A new dimeric bidentated NHC-Pd(II) complex from trans-cyclohexane-1,2- diamine for Suzuki reaction and Heck reaction

Article abstract of DOI:10.1016/j.tet.2005.03.043

A new dimeric bidentated NHC-Pd(II) complex was synthesized from trans-cyclohexane-1,2-diamine and its structure was characterized by MALDI Mass spectrum. This NHC-Pd(II) complex was fairly effective in Suzuki and Heck reactions to give the products in good to excellent yields in most cases.

Full text of DOI:10.1016/j.tet.2005.03.043

Tetrahedron 61 (2005) 4949–4955
A new dimeric bidentated NHC–Pd(II) complex from
trans-cyclohexane-1,2-diamine for Suzuki reaction
and Heck reaction
*
Min Shi and Heng-xin Qian
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science,
354 Fenglin Lu, Shanghai 200032, China
Received 18 January 2005; revised 8 March 2005; accepted 10 March 2005
Available online 31 March 2005
Abstract—A new dimeric bidentated NHC–Pd(II) complex was synthesized from trans-cyclohexane-1,2-diamine and its structure was
characterized by MALDI Mass spectrum. This NHC–Pd(II) complex was fairly effective in Suzuki and Heck reactions to give the products in
good to excellent yields in most cases.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
the synthesis of a novel dimeric bidentated NHC–Pd(II)
complex derived from trans-cyclohexane-1,2-diamine and
its application in Suzuki–Miyaura reaction and Heck
reaction.
With its unique ligating properties, N-heterocyclic carbenes
(NHC) have long been the subject of both synthetic and
catalytic studies. Owing to the high stability of the metal
complexes of NHC’s toward heat, moisture and oxygen,
these complexes have been successfully used in a wide
range of catalytic reactions.1 Among these versatile
catalysts, bis(carbene)palladium(II)dihalide complexes are
the efficient catalysts for various C–C and C–N coupling
reactions.2
2. Results and discussion
The synthesis of dimeric bidentated NHC–Pd(II) complex 5
is shown in Scheme 1. Using trans-cyclohexane-1,2-
diamine (1.0 equiv) as starting material to react with
2-fluoronitrobenzene (2.0 equiv) in anhydrous ethanol,
N,N0-bis(2-nitrophenyl)cyclohexane-1,2-diamine 1 was
obtained in 77% yield under reflux in the presence of
NaHCO3 for 10 h. The reduction of 1 with Pd–C in
dichloromethane under H2 atmosphere (1.0 atm) at room
temperature for 2 days afforded N,N0-bis(2-nitrophenyl)-
cyclohexane-1,2-diamine 2 in 92% yield. The treatment of
2 by triethyl orthoformate containing a catalytic amount of
TsOH gave compound 3 in 77% yield at room temperature
for 2 days. The benzimidazolium iodide 4 was obtained
by stirring compound 3 with CH3I in CH2Cl2 at room
temperature as a yellow precipitate in 79% yield. Reaction
of NHC precursor 4 with Pd(OAc)2 in DMSO at 85 8C for
6 h afforded the desired NHC–Pd(II) complex 5 in 80%
yield. This pale green powder is stable even at more than
350 8C under ambient atmosphere. We can not obtain its
single crystals for X-ray diffraction because it could only
be poorly dissolved in DMA or DMF and could hardly be
recrystallized from organic solvents. Therefore, its structure
was determined by 1H NMR, MALDI Mass spectrum
The palladium-catalyzed biaryl fomation of aryl halides
with arylboronic acids (the Suzuki reaction)3 and arylation
and alkenylation of olefins (the Heck reaction)4 have turned
out to be two of the most powerful methods for the
formation of carbon–carbon bond in organic synthesis.
