Synthesis of bis(N-heterocyclic carbene) palladium complexes derived from (S,S)-1,2-bis(1-hydroxypropyl)benzene

Article abstract of DOI:10.1016/j.jorganchem.2011.03.001

New o-xylylene-linked bis(benzimidazolium) salts were synthesized in six-steps from C₂-symmetric chiral 1,4-diol, 1,2-bis(1-hydroxypropyl) benzene, as a starting material. The silver complex of bis(benzimidazol-2- ylidene) was obtained on treat

Full text of DOI:10.1016/j.jorganchem.2011.03.001

Journal of Organometallic Chemistry 696 (2011) 2399e2405
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of bis(N-heterocyclic carbene) palladium complexes derived from
(S,S)-1,2-bis(1-hydroxypropyl)benzene
*
Satoshi Anezaki, Yoshitaka Yamaguchi, Masatoshi Asami
Department of Advanced Materials Chemistry, Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
New o-xylylene-linked bis(benzimidazolium) salts were synthesized in six-steps from C2-symmetric
chiral 1,4-diol, 1,2-bis(1-hydroxypropyl)benzene, as a starting material. The silver complex of bis(ben-
zimidazol-2-ylidene) was obtained on treatment of bis(benzimidazolium) salt with silver oxide. The
reaction of the silver bis-NHC with [PdCl2(PhCN)2] afforded the bis-NHC complex of palladium. The X-ray
diffraction studies on Pd complexes revealed that these complexes have distorted square planar
geometry around the Pd center coordinating the NHC ligand in mutually cis-position. The arene ring of
o-xylylene unit hanged over the Pd center and thus these complexes showed C1-symmetric structures.
The variable temperature NMR spectroscopy revealed that these Pd complexes showed fluxional
behavior between C1- and C2-symmetric structures in solution state.
Received 17 January 2011
Received in revised form
19 February 2011
Accepted 1 March 2011
Keywords:
N-Heterocyclic carbenes
Chiral o-xylylene framework
C2-Symmetry
Ó 2011 Elsevier B.V. All rights reserved.
Palladium complexes
1. Introduction
the complex showed C1-symmetric structure because the Pd complex
had strained 7-membered chelate ring. On the other hand, Shi and co-
N-heterocyclic carbenes (NHCs) are now established as an impor-
tant class of ligands forhomogeneouscatalysts, because NHCs provide
an electron-rich metal center with a thermally robust metal-ligand
bond [1]. The use of polydentate NHC ligands has allowed the prep-
aration of novel complexes whosestabilityisentropically increased by
the chelate effect [2]. Most of these polydentate NHCs reported so far
are bidentate NHC ligands and there are numerous studies on their
late transition metal complexes of Rh, Ir, Ni, Pd, and so on [3,4]. The C2-
symmetric chiral bidentate NHC ligands were reported firstly by
Trudell [5] and RajanBabu [6], independently, in 2000, and have
received much attention as promising ligands in catalytic enantiose-
lective organic transformations [7]. The first chiral bidentate NHC
ligand was derived from 2,20-di(bromomethyl)-1,10-binaphthyl, and
C2-symmetric structure was observed in only the trans isomer (A in
Chart 1) in complexation of the ligand with transition metals [5,6].
Similar geometrical features were observed in Pd complexes derived
from (R,R)-2-chloro-N-[2-(chloroacetylmethylamino)cyclohexyl]-N-
methylacetamide [8] or 4,5-bis(bromoethyl)-2,2-dimethyl-1,3-diox-
olane[9]probablyduetotheformationof11-or13-memberedchelate
ring in thesecomplexes. Douthwaite forced a cis coordination modeby
building the NHC moiety directly adjacent to a cyclohexane backbone
and metalated with Pd to form cis-coordinate Pd complex derived
from N,N0-diethylidene-(1R,2R)-diamino cyclohexane [10]. However,
workers have reported a series of axially chiral bidentate NHC ligands
derived from 1,10-binaphthyl-2,20-diamine and its partially saturated
framework (B in Chart 1) [11]. In contrast to the bis-NHC ligand
reported by Trudell and RajanBabu featuring 11-membered chelate
ring, Shi’s complex consisted of 9-membered chelate ring and thus
two NHCswere cis-orientationwith C2-symmetry. Thesecomplexes of
Rh, Ir, andPdworkedasexcellentcatalystsforenantioselectiveorganic
transformations.
Recently, a novel family of C2-symmetric bidentate NHCs with
dihydroethanoanthracene unit has been reported by Veige and
co-workers [12]. Although two NHCs in these ligands located in cis-
position, the complex showed C1-symmetric structure. We have
also reported novel chiral o-xylylene-bridged bis(imidazol-2-yli-
dene) ligands derived from enantiomerically pure 1,4-diol, 1,2-bis
(1-hydroxypropyl)benzene (3), and their molybdenum complexes
[13]. These molybdenum complexes showed cis-C2-symmetric
structure in octahedral geometry. In this paper, we wish to report
a preparation of the benzimidazol-2-ylidene derivatives of the
chiral o-xylylene-bridged bis-NHCs and their palladium complexes.
2. Results and discussion
2.1. Synthesis of ligands and complexes
As described in the Introduction, we have recently reported
the preparation of chiral o-xylylene-bridged bis(imidazolium)
* Corresponding author. Tel./fax: þ81 45 339 3968.
E-mail address: m-asami@ynu.ac.jp (M. Asami).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.03.001

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S. Anezaki et al. / Journal of Organometallic Chemistry 696 (2011) 2399e2405
The nitro groups in (R,R)-6 was reduced with H2 in mixed solvents
of CH2Cl2 and MeOH to afford tetraamine (R,R)-7 in 83% yield.
Tetraamine (R,R)-7 was reacted with excess of HC(OEt)3 (64 equiv)
in the presence of catalytic amount of p-toluenesulfonic acid
monohydrate to give bisbenzimidazole derivative (R,R)-8 in 84%
yield. The resulting (R,R)-8 was alkylated with excess of alkyl halide
(20 equiv) to give bis(benzimidazolium) salt (R,R)-9a and (R,R)-9b,
respectively.
