Synthesis, structure, and catalytic activity of palladium complexes with new chiral cyclohexane-1,2-based di-NHC-ligands

Article abstract of DOI:10.1016/j.jorganchem.2011.12.008

A pair of novel palladium complexes {[(1S,2S)-L]PdBr₂} and {[(1R,2R)-L]PdBr₂} of chiral di-N-heterocyclic carbene ligands derived from chiral 1,2-cyclohexanediamine have been prepared in moderate to good yields and characterized by NMR, HRMS and X-ray single crystal diffraction studies. The obtained chiral NHC-Pd compound was able to catalyze the asymmetric Suzuki-Miyaura couplings of aryl bromides with arylboronic acids in good yields and moderate enantioselectivities (up to 61% ee).

Full text of DOI:10.1016/j.jorganchem.2011.12.008

Journal of Organometallic Chemistry 700 (2012) 223e229
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis, structure, and catalytic activity of palladium complexes with new chiral
cyclohexane-1,2-based di-NHC-ligands
*
Gongwo Shigeng, Junkai Tang, Dao Zhang , Quanrui Wang, Zhenxia Chen, Linhong Weng
Department of Chemistry, Fudan University, Handan Road 220 Shanghai, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A pair of novel palladium complexes {[(1S,2S)-L]PdBr2} and {[(1R,2R)-L]PdBr2} of chiral di-N-heterocyclic
carbene ligands derived from chiral 1,2-cyclohexanediamine have been prepared in moderate to good
yields and characterized by NMR, HRMS and X-ray single crystal diffraction studies. The obtained chiral
NHCePd compound was able to catalyze the asymmetric SuzukieMiyaura couplings of aryl bromides
with arylboronic acids in good yields and moderate enantioselectivities (up to 61% ee).
Ó 2012 Elsevier B.V. All rights reserved.
Received 12 September 2011
Received in revised form
27 November 2011
Accepted 6 December 2011
Keywords:
Chiral N-heterocyclic carbene complex
Palladium
Crystal structure
Asymmetric SuzukieMiyaura coupling
1. Introduction
A substantial amount of work on the preparation of chiral NHC
ligands has been published [40e63]. However, little attention has
Axially chiral biaryls are common structure motifs in natural
products [1] and are the core for many of the most effective chiral
ligands [2], such as the well-known BINAP [3,4], MOP [5] and BINOL
[6] ligands. The palladium-catalyzed asymmetric SuzukieMiyaura
coupling has recently emerged as an attractive alternative to
standard methods for the control of biaryl axial chirality [7]. Until
now there are few reports of asymmetric SuzukieMiyaura
reactions [8e15]. One of the challenges is to find suitable chiral
ligands to support catalysts that are able to induce high levels of
enantioselectivities.
N-Heterocyclic carbene (NHC) ligands have been studied
intensely recently and are now used widely as strongly basic
phosphine analogues, to support early and late transition metal
complexes [16e27]. Late transition metal complexes of NHC ligands
usually reveal higher air- and moisture-stability and have attracted
considerable attention as homogeneous catalysts [28,29]. Palladium
complexes have been widely used as catalysts for carbonecarbon
bond forming reaction [30,31]. The NHC complexes of palladium
show excellent catalytic properties for SuzukieMiyaura coupling
reactions of aryl halides and arylboronic acids [32e39]. Among
these, an early example of the use of a well-defined PdeNHC cata-
lyst used for SuzukieMiyaura cross-coupling was reported by
Herrmann in 2002 [32].
been focused on the asymmetric catalytic reaction based on these
chiral NHCePd species. Quite recently, Labande and Poli have re-
ported the first example of asymmetric SuzukieMiyaura coupling
utilizing an NHCePd pre-catalyst (ee% value is less than 40%) [64].
Although direct construction of chiral biaryls has provided some
limited success, there are much room for the improvement of the
enantioselectivity.
