results among those single solvents (Table 1, entry 6).
For the metal source, Pd(dba)2 gave the best result among
Pd(OAc)2, Pd(dba)2, and Pd2(dba)3 (Table 1, entries 7, 8, 12, 13).
Upon varying the metal/ligand ratio from 1:1 to 1:4, the ratio of
1:2 provided the highest yield (Table 1, entries 7, 9–11). The use
of Pd(dba)2 and metal/ligand ratio 1:2 was then chosen for further
screening process. Several common inorganic bases were
R
N
Ph
N
N
N
Fe
Ph2P
NH2
Pt-Bu2
PPh2
PPh2
PPh2
Figure 2. Recent developments on P,N ligands for Suzuki coupling reaction.
Previous work
This work
Efficient indolyl phosphine ligands for
aryl halide & ArOTs & ArOMs couplings
Benzimidazolyl
phosphine ligands
R'
N
MeN
MeN
N
PCy2
PR2
PCy2
N
PCy2
CM-phos
Figure 3. Efficient indolyl phosphine ligands for coupling reactions.
O
NH
PPA
NH2
NH2
+
HO
Br
N
Br
1, 70%
Me
Me
N
N
(i) NaH
(ii) Me2SO4
(i) n-BuLi
N
N
(ii) ClPR2
R2P
Br
2, 75%
L1, R = Ph, 80%
L2, PCy PhMezole-phos
R = Cy, 70%
L3, R = i-Pr, 83%
Scheme 1. Synthetic pathway of benzimidazolyl phosphine ligands.
Table 1
Optimization of reaction conditionsa
Me
Cl
Me
Me
Pd-L
PR2
N
Ph-BF3K
N
Base, Solvent
135oC, 20h
L1: R=Ph
L2: R=Cy
L3: R=iPr
Entry
Pd source
Mol % Pd (%)
M:L
Solvent
Base
Yeildb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20c
21d
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd2(dba)3
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(dba)2
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:3
1:4
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
t-BuOH
Toluene
Dioxane
DMF
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
K3PO4ꢀH2O
t-BuONa
70
16
16
15
60
99
99
34
44
55
37
82
28
12
10
56
3
t-Amyl alcohol
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
t-BuOH/toluene (1:1)
Cs2CO3
K3PO4
CsF
K2CO3
49
44
1
Na2CO3
K3PO4ꢀH2O
K3PO4ꢀH2O
80
a
b
c
Reaction conditions: Pd-L2, 2-chlorotoluene (1.0 mmol), PhBF3K (1.5 mmol), base (3.0 mmol), solvent (3.0 mL) were stirred for 20 h at 135 °C under nitrogen.
Calibrated GC yields were reported using dodecane as the internal standard.
L1 used as ligand.
L3 used as ligand.
d

3756
S. M. Wong et al. / Tetrahedron Letters 53 (2012) 3754–3757
examined in the presence of ligand L2. K3PO4ꢀH2O was found to be
the best base of choice in this catalytic system (Table 1, entries 12,
14–19). Ligand L1 with a diphenylphosphino moiety provided
trace conversion while the dicyclohexylphosphino analogue,
were compatible under these mild reaction conditions. The func-
tionalized aryl chlorides containing keto, ester, and nitrile groups
were found to be effective electrophiles (Table 2, entries 5–8).
Apart from a variety of aryl chlorides, heteroaryl chlorides were
also examined. Quinolyl and pyridyl substrates could couple with
potassium aryltrifluoroborate to furnish products in moderate to
excellent yields. (Table 2, entries 9–12).
In conclusion, we have developed a series of efficient P,N-type
benzimidazolyl phosphine ligands, which can be easily synthesized
from inexpensive and commercially available starting materials.
Palladium complexes derived from these ligands provide highly
active catalysts for Suzuki–Miyaura coupling of aryl chlorides with
potassium aryltrifluoroborates. Considering the simplicity of the
ligand synthesis as well as the easy modification of the ligand
L2, gave the best catalytic activity. Ligand L3 bearing
a
diisopropylphosphino moiety showed a low catalytic activity
toward the Suzuki coupling reaction (Table 1, entries 20 and 21).
The scope of this reaction was then investigated under the opti-
mized conditions. A range of aryl chlorides were examined and the
results are listed in Table 2. The deactivated aryl chloride contain-
ing a para-methoxy substituted group was found to be a feasible
coupling partner (Table 2, entry 4). ortho-Substituted aryl chlorides
and aryltrifluoroborates were also good substrates for this reaction
(Table 2, entries 2, 3, 5). A variety of common functional groups
Table 2
Palladium-catalyzed Suzuki coupling of ArCl with ArBF3Ka
Cy2P
R
N
Pd(dba)2/L2
R
Ar-BF3K
t-BuOH/toluene
K3PO4಺ H2O,
135oC
N
Me
L2
Cl
R'
Entry
1
ArCl
Ar0-BF3K
Product
Mol % Pd
0.2
Yeildb (%)
94
Me
Me
1
KF3B
KF3B
Cl
Me
Me
2
3
4
0.2
0.2
0.2
98
98
79
Cl
Me
Me
Me
Me
KF3B
KF3B
Cl
MeO
MeO
Cl
O
Me
Me
O
Ph
5
6
7
0.05
0.05
0.05
99
92
85
KF3B
Ph
Cl
O
KF3B
O
Me
Me
Cl
Me
Me
Cl
MeOOC
KF3B
KF3B
MeOOC
8
9
0.05
0.05
83
88
NC
N
Cl
NC
Me
Me
KF3B
KF3B
N
Cl
Cl
N
N
10
0.05
80
Me
Me
N
N
KF3B
KF3B
11
12
0.05
0.05
98
99
Cl
MeO
MeO
N
N
Cl
a
Reaction conditions: Pd(dba)2-L2 = 1:2, ArCl (1.0 mmol), ArBF3K (1.5 mmol), K3PO4ꢀH2O (3.0 mmol), t-BuOH/toluene (1:1) (3.0 mL) were stirred for 24 h at 135 °C under
nitrogen. Catalyst loading and time for the reactions were not optimized.
b
Isolated yield.

S. M. Wong et al. / Tetrahedron Letters 53 (2012) 3754–3757
3757
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Acknowledgments
We thank the Research Grants Council of Hong Kong (GRF:
PolyU 5010/11P), PolyU Internal Competitive Research Grant
(A-PG13) and the University Grants Committee Areas of Excellence
Scheme (AoE/P-10/01) for their financial support. We are grateful
to Professor Albert S. C. Chan’s research group (PolyU Hong Kong)
for kindly sharing their GC instruments.
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