Novel monoligated imine-Pd-NHC complexes: Extremely active pre-catalysts for Suzuki-Miyaura coupling of aryl chlorides

Article abstract of DOI:10.1016/j.tetlet.2014.04.044

Novel monoligated imine-Pd-NHC pre-catalysts with extremely high activity for the coupling of aryl chlorides and aryl boronic acids have been well explored. Three diffident catalysts could be obtained through one reaction. Changes in imine ligands would lead to remarkable variation on catalytic activity. Under mild reaction conditions, high reaction yields were achieved. A wide range of biphenyls could be efficiently obtained at ultra low catalyst loadings of 0.005 mol %.

Full text of DOI:10.1016/j.tetlet.2014.04.044

Tetrahedron Letters 55 (2014) 3278–3282
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Tetrahedron Letters
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Novel monoligated imine–Pd–NHC complexes: extremely active
pre-catalysts for Suzuki–Miyaura coupling of aryl chlorides
a,b
An Shen a,b, Chen Ni b, Yu-Cai Cao a,b, , Hui Zhou a,b, Gong-Hua Song c, , Xiao-Feng Ye
⇑
⇑
a Shanghai Key Laboratory of Catalysis Technology for Polyolefins (Shanghai Research Institute of Chemical Industry), 345 East Yunling Road, Shanghai 200062, China
b Organic Chemistry Division, Shanghai Research Institute of Chemical Industry (SRICI), 345 East Yunling Road, Shanghai 200062, China
c Department of Pharmaceutical Engineering and Technology, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 February 2014
Revised 9 April 2014
Accepted 15 April 2014
Available online 23 April 2014
Novel monoligated imine–Pd–NHC pre-catalysts with extremely high activity for the coupling of aryl
chlorides and aryl boronic acids have been well explored. Three diffident catalysts could be obtained
through one reaction. Changes in imine ligands would lead to remarkable variation on catalytic activity.
Under mild reaction conditions, high reaction yields were achieved. A wide range of biphenyls could be
efficiently obtained at ultra low catalyst loadings of 0.005 mol %.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
NHC–Pd complexes
Imine ligands
Suzuki–Miyaura coupling
Aryl chlorides
Low catalyst loadings
Since the first report in 1979, the versatility of Suzuki–Miya-
ura coupling reaction has greatly progressed, especially inspired
by 2010 Nobel Prize in chemistry.1 It has become one of the
most powerful and convenient carbon–carbon bond-forming pro-
cess in agrochemistry, pharmaceutical chemistry, materials and
synthetic chemistry.2,3 Despite the successful application of aryl
iodides and bromides in the reaction, the use of aryl chlorides
is more significant and attractive due to their low cost, wide
availability, and good stability. However, aryl chlorides are more
difficult to be activated because of high C–Cl bond strength.
Thus, highly active catalysts are in great demand to improve this
transformation. Novel catalysts have been investigated most
intensively since 1998 when Fu first demonstrated Pd2(dba)3/Pt-
Bu3 catalytic system to facilitate the activation of aryl chlorides
in Suzuki–Miyaura cross coupling.4 Varieties of excellent cata-
lysts including phosphine palladium complexes, palladacycle
complexes and N-heterocyclic carbene palladium complexes
have been established, while relatively high catalyst loadings
were often required to attain satisfactory results.5 Very few liter-
atures revealed their achievements in Suzuki–Miyaura coupling
of aryl chlorides based on lower catalyst loadings (<0.5 mol %)
and milder reaction conditions.5f,6
Recently, a number of monoligated Pd–NHC complexes have
been prepared and show high levels of activity including phos-
phine ligands, allyl-palladium, acac-palladium, and Pd–PEPPSI–
NHC complexes (Fig. 1).5e–h Despite stability coming from NHC
ligands, it is known that palladacycles would exhibit higher air
and thermal stability. For the coupling of aryl halides with organo-
boronic acids, palladacycle catalysts are adequate catalysts with
TONs between 102 and 106.7 Thus, it would be quite attractive to
use both NHC ligands and palladacycles as catalysts for Suzuki–
Miyaura coupling of aryl chlorides.