Chart 1 gives two reported examples of metal complexes
bearing chiral di-NHC ligands derived from 1,2-cyclohex-
anediamine [51,63]. These chelating coordination bidentate bis-
NHC ligands are expected to form higher stable catalysts capable
of tolerating harsher reaction conditions than monodentate NHCs.
Herein we wish to report the synthesis and structural character-
ization of a pair of novel palladium species with chiral di-N-
heterocyclic-carbene ligands (Scheme 1). Their catalytic behavior
for asymmetric SuzukieMiyaura coupling reaction was also
examined.
2. Experimental section
2.1. General considerations
All manipulations were performed under an inert atmosphere of
dry argon by using vacuum line and Schlenk tube techniques.
Solvents for syntheses were dried and degassed by standard
methods before use. Commercial chemicals were from Acros,
* Corresponding author. Tel.: þ86 21 65643021; fax: þ86 21 65641740.
E-mail address: daozhang@fudan.edu.cn (D. Zhang).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.12.008

224
G. Shigeng et al. / Journal of Organometallic Chemistry 700 (2012) 223e229
Fig. 1. Molecular structure of complex Pd[(1S,2S)-L]Br2. All hydrogen atoms and solvent molecules are omitted for clarity. Selected distances (Å) and angles (deg): Pd(1)e
C(1) 1.987(8), Pd(1)eC(14) 1.982(7), Pd(1)eBr(1) 2.4642(10), Pd(1)eBr(2) 2.4716(12). C(14)ePd(1)eC(1) 84.3(3), C(14)ePd(1)eBr(1) 172.4(2), C(1)ePd(1)e
Br(1) ¼ 88.98(18), C(14)ePd(1)eBr(2) ¼ 92.3(2), C(1)ePd(1)eBr(2) ¼ 176.48(18), Br(1)ePd(1)eBr(2) ¼ 94.52(4).
¼
¼
¼
¼
¼
Aldrich, Alfa Aesar, or Fluka and used as received. (1S,2S)-(þ)-1,2-
cyclohexanediamine [65] and 2-(benzyloxy)-1-(bromomethyl)-
3,5-di-tert-butylbenzene [67] were prepared according to previ-
ously reported procedures.
High-resolution mass spectra (HRMS) were recorded on a TOFLC/
MS spectrometer. Optical rotations were measured in a 50 mm
cell with a PerkineElmer 241 photopolarimeter. GC chromato-
grams were recorded on a HP 4890A GC equipped with a DB-5 MS
UI capillary column and the products were identified by
comparison with authentic samples. HPLC chromatograms were
recorded on a Shimadzu LC-2010A (HT) equipped with a Chiralcel
OJ column.
NMR spectra were recorded on Bruker AV500 spectrometers.
1H and 13C chemicals shifts (
d) are given in ppm (the residual peak
of deuterated solvents was used as reference). Peaks are labeled as
singlet (s), doublet (d), triplet (t), multiplet (m) and broad (br).
Scheme 1. Synthesis of palladium complex {Pd[(1S,2S)-L]Br2}.

G. Shigeng et al. / Journal of Organometallic Chemistry 700 (2012) 223e229
225
2.2. Synthesis
CH2), 2.18 (m, 1H, CH2), 2.03 (m, 1H, CH2), 1.80 (m, 1H, CH2), 1.43 (br
m,18H, CH3), 0.85 ppm (br m,18H, CH3). 13C NMR (100 MHz, CDCl3):
2.2.1. Synthesis of palladium complex {Pd[(1S,2S)-L]Br2}
d 25.1, 25.7, 31.1, 31.2, 34.0, 35.5, 48.0, 61.0, 61.2, 63.1, 76.8, 77.1, 77.4,
2.643 g (10 mmol) of (1S,2S)-(ꢀ)-1,2-cyclohexane diamine
D
-
109.8, 111.5, 113.0, 113.4, 123.6, 123.8, 128.5, 132.5, 133.7, 134.2,
134.8, 146.1, 146.2, 146.6, 153.9, 154.0, 154.1, 169.8, 170.2, 176.2 ppm
(NCN). HRMS (positive ions): m/z 1223.326 (calcd for [MC þ Naþ]þ
1223.33), 1119.417 (calcd for [MCeBr]þ 1119.42). Anal. Calcd (%) for
C64H76Br2N4O2Pd (1199.54): C, 64.08; H, 6.39; N, 4.67. Found: C,
64.25; H, 6.45; N, 4.75.
