N-heterocyclic benzhydrylamines as new N,N-bidentate ligands in palladium complexes: Synthesis, characterization and catalytic activity

Article abstract of DOI:10.1002/ejic.200800154

The synthesis of new N,N-bidentate ligands based on a π-deficient N-heterocyclic benzhydrylamine core is described. The corresponding air-stable palladium complexes are obtained in high yields and fully characterized by NMR spectroscopy and X-ray structur

Full text of DOI:10.1002/ejic.200800154

FULL PAPER
DOI: 10.1002/ejic.200800154
N-Heterocyclic Benzhydrylamines as New N,N-Bidentate Ligands in Palladium
Complexes: Synthesis, Characterization and Catalytic Activity
Vincent Terrasson,[a] Damien Prim,*[a] and Jérome Marrot[a]
Keywords: Nitriles / Benzhydrylamines / N,N-Ligands / Palladium
The synthesis of new N,N-bidentate ligands based on a π-
in good yields, also for reactions with o-substituted phenyl
deficient N-heterocyclic benzhydrylamine core is described.
chlorides as starting materials. The effects of both electronic
The corresponding air-stable palladium complexes are ob- and steric modulations at the benzhydrylamine core on the
tained in high yields and fully characterized by NMR spec-
troscopy and X-ray structure determination. These new
phosphane-free catalytic systems proved efficient in the Su-
zuki–Miyaura reaction, enabling us to obtain biaryl products
catalytic activity are also described.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
tion-metal complex, common starting materials, as well as
ease of purification should also be taken into account. Even
if significant progresses have been made, there is a steady
demand for alternative, new, easy to prepare, stable ligands,
and efficient catalytic systems. Herein, we report on a new
series of N,N-bidentate ligands and their corresponding pal-
ladium complexes based on a N-heterocyclic benzhy-
drylamine framework (Figure 1).
Introduction
Transition-metal-catalyzed bond formation has become a
powerful chemical tool over the last decades. Among these
catalyzed transformations, the Suzuki–Miyaura reaction
has emerged as one of the most popular and general way
to create carbon–carbon bonds and has thus found wide-
spread applications in organic synthesis.[1] This Pd-assisted
process offers many advantages over other transition-metal-
catalyzed reactions, such as commercial availability of a
large panel of boronic acid derivatives, easy handling, easy
removal of boron residues, and high functional group toler-
ance, explaining the increasing interest not only in academic
research but also in the industrial scientific community. In-
deed, numerous drugs, materials, optical devices, or prod-
ucts of industrial significance, prepared by using at least
one catalyzed C–C bond formation, have been recently pat-
ented and commercialized.[2] The catalytic system (ligand–
Pd combination), of course, plays a crucial role in such pro-
cesses.[3] The most often used ligands, which paved the way
to the current success of Pd-catalyzed reactions, are tertiary
phosphanes. Recent investigations[4] have shown that phos-
phane-free catalytic systems efficiently compete with P-
based catalytic systems and overcome some of their known
drawbacks such as cost, air-stability, degradation, or P-
based waste management. The selection of a robust ligand
involves an overall compromise between several significant
criteria. If the whole efficiency of the catalytic system (cata-
lyst loading, yields, conversion, etc.) is a key parameter, ra-
pid access to the ligand, stability of both ligand and transi-
Figure 1. N-Heterocyclic benzhydrylamine-based Pd complexes.
Such ligands are readily obtained in high yields from
commercially available common reactants, and the access
methodology allows facile structural modulation of the li-
gand. In addition, the corresponding Pd complexes are air-
stable and reveal highly efficiency in the Suzuki–Miyaura
reaction.