R
N
N
N
L
N
N
N
L
M
R
R
L
R
M
L
N
N
As the pro-ligands (R,R)-9 were obtained, a preparation of cor-
responding Pd complex was examined (Scheme 3). The treatment
of (R,R)-9a with 1.0 equiv of Ag2O afforded the silver complex of bis-
NHC (R,R)-10a as a pale brown powder in 79% yield. The corre-
sponding N-benzyl derivative (R,R)-10b was also obtained in
excellent yield from (R,R)-9b. In the 1H NMR spectrum of (R,R)-10a,
the characteristic signal of (R,R)-9a at 9.85 ppm assigned to ben-
zimidazolium protons were disappeared. Furthermore, in the 13C
NMR spectrum of (R,R)-10a the signal at 200.1 ppm assigned to the
carbene carbon was observed. Similar 1H and 13C NMR spectra were
also obtained for (R,R)-10b. These spectroscopic features suggested
the formation of silver NHC complexes (R,R)-10. In the reactions of
(R,R)-10a and (R,R)-10b with [PdCl2(PhCN)2], the NHC-transfer
from silver to palladium took place to give palladium complexes
(R,R)-11a and (R,R)-11b, respectively. In the 1H NMR spectrum of
(R,R)-11a, only one signal assignable to the methyl groups on the
nitrogen atoms was observed at 4.29 ppm. The ethyl groups
attached to the benzyl carbons showed one triplet and one broad
singlet signals assignable to the methyl and methylene protons at
1.27 ppm and 2.20 ppm, respectively. The 13C NMR spectrum of
(R,R)-11a showed one signal assignable to the carbene carbons
(172.5 ppm). The signals for methyl and methylene carbons of the
ethyl groups were observed at 12.1 ppm and 25.1 ppm, respectively.
These observations indicated that Pd complex has C2-symmetric
structure in solution. The 1H NMR spectrum of complex (R,R)-11b
showed somewhat broadening signals at room temperature. The
variable temperature NMR spectroscopy for (R,R)-11a and (R,R)-11b
was carried out. These results are discussed in the Section 2.3.
cis-C -symmetry
trans-C -symmetry
2
2
A
B
Chart 1. C2-Symmetric chiral bis-NHC complexes.
2I
Ag
I Ag
I
N
N
N
N
Me
Me
Ag2O (1.0 eq.)
N
N
N Me
Me
N
CH2Cl2, rt, overnight
(R,R)-2
86%
(R,R)-1
Scheme 1. Synthesis of Ag complex with imidazol-2-ylidene.
pro-ligands, which were obtained in two-steps from 3, and their
molybdenum complexes [13]. Then, we next examined the prepa-
ration of the bis(imidazol-2-ylidene) complex of palladium. The
silver complex (R,R)-2 was obtained in a good yield by reaction of
bis(imidazolium) pro-ligand (R,R)-1 with Ag2O (Scheme 1).
Although the reaction of (R,R)-2 with [PdCl2(PhCN)2] afforded
a yellow solid, the 1H NMR spectrum of the product showed the
complicated result and suitable crystals for the X-ray analysis were
not obtained.
Thus we began the synthesis of bis(benzimidazolium) pro-
ligands (R,R)-9. Multi-step procedure for the synthesis of (R,R)-9 is
shown in Scheme 2. In the first place, two hydroxy groups in (S,S)-3
was converted to diazide (R,R)-4 by Mitsunobu reaction in 69% yield
(2.5 equiv of diethyl azodicarboxylate (DEAD), PPh3, and diphe-
nylphosphoryl azide (dppa)). Then, (R,R)-4 was hydrogenated to
diamine (R,R)-5, which was used without purification in the next
reaction. Dinitrodiamine (R,R)-6 was obtained by the reaction of the
resulting (R,R)-5 and 2.2 equiv of o-fluoronitrobenzene in the
presence of K2CO3 in CH3CN at 80 ꢀC for 36 h in 70% from (R,R)-4.
2.2. X-ray crystal structures of Pd complexes 11a and 11b
Unfortunately, the X-ray analyses of complexes (R,R)-11a and
(R,R)-11b could not be undertaken due to the rapid deterioration of
these crystals during separation of crystals and the mother liquid.
However, the structures of racemic complexes 11a and 11b were
determined by the X-ray analyses. Complexes 11a and 11b con-
tained solvent molecules of CH2Cl2 and CHCl3, respectively. The
NO2
PPh3 (2.5 eq.)
DEAD (2.5 eq.)
(2.2 eq.)
F
dppa (2.5 eq.)
H2, Pd/C
K2CO3 (2.2 eq.)
NH NO2
NH NO2
OH
OH
N3
N3
NH2
NH2
THF, rt, 24 h
MeOH, rt, 2 h
CH3CN, 80 °C, 36 h
(S,S)-3
(R,R)-4
69%
(R,R)-5
(R,R)-6
70% (from (R,R)-4)
2X
HC(OEt)3 (64 eq.)
N
N
N
R-X (20 eq.)
N
N
R
R
H2, Pd/C
p-TsOH H2O(0.2 eq.)
NH NH2
NH NH2
CH2Cl2-MeOH,
rt, 24 h
rt, 48 h
DME, reflux, 5 h
N
N
N
(R,R)-8
84%
(R,R)-9a; 98% (R = Me, X = I)
(R,R)-9b; quant. (R = Bn, X = Br)
(R,R)-7
83%
Scheme 2. Synthesis of chiral bis(benzimidazolium) salts.

S. Anezaki et al. / Journal of Organometallic Chemistry 696 (2011) 2399e2405
2401
R
N
Ag
XAg
X
Et
N
Cl
(R,R)-9a
or
(R,R)-9b
Pd(PhCN)2Cl2 (1.0 eq.)
CH2Cl2, rt, overnight
Ag2O (1.0 eq.)
H
H
N
N
N R
R
N
Pd
CH2Cl2, rt, overnight
Cl
N
Et
N
R
(R,R)-10a; 79% (R = Me, X = I)
(R,R)-10b; 92% (R = Bn, X = Br)
(R,R)-11a; 68% (R = Me)
(R,R)-11b; 72% (R = Bn)
Scheme 3. Synthesis of Ag and Pd complexes with benzimidazol-2-ylidene.
ORTEP drawings of 11a and 11b are shown in Fig. 1. The selected
bond distances and angles are listed in Table 1. These complexes
have distorted square planar geometry around the Pd center
coordinating the bis-NHC ligand in mutually cis-position, whereas
the bis-NHC ligands do not adopt C2-symmetric coordination mode
suggested by the NMR spectra mentioned in the Section 2.1. The
arene ring of o-xylylene unit hangs over the Pd center and thus
these complexes show C1-symmetric structures.