tartrate, 2.822 g (20 mmol) of 2-fluoronitrobenzene and 3.528 g
(42 mmol) of NaHCO3 are mixed in anhydrous ethanol (15 mL) and
stirred under reflux for 18 h. The reaction is quenched with slow
addition of ice water. The organic layer is separated, the aqueous
layer is extracted with CH2Cl2 three times and the combined
organic layers are dried with anhydrous Na2SO4. The solvent was
removed under reduced pressure and the remaining solid was
purified by flash silica gel column chromatography (eluent: hexane/
CH2Cl2 ¼ 1/1) to afford (1S,2S)-N,N0-bis(2-nitrophenyl)cyclohexane
2.2.2. Synthesis of palladium complex {Pd[(1R,2R)-L]Br2}
Complex Pd[(1R,2R)-L]Br2 was prepared by a similar way to {Pd
[(1S,2S)-L]Br2} using ligand H2[(1R,2R)-L]Br2 (400 mg, 0.366 mmol).
20
-1,2-diamine [(1S,2S)-1] as an orange solid. Yield: 3.149
(8.9 mmol, 89%); [
20 209.91 (c 0.01, CHCl3).
Under 1.0 atm of pure hydrogen atmosphere, 3.887
g
Yield: 394 mg (0.32 mmol, 86%). [
a
]
ꢀ31.98 (c 0.1, CHCl3). 1H
D
a]
NMR(500 MHz, CDCl3): d 7.63 (m, 2H, CH), 7.51e7.38 (m, 4H, CH),
D
g
7.30e7.12 (m, 10H, CH), 6.95e6.93 (m, 2H, CH), 6.57 (d,
JHH ¼ 8.25 Hz, 1H, CH), 6.48 (d, JHH ¼ 8.25 Hz,1H, CH), 6.30e5.30 (br
m, 3H, CH2), 4.92 (m, 2H, CH2), 4.79e4.69 (m, 3H, CH2), 2.94 (m, 1H,
CH2), 2.77 (m, 1H, CH2), 2.58 (m, 1H, CH2), 2.28 (m, 2H, CH2), 2.18
(m, 1H, CH2), 2.03 (m, 1H, CH2), 1.80 (m, 1H, CH2), 1.42 (br m, 18H,
(10.91 mmol) of (1S,2S)-1 and 10 wt% Pd/C (612 mg, 0.58 mmol) are
stirred in anhydrous CH2Cl2 at room temperature for 2 days. After
filtration, the solvent is removed under reduced pressure and the
residue was purified by flash silica gel column chromatography
(eluent: CH2Cl2) to afford (1S,2S)-N,N0-bis(2-aminophenyl)cyclo-
CH3), 0.85 ppm (br m, 18H, CH3). 13C NMR (100 MHz, CDCl3):
d 25.1,
hexane-1,2 -diamine [(1S,2S)-2] as a white solid. Yield: 2.063 g
25.7, 31.1, 31.2, 34.0, 35.5, 48.0, 61.0, 61.2, 63.1, 76.8, 77.1, 77.4, 109.8,
111.5, 113.0,113.4, 123.6, 123.8, 128.5, 132.5, 133.7, 134.2, 134.8, 146.1,
146.2, 146.6, 153.9, 154.0, 154.1, 169.8, 170.2, 176.2 ppm (NCN).