Results and Discussion
We first focused our attention on model compound 1a,
whose structure is based on a benzylamine core substituted
by a pyridine ring. Although access to arylmethylamines
can be envisioned through several metal-catalyzed pro-
cesses,[5] methylamine 1a was prepared in a one-pot se-
quence involving addition of phenylmagnesium chloride to
2-cyanopyridine and subsequent reduction according to our
previously described methodology.[6] Benzylation of the pri-
mary amine proved judicious in increasing the solubility of
[a] Institut Lavoisier UMR CNRS 8180, Université de Versailles-
Saint-Quentin-en-Yvelines,
45, avenue des Etats-Unis, 78035 Versailles, France
Fax: +33-1-39254452
E-mail: prim@chimie.uvsq.fr
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V. Terrasson, D. Prim, J. Marrot
FULL PAPER
the complex in most common solvents (CH2Cl2, EtOAc,
etc.), thus making isolation, purification, and characteriza-
tion easier. With the aim to develop new ligand series, we
chose to modulate both electronic and steric effects over
model compound 1a. Indeed, using the same straightfor-
ward methodology, we were able to prepare new pyrimidine
and pyrazine analogues, 1b and 1c, respectively (Scheme 1).
The question was whether the presence of a second nitrogen
atom and its relative position in the π-deficient heterocycle
could influence complexation properties, structures of the
complexes, and their catalytic activities. Substitution at the
methylamine nitrogen atom was also modified by further
alkylation, leading to a tertiary amine.
Scheme 2. Preparation of Pd complexes 2.
for all complexes indicates that the complexation process
selectively afforded only one of the possible (rac-)dia-
1
stereomers. Selected H NMR chemical shifts are gathered
in Table 1.
Characteristic positive shieldings ranging from 0.46 to
0.82 ppm for benzyl protons H(1) and H(1Ј) are observed
between ligands 1a–1d and their complexes 2a–2d, thus
confirming coordination by the metal center. However, it is
worth noting that close chemical shifts of benzyl protons
H(1) and H(1Ј) in both ligands 1a–1c and the correspond-
ing complexes 2a–2c are observed, regardless of the hetero-
cyclic substituent. Interestingly, H(6) protons exhibit only
poor shielding regardless of the heterocycle and the pres-
ence of additional substituents at the methylamine frag-
Scheme 1. Preparation of hetero-biarylmethylamine ligands.
In order to prepare the corresponding Pd complexes 2a– ment.
2d, complexation reactions were next examined. Com-
Single crystals suitable for X-ray analysis were obtained
pounds 1a–d were treated with Na2PdCl4 under classical by slow evaporation of dmso solutions of complexes 2a, 2c,
conditions[7] to give the expected PdII complexes 2 in high and 2d (Figure 2).[8]
yields ranging from 92% to quantitative (Scheme 2). The
It is worth noting that all structures present similar ar-
reaction proceeded smoothly at room temperature in freshly rangements. Selected bond lengths and angles are presented
distilled methanol. After several hours, precipitation of in Table 2. For example, Pd–N(1) and Pd–N(8), as well as
complexes 2a–c was observed. In contrast, after 16 h, 2d Pd–Cl(1) and Pd–Cl(2) bond lengths in all complexes are
remained soluble. In all cases, the solvent was removed, and very close. Similarly, C(2)–N(1)–Pd and C(7)–N(8)–Pd
the solid residue was purified by simple filtration through a angles range from 113.7 to 115.3° and from 105.4 to 108.9°,
silica gel pad to afford the corresponding palladium com- respectively. Only a small variation of the Pd-plane devia-
plex. These complexes are perfectly air-stable, both in the tion between 2a, 2c, and 2d has been observed.
solid state and in solution.
It is also worth to note that large substituents at C(7) and
It is worth noting that increasing nitrogen atom substitu- N(8) arranged in an anti fashion during the complexation
tion from 1a to 1d did not affect complexation yields. Simi- process. Minimization of steric factors between these sub-
larly, moving from pyridine in 1a to pyrimidine and pyr- stituents in the corresponding ligands probably account for
azine in 1b and 1c, respectively led to no significant varia- these preferred relative configurations at the rigid diazamet-
tion in isolated yields for the corresponding complexes. allacyclopentane in 2a, 2c, and 2d. Thus, obtention of one
1
NMR spectroscopic data of complexes 2a–2d are in good (rac-)diastereomer is strongly supported by both H NMR
agreement with their respective structures. In addition, the spectroscopic data in solution (Table 2) and crystallo-
1
presence of only one set of signals in the H NMR spectra graphic data in the solid state.