The bond distances of PdeC (carbene) (1.990(4) and 1.949(4) Å for
11a and 1.967(12) and 1.953(9) Å for 11b) and PdeCl (2.3998(9) and
2.3715(10) Å for 11a and 2.405(3) and 2.379(3) Å for 11b) fall within
the range found for the chelating bis-NHC palladium(II) complexes
[4d]. Bite angles defined by C (carbene)ePdeC (carbene) are 86.54
(14)ꢀ for 11a and 87.7(4)ꢀ for 11b, which are similar to those of the
chelating bis-NHC complexes of palladium [4g]. The most character-
istic feature of these complexes is the arene ring of o-xylylene unit
locating over the Pd. The distances of Pd.C (ipso carbon of o-xyly-
lene) are 3.092(4) and 2.949(4) Å for 11a and 3.133(9) and 2.998(6) Å
for 11b. The similar structural feature was observed in the cyclo-
phane-type bis-NHC complexes of palladium reported by Veige [12d]
and Baker [4d], respectively. The arene ring containing the linker unit
of bis-NHC ligands usually directed away from the metal center
to the methyl protons on the nitrogen atoms was observed at
4.29 ppm and one signal due to the carbene carbons was observed at
172.5 ppm. As the temperature gradually decreased, the singlet
assignable to NeCH3 protons became broad and coalesced at 4 ꢀC
(277 K). The signals assignable to the hanged structure observed in
the solid state became sharpe under ꢁ30 ꢀC, and a new set of signals
appeared. The newsignals showed two sets of NeCH3 protons at 4.16
and 4.44 ppm as a singlet. Furthermore, the ethyl groups attached to
the benzyl carbons were observed as two triplets (1.28 and
1.30 ppm) and four multiplets (1.57, 1.99, 2.41, and 2.82 ppm)
assignable to the methyl and methylene protons, respectively.
Application of the coalescence formula [15] tothe fluxional behavior
of complex (R,R)-11a gave a D
Gz (277 K) value of 13.2 kcal molꢁ1 for
the methyl group exchange. Low temperature 13C NMR spectrum
(ꢁ70 ꢀC) showed two signals in the region of the carbene carbons at
169.9 and 170.1 ppm. Other signals observed at ꢁ70 ꢀC were
consistent with the formation of the C1-symmetric conformation
revealed by the X-ray analysis of 11a. In complex (R,R)-11b, similar
spectroscopic features were observed in variable temperature NMR
measurements [16]. These results indicated that the complexes
(R,R)-11a and (R,R)-11b showed the fluxional process which may be
interpreted in terms of the flipping of the o-xylylene unit. Similar
fluxional process has been reported independently by Baker [4d,17]
and Veige [12d,18], respectively (Fig. 2).
[4c,14]. Therefore, the existence of the interaction between
p-elec-
trons on the arene ring and the palladium metal might allow to adopt
the hanged structure, in complexes 11a and 11b.
3. Conclusion
2.3. Spectroscopic and structural properties of Pd complexes (R,R)-
11a and (R,R)-11b
In a previous paper, we have reported the synthesis of the new
type of chiral C2-symmetric bis(imidazolium) pro-ligands derived
from enantiomerically pure 1,4-diol. In this study, the extension to
the synthesis of chiral bis(benzimidazolium) pro-ligands and the
preparation of their palladium complexes were demonstrated. In
a solid state, these Pd complexes showed the interaction of the arene
As mentioned above, the conformation of the complex (R,R)-11a
(and/or (R,R)-11b) in the solid state was not consistent with that in
the solution state. Then, the variable temperature NMR measure-
ments for complexes (R,R)-11a and (R,R)-11b were carried out.
The 1H and 13C NMR spectra of (R,R)-11a at 25 ꢀC suggested the
formation of the C2-symmetric structure; only one signal assignable
Table 1
Selected bond distances (Å) and angles (ꢀ) for 11a and 11b.
11a
11b
C1ePd1
C21ePd1
Cl1ePd1
Cl2ePd1
C1eN1
1.990(4)
1.949(4)
2.3998(9)
2.3715(10)
1.350(5)
1.367(5)
1.353(5)
1.354(5)
3.092(4)
2.949(4)
C1ePd1
C27ePd1
Cl1ePd1
Cl2ePd1
C1eN1
1.967(12)
1.953(9)
2.405(3)
2.379(3)
1.360(10)
1.397(13)
1.374(9)
1.364(9)
3.133(9)
2.998(6)
C1eN2
C1eN2
C21eN3
C21eN4
C12.Pd1
C16.Pd1
C27eN3
C27eN4
C18.Pd1
C23.Pd1
C1ePd1eC21
Cl1ePd1eCl2
C1ePd1eCl1
C1ePd1eCl2
C21ePd1eCl1
C21ePd1eCl2
N1eC1eN2
86.54(14)
95.48(3)
C1ePd1eC27
Cl1ePd1ePd2
C1ePd1eCl1
C1ePd1eCl2
C27ePd1eCl1
C27ePd1eCl2
N1eC1eN2
87.7(4)
91.81(11)
175.0(2)
91.8(3)
173.19(10)
89.97(10)
87.93(10)
176.38(10)
107.0(3)
88.5(4)
177.04(17)
104.3(9)
106.1(7)
Fig. 1. ORTEP drawings of 11a (left) and 11b (right). Thermal ellipsoids are drawn at
50% (11a) and 20% (11b) probability level. Hydrogen atoms and solvent molecules were
omitted for clarity.
N3eC21eN4
107.0(3)
N3eC27eN4

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S. Anezaki et al. / Journal of Organometallic Chemistry 696 (2011) 2399e2405
Fig. 2. Variable temperature NMR measurements of (R,R)-11a in CD2Cl2. ꢂ ¼ solvent or H2O.
ring on the linker unit with the Pd center. Variable temperature NMR
spectroscopy revealed that these Pd complexes showed the fluxional
behavior on the basis of the flipping motion of the o-xylylene unit
(Chart 2). There is a possibility that these Pd complexes adopt the
chiral C2-symmetric structure in a solution state. Further investiga-
tion on the enantioselective carbonecarbon bond forming reactions
catalyzed by these complexes is now in progress in our laboratory.