HRMS (positive ions): m/z 1223.326 (calcd for {Pd[(1R,2R)-L]
Br2 þ Na}þ 1223.33),1119.417 (calcd for {Pd[(1S,2S)-L]Br}þ 1119.42).
Anal. Calcd (%) for C64H76Br2N4O2Pd (1199.54): C, 64.08; H, 6.39; N,
4.67. Found: C, 64.21; H, 6.43; N, 4.71.
20
(7.0 mmol, 64%); [
a
]
D
85.961 (c 0.1, CHCl3).
A mixture of (1S, 2S)-2 (1.049 g, 3.54 mmol), TsOH$H2O
(15.9 mg), and HC(OEt)3 (14.4 mL) are stirred at room temperature
for 2 days. The solvent is removed under reduced pressure and the
residue is purified by flash silica gel column chromatography
(eluent: EtOAc/CH2Cl2/CH3OH/Et3N
((1S,2S)-2-(1H-benzo[d] imidazol-1-yl)cyclohexyl)-1H-benzo[d]
imidazole [(1S,2S)-3] as a gray solid. Yield: 0.784 g (2.5 mmol, 70%);
¼
8/2/1/0.2) to afford 1-
2.2.3. Synthesis of palladium complex {Pd[Rac-L]Br2}
Complex Pd[Rac-L]Br2 was prepared by a similar way to {Pd
[(1S,2S)-L]Br2} in 74% yield.
20
[
a]
43.974 (c 0.1, CHCl3).
D
A mixture of (1S,2S)-3 (126 mg, 0.40 mmol) and 2-(benzyloxy)-
1-(bromomethyl) -3,5-di-tert-butylbenzene (4) (327 mg,
0.84 mmol) are stirred at 100 ꢁC in 1,4-dioxane (5 mL) for 18 h. Then
the solvent is removed under reduced pressure and the residue is
purified by a silica gel column chromatography (eluent: EtOAc/
CH2Cl2/CH3OH/Et3N ¼ 8/2/1/0.2) to afford the di-imidazolium salt
{H2[(1S,2S)-L]Br2} as a white solid. Yield: 414 mg (0.38 mmol, 94%).
2.3. General procedure for asymmetric SuzukieMiyaura coupling
1-Bromo-2-methoxynaphathalene (47 mg, 0.2 mmol), 1-
naphthylboronic acid (42 mg, 0.24 mmol), K2CO3 (66 mg,
0.48 mmol), {Pd[(1S,2S)-L]Br2} (5 mg, 0.004 mmol) and dimethy-
lacetamide (DMA, 3 mL) are mixed in a Schlenk tube under ambient
atmosphere. The mixture is frozen by liquid nitrogen and the
atmosphere in the tube is exchanged with pure nitrogen while
cooled for 2e3 times to remove the dissolved oxygen. After
warming to room temperature, the airtight tube is placed in
a 140 ꢁC oil bath for 24 h. Then the solvent is immediately removed
under reduced pressure and the residue is diluted with CH2Cl2 and
water. The organic layer is separated and the aqueous layer is
washed with CH2Cl2 for 3 times. The combined organic layers are
dried by anhydrous Na2SO4, concentrated by a rotatory evaporator.
The product is obtained by silica gel column chromatography or
preparative TLC (eluent: hexane). The crude product is weighed and
analyzed by GCeMS. The ee% value of the obtained product was
determined by HPLC on a chiral stationary phase.