A number of palladium/N-heterocyclic carbene systems
have been employed in these processes with great success.5
Most of them used in situ formed palladium-NHC species
from imidazolium salts and palladium sources in the
presence of base.5c,e,f,k,l There are only a few examples
using prepared bis(carbene)palladium(II)dihalide com-
plexes as catalysts in above two coupling reactions.2,6
These carbene–Pd(II) complexes have fixed ligand/metal
ratio and are easy to handle. In this paper, we wish to report
Keywords: trans-Cyclohexane-1,2-diamine; Suzuki–Miyaura cross-coup-
ling reaction; Heck reaction; Dimeric bidentated NHC–Pd(II) complex;
Benzimidazolium iodide.
Corresponding author. Tel.: C86 21 5492 5137; fax: C86 21 64166128;
e-mail: mshi@pub.sioc.ac.cn
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.03.043

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M. Shi, H. Qian / Tetrahedron 61 (2005) 4949–4955
Scheme 1. Synthesis of the benzimidazolium salt and Pd(II)–NHC complex 5.
(Matrix-assisted laser desorption/ionization)7 and elemental
analysis. On the basis of MALDI Mass spectrum (Fig. 1),
a molecular ion peak was identified at 1283 (MCKI) along
with a ion peak at 577 (1/2MCKH–I) by this soft and
sensitive ionization technique. Thus, this NHC–Pd(II)
complex 5 should be a dimeric bidentated NHC–Pd(II)
complex.
Using these optimized reaction conditions, the Suzuki–
Miyaura reaction of a variety of arylhalides, including
arylbromides and phenyliodide, with phenylboronic acid
were studied. The results are summarized in Table 3. As can
be seen, arylbromides with different substituents on the
benzene ring afforded coupling products 6 in 54–99% yield
under ambient atmosphere within 2–10 h (Table 2, entries
1–6).
The application of Pd(II)–NHC complex 5 as a catalyst for
Suzuki–Miyaura cross-coupling reaction was first examined
where base, reaction temperature and additive effects
were carefully examined in the reaction of phenylboronic
acid with bromobenzene under ambient atmosphere in
N,N-dimethylacetamide (DMA). This is because Pd(II)–
NHC complex 5 could only be poorly dissolved in DMA.
The results are summarized in Tables 1 and 2, respectively.
The use of K2CO3 or Cs2CO3 (1.5 equiv) as the base in
DMA (2.0 mL) at 120 8C gave the coupled product biphenyl
6a in 94 and 92% yield after 2 h, respectively in the
presence of additive C16H33Me3NBr (1.0 mmol%)8 (Table
1, entries 1 and 2). Under the identical conditions, 6a was
obtained in 84% yield using KOAc or K3PO$43H2O as a base
(Table 1, entries 3 and 5). The other bases such as KF$2H2O,
NaHCO3 and KOH are not as effective as K2CO3 and
Cs2CO3. In the presence of organic bases such as DMAP
and DBU, no reaction occurred (Table 1, entries 6 and 7).
Thus, the relatively cheap K2CO3 was the preferred base
for this reaction. We also examined the effect of the reac-
tion temperature and the additive C16H33Me3NBr on this
reaction. The results are summarized in Table 2. We found
that under identical conditions, the coupled product 6a was
obtained in higher yield at higher temperature after 2 h and
no reaction occurred at temperature 15 8C (Table 1, entries
10–13). Without addition of additive C16H33Me3NBr, 6a
was obtained in similar yield (95%) at 120 8C after 12 h
(Table 1, entries 13 and 14).
On the other hand, we also examined the Suzuki–Miyaura
cross-coupling reaction of arylchlorides with phenylboronic
acid using this novel dimeric bidentated NHC–Pd(II)
complex 5 in DMA. The results are elucidated in Table 3.
We found that under the similar conditions as those shown
in Table 2 with KOBut as a base, 6a was obtained in only
15% (Table 3, entry 1). Previously, Nolan et al. reported that
using alcohols as solvent, the Suzuki–Miyaura cross-
coupling reaction of arylchlorides with phenylboronic acid
could be achieved at room temperature in the presence of
NaOtBu.9 Therefore, we examined the additive effects of
iso-propanol and tert-amyl alcohol (1.5 mL) on this type
of Suzuki–Miyaura cross-coupling reaction. We found that
with the addition of tert-amyl alcohol (1.5 mL), 6a can be
obtained in 55% yield under identical conditions. For two
other arylchlorides such as 4-methylphenylchloride and
4-acetylphenylchloride, the corresponding coupled products
6d and 6e were obtained in 45 and 62% yields, respectively
(Table 3, entries 4 and 5).