In this context, we set out to develop monoligated imine–
Pd–NHC complexes as pre-catalysts with extremely high cata-
lytic activity for Suzuki–Miyaura coupling reaction of aryl
chlorides under relatively mild conditions. It is the first time
N
N
Ar
Ar
Cl
N
N
O
N
N
N
N
Ar
Ar
Ar
Ar
Ar
Ar
Cl Pd Cl
N
Pd
O
Pd
Pd
Cl
R
Cl
P(o-tolyl)3
1
2
3
4
(2001)
(2002-2006)
(2005)
(2006-2012)
⇑
Corresponding authors. Tel.: +86 021 52817942; fax: +86 021 52800850.
E-mail address: caoyc@srici.cn (Y.-C. Cao).
Figure 1. A selection of monoligated Pd–NHC complexes.
http://dx.doi.org/10.1016/j.tetlet.2014.04.044
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

A. Shen et al. / Tetrahedron Letters 55 (2014) 3278–3282
3279
N
N
R
R
N
N
N
R
N
N
R
PdCl2, LiCl
NaOAc
R
R
OMe
Me
OMe
Me
Cl Pd
N
Cl
KOtBu, THF, rt
OMe
Ph
Cl Pd
N
+
MeOH, rt
Ph
Ph
5a
6a
6c
N
N
R
R
N
iPr
iPr
N
N
R
PdCl2, LiCl
NaOAc
R
OH
Me
OH
Me
Cl Pd
N
Cl
KOtBu, THF, rt
R=
MeOH, rt
5b
6b
Scheme 1. Synthesis of Pd–NHC complexes.
to utilize imine-based ligands in Pd–NHC complexes which
could be prepared straightforward within only two steps
(Scheme 1).
Acetophenone oxime 5a reacted with PdCl2 at room tempera-
ture to give the corresponding palladacycle dimer,8a which was
fragmentized by IPr [1,3-bis(2,6-diisopropylphenyl) imidazolium
chloride] to form Pd–NHC complex 6 in the presence of KOtBu
in dry THF.8b Compounds 6a and 6c could be isolated by column
chromatography and recrystallized respectively as a yellow crys-
tals from hexane/ether, whose structures could be determined by
X-ray crystallographic study (Figs. 2 and 3). X-ray diffraction
analysis reveals that complex 6a crystallizes in triclinic system,
space group P-1 and the asymmetrical unit of the unit cell con-
tains two Pd atom (Pd1 and Pd2), two IPr ligands, and two imine
ligands. The Pd1 center is in a distorted quadrilateral coordination
environment, ligated by one carbon atom (C1) from IPr ligand,
one terminal chlorine atom (Cl1) and the other two atoms (C28
and N3) from the same imine ligand. The Pd2 center is also quad-
rilateral coordinated by two carbon atoms (C37, C64), one nitro-
gen atom (N6) and one chlorine atom (Cl2), which is very
similar to Pd1. Palladium is found to be well coordinated in the
X-ray crystal structure. While Pd–NHC complex 6c crystallizes
in orthorhombic system, space group P2(1)2(1)2(1) and the
asymmetrical unit of the unit cell contains one Pd atom (Pd1),
one IPr and one imine ligand. Aza-allyl scaffold coordinated to
Pd1 center is similar to the reported structures.5f But in com-
pound 6c, the distances of Pd1-C35 and Pd1-N3 are shorter than
the corresponding distances, while the distance of Pd1-C34 is
much longer. When acetophenone oxime 5b was employed as
substance, only one Pd–NHC complex 6b was obtained as product
in 79% yield.
Figure 3. X-ray crystal structure of Pd–NHC complex 6c.
With the catalysts in hand, we set out to test their catalytic
activities in Suzuki–Miyaura coupling especially at low catalyst
loadings. The coupling of 4-chlorotoluene and phenyl boronic acid
in the presence of 0.1 mol % of 6a was examined as a model system
to identify an effective reaction condition (Table 1). It is worthy to
note that catalyst loadings in the model system were decreased by
one order of magnitude as compared with the normal. Several
bases were investigated including K2CO3, K3PO4, KOH, and KOtBu.