[
a]
20 ꢀ11.99 (c 0.1, CHCl3). 1H NMR(500 MHz, CDCl3):
d 12.38 (s, 2H,
D
NCH]N), 8.57 (d, JHH ¼ 8.25 Hz, 2H, CH), 7.51 (d, JHH ¼ 7.35 Hz, 4H,
CH), 7.44 (m, 4H, CH), 733e7.38 (m, 6H, CH), 7.16 (m, 4H, CH), 7.03
(m, 4H, CH), 6.17 (d, JHH ¼ 8.25 Hz, 2H, CH), 5.83 (d, JHH ¼ 14.65 Hz,
2H, CH2), 5.36 (d, JHH ¼ 14.60 Hz, 2H, CH2), 5.04 (d, JHH ¼ 23.35 Hz,
2H, CH2), 4.85 (d, JHH ¼ 12.40 Hz, 2H, CH2), 2.82 (s, 2H, CH2), 2.20 (d,
JHH ¼ 8.25 Hz, 2H, CH2), 2.08 (s, 4H, CH2), 1.40 (18H, m, CH3),
1.09 ppm (18H, m, CH3). 13C NMR (100 MHz, CDCl3):
d 23.6, 24.2,
31.2, 31.3, 35.6, 42.0, 47.2, 51.8, 61.3, 76.8, 77.1, 77.4, 113.0, 113.3,
126.2, 127.0, 128.3, 128.9, 136.6, 142.9, 143.0, 148.1, 153.8, 153.9 ppm.
HRMS (positive ions): m/z 1015.5263 (calcd for [MLeBrꢀ]þ
1015.5263), 467.3066 (calcd for [MLeBrꢀeBrꢀ]2þ 467.305),
625.3897 (calcd for [MLeBrꢀeBrꢀeRCH2þ]þ (RCH2þ ¼ [2-PhCH2O-
3,5-(tert-Bu)2 (C6H2)CHþ2 ]) 625.39).
A mixture of ligand H2[(1S,2S)-L]Br2 (401 mg, 0.366 mmol) and
98% Pd(OAc)2 (83 mg, 0.369 mmol) in DMSO (3 mL) are stirred at
50 ꢁC for 2 h and then at 110 ꢁC for 3 h. After the solvent is removed
by reduced pressure while heating, the residue is purified by a silica
gel column chromatography (eluent: EtOAc/CH2Cl2 ¼ 1:9) to afford
a brown crude product. The crude product is washed with toluene
2.4. X-ray crystallography
The single crystals suitable for X-ray analysis was sealed into
a glass capillary and the intensity data of the single crystal were
collected on the CCD Bruker Smart APEX system. Data obtained
to afford the palladium complex {Pd[(1S,2S)-L]Br2} as a white solid.
with the
CCD diffractometer with graphite-monochromated Mo K
tion (
direct methods, while further refinement with full-matrix least
square on F2 was obtained with the SHELXTL program package [68].
All non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were introduced in calculated positions with the
u
-2
q
scan mode were collected on a Bruker SMART 1000
27.99 (c 0.1, CHCl3). 1H
a
radia-
20
Yield: 328 mg (0.27 mmol, 74%). [
a
]
D
NMR(500 MHz, CDCl3):
d
7.63 (m, 2H, CH), 7.50(m, 2H, CH), 7.38 (m,
l
¼ 0.710 73 Å) at 293 K. The structures were solved using
2H, CH), 7.28e7.12 (m, 10H, CH), 6.95e6.92 (m, 2H, CH), 6.57(d,
JHH ¼ 8.25 Hz, 1H, CH), 6.49(d, JHH ¼ 8.25 Hz, 1H, CH), 6.30e5.30 (br
m, 3H, CH2), 4.92 (m, 2H, CH2), 4.77 (m, 1H, CH2), 4.67 (m, 2H, CH2),
2.94 (m, 1H, CH2), 2.77 (m, 1H, CH2), 2.58 (m, 1H, CH2), 2.28 (m, 2H,

226
G. Shigeng et al. / Journal of Organometallic Chemistry 700 (2012) 223e229
Table 1
carbene)-based palladium complex Pd[(1S,2S)-L]Br2 was shown in
Scheme 1. (1S,2S)-(ꢀ)-1,2-diaminocyclohexane -tartrate instead of
Crystallographic data and refinement for palladium complexes.