Heck reaction was also examined in DMA by the reaction of
bromobenzene with butyl acrylate in the presence of various
bases and additives (Table 4, entries 1–6). We found
that K2CO3 afforded the best results for this reaction and
allowed the coupling product 7a to be obtained in 80%
under ambient atmosphere at 160 8C in the presence of
C16H33Me3NBr (1.0 mol%) (Table 4, entry 6).

M. Shi, H. Qian / Tetrahedron 61 (2005) 4949–4955
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M. Shi, H. Qian / Tetrahedron 61 (2005) 4949–4955
Table 1. Screening for base, additive and reaction temperature in the NHC–Pd(II) complex 5 catalyzed Suzuki-cross coupling reaction of bromobenzene
(1.0 equiv) with phenylboronic acid (1.2 equiv) in DMA (2.0 mL)
Entry
Temp. (8C)
Base
Time (h)
6a Yield (%)a
1
2
3
4
5
6
7
8
120
120
120
120
120
120
120
120
120
90
K2CO3
Cs2CO3
KOAc
KF$2H2O
K3PO4$3H2O
DMAP
DBU
2
2
2
2
2
2
2
2
94
92
84
58
84
N.Rb
N.Rb
65
NaHCO3
KOH
9
2
32
84
32
N.R.
94
95
10
11
12
13
14c
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
12
12
12
12
12
50
15
120
120
a Isolated yields.
b No reaction occurred.
c Without addition of additive C16H33Me3NBr.
Using these reaction conditions, we next examined the Heck
coupling of a variety of arylhalides with butyl and methyl
acrylate. The results are summarized in Table 5. We found
that the Heck reaction products 7 were obtained in good to
high yields in most cases under ambient atmosphere (Table
5, entries 1–3 and 5–10). For methyl acrylate, the coupled
product 7f was obtained in moderate yield (Table 5, entry
7). Product structures were determined by 1H and 13C NMR
spectroscopy (see Supporting information).
In conclusion, we disclosed a novel dimeric bidentated
NHC–Pd(II) complex 5 as an effective catalyst for Suzuki–
Miyaura reaction and Heck cross-coupling reaction. The
corresponding coupled products were obtained in good to
high yields in most cases by this Pd(II)-catalyst under
normal conditions. Efforts are underway to elucidate the
mechanistic details of this C–C bond forming reaction
catalyzed by Pd(II)–NHC complex and the use of 5 to
catalyze other C–C bond forming transformations.
Table 2. NHC–Pd(II) complex 5 catalyzed Suzuki-cross coupling reaction of arylbromide (1.0 equiv) with phenylboronic acid (1.2 equiv)
Entry
1
Ar
Time (h)
10
6 Yield (%)a
6b, 80
2
3
6c, 92
3
4
5
3
6d, 99
6e, 73
6a, 94
10
2
6
10
3
6f, 54
6a, 85
7
a Isolated yields.

M. Shi, H. Qian / Tetrahedron 61 (2005) 4949–4955
4953
Table 3. NHC–Pd(II) complex 5 catalyzed Suzuki-cross coupling reaction of arylchloride (1.0 equiv) with phenylboronic acid (1.2 equiv)
Entry
1
Ar
Base
Additive
None
6a Yield (%)a
6a, 15
KOC(CH3)3
2
3
4
KOC(CH3)3
KOC(CH3)3
KOC(CH3)3
iso-Propanol
6a, 42
6a, 55
6d, 45
tert-Amyl alcohol
tert-Amyl alcohol
5
KOC(CH3)3
tert-Amyl alcohol
6e, 62
a Isolated yields.