INC.) or Chromatorex (NH, 100e200 mesh, Fuji Silysia Chemical,
Ltd.). Specific rotations were measured on a JASCO P-1010 digital
polarimeter using 1-dm cell at sodium D line (589 nm) in the
indicated solvent. Elemental analyses were carried out on a Vario EL
Elemental analyzer. High-resolution mass spectra were recorded on
a Hitachi Nano Frontier LD spectrometer.
4.2. Silver complex (R,R)-2
4. Experimental
The solution of (R,R)-1 (0.202 g, 0.350 mmol) and Ag2O (0.081 g,
0.354 mmol) in CH2Cl2 (5 mL) was stirred overnight at room
temperature. The mixture was filtered through Celite and the
filtrate was concentrated under reduced pressure to give (R,R)-2 as
a white powder (0.239 g, 0.302 mmol, 86%). mp > 200 ꢀC (Dec.).
4.1. General
All air- and moisture-sensitive experiments were carried out
under an atmosphere of dry argon or nitrogen, which was purified
SICAPENT (Merck Co., Inc.) by using a standard Schlenk tube or high
vacuum techniques. All solvents were distilled over appropriate
drying agents prior to use. All reagents were commercially available
and used without further purification. Compound (R,R)-1 was
prepared according to the literature method [13]. Melting points
were determined on a Stuart Scientific melting point apparatus
SMP3 and are uncorrected. IR spectra were recorded on a HORIBA
FT-730 spectrometer. 1H and 13C NMR spectra were measured on
JEOL JNM-EX-270, Bruker DRX-300, JEOL AL-400, and Bruker DRX-
500 spectrometers at ambient temperature unless otherwise
mentioned. 1H and 13C NMR chemical shifts were recorded in ppm
relative to internal Me4Si. Multiplicity is indicated by s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad).
Thin layer chromatography (TLC) was performed using Merck
silica-gel 60F-254 plates. Column chromatography was performed
[a
]
D
21 þ54.6 (c 0.60, CH2Cl2). 1H NMR (500 MHz, CD2Cl2)
d 0.73 (t,
J ¼ 7.6 Hz, 6H, 2 ꢂ CH3), 2.00e2.12 (m, 2H, CH2), 2.22e2.31 (m, 2H,
CH2), 3.91 (s, 6H, 2 ꢂ NeCH3), 5.86 (t, J ¼ 7.6 Hz, 2H, 2 ꢂ CH), 6.65 (d,
J ¼ 1.3 Hz, 2H, ImH), 6.90 (d, J ¼ 1.3 Hz, 2H, ImH), 7.53e7.56 (m, 2H,
ArH), 7.66e7.69 (m, 2H, ArH). 13C NMR (125 MHz, CD2Cl2)
d 11.4
(CH3), 29.4 (CH2), 39.5 (NeCH3), 61.4 (CH), 118.6, 118.9, 128.6, 129.6
(aromatic), 186.0 (NeCeN). HRMS (ESI): calcd for C20H27N4
[Me2AgI þ H]þ 323.2226, found 323.2230.
4.3. (R,R)-1,2-bis(1-azidopropyl)benzene (R,R)-4
DEAD (25.7 mL, 11.4 g, 65.2 mmol) was slowly added via drop-
ping funnel to a solution of (S,S)-3 (5.07 g, 26.1 mmol) and PPh3
(17.1 g, 65.2 mmol) in THF (65 mL) at 0 ꢀC and the mixture was
stirred for 10 min at the temperature. A solution of dppa (17.9 g,
65.2 mmol) in THF (25 mL) was added to the reaction mixture and
using Silica Gel 60N (neutral, 63e210
mm, KANTO CHEMICAL CO.,
Et
R
R
N
Et
N
N
Cl
N
Et
R
N
Pd
N
N
R
Cl
Cl
flipping
flipping
H
H
Pd
Cl
Pd
N
Et
Cl
Cl
N
N
Et
N
N
R
Et
C -symmetry
R
C -symmetry
C -symmetry
2
1
1
Chart 2. Flipping motion of the o-xylylene unit.

S. Anezaki et al. / Journal of Organometallic Chemistry 696 (2011) 2399e2405
2403
stirring was continued for 24 h at room temperature. The solvent
was removed under reduced pressure and the resulting residue was
purified by silica-gel column chromatography (Silica Gel 60N,
hexane/Et2O/CH2Cl2 ¼ 70/1/2). After the solvent was removed
under reduced pressure, (R,R)-4 was obtained as a yellow oil (4.37 g,
temperature. Excess HC(OEt)3 was removed under reduced pres-
sure and the resulting residue was purified by silica-gel column
chromatography (Chromatorex, hexane/EtOAc ¼ 1/3) to give (R,R)-
8 as a pale yellow green solid (1.65 g, 4.18 mmol, 84%). mp
215.8e216.8 ꢀC [
CDCl3)
a
]
27 þ50.5 (c 1.00, CHCl3). 1H NMR (300 MHz,
D
17.9 mmol, 69%). [
a
]
27 þ95.0 (c 1.00, CHCl3). IR (neat): 2098 (nN3
)
d
0.62 (t, J ¼ 7.3 Hz, 6H, 2 ꢂ CH3), 1.80e1.90 (m, 2H, CH2),
D
cmꢁ1. 1H NMR (270 MHz, CDCl3)
d
0.99 (t, J ¼ 7.3 Hz, 6H, 2 ꢂ CH3),
2.14e2.24 (m, 2H, CH2), 5.48 (t, J ¼ 7.3 Hz, 2H, 2 ꢂ CH), 7.26e7.35
(m, 6H, ArH), 7.47e7.51 (m, 2H, ArH), 7.56e7.60 (m, 2H, ArH), 7.76
(s, 2H, 2 ꢂ N]CHeN), 7.82e7.87 (m, 2H, ArH). 13C NMR (75.0 MHz,
1.73e2.01 (m, 4H, 2 ꢂ CH2), 4.65 (dd, J ¼ 8.2, 6.3 Hz, 2H, 2 ꢂ CH),
7.35e7.43 (m, 4H, ArH). 13C NMR (67.5 MHz, CDCl3)
d
11.2 (CH3),
29.3 (CH2), 63.3 (CH), 127.1, 128.4, 137.3 (aromatic). Anal. Calc. for
C12H16N6: C, 59.00; H, 6.60; N, 34.40. Found: C, 58.48; H, 6.69; N,
34.01%.