D
the free base (1S,2S)-(ꢀ)-1,2-diamino cyclohexane was used as
a chiral material to prepare chiral (1S,2S)-cyclohexanediylbis-
benzimidazole (1S,2S-3) [69]. In order to improve the solubility in
common organic solvent, 2-(benzyloxy)-1-(bromomethyl)-3,5-di-
tert-butylbenzene (4) [67] was prepared and reacted with (1S,2S)-3
to afford the chiral di-NHC pro-ligand H2[(1S,2S)-L]Br2 which was
allowed to combine with Pd(OAc)2 to give chiral di-NHC palladium
Pd[(1S,2S)-L]Br2. Another chiral compound Pd[(1R,2R)-L]Br2 and
racemic complex Pd[(Rac)-L]Br2 were synthesized in a similar way.
All of these three Pd complexes are air- and moisture-stable
both in the solid state and in solution. They are only sparingly
soluble in toluene and diethyl ether, but can be easily dissolved in
other common solvents like CH2Cl2, CHCl3, DMF and DMA. The
structure has been fully characterized by elemental analysis, 1H
NMR, 13C NMR, specific rotation and HRMS spectra. The formation
of the NHC complexes was confirmed by the single resonance at
about 176.2 ppm (for NCN) in the 13C NMR spectra and the absence
of the singlet corresponding to the proton of NCHN group of the
imidazoline ring in the 1H NMR spectra. Optical rotations of the
chiral palladium complexes [27.99 and ꢀ31.98ꢁ, respectively] is
further proof that enantiomerically pure bis(imidazolium) salts
were formed and the absolute configuration remained unchanged.
All chiral proligands and corresponding palladium complexes
were in good agreement with the results by high-resolution mass
spectra (HRMS, see Supporting Information). For bis-imidazolium
salt, although no peak was observable at the position correspond-
ing to the parent molecule ML, there are three dominant fragment
ions in the mass spectra. The gradual loss of Brꢀ from ML afforded
monocation [MLeBrꢀ]þ at m/z 1015.5 and double-charge molecular
ion [MLeBrꢀeBrꢀ]2þ at 467.3. The fragment ion at m/z 625.4 (100)
showed the most dominant intensity as the base peak. It was
formed by bond cleavage between alkyl carbon and nitrogen in the
fragment ion [MLeBrꢀeBrꢀ]2þ to lose {2-PhCH2O-3,5-(tert-
Bu)2(C6H2)CH2}þ (for short, RCHþ2 ) moiety which was characterized
Pd[(1S,2S)-L]Br2
Pd[(1R,2R)-L]Br2
Empirical formula
Formula wt
Temp (K)
Cryst size [mm]
Cryst syst
Space group
a (Å)
C64H76Br2N4O2Pd
1199.51
296(2)
C64H76Br2N4O2Pd
1199.51
293(2)
0.20 ꢂ 0.10 ꢂ 0.08
Monoclinic
P2(1)
17.606(6)
11.613(4)
18.482(6)
116.805(6)
3373(2)
0.20 ꢂ 0.10 ꢂ 0.10
Monoclinic
P2(1)
17.591(7)
11.627(5)
18.454(8)
116.775(6)
3370(2)
b (Å)
c (Å)
b
(deg)
V (Å3)
Z
2
2
Ds (Mg/m3)
Abs coeff (mmꢀ1
F(000)
1.181
0.079
1240
1.182
1.501
1240
)
q
range (deg)
2.18e22.89
13 035/9619
(Rint ¼ 0.0493)
13 035/37/658
1.045
0.0604, 0.1463
0.0881, 0.1610
0.000/0.001
2.14e25.01
10 492/6435
(Rint ¼ 0.0492)
10 492/1/658
0.943
0.0610, 0.1500
0.1002, 0.1655
0.000/ꢀ0.003
No. of reflns collected/unique
No. of data/restraints/params
Goodness of fit on F2
Final R indices (I > 2
s
(I))a
R indices (all data)a
Largest diff peak and hole (e/Åꢀ3
)
a
2
2
R1 ¼ SjjFojꢀjFcjj/SjFoj. wR2 ¼ [S(jFoj ꢀjFcj )2/S(Fo2)]1/2
.
displacement factors of the host carbon atoms. Crystallographic
data are summarized in Table 1.