It seems likely that the reaction would perform better with a stron-
ger base as an additive (Table 1, entries 1–3), but with KOtBu as an
exception. Only moderate yield was obtained, and ca. 30% of the
chloride was converted to toluene (Table 1, entries 4 and 13). It
was also noteworthy that the dechlorination byproduct was
detected to be ca. 5% in the case of KOH. Though in the cases of
K2CO3 and K3PO4, dechlorination did not appear, the conversion
of chloride was much lower. Screening of several solvents revealed
that alcoholic solvents would be beneficial to the reaction with
imine–Pd–NHC complexes 6a (Table 1, entries 3 and 5–8). To our
i
delight, PrOH mixed with 10% water could reduce the amount of
dechlorination byproduct effectively to ca. 1–2% and increase the
Figure 2. X-ray crystal structure of Pd–NHC complex 6a.

3280
A. Shen et al. / Tetrahedron Letters 55 (2014) 3278–3282
Table 1
Various reaction parameters for Suzuki–Miyaura couplinga
Cat.6 (0.1 mol%)/ base
PhB(OH)2
Cl +
Ph
Solvent, Temp
Entry
Cat.
Solvent
Base
Time (h)
Temp (°C)
Conv.b (%)
Yieldc (%)
1
2
3
4
5
6
7
8
6a
6a
6a
6a
6a
6a
6a
6a
6a
6ae
6ae
6ae
6a
6b
6b
6ce
iPrOH
iPrOH
iPrOH
iPrOH
MeOH
Toluene
DMF
Dioxane
iPrOHd
iPrOHd
iPrOHd
iPrOHd
iPrOH
iPrOH
iPrOH
iPrOHd
K2CO3
K3PO4
KOH
2
2
2
2
2
2
2
2
2
2
6
—
6
2
5
2
80
80
80
80
80
80
80
80
80
80
50
rt
6
26
86
100
63
27
4
6
26
81
66
62
27
4
KOtBu
KOH
KOH
KOH
KOH
KOH
KOH
KOH
4
4
9
90
89
88
—
99
82
56
85
88
87
87
—
67
67
18
85
10
11
12
13
14
15
16
KOH
KOtBu
KOH
50
80
80
80
KOtBu
KOH
a
b
c
Reaction condition: 4-chlorotoluene (3.0 mmol), PhB(OH)2 (3.15 mmol, 1.05 equiv), base (4.5 mmol, 1.5 equiv), solvent (5 mL).
Determined by GC.
GC yield of coupling product.
Mixed with water (iPrOH/H2O = 10:1).
Catalyst loading = 0.05 mol %.
d
e
product yield to 88% at the same time (Table 1, entry 9). Interest-
ingly, the same yield could be obtained when the catalyst loading
was decreased to 0.05 mol % (Table 1, entry 10). Moreover, the
reaction temperature can be lowered to 50 °C although a prolonged
time was needed to achieve a similar yield (Table 1, entry 11). But,
when the reaction was carried out under room temperature, the
result was not satisfactory (Table 1, entry 12).
As compared with 6a, pre-catalyst 6b led to an unsatisfactory
result because of the absence of one methyl group (Table 1, entries
3 vs 14 and entries 4 vs 15).9 On the other hand, pre-catalyst 6c
could perform as well as 3a, though the structure of imine ligand
was quite different (Table 1, entries 10 vs 16). Thus, we reasoned
that using 0.05 mol % 6a or 6c as pre-catalyst in combination with
iPrOH–H2O as solvent and 1.5 equiv KOH as base might be opti-
mized reaction conditions (Table 1, entries 10 and 16).
dine and 2-chlorothiophene as representative substrates, giving
the corresponding products in 93% yields. And the yields could
be increased to 99% when catalyst 6c was used (Table 2, entries
6–7). In addition, even better results could be afforded when aryl
boronic acids with 4-tBu and 4-F substituent were used (Table 2,
entries 12, 13 and 15, 16). It was also noted that catalysts were
quite active that most of the reactions completed in 2 h with
84–97% isolated yields (in the presence of catalyst 6a), which
implies the feasibility of using lower catalyst loadings in this
reaction.