1
Table 1. Selected H NMR chemical shifts.
Entry
Series
δ(H1Ј) [ppm]
δ(H1) [ppm]
δH(complex) – δH(ligand)
δ(H6) [ppm]
δH(complex) – δH(ligand)
1
2
δH(complex) – δH(ligand)
1
2
1
2
1
2
3
4
a
b
c
3.62
3.64
3.65
3.43
4.09
4.14
4.11
4.17
0.47
0.50
0.46
0.74
4.85
4.94
4.95
4.63
5.42
5.51
5.57
5.45
0.57
0.57
0.62
0.82
8.47
8.75
8.49
8.46
8.53
8.75
8.57
8.67
0.06
0
0.08
0.21
d
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N-Heterocyclic Benzhydrylamines as N,N-Bidentate Ligands in Pd Complexes
Figure 2. X-ray structures of complexes 2a, 2c, and 2d.
Table 2. Selected crystallographic data for complexes 2a, 2c, and 2d.
Complex
Bond lengths [Å]
Angles [°]
Pd-plane
deviation
N(1)–Pd
N(8)–Pd
Cl(1)–Pd
Cl(2)–Pd
C(2)–N(1)–Pd
C(7)–N(8)–Pd
2a
2c
2d
2.021
2.023
2.020
2.031
2.043
2.083
2.303
2.289
2.293
2.282
2.281
2.300
114.9
113.7
115.3
108.9
105.4
107.1
0.062
0.043
0.039
At this stage, we started to evaluate the catalytic activity azine heterocycles, respectively. Interestingly, the use of the
of methylamine-based complexes 2a–2d. The preliminary latter catalytic systems afforded higher conversions in the
study and comparison of our four catalytic systems for the same 30-minute time frame (entries 9 and 10). Moreover,
preparation of 4-methyl-4Ј-nitro-1,1Ј-biphenyl (3) are the relative positions of the two nitrogen atoms in the het-
shown in Table 3. The behavior of complex 2a was first ex- erocycle seemed to affect the overall coupling process. In-
amined in the Suzuki reaction shown in Table 3. The model deed, pyrazine-based complex 2c compared favorably to
compound could be easily obtained in quantitative yield by
using classical reaction conditions (Table 3, entry 1). When
the catalyst loading was lowered to 0.01 mol-%, longer reac-
tion courses (4 h) were required to reach quantitative con-
versions (compare entries 2 and 3). Attempts to further de-
crease the catalyst loading to 10–3 mol-% failed, leading to
poor conversion even after prolonged heating times. The
nature of the base was next examined. Under the same con-
ditions, moving from K2CO3 to K3PO4 (entry 4) and
Cs2CO3 (entry 5) gave various results. The use of K3PO4
afforded a lower conversion rate. In contrast, Cs2CO3 as the
base was found to be superior, significantly enhancing the
reaction rate and allowing quantitative access to the model
compound within 2 h (entry 7). These preliminary results
clearly demonstrate that this first catalytic system represents
a good compromise between the overall catalytic activity
and the easy obtention of the methylamine ligand. It was
then debatable, whether electronic and steric modifications
of catalyst 2a would modulate the catalytic activity and im-
prove the results we already described. The steric effect was
next examined. Comparison of catalysts 2a and 2d under
identical reaction conditions (entries 5 and 8) led to similar
conversion after 30 min, indicating that an increase in sub-
stitution at the methylamine nitrogen atom only poorly af-
fected the catalytic process. The same procedure was ap-
Table 3. Evaluation of catalytic activity.