3. Results and discussion
3.1. Synthesis and characterization of di-NHC palladium complexes
In general, the imidazolium salts could be easily obtained by the
reaction of imidazole with the halogenated hydrocarbon. The
modified synthetic route for chiral bidentate bis(N-heterocyclic
Fig. 2. Molecular structure of complex Pd[(1R,2R)-L]Br2. All hydrogen atoms and solvent molecules are omitted for clarity. Selected distances (Å) and angles (deg): Pd(1)e
C(1) ¼ 1.980(10), Pd(1)eC(36) ¼ 2.002(9), Pd(1)eBr(2) ¼ 2.4690(14); Pd(1)eBr(1) ¼ 2.4731(16). C(1)ePd(1)eC(36) ¼ 83.2(4), C(1)ePd(1)eBr(2) ¼ 89.2(3), C(36)ePd(1)e
Br(2) ¼ 171.8(3), C(1)ePd(1)eBr(1) ¼ 176.3(3), C(36)ePd(1)eBr(1) ¼ 93.1(3), Br(2)ePd(1)eBr(1) ¼ 94.52(6).

G. Shigeng et al. / Journal of Organometallic Chemistry 700 (2012) 223e229
227
Table 2
by three other fragment ions at m/z 309.2, 253.2, 197.1 and 105.1.
Substrate scope of Suzuki cross-coupling by Pd catalysts.a
For the corresponding palladium complex (Pd[(1S,2S)-L]Br), the
molecular ion peak at 1223.3 ([MC þ Na]þ) along with two ion
peaks at 1119.4 ([MCꢀBrꢀ]þ) and 1187.4 ([MCeCH3]þ) was observed.
R'
R
[Pd]
K2CO3, DMA
140 oC, 24h
R
R'
3.2. X-ray single crystal structural analysis of di-N-heterocyclic
carbene palladium complexes
B(OH)2
Br
The structures of chiral complexes Pd[(1S,2S)-L]Br2 and Pd
[(1R,2R)-L]Br2 were also undoubtedly determined by X-ray crys-
tallography. The molecular structures of the square-planar Pd(II)
complexes Pd[(1S,2S)-L]Br2 and Pd[(1R,2R)-L]Br2 in the solid state
are shown in Fig. 1 and Fig. 2, respectively. Selected bond lengths
and angles are denoted in the figure caption.
Entry
1
Boronic acid
Aryl halide
Yield (%)b
85
B(OH)2
I
B(OH)2
B(OH)2
B(OH)2
Br
The two palladium complexes show similar crystal structure of
a distorted square planar coordinated palladium center. The chiral
biscarbene ligand chelates Pd(II) center in a cis-arrangement and
the remaining two coordination sites are occupied by bromine
anions. The seven-membered palladacycle adopts a pseudo-boat-
like conformation, and the di-NHC CePdeC bite angles are
84.3(3) and 83.2(4)ꢁ, respectively. The PdeC distances [1.987(8) and
1.982(7) Å for Pd[(1S,2S)-L]Br2; 1.980(10) and 2.002(9) Å for Pd
[(1R,2R)-L]Br2] fall comfortably within the range found for the
neutral chiral chelating bis(carbene)palladium(II) complexes (A,
Chart 1) [1.987(5) and 2.003(5) Å] [51] and dibromo{1,10-di[(R)-100-
phenylethyl]-3,3-methylenediimidazolin-2,20-diylidene}palladium
(II) [1.992(3) and 1.974(3) Å] [38] as well as with those reported
for the related non-chiral neutral and dicationic bis-NHC
palladium complex cis-CH2{NC(H)]C(H)N(CH3)C}2PdI2 [1.990(3)
and 1.997(3) Å] [66] and cis-CH2{NC(H)]C(H)N(CH3)C}2 Pd
(NCCH3)2]2ꢀ [BF4ꢀ]2 [1.966(2) and 1.972(3) Å] [34]. This structure
exhibited that the synthesis of such chiral palladium (II) complexes
occurred with no appreciable racemization. The absolute configu-
ration at each chiral center is assigned to be (1S,2S) or (1R,2R),
which is in accordance with the absolute configuration of the
primarily employed chiral 1,2-cyclohexanediamine.