CDCl3) d 11.1 (CH3), 27.8 (CH2), 56.4 (CH), 109.6, 120.7, 122.5, 123.1,
127.3, 129.2, 133.4, 136.8, 141.1, 143.7 (aromatic). Anal. Calc. for
C26H26N4: C, 79.16; H, 6.64; N, 14.20. Found: C, 78.85; H, 6.68; N,
13.87%.
4.4. (R,R)-1,2-bis(1-(2-nitroanilino)propyl)benzene (R,R)-6
4.7. Bis(benzimidazolium) salt (R,R)-9a
To a solution of (R,R)-4 (4.15 g, 17.0 mmol) in MeOH (55 mL) was
added 5% Pd/C (1.25 g) and the mixture was stirred under H2
atmosphere (1 atm balloon) for 2 h. The reaction mixture was
filtered through Celite to remove Pd/C. The filtrate was concen-
trated under reduced pressure to give (R,R)-5 as a purple oil. The
diamine (R,R)-5 was used without further purification. Then o-
fluoronitrobenzene (5.39 g, 37.4 mmol) and K2CO3 (5.17 g,
37.4 mmol) were added to a solution of (R,R)-5 in CH3CN (20 mL)
and the mixture was heated at 80 ꢀC for 36 h. The mixture was
cooled to room temperature, and water (35 mL) was added. The
resulting red brown solid was collected by filtration and the solid
was washed with water. Then red brown solid was dissolved in
EtOH (75 mL) and the solution was stirred at 70 ꢀC for 20 min. After
cooling to 0 ꢀC the resulting precipitate was collected by filtration
and the precipitate was washed with cold EtOH to give (R,R)-6 as
a bright yellow powder (5.17 g, 11.9 mmol, 70% for 2 steps). mp
Methyl iodide (1.62 mL, 3.69 g, 26.0 mmol) was added to
a solution of (R,R)-8 (0.513 g, 1.30 mmol) in DME (15 mL) and the
mixture was refluxed for 5 h. After the solvent was removed under
reduced pressure, the residue was washed with hexane (10 mL ꢂ 3)
and Et2O (10 mL ꢂ 3), and dried in vacuo to give (R,R)-9a as a pale
yellow powder (0.863 g, 1.27 mmol, 98%). mp 186.3e188.3 ꢀC.
[a
]
28 ꢁ6.2 (c 1.03, MeOH). 1H NMR (400 MHz, DMSO-d6)
d 0.88 (t,
D
J ¼ 7.3 Hz, 6H, 2 ꢂ CH3), 1.92e1.99 (m, 2H, CH2), 2.41e2.50 (m, 2H,
CH2), 4.06 (s, 6H, 2 ꢂ NeCH3), 6.54 (t, J ¼ 7.3 Hz, 2H, 2 ꢂ CH),
7.53e7.55 (m, 2H, ArH), 7.62e7.65 (m, 2H, ArH), 7.71e7.75 (m, 4H,
ArH), 8.02e8.04 (m, 4H, ArH), 9.85 (s, 2H, 2 ꢂ N]¼CHeN) 13C NMR
(100 MHz, DMSO-d6)
d 10.4 (CH3), 27.9 (CH2), 33.6 (NeCH3), 58.0
(CH), 113.7, 113.8, 126.6, 126.8, 127.9, 130.1, 130.4, 31.9, 135.0, 141.8
(aromatic). HRMS (ESI): calcd for C28H32N4I [MeI]þ 551.1646, found
551.1666.
136.1e136.6 ꢀC [
a]
22 ꢁ1059.6 (c 1.00, CHCl3). IR (neat): 1438, 1570
D
(
nNO2), 3344, 3359 (nNH) cmꢁ1. 1H NMR (300 MHz, CDCl3)
d
1.07 (t,
4.8. Bis(benzimidazolium) salt (R,R)-9b
J ¼ 7.5 Hz, 6H, 2 ꢂ CH3), 2.04 (m, 4H, 2 ꢂ CH2), 4.84 (q, J ¼ 6.4 Hz, 2H,
2 ꢂ CH), 6.58e6.64 (m, 4H, ArH), 7.18e7.24 (m, 2H, ArH), 7.30e7.34
(m, 2H, ArH), 7.42e7.46 (m, 2H, ArH), 8.12 (dd, J ¼ 8.9, 1.7 Hz, 2H,
ArH), 8.39 (br d, J ¼ 5.6 Hz, 2H, 2 ꢂ NH). 13C NMR (75.0 MHz, CDCl3)
Compound (R,R)-9b was obtained from (R,R)-8 (0.197 g,
0.500 mmol), benzyl bromide (1.71 mL, 2.46 g, 10.0 mmol), and
DME (10 mL) in the same manner as that for (R,R)-9a. Compound
(R,R)-9b was isolated as a pale yellow powder (quantitative yield).
d
11.3 (CH3), 30.3 (CH2), 55.9 (CH), 114.2, 115.6, 126.7, 126.8, 128.4,
132.0, 135.9, 138.8, 144.3 (aromatic). Anal. Calc. for C24H26N4O4: C,
66.34; H, 6.03; N, 12.89; O, 14.73. Found: C, 66.17; H, 6.07; N,12.83%.
mp 175.2e177.7 ꢀC [
CDCl3)
a
]
27 þ50.5 (c 1.00, CHCl3). 1H NMR (300 MHz,
D
d
0.78 (t, J ¼ 7.1 Hz, 6H, 2 ꢂ CH3), 2.00e2.09 (m, 2H, CH2),
2.45e2.52 (m, 2H, CH2), 5.83 (d, J ¼ 5.4 Hz, 4H, 2 ꢂ NeCH2-Ph), 6.65
(t, J ¼ 7.1 Hz, 2H, 2 ꢂ CH), 7.31e7.45 (m, 6H, ArH), 7.53e7.55 (m, 4H,
ArH), 7.58e7.62 (m, 2H, ArH), 7.66e7.73 (m, 6H, ArH), 7.97e8.00 (m,
2H, ArH), 8.09e8.11 (m, 2H, ArH), 10.34 (s, 2H, 2 ꢂ N]¼CHeN) 13C
4.5. (R,R)-1,2-bis(1-(2-aminoanilino)propyl)benzene (R,R)-7
To a solution of (R,R)-6 (4.35 g, 10.0 mmol) in CH2Cl2 (25 mL) and
MeOH (25 mL) was added 5% Pd/C (1.30 g) and the mixture was
stirred under H2 atmosphere (1 atm balloon) for 24 h. The reaction
mixture was filtered through Celite to remove Pd/C. The filtrate was
concentrated under reduced pressure and the resulting residue was
purified by silica-gel column chromatography (Silica Gel 60N,
hexane/EtOAc ¼ 2/1) to give (R,R)-7 as a pale purple powder (3.11 g,
NMR (75.0 MHz, CDCl3)
d 10.3 (CH3), 27.9 (CH2), 50.3 (NeCH2ePh),
58.6 (CH), 114.3, 114.4, 126.9, 127.2, 127.9, 128.1, 129.0, 130.3, 130.9,
134.0, 135.0, 141.8 (aromatic). HRMS (ESI): calcd for C40H40N4Br
[MeBr]þ 655.2440, found 655.2431.