Inspired by these results and our continuous interest in explor-
ing the limits of the catalytic activity, the amount of pre-catalyst 6
was reduced one step further to only 0.005 mol %. An ideal catalyst
6d explored by Nolan5f was chosen as a contrast.10 Some represen-
tative substrates presented in Table 3 were investigated under the
same condition in the presence of 6a, 6c, and 6d. To our delight, in
most cases, the reaction was carried out smoothly and approached
to corresponding products in very high yields. Notably, the activity
of catalyst 6c was much better than that of catalysts 6a and 6d. The
contrast of catalyst 6c and catalyst 6d was quite interesting. For
ortho-substituted aryl chlorides, catalyst 6c performed better in
all cases (Table 3, entries 1–7). It is implied that catalyst 6d is
not effective enough in our milder reaction conditions. While sub-
strates with electron-withdrawing groups were selected, both of
the catalysts achieved the same level of activity (Table 3, entries
8–10).
With optimized conditions in hand, we set out to explore the
scope of the cross coupling of aryl chlorides and aryl boronic
acids. As summarized in Table 2, a series of aryl chlorides 7
i
and aryl boronic acids 8 were investigated in PrOH/H2O in the
presence of 0.05 mol % pre-catalyst
6 and 1.5 equiv KOH at
80 °C (Table 2).
Gratifyingly, all of the coupling proceeded rapidly and effi-
ciently to afford the desired biphenyls in remarkable yields, what-
ever catalyst 6a or 6c was used. Both electron-donating and -
withdrawing groups on the phenyl ring of aryl chlorides did not
seem to affect the product yields apparently (Table 2, entries 1–
10). It was illustrated that the reaction was quite feasible with sin-
gle ortho-substituted aryl chlorides whatever the aryl boronic acids
were used (Table 2, entries 2, 4, 11, 12, and 15). A more sterically
hindered double substitution in ortho positions of aryl chlorides
was also tolerated in this reaction condition when catalyst 6a
was used, though a slightly lower yield was observed (Table 2,
entries 3, 14, and 16). However, when catalyst 6c was employed,
the reaction of double substitution aryl chlorides proceeded better
(Table 2, entries 14 and 16).
In conclusion, we have developed several novel monoligated
imine–Pd–NHC complexes which showed extremely high activity
in the coupling of aryl chlorides and aryl boronic acids. Our
observation of dramatic methyl group effect implied a crucial
factor between catalytic activity and the structure of imine
i
ligand. The use of PrOH–H2O as the solvent and KOH as the base
at 80 °C proved to be an efficient and mild condition for the syn-
thesis of biphenyls in excellent yields with only 0.05 mol % cata-
lyst loadings. More impressively, almost the same level yields
could also be achieved even at the condition of 0.005 mol %
catalyst loadings, especially with catalyst 6c. Further studies
The compatibility of heteroaromatic aryl chlorides in this pro-
cess was also proven to be feasible by employing 3-chloropyri-