Table 4. NHC–Pd(II) complex 5 (0.5 mol%) catalyzed Heck-cross coupling reaction of phenylbromide (1.0 equiv) with butyl acrylate (1.5 equiv)
Entry
Additive
Basea
Temp (8C)
Time (h)
7a Yield (%)b
1
2
3
4
5
6
None
None
4A MS
No
No
C16H33(CH3)3NBr
K2CO3
160
160
160
160
160
160
12
12
12
12
12
12
15
Cs2CO3
K3PO4$3H2O
KOAc
Et3N
K2CO3
Trace
!10
!10
N.R.
80
a 1.5 equiv of base.
b Isolated yields.
Table 5. NHC–Pd(II) 5 (0.5 mol%) catalyzed Heck-cross coupling reaction of arylhalide (1.0 equiv) with butyl acrylate (1.5 equiv)
Entry
1
Ar–X
R
Temp (8C)
Time (h)
6
7 Yield (%)a
7a, 80
Bun
160
2
3
Bun
Bun
160
160
6
5
7a, 99
7b, 90
4
Bun
Bun
160
160
5
3
7c, 88
7d, 88
5
6
Bun
Me
160
160
10
6
7e, 80
7f, 45
7
a Isolated yield.

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M. Shi, H. Qian / Tetrahedron 61 (2005) 4949–4955
1
3. Experimental
(CH2Cl2): n 3443, 2941, 1655, 1492, 741 cmK1; H NMR
(CDCl3, TMS, 300 MHz): d 1.65–1.80 (2H, m, CH2), 2.12–
2.19 (2H,m, CH2), 2.20–2.40 (4H, m, CH2), 4.60–4.75 (2H,
m, CH), 7.00–7.15 (6H, m, ArH), 7.50–7.65 (2H, m, ArH),
7.73 (2H, s, N]CH); 13C NMR (CDCl3, 75 MHz): d 25.0,
32.2, 59.1, 109.1, 120.2, 122.0, 122.7, 132.5, 140.5, 143.3;
MS (EI) m/z: 317 (MHC, 25.32), 316 (MC, 100), 118
(MCK198, 46.32), 171 (MCK145, 40.75); HRMS
(MALDI/DHB) Calcd for C20H21NC4 1(MCHC) 317.1761;
found: 317.1752.
3.1. General remarks
All reactions were conducted in oven (135 8C) and flame-
dried glassware under ambient atmosphere of air and
moisture; dichloromethane was distilled from calcium
hydride; diethyl ether, tetrahydrofuran were distilled from
sodium metal/benzophenone ketyl. 1H and 13C NMR
spectra were recorded at 300 and 75 MHz, respectively.
Mass spectra were recorded by the EI method. All of the
solid compounds reported in this paper gave satisfactory
CHN microanalyses. Commercially obtained reagents were
used without further purification. All reactions were
monitored by TLC with silica gel coated plates.
3.1.4. Synthesis of compound 4. A mixture of 3 (200 mg,
0.63 mmol) and iodomethane CH3I (0.2 mL, 3.2 mmol)
were stirred in anhydrous CH2Cl2 at room temperature for 2
days. A white precipitation was formed. The solid
compound was obtained after filtration and washed with
diethyl ether (2!20 mL). The compound 4 was obtained as
a white powder (300 mg, yield: 79%). mp 305–307 8C; IR
3.1.1. Synthesis of N,N0-bis(2-nitrophenyl)cyclohexane-
1,2-diamine 1. trans-Cyclohexanediamine (a racemic
compound, 228 mg, 2 mmol), 2-fluoronitrobenzene
(564 mg, 4 mmol), and NaHCO3 (370 mg, 4.4 mmol) were
stirred in refluxing anhydrous ethanol for 10 h. Then the
reaction mixture was poured into 15 mL of ice water when
hot and was further stirred for 20 min. The organic layers
were extracted with CH2Cl2 (2!20 mL) and dried over
anhydrous Na2SO4. The solvent was removed under
reduced pressure and the residue was purified by a silica
gel flash column chromatography (eluent: hexane/
CH2Cl2Z1/1) to give 1 as an orange solid (547 mg, yield:
77%). This is a known compound.10
(KBr): n 3432, 3022, 1565, 769 cmK1; H NMR (DMSO,
1
300 MHz): d 1.68–1.72 (2H, m, CH2), 2.06–2.09 (4H, m,
CH2), 2.41–2.42 (2H, m, CH2), 3.96 (6H, s, CH3), 5.33–5.39
(2H, m, CH), 7.42–7.48 (4H, m, ArH), 7.63–7.71 (4H,
m, ArH), 10.00–10.03 (2H, s, CH); 13C NMR (DMSO,
75 MHz): d 24.7, 31.2, 34.4, 61.3, 112.4, 114.1, 127.4,
127.6, 131.5, 142.9; MS (MALDI): 473 (MCKI), 345
(MCKH-2I). Anal. Calcd for C22H26I2N4 C, 44.02; H, 4.37;
N, 9.33; found: C, 44.15; H,4.21; N, 9.36%.