Entry Cat. (mol-%) Base
Time [h]
Conversion[b] [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
2a (1)
K2CO3
K2CO3
K2CO3
K3PO4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
0.5
1
quant.
43
quant.
31
34
62
96
28
2a (0.01)
2a (0.01)
2a (0.01)
2a (0.01)
2a (0.01)
2a (0.01)
2d (0.01)
2b (0.01)
2c (0.01)
2c (0.01)
4
1
0.5
1
2
0.5
0.5
0.5
1
50
70
quant. (92)[c]
82
2c (0.01)[d] K2CO3
2c (0.1)[e]
Cs2CO3
2
18
quant.
[a] Conditions: Bromonitrobenzene (0.5 mmol), 4-methylben-
zeneboronic acid (0.6 mmol, 1.2 equiv.), base (1.25 mmol,
2.5 equiv.), dmf/H2O (95:5, 1 mL), 100 °C. [b] Based on H NMR
1
spectra. [c] Isolated yield [%]. [d] Reaction conducted in water. [e]
plied for complexes 2b and 2c, bearing pyrimidine and pyr- Reaction conducted at 50 °C.
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V. Terrasson, D. Prim, J. Marrot
FULL PAPER
pyrimidine-based complex 2b and quantitatively afforded versions required increased reaction times and higher cata-
the expected coupling product 3 within 1 h (entry 11). At lyst loadings (entries 9–10). Biphenyl 12, bearing two o,oЈ-
this stage, the exact role of the two nitrogen atoms and the substituents, was successfully prepared and isolated in 89%
influence of their relative position on the outcome of the yield by using 0.05 mol-% of catalyst 2c (entry 11). It is
catalytic process are not fully stated. Also worth noting are worth noting that the use of pure water instead of aqueous
the results obtained in pure aqueous media and at lower dmf may afford increased conversions and yields. Indeed,
temperatures. Interestingly, biphenyl 3 could be obtained in biphenyls 9 and 11 were readily obtained in 93 and 89%
high yield (82%) by using complex 2c as the catalyst (entry respective yield in inorganic media (compare entries 6–7
12) in environmentally more friendly pure aqueous media. and 9–10). Finally, catalytic system 2c (0.2 mol-%) also
In addition, all coupling reactions were run at 100 °C in proved to be efficient in the coupling reaction of aromatic
aqueous dmf, which potentially hampers the use of ther- chlorides and boronic acids substituted in an ortho or para
mally sensitive substrates. Thus, we envisioned decreasing fashion (entries 12–14).
the reaction temperature. Although no coupling reaction
could be observed at 25 °C, even for prolonged reaction
times, quantitative conversions have been obtained in 18 h
at 50 °C (entry 13).
Conclusion
We have developed efficient and robust phosphane-free
catalytic systems based on a N-heterocyclic benzhy-
drylamine core for Suzuki couplings. Such new, stable PdII
complexes represent a useful compromise between catalyst
efficiency (catalyst loading, yields), facile and short prepa-
ration of ligands and complexes, compatibility with chloride
substrates, ortho-substituted coupling partners, and pure
aqueous and mild reaction conditions.
Table 4 gathers the results of an aromatic substitution
survey for this reaction. Variously substituted bromoben-
zenes were successfully coupled with substituted phenyl-
boronic acids by using 2c as the catalyst. Disappearance of
the starting aromatic halides was monitored by TLC (times
indicated in Table 4) and confirmed by 1H NMR spec-
troscopy of the crude material. Compounds 4 to 15 were
obtained in high yields after easy isolation and filtration of
the crude product over a silica gel pad.
Experimental Section
Table 4. Suzuki reaction.
General Remarks: Reactions were carried out in round-bottomed
flasks equipped with a magnetic stirring bar and capped with a
septum. Diethyl ether was distilled from Na/benzophenone and
methanol over Mg/I2. TLC analyses were performed on Merck sil-
ica gel 60 F254 TLC plates (0.5 mm thickness). FTIR spectra were
1
recorder with a Perkin–Elmer Spectrum BX spectrometer, and H
and 13C spectra were recorded with Bruker 200 and Advance-300
spectrometers and referenced to CDCl3 or [D6]dmso. Mass spectra
were obtained at the mass spectrometry facilities operated by the
ENSC Clermont-Ferrand.