4.9. Silver complex (R,R)-10a
8.31 mmol, 83%). mp 122.7e124.1 ꢀC [
a
]
24 ꢁ79.1 (c 1.00, CHCl3). IR
D
(neat): 1598, 1621, 3322, 3392 (nNH, NH2) cmꢁ1. 1H NMR (400 MHz,
The solution of (R,R)-9a (0.678 g, 1.00 mmol) and Ag2O (0.238 g,
1.03 mmol) in CH2Cl2 (10 mL) was stirred overnight at room
temperature. The mixture was filtered through Celite and the
filtrate was concentrated under reduced pressure to give (R,R)-10a
as a pale brown powder (0.709 g, 0.795 mmol, 79%). mp >175.0 ꢀC
CDCl3)
d
0.96 (t, J ¼ 7.3 Hz, 6H, 2 ꢂ CH3), 1.83e2.00 (m, 4H, 2 ꢂ CH2),
3.18 (br s, 4H, 2 ꢂ NH2), 3.69 (br s, 2H, 2 ꢂ NH), 6.46e6.48 (m, 2H,
ArH), 6.59e6.70 (m, 6H, ArH), 7.22e7.26 (m, 2H, ArH), 7.45e7.49 (m,
2H, ArH). 13C NMR (100 MHz, CDCl3)
d 11.2 (CH3), 29.6 (CH2), 55.8
(CH), 112.6, 116.9, 118.3, 120.7, 126.5, 127.4, 134.2, 137.1, 140.8
(aromatic). Anal. Calc. for C24H30N4: C, 76.97; H, 8.07; N, 14.96.
Found: C, 77.12; H, 8.13; N, 14.88%.
(Dec.). [
a
]
23 þ57.1 (c 1.00, CH2Cl2). 1H NMR (270 MHz, CDCl3)
d 0.64
D
(t, J ¼ 7.3 Hz, 6H, 2 ꢂ CH3), 1.84e1.97 (m, 2H, CH2), 2.25e2.42 (m,
2H, CH2), 4.14 (s, 6H, 2 ꢂ NeCH3), 6.42 (dd, J ¼ 7.3, 5.9 Hz, 2 ꢂ CH),
7.04e7.13 (m, 2H, ArH), 7.16e7.23 (m, 2H, ArH), 7.27e7.36 (m, 3H,
ArH), 7.38e7.49 (m, 3H, ArH), 7.81e7.84 (m, 2H, ArH). 13C NMR
4.6. (R,R)-1,2-bis(1-(2-benzimidazolyl)propyl)benzene (R,R)-8
(67.5 MHz, CDCl3)
d 11.8 (CH3), 25.7 (CH2), 36.1 (NeCH3), 62.3 (CH),
p-Toluenesulfonic acid monohydrate (0.190 g, 1.00 mmol) was
added to a solution of (R,R)-7 (1.87 g, 5.00 mmol) in HC(OEt)3
(47.4 g, 320 mmol) and the mixture was stirred for 48 h at room
111.1, 112.6, 122.9, 123.0, 128.4, 128.6, 132.3, 135.6, 137.8 (aromatic),
200.1 (NeCeN). HRMS (ESI): calcd for C28H31N4 [Me2AgI þ H]þ
423.2553, found 423.2543.

2404
S. Anezaki et al. / Journal of Organometallic Chemistry 696 (2011) 2399e2405
4.10. Silver complex (R,R)-10b
2 ꢂ NeCH2ePh), 6.44 (d, J ¼ 8.2 Hz, 1H, ArH), 6.51 (d, J ¼ 7.2 Hz, 2H,
ArH), 6.68 (d, J ¼ 15.4 Hz, 1H, 1/2 ꢂ NeCH2ePh), 6.87 (t, J ¼ 7.6 Hz, 2H,
ArH), 7.00e7.03 (m, 4H, ArH), 7.15 (t, J ¼ 7.6 Hz, 1H, ArH), 7.20e7.28 (m,
5H, ArH), 7.42 (t, J ¼ 7.9 Hz, 1H, ArH), 7.45 (d, J¼ 7.9 Hz, 2H, ArH), 7.62 (d,
J ¼ 8.2 Hz, 1H, ArH), 7.66 (t, J ¼ 7.6 Hz, 1H, ArH), 7.77 (t, J ¼ 7.6 Hz, 1H,
ArH), 8.13 (d, J ¼ 8.2 Hz,1H, ArH). (DMSO-d6) (70ꢀC) Many signals were
Compound (R,R)-10b was obtained from (R,R)-9b (0.221 g, 0.300
mmol), Ag2O (0.080 g, 0.345 mmol), and CH2Cl2 (10 mL) in the same
manner as that for (R,R)-10a. Compound (R,R)-10b was isolated as
a pale brown powder (0.263 g, 0.277 mmol, 92%). mp > 155.4 ꢀC (Dec.).