A. Shen et al. / Tetrahedron Letters 55 (2014) 3278–3282
3281
Table 2
Table 3
Suzuki–Miyaura coupling of various aryl chlorides and aryl boronic acids in the
presence of 6a and 6c
Suzuki–Miyaura coupling of aryl chlorides and aryl boronic acids at 0.005 mol %
catalyst loading11
Cat.6 (0.05 mol%)
KOH (1.5 equiv)
Cat.6 (0.005 mol%)
KOH (1.5 equiv)
i PrOH-H2O, 80 o
C
ArCl
+
Ar'B(OH)2
ArCl
+
Ar'B(OH)2
Ar Ar'
9
Ar Ar'
iPrOH-H2O, 80 o
C
7
8
7
8
9
i Pr
N
i Pr
i Pr
N
i Pr
N
i Pr
N
i Pr
N
Entrya Aryl chlorides
Boronic acids
Product
Yieldb (%)
6a 6c
N
iPr
i Pr
i Pr
i Pr
N
iPr
i Pr
OMe
Me
87
(85)
OMe
Cl Pd
Pd
Cl Pd
N
Cl
Cl
1
2
9a
9b
85
(HO)2B
(HO)2B
6a
6c
6d
Ph
>99
(92)
>99
94
Cl
Cl
Entrya Aryl chlorides
Boronic acids
Product
Yieldb (%)
(HO)2B
6a 6c 6d
92
(86)
3
4
5
9c
9d
9e
(HO)2B
1
9b
9k
82 98 88
Cl
OMe
(HO)2B
(HO)2B
96
(92)
94
87
Cl
Cl
Cl
2
88 97 81
(HO)2B
(HO)2B
(HO)2B
88
(84)
F
3
4
9l
37 99 98
56 92 39
Cl
Cl
Cl
(HO)2B
(HO)2B
93
(89)
6
7
9f
97
99
OMe
N
9d
Cl
OMe
93
(86)
9g
S
t Bu
(HO)2B
5
6
9o
9n
69 92 45
73 90 80
>99
(96)
F3C
Cl
Cl
Cl
8
9h
9i
>99
98
Cl
(HO)2B
(HO)2B
(HO)2B
95
(91)
OHC
Ac
9
Cl
(HO)2B
(HO)2B
95
(88)
10
9j
95
t Bu
96
(90)
Cl
7
9p
75 91 88
Cl
11
9k
99
(HO)2B
(HO)2B
(HO)2B
(HO)2B
(HO)2B
F3C
Ac
Cl
Cl
Cl
8
9h
9j
93 98 96
87 92 94
93 96 96
(HO)2B
(HO)2B
(HO)2B
F
F
>99
(91)
12
13
9l
>99
>99
Cl
Cl
9
99
(97)
F3C
Cl
9m
F
F3C
10
9m
91
(86)
a
Reaction condition: catalyst 6 (0.005 mol %), ArCl 7 (3.0 mmol), ArB(OH)2
8
14
15
16
9n
9o
9p
97
96
99
(3.15 mmol, 1.05 equiv), KOH (4.5 mmol, 1.5 equiv), iPrOH/H2O = 10:1 (5 mL).
Reactions were completed in 2 h.
OMe
b
GC yield based on ArCl 7.
t Bu
t Bu
>99
(93)
Cl
Acknowledgements
(HO)2B
90
(84)
Cl
Financial supports from Shanghai Municipal Science and Tech-
nology Commission (12NM0504500 and 13XD1421700) are grate-
fully acknowledged. We also acknowledge Prof. Guo-Qiang Lin
from Shanghai Institute of Organic Chemistry, CAS and Prof.
Xing-Wen Sun from Fudan University for their analysis of X-ray
crystal and HRMS.
Cl
B(OH)2
93
(85)
17
9q
96
a
Reaction condition: catalyst 6a (0.05 mol %), ArCl 7 (3.0 mmol), ArB(OH)2
8
(3.15 mmol, 1.05 equiv), KOH (4.5 mmol, 1.5 equiv), iPrOH/H2O = 10:1 (5 mL).
Reactions were completed in 2 h.
Supplementary data
b
GC yield based on ArCl 7 (isolated yield).
Supplementary data (detailed description of all experimental
procedures, spectroscopic data and crystallographic data of CCDC
945633 & CCDC 977586) associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.04.
044. These data include MOL files and InChiKeys of the most
important compounds described in this article.
concerning the function of the substitution on imine ligand, the
application of the pre-catalyst in other coupling reactions, and
some more similar pre-catalysts with better catalytic activity
are underway.

3282
A. Shen et al. / Tetrahedron Letters 55 (2014) 3278–3282
Pompeo, M.; Froese, R. D. J.; Hadei, N.; Organ, M. G. Angew. Chem., Int. Ed. 2012,
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