Entry
R1
X
R2
Cat. [mol-%] - Time Comp. Conv. (%)[b]
Conditions[a] [h]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
4-NO2 Br 2,4-(MeO)2
0.01 - A
0.01 - A
0.01 - A
0.01 - A
0.01 - A
0.01 - A
0.01 - B
0.01 - B
0.1 - A
4
5
2
3
5
2
2
4
72
18
12
24
24
24
4
5
6
7
8
quant. (95)
quant. (95)
95 (90)
quant. (95)
quant. (97)
44
quant. (93)
quant. (95)
85
4-NO2 Br
4-CHO Br
4-CHO Br
4-CHO Br
4-Ac Br
4-Ac Br
4-OH Br
4-OMe Br
4-OMe Br
2-CN Br
4-CN Cl
2-CHO Cl
2-NO2 Cl
4-CF3
H
2-Me
3-NO2
H
H
H
H
H
2-Me
2-Me
4-MeO
4-MeO
Synthesis of the Phenyl(pyrazin-2-yl)methylamine: Biarylmethyl-
amines were synthesized by following our previously described pro-
cedure:[6] to a stirred solution of pyrazine-2-carbonitrile (1 g,
9.50 mmol) in freshly distilled diethyl ether (25 mL) at 0 °C under
argon was added dropwise a solution of phenylmagnesium chloride
(7.50 mL, 1.9 , 14.27 mmol) in thf. The reaction mixture was
stirred at room temperature for 16 h, and the solvent was removed
by evaporation. The residue was then dissolved in MeOH (25 mL),
NaBH4 (360 mg, 9.50 mmol) was added at 0 °C, and the mixture
was stirred at room temperature for 4 h. After concentration of the
solution under vacuum, saturated aqueous NH4Cl solution
(25 mL) was added, and the aqueous phase was extracted with
EtOAc (3ϫ25 mL). The combined organic layers were dried
(MgSO4), filtered, and concentrated under vacuum. The crude
product was purified by flash chromatography on silica gel to give
the pure methylamine (1.11 g, 5.99 mmol, 63%) as an orange oil.
9
9
10
11
11
12
13
14
15
0.1 - B
0.05 - A
0.2 - A
0.2 - A
0.2 - A
quant. (89)
quant. (89)
quant. (75)
quant. (74)
quant. (78)
[a] Conditions: Aromatic halide (0.5 mmol), boronic acid
(0.6 mmol, 1.2 equiv.), base (1.25 mmol, 2.5 equiv.), solvent (1 mL),
100 °C, A: Cs2CO3, dmf/H2O (95:5), B: K2CO3, H2O. [b] Conver-
sion based on 1H NMR spectra, isolated yields are within brackets.
1
Rf = 0.15 (5% MeOH in EtOAc). H NMR (300 MHz, CDCl3): δ
A first set of aromatic bromides reacted with phenylbo-
ronic acids bearing either electronic donating or with-
drawing groups under the aforementioned catalytic condi-
tions (entries 1–10). For reaction times of 2 to 5 h, regard-
less of the substituent on the phenylboronic acid (entries 1–
8), the presence of a strong donating group at the halide
= 2.31 (s, 2 H, NH2), 5.20 (s, 1 H), 7.15–7.27 (m, 3 H), 7.29–7.33
(m, 2 H), 8.32 (d, J = 2.50 Hz, 1 H), 8.41 (dd, J = 1.50 and 2.50 Hz,
1 H), 8.49 (d, J = 1.50 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 59.0, 126.8 (2 C), 127.5, 128.7 (2 C), 142.9, 143.4, 143.6, 143.7,
158.6 ppm. IR (film): ν = 3360, 3288, 3058, 3027, 2919, 1685, 1491,
˜
1470, 1450, 1399, 1143, 1050, 1020, 851, 753, 697, 600 cm–1. MS
partner affected the reaction course. Indeed, complete con- (CI, CH4): m/z (%) = 371 (20) [2M + H]+, 352 (35), 186 (95) [M +
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N-Heterocyclic Benzhydrylamines as N,N-Bidentate Ligands in Pd Complexes
H]+, 169 (100) [M – NH2]+. HRMS (ESI): calcd. for [M + H]+
Methylation of Amine 1a: To a stirred solution of methylamine 1a
186.1031; found 186.1026.