2
3
4
48
30
28
Cl
Br
Br
B(OH)2
B(OH)2
5
6
99
67
Br
O
I
B(OH)2
7
8
86
21
99
3.3. Catalysis results of SuzukieMiyaura coupling
Br
Br
B(OH)2
B(OH)2
For the catalytic studies we firstly performed a simple substrate
scope screening test of the Suzuki reactions with {Pd[(1S,2S)-L]Br2}
as the catalyst. The results are summarized in Table 2. Under the
optimal (or representative) condition (K2CO3, DMA, 140 ꢁC, 24 h)
a number of aryl iodides, bromides, and boronic acids have been
successfully coupled with arylboronic acids. Specifically, naphthyl
bromide was found to give the best results from the point of the
isolated yields (Table 2, entry 5 & 9).
9
a
Conditions: 3 mol% {Pd[(1S,2S)-L]Br2}, 1.0 equiv of aryl halide, 1.2 equiv of
ArB(OH)2, 2.4 equiv of K2CO3. 30 mL of DMA as solvent, 140 ꢁC, 14 h.
b
Isolated yield.
R
N
Pri
iPr
N
N
Naphthylboronic acid was chosen to couple with 2-methoxyl
naphthyl bromide to investigate the asymmetric SuzukieMiyaura
coupling by the chiral Pd catalyst {Pd[(1S,2S)-L]Br2} (Table 3). A
number of observations could be made from the examinations.
Firstly, Cs2CO3 is an effective base for the coupling to achieve higher
ee% values than K2CO3 (Table 3, entries 1, 2, 4 vs. 5e8). Secondly, the
suitable reaction temperature is ca. 80 ꢁC with 73% isolated yield
and 61% ee value (Table 3, entry 6 vs. 4, 5 and 7). At higher
temperature the isolated yields were diminished by the formation
of byproduct and the levels of enantioselectivities were lowered
due to racemization of two atropisomeric compounds [70,71].
Thirdly, the other chiral Pd enantiomer {Pd[(1R,2R)-L]Br2} has also
Rh
N
N
N
N
Pd
N
R
R
R
Cl
Cl
(1R,2R)
(1R,2R)
R = Me2CH,
R. E. Douthwaite et al 51
W. A. Herrmann et al 63
Chart 1. Two typical cyclohexane-1,2-based di-NHC-metal complexes.

228
G. Shigeng et al. / Journal of Organometallic Chemistry 700 (2012) 223e229
Table 3
Appendix A. Supplementary material
Asymmetric SuzukieMiyaura coupling between naphthyl bromides and naphthyl
acids by the chiral Pd catalyst.a
CCDC 842556 and 846557 contain the supplementary crystal-
lographic data for complete HRMS spectra and analysis of
compounds {[H2(1S,2S)-L]Br2} and {[(1S,2S)-L]PdBr2}, selected
chiral HPLC of biaryl compounds catalyzed by {[(1S,2S)-L]PdBr2}.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
R
[Pd]
R
B(OH)2
Br
Yield (%)b
ee (%)c
Appendix. Supplementary data
Entry
R
Base
Temperature (ꢁC)
1
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
Me
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Cs2CO3
Cs2CO3
Cs2CO3
140
140
140
100
140
80
60
60
80
100
80
52
67
n.d.f
69
41
73
39
50
72
66
46
99
19
<5
<5
0
<5
39
61
52
36
59
<10
<10
<10
70
2d
3e
4
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.12.008.
5
6
7
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