[
a D
]
27 þ18.7 (c 1.00, CH2Cl2). 1H NMR (500 MHz, CDCl3)
d
0.71 (t,
observed asbroad signal.
d
1.21 (t,J¼ 7.3 Hz, 6H, 2 ꢂ CH3), 2.15e2.36 (br,
J ¼ 7.3 Hz, 6H, 2 ꢂ CH3),1.07e1.88 (m, 2H, CH2), 2.45e2.51 (m, 2H, CH2),
5.78 (m, 4H, 2 ꢂ NeCH2ePh), 6.47 (dd, J ¼ 7.7, 5.7 Hz, 2H, 2 ꢂ CH),
7.18e7.31 (m, 10H, ArH), 7.33e7.41 (m, 6H, ArH), 7.49e7.51 (m, 4H,
4H, 2 ꢂ CH2), 5.60e5.68 (br, 2H, NeCH2ePh), 5.90e5.91 (br, 2H,
2 ꢂ CH), 6.04e6.11 (br, 2H, NeCH2ePh), 6.81e6.85 (m, 2H, ArH),
6.97e7.02 (br, 2H, ArH), 7.10e7.20 (m, 10H, ArH), 7.33 (t, J ¼ 7.9 Hz, 2H,
ArH), 7.50e7.54 (br, 2H, ArH), 7.65e7.72 (br, 2H, ArH), 7.81 (d, J ¼ 7.9 Hz,
ArH), 7.89e7.91 (m, 2H, ArH). 13C NMR (125 MHz, CDCl3)
d 11.6 (CH3),
27.1 (CH2), 63.1 (NeCH2ePh), 72.2 (CH),112.7,113.4,124.2,128.1,128.5,
129.0,129.2,129.5,133.9,134.9,136.3,138.2 (aromatic),196.2 (NeCeN).
HRMS (ESI): calcd for C40H39N4BrAg [MeAgBr þ H]þ 761.1404, found
761.1404.
2H, ArH). 13C NMR (125 MHz, CD2Cl2) (ꢁ30 ꢀC)
d 12.4 (CH3),12.5 (CH3),
23.8 (CH2), 26.2 (CH2), 52.9 (NeCH2ePh), 54.5 (NeCH2ePh), 60.7 (CH),
68.0(CH),110.9,111.5,112.2,113.3,123.6,123.8,124.3,124.5,125.7,127.9,
128.0, 128.5, 129.0, 129.9, 130.6, 131.5, 133.5, 134.0, 134.3, 134.5, 134.6,
135.1,135.3,136.4 (aromatic),171.2 (NeCeN),172.5 (NeCeN). (DMSO-
4.11. Pd complex (R,R)-11a
d6) (70 ꢀC)
d 11.5 (CH3), 24.3 (CH2), 53.3 (NeCH2ePh), 63.5 (CH), 111.8,
112.1, 123.5, 127.0, 127.5, 128.2, 129.4, 133.5, 133.9, 135.2 (aromatic),
172.6 (NeCeN). Anal. Calc. for C40H38N4Cl2Pd: C, 63.88; H, 5.09; N, 7.45.
Found: C, 63.93; H, 5.29; N, 7.30%.
The solution of (R,R)-10a (0.170 g, 0.120 mmol) and PdCl2(PhCN)2
(0.046 g, 0.120 mmol) in CH2Cl2 (5 mL) was stirred overnight at room
temperature. The mixture was filtered through Celite and the filtrate
was concentrated under reduced pressure. The residuewas purified by
silica-gel column chromatography (Silica Gel 60N, acetone/EtOAc ¼ 1/
1) to give (R,R)-11a as a white solid (0.049 g, 0.082 mmol, 68%).
4.13. Experimental procedure for X-ray analyses
mp > 300.0 ꢀC (Dec.). [a D24 þ70.3 (c 0.30, CH2Cl2). 1H NMR (500 MHz,
]
Suitable single crystals were obtained by recrystallization from
CH3CN (11a) or CHCl3/hexane (11b) and are mounted on glass
fibers, respectively. The measurement of complex 11a was made on
a Rigaku Saturn diffractometer using graphite monochromated Mo-
CD2Cl2) (ꢁ70 ꢀC)
1.20e1.41 (2 ꢂ br, 6H, 2 ꢂ CH3),1.46e1.62 (br m, 1H,
d
1/2 ꢂ CH2), 1.91e2.11 (br m, 1H, 1/2 ꢂ CH2), 2.35e2.52 (br m, 1H, 1/
2 ꢂ CH2), 2.71e2.93 (br m, 1H, 1/2 ꢂ CH2), 4.17 (s, 3H, NeCH3), 4.47 (s,
3H, NeCH3), 5.70 (br d, J ¼ 10.7 Hz, 1H, CH), 5.75 (br d, J ¼ 10.7 Hz, 1H,
CH), 7.40e7.76 (m, 11H, ArH), 8.07e8.08 (m,1H, ArH). (CD2Cl2)(ꢁ30 ꢀC)
K
a
radiation at ꢁ180 ꢃ 1 ꢀC. Complex 11b was measured by the use
of a Rigaku R-AXIS RAPID diffractometer using graphite mono-
chromated Cu-K
a
radiation at 23 ꢃ 1 ꢀC.
d
1.28 (t, J ¼ 7.3 Hz, 3H, CH3), 1.30 (t, J ¼ 7.3 Hz, 3H, CH3), 1.54e1.60 (m,
Crystallographic data and the results of measurements are
summarized in Table 2. The structures were solved by direct
methods (SIR97) [19] and expanded using Fourier techniques.
Least-square refinements were carried out using SHELXL97 [20]. All
of the non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were introduced at the ideal positions and refined using the
riding model. All calculations were performed using the Crystal-
Structure crystallographic software package [21].
1H, 1/2 ꢂ CH2), 1.96e2.02 (m, 1H, 1/2 ꢂ CH2), 2.38e2.44 (m, 1H, 1/
2 ꢂ CH2), 2.79e2.86 (m,1H,1/2 ꢂ CH2), 4.16 (s, 3H, NeCH3), 4.44 (s, 3H,
NeCH3), 5.70 (br s, 1H, CH), 5.72 (br s, 1H, CH), 7.35e7.46 (m, 8H, ArH),
7.55 (d, J¼ 8.2 Hz,1H, ArH), 7.60 (t, J¼ 7.9 Hz, 1H, ArH), 7.73 (t, J¼ 7.9 Hz,
1H, ArH), 8.06 (d, J ¼ 8.2 Hz, 1H, ArH). (CD2Cl2) (25 ꢀC)
d 1.27 (t,
J ¼ 7.3 Hz, 6H, 2 ꢂ CH3), 2.20 (br, 4H, 2 ꢂ CH2), 4.29 (s, 6H, 2 ꢂ NeCH3),
5.68e5.71 (m, 2H, 2 ꢂ CH), 7.31e7.77 (m, 12H, ArH). (DMSO-d6) (70 ꢀC)
d
1.19 (t, J ¼ 7.3 Hz, 6H, 2 ꢂ CH3), 2.10e2.20 (br m, 4H, 2 ꢂ CH2), 4.25 (s,
6H, 2 ꢂ NeCH3), 5.83 (dd, J ¼ 7.4, 4.7 Hz, 2H, 2 ꢂ CH), 7.35e7.40 (m, 4H,
ArH), 7.48e7.49 (m, 2H, ArH), 7.58e7.61 (m, 4H, ArH), 7.77e7.78 (m, 2H,
Table 2
Summary of crystal data for complexes 11a and 11b.