(362 mg, 1.32 mmol) and K2CO3 (274 mg, 1.98 mmol) in MeCN
(10 mL) was added dropwise methyl iodide (82 µL, 187 mg,
1.32 mmol). The mixture was stirred at room temperature for 16 h,
and the reaction was quenched by addition of saturated aqueous
NaHCO3 solution (15 mL). The aqueous phase was extracted with
EtOAc (3ϫ25 mL), and the combined organic layers were dried
(MgSO4), filtered, and concentrated under vacuum. The crude
product was purified by flash chromatography on silica gel to give
tertiary amine 1d (288 mg, 1 mmol, 76%) as a yellow oil.
Benzylation of Primary Amines: To a stirred solution of biarylme-
thylamine (2 mmol) and MgSO4 (481 mg, 4 mmol) in MeOH
(20 mL) was added benzaldehyde (202 µL, 212 mg, 2 mmol). The
mixture was stirred at room temperature for 12 h, then cooled to
0 °C, and NaBH4 (76 mg, 2 mmol) was added. The reaction mix-
ture was stirred at room temperature for 4 h. The solution was then
concentrated under vacuum, and saturated aqueous NH4Cl solu-
tion (25 mL) were added. The aqueous phase was extracted with
EtOAc (3ϫ25 mL). The combined organic layers were dried
(MgSO4), filtered, and concentrated under vacuum. The crude
product was purified by flash chromatography on silica gel.
N-Benzyl-N-methyl-N-[phenyl(pyridin-2-yl)methyl]amine (1d): Rf =
1
0.45 (20% EtOAc in cyclohexane). H NMR (300 MHz, CDCl3):
δ = 1.98 (s, 3 H), 3.34 (d, J = 13.5 Hz, 1 H), 3.49 (d, J = 13.5 Hz,
1 H), 4.55 (s, 1 H), 6.90 (ddd, J = 1.20, 4.90 and 7.40 Hz, 1 H),
7.05–7.22 (m, 6 H), 7.26–7.29 (m, 2 H), 7.44–7.48 (m, 3 H), 7.59
N-Benzyl-N-[phenyl(pyridin-2-yl)methyl]amine (1a): Yield 490 mg
(90%); yellow oil; Rf = 0.20 (20% EtOAc in cyclohexane). 1H
NMR (300 MHz, CDCl3): δ = 2.88 (s, 1 H, NH), 3.60 (d, J =
13.3 Hz, 1 H), 3.67 (d, J = 13.3 Hz, 1 H), 4.86 (s, 1 H), 6.93 (ddd,
J = 1.0, 4.9, and 7.4 Hz, 1 H), 7.07–7.13 (m, 2 H), 7.16–7.24 (m, 7
H), 7.34–7.37 (m, 2 H), 7.40 (td, J = 1.8 and 7.6 Hz, 1 H), 8.42 (d,
1