3.1.5. Synthesis of NHC–Pd(II) complex 5. The compound
4 (70 mg, 0.12 mmol) was dissolved in 8 mL of DMSO and
was stirred at 65 8C, then Pd(OAc)2 (30 mg, 0.13 mmol)
was added. The oil bath was elevated to 85 8C during a
period of 1 h. A pale green precipitation was formed.
The solid was filtered and washed with CH2Cl2 (2!10 mL)
and dried in a dry box under reduced pressure to give NHC–
Pd(II) complex 5 (70 mg, yield: 80%). mp 360–362 8C
(dec.); IR (KBr): n 3483, 2940, 2854, 1477, 1448, 1342,
749 cmK1; 1H NMR (DMF-d7, 300 MHz): d 2.03–2.05 (3H,
m, CH2), 2.20–2.25 (1H, m, CH2), 2.57–2.59 (3H, m, CH2),
2.90–2.95 (1H, m, CH2), 4.13 (3H, s, CH3), 4.29 (3H, s,
CH3), 5.27–5.40 (2H, m, CH), 7.31–7.36 (5H, m, ArH),
7.64–7.79 (3H, m, ArH); MS (MALDI): 1283 (MCKI), 577
(1/2 MCKH–I); Anal. Calcd for C44H48I4N8Pd2 C, 37.50;
H, 3.43; N, 7.95; found: C, 37.56; H, 3.37; N, 7.80%.
1H NMR (CDCl3, TMS, 300 MHz): d 1.30–1.70 (4H, m,
CH2), 1.72–2.00 (2H, m, CH2), 2.18–2.25 (2H, m, CH2),
3.65–3.73 (2H, m, CH), 6.52 (2H, t, JZ7.5 Hz, ArH), 6.94
(2H, d, JZ8.4 Hz, ArH), 7.28 (2H, t, JZ7.5 Hz, ArH), 7.95
(2H, d, JZ8.4 Hz, ArH), 8.15 (2H, d, JZ5.7 Hz, NH).
3.1.2. Synthesis of N,N0-bis(2-aminophenyl)cyclohexane-
1,2-diamine 2. Under 1.0 atm of hydrogen atmosphere, a
mixture of 1 (2.86 g, 8.03 mmol) and 10 wt% Pd/C
(450 mg, 0.43 mmol) were stirred in anhydrous CH2Cl2 at
room temperature for 2 days. After filtration, the solvent
was removed under reduced pressure and the residue was
purified by a silica gel flash column chromatography
(eluent: CH2Cl2) to give 2 as a white solid (2.18 g, yield:
92%). mp 120–122 8C; IR (CH2Cl2): n 3296, 2933, 1597,
1508, 1267, 737 cmK1; 1H NMR (CDCl3, TMS, 300 MHz):
d 1.10–1.26 (2H, m, CH2), 1.36–1.43 (2H, m, CH2), 1.56
(2H, s, NH), 1.71–1.78 (2H, m, CH2), 2.33–2.41 (2H, m,
CH2), 3.10–3.30 (2H, m, CH), 3.37 (4H, s, NH2), 6.70–6.82
(8H, m, ArH); 13C NMR (CDCl3, 75 MHz): d 24.8, 32.2,
57.4, 113.4, 116.6, 119.1, 120.1, 135.6, 136.4; MS (EI) m/z:
296 (MC), 189 (86.80), 145 (15.66), 119 (100); Anal. Calcd
for C18H24N4 C, 72.94; H, 8.16; N, 18.90; found: C, 72.56;
H, 7.81; N, 18.64%. HRMS (MALDI/DHB) Calcd for
C18H25NC4 1(MCHC) 297.2074; found: 297.2076.