ArH).13C NMR (125 MHz, CD2Cl2)(ꢁ70 ꢀC)
d 12.4 (CH3),12.6 (CH3), 23.7
11a
11b
(CH2), 26.2 (CH2), 36.2 (NeCH3), 37.7 (NeCH3), 60.2 (CH), 67.4 (CH),
110.8, 111.15, 111.21, 111.3, 123.93, 123.97, 124.00, 124.3, 129.6, 130.4,
131.5, 133.2, 133.4, 134.2, 134.6, 135.17, 135.19, 136.3 (aromatic), 169.9
Emprical formula
Formula weight
Crystal color, habit
Crystal size (mm)
Crystal system
Space group
Lattice parameters
a (Å)
C30H34Cl6N4Pd
769.74
Pale yellow, prism
C41H39Cl5N4Pd
871.45
Yellow, prism
(NeCeN),170.1 (NeCeN). (CD2Cl2)(25ꢀC)
d
12.1 (CH3), 25.1 (CH2), 36.8
(NeCH3), 111.0, 111.2, 124.0, 124.3, 230.4, 134.2, 135.5 (aromatic), 172.5
(NeCeN). (DMSO-d6) (70 ꢀC)
11.0 (CH3), 23.9 (CH2), 35.8 (NeCH3),
0.120 ꢂ 0.080 ꢂ 0.060 0.180 ꢂ 0.160 ꢂ 0.120
Triclinic
P1 (no. 2)
Monoclinic
C2/c (no. 15)
d
62.8 (CH), 110.7, 111.0, 123.0, 123.2, 128.9, 132.9, 133.2, 133.5, 134.2
(aromatic), 171.4 (NeCeN). Anal. Calc. for C28H30N4Cl2Pd: C, 56.06; H,
5.04; N, 9.34. Found: C, 55.72; H, 5.30; N, 8.99%.
10.423(2)
10.847(2)
14.067(3)
96.062(3)
93.107(4)
90.772(3)
1578.9(6)
2
26.724(2)
21.4243(8)
17.6792(10)
b (Å)
c (Å)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
129.564(2)
4.12. Pd complex (R,R)-11b
V (Å3)
7803.2(7)
8
1.483
Compound (R,R)-11b was obtained from (R,R)-10b (0.128 g,
0.135 mmol), PdCl2(PhCN)2 (0.052 g, 0.135 mmol), and CH2Cl2 (6 mL) in
the same manner as that for (R,R)-11a. The residue was purified by
silica-gel column chromatography (Silica Gel 60N, hexane/EtOAc ¼ 2/
Z
Dc (g cmꢁ3
)
1.619
F000
m
780.00
3552.00
(cmꢁ1
)
(Mo-K
a
) 11.236
(Cu-Ka) 72.546
Reflections measured
Independent reflections (Rint
Number of variables
Reflection/parameter ration
Residuals: R; Rw
12809
7033 (0.0229)
370
19.01
0.0509; 0.1137
0.0413
36824
6693 (0.1411)
461
14.52
0.1110; 0.2905
0.0913
3wEtOAc) to give(R,R)-11b as apaleyellowsolid(0.073g, 0.097 mmol,
)
20
72%). mp > 220.0 ꢀC (Dec.). [a D
]
þ183.9 (c 1.00, CH2Cl2). 1H NMR
(500 MHz, CD2Cl2) (ꢁ30 ꢀC)
d
1.33 (t, J ¼ 7.3 Hz, 3H, CH3), 1.38 (t,
J ¼ 7.3 Hz, 3H, CH3), 1.70e1.77 (m, 1H, 1/2 ꢂ CH2), 2.15e2.20 (m, 1H, 1/
2 ꢂ CH2), 2.46e2.50 (m, 1H, 1/2 ꢂ CH2), 2.93e3.00 (m, 1H, 1/2 ꢂ CH2),
4.22 (d, J ¼ 15.4 Hz, 1H, 1/2 ꢂ NeCH2ePh), 5.78e5.81 (m, 2H, 2 ꢂ CH),
5.85 (d, J ¼ 16.7 Hz, 1H, 1/2 ꢂ NeCH2ePh), 6.12 (d, J ¼ 16.7, 1H, 1/
Residuals: R1
Goodness-of-fit indicator
1.046
1.18, ꢁ1.25
1.142
1.31, ꢁ1.30
drmax,min (e Å3)

S. Anezaki et al. / Journal of Organometallic Chemistry 696 (2011) 2399e2405
2405
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Acknowledgment
The authors are grateful to Dr. Hiroyasu Sato (Rigaku Co., Ltd.)
and Prof. Shinya Matsumoto for help in X-ray analyses, Mr. Shinji
Ishihara for elemental analyses and Dr. Takeo Kaneko (Yokohama
Natl. Univ.) for HRMS. This work was financially supported by the
Japan Society for the Promotion of Science (Grant-In-Aid for
Scientific Research 20550057).
(n) X.-Q. Xiao, A.-Q. Jia, Y.-J. Lin, G.-X. Jin, Organometallics 29 (2010) 4842.
[5] C. Zhang, M.L. Trudell, Tetrahedron Lett. 41 (2000) 595.
[6] D.S. Clyne, J. Jin, E. Genest, J.C. Gallucci, T.V. RajanBabu, Org. Lett. 2 (2000)
1125.
[7] Selected reviews: (a) M.C. Perry, K. Burgess, Tetrahedron: Asymmetry 14
(2003) 951;
Appendix. Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.jorganchem.2011.03.001.
(b) V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004) 619;
(c) R.E. Douthwaite, Coord. Chem. Rev. 251 (2007) 702;
(d) L.H. Gade, S. Bellemin-Laponnaz, Coord. Chem. Rev. 251 (2007) 718.
[8] M.C. Perry, X. Cui, K. Burgess, Tetrahedron: Asymmetry 13 (2002) 1969.
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