(d, J = 7.90 Hz, 1 H), 8.36 (d, J = 4.90 Hz, 1 H) ppm. H NMR
(300 MHz, [D6]dmso): δ = 1.96 (s, 3 H), 3.38 (d, J = 13.5 Hz, 1 H),
3.48 (d, J = 13.5 Hz, 1 H), 4.63 (s, 1 H), 7.19–7.24 (m, 3 H), 7.26–
7.40 (m, 6 H), 7.52–7.55 (m, 2 H), 7.71–7.80 (m, 2 H), 8.46 (d, J
= 4.60 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 40.2, 59.6,
77.4, 121.8, 121.9, 126.7, 127.1, 128.1 (4 C), 128.4 (2 C), 128.5 (2
1
J = 4.8 Hz, 1 H) ppm. H NMR (300 MHz, [D6]dmso): δ = 3.26
(s, 1 H, NH), 3.62 (t, J = 14.6 Hz, 2 H), 4.85 (s, 1 H), 7.19–7.24
(m, 3 H), 7.25–7.32 (m, 6 H), 7.39–7.43 (m, 2 H), 7.50 (d, J =
7.9 Hz, 1 H), 7.73 (td, J = 1.8 and 7.7 Hz, 1 H), 8.47 (d, J = 4.8 Hz,
1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 50.6, 66.6, 121.4,
122.0, 126.6, 126.9, 127.4 (2 C), 128.0 (2 C), 128.1 (2 C), 128.3 (2
C), 136.5, 139.3, 141.6, 148.8, 162.6 ppm. IR (film): ν = 3083, 3058,
˜
3027, 307, 2955, 2929, 2843, 2786, 1665, 1588, 1496, 1455, 1429,
1588, 1312, 1281, 1132, 1020, 917, 779, 743, 697, 615, 554 cm–1.
MS (CI, CH4): m/z (%) = 289 (100) [M + H]+. HRMS (ESI): calcd.
for [M + H]+ 289.1705; found 289.1698.
C), 136.7, 140.5, 143.1, 148.7, 162.8 ppm. IR (film): ν = 3324, 3083,
˜
3063, 3027, 2832, 1670, 1583, 1568, 1496, 1475, 1450, 1434, 1117,
Preparation of Palladium Complexes: To a stirred solution of amine
1a–d (0.25 mmol) in freshly distilled MeOH (5 mL) was added
1025, 989, 743, 697, 615, 543 cm–1. MS (CI, NH3): m/z (%) = 275
(100) [M + H]+. HRMS (ESI):calcd. for [M + H]+ 275.1548; found Na2PdCl4 (74 mg, 0.25 mmol). The mixture was stirred at room
275.1551.
temperature for 16 h, and the solvent was removed by evaporation
under vacuum. The residue was then filtered through a silica gel
pad [firstly eluting with cyclohexane/EtOAc (7:3) to remove traces
of free amine, then eluting with EtOAc] to give the corresponding
palladium complexes.
N-Benzyl-N-[phenyl(pyrimidin-2-yl)methyl]amine (1b): Yield 436 mg
(79%); brown solid; Rf = 0.30 (50% EtOAc in cyclohexane); m.p.