3.2. General procedure for the Suzuki reaction of aryl
halides with boronic acids
A typical procedure is given below on the reaction
expressed in entry 1 of Table 1. NHC–Pd(II) complex 5
(7.0 mg, 0.005 mmol), potassium carbonate (200 mg,
1.5 mmol), benzenebromide (110 mL, 1.0 mmol), phenyl-
boronic acid (148 mg, 1.2 mmol), and DMA (2.0 mL) were
introduced to a Schlenk tube under ambient atmosphere.
The mixture was stirred at 120 8C for 2 h. The reaction
mixture was diluted with H2O (15 mL) and Et2O (15 mL),
followed by extraction twice with Et2O. The organic layers
were dried over anhydrous MgSO4, filtered, and evaporated
under reduced pressure to give crude product. The pure
product was isolated by column chromatography (eluent:
hexane) on silica gel to give 147 mg (95%) of biphenyl as
3.1.3. Synthesis of compound 3. A mixture of 2 (2.18 g
7.36 mmol), TsOH (30 mg), and HC(OEt)3 (30 mL) were
stirred at room temperature for 2 days. The volatiles were
removed under reduced pressure and the residue was
purified by a silica gel flash column chromatography
(eluent: EtOAc/CH2Cl2/CH3OH/Et3N Z8/2/1/0.2) to give
3 as a gray solid (1.8 g, yield: 77%). mp 252–254 8C; IR
1
a colorless solid and was analyzed by H NMR and IR
spectroscopy.

M. Shi, H. Qian / Tetrahedron 61 (2005) 4949–4955
4955
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3.3. Typical reaction procedure for Heck reaction
In a Schlenk tube fitted with a septum and reflux condenser
were placed aryl halide (1.0 mmol), butyl acrylate
(1.5 mmol), potassium carbonate (200 mg, 1.5 mmol),
cetyltrimethylammonium bromide (4 mg, 0.01 mmol),
N,N-dimethylacetamide (DMA, 2.0 mL) and then NHC–
Pd(II) complex 5 (7.0 mg, 0.005 mmol) was added. The
reaction mixture was stirred at 160 8C for 6 h. The reaction
mixture was diluted with H2O (15 mL) and Et2O (15 mL),
followed by extraction twice with Et2O. The organic layers
were dried over anhydrous MgSO4, filtered, and evaporated
under reduced pressure to give crude product. A pure
product was isolated by column chromatography (eluent:
15/1 hexanes/ethyl acetate) on silica gel. The purified
1
product was analyzed by H NMR and IR spectroscopy.
Acknowledgements
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We thank the State Key Project of Basic Research (Project
973) (No. G2000048007), Shanghai Municipal Committee
of Science and Technology, and the National Natural
Science Foundation of China for financial support
(203900502, 20025206 and 20272069).
¨
¨
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V. P. W.; Herdtweck, E.; Grosche, M.; Herrmann, W. A.
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.03.
043
¨
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soft and sensitive ionization technique than others. Among the
chemical and physical ionization pathways suggested for
MALDI are gas-phase photoionization, ion-molecule reac-
tions, etc. The most widely accepted ion formation mechanism
involves gas-phase proton transfer or alkali metal transfer in
the expanding plume with photoionized matrix molecules. The
matrix will serve to minimize sample damage from the laser
pulse and enhance the appearance of molecular ionsde
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