1
73 °C. H NMR (300 MHz, CDCl3): δ = 2.48 (s, 1 H, NH), 3.64
(d, J = 13.1 Hz, 1 H), 3.73 (d, J = 13.1 Hz, 1 H), 5.03 (s, 1 H),
7.06 (t, J = 5.0 Hz, 1 H), 7.15–7.30 (m, 8 H), 7.38–7.41 (m, 2 H),
8.62 (d, J = 5.0 Hz, 2 H) ppm. H NMR (300 MHz, [D6]dmso): δ
Complex 2a: Yield 103 mg (92%); yellow powder; Rf = 0.60
(EtOAc). 1H NMR (300 MHz, [D6]dmso): δ = 3.95 (dd, J = 2.5
and 13.1 Hz, 1 H), 4.30 (dd, J = 4.2 and 13.1 Hz, 1 H), 5.46 (s, 1
H), 6.99 (s, 1 H, NH), 7.23–7.32 (m, 3 H), 7.34–7.39 (m, 2 H),
1
= 3.36 (s, 1 H, NH), 3.64 (s, 2 H), 4.94 (s, 1 H), 7.21–7.24 (m, 2
H), 7.27–7.32 (m, 6 H), 7.36 (t, J = 4.9 Hz, 1 H), 7.40–7.43 (m, 2
H), 8.77 (d, J = 4.9 Hz, 2 H) ppm. 13C NMR (75 MHz, CDCl3): δ 7.46–7.58 (m, 3 H), 7.88 (td, J = 1.4 and 7.7 Hz, 1 H), 7.95 (d, J
= 51.3, 67.5, 119.2, 127.0, 127.5, 127.8 (2 C), 128.4 (2 C), 128.5 (2
C), 128.6 (2 C), 139.9, 141.3, 157.2 (2 C), 170.9 ppm. IR (in KBr):
= 7.1 Hz, 4 H), 8.57 (d, J = 5.4 Hz, 1 H) ppm. 13C NMR (75 MHz,
[D6]dmso): δ = 57.8, 73.0, 123.4, 124.2, 128.6 (2 C), 128.8 (3 C),
129.5 (2 C), 129.6, 131.7 (2 C), 134.7, 137.9, 140.5, 148.9, 166.2
ν = 3304, 3083, 3053, 3017, 2940, 2843, 1783, 1562, 1491, 1460,
˜
1419, 1117, 1020, 820, 748, 697, 595, 549 cm–1. MS (CI, CH4): m/z
(%) = 276 (100) [M + H]+. HRMS (ESI): calcd. for [M + H]+
276.1501; found 276.1519.
ppm. IR (in KBr): ν = 3452, 3109, 3058, 3032, 2930, 2858, 1609,
˜
1496, 1475, 1455, 1440, 1424, 1163, 1143, 912, 764, 692, 554 cm–1.
MS [ESI, MeCN/MeOH (4:1)]: m/z (%) = 462–453 (100) [M – Cl
+ MeCN]+, 426–417 (50) [M – Cl – HCl + MeCN]+, 416–407 (10),
386–377 (15) [M – Cl – HCl]+. HRMS (ESI): calcd. for ([M – Cl –
HCl]+, 104Pd) 377.0432; found 377.0444. C19H18Cl2N2Pd (451.69):
calcd. C 50.52, H 4.02, N 6.20; found C 50.59, H 4.10, N 6.28.
N-Benzyl-N-[phenyl(pyrazin-2-yl)methyl]amine (1c): Yield 414 mg
(75%); yellow solid; Rf = 0.30 (30% EtOAc in cyclohexane); m.p.
1
79 °C. H NMR (300 MHz, CDCl3): δ = 2.59 (s, 1 H, NH), 3.62
(d, J = 13.2 Hz, 1 H), 3.69 (d, J = 13.2 Hz, 1 H), 4.90 (s, 1 H),
7.13–7.18 (m, 2 H), 7.20–7.26 (m, 6 H), 7.32–7.36 (m, 2 H), 8.28
(d, J = 2.7 Hz, 1 H), 8.37 (dd, J = 1.5 and 2.5 Hz, 1 H), 8.52 (d, J
Complex 2b: Yield 107 mg (95%); yellow powder; Rf = 0.65
(EtOAc). 1H NMR (300 MHz, [D6]dmso): δ = 3.97 (d, J = 12.7 Hz,
1 H), 4.31 (dd, J = 3.7 and 12.7 Hz, 1 H), 5.51 (s, 1 H), 7.08 (s, 1
1
= 1.50 Hz, 1 H) ppm. H NMR (300 MHz, [D6]dmso): δ = 3.41 (s,
1 H, NH), 3.62 (d, J = 13.7 Hz, 1 H), 3.68 (d, J = 13.7 Hz, 1 H), H, NH), 7.20–7.28 (m, 3 H), 7.39 (t, J = 5.3 Hz, 1 H), 7.45–7.54
4.95 (s, 1 H), 7.20–7.26 (m, 2 H), 7.30–7.35 (m, 6 H), 7.43–7.46 (m,
2 H), 8.49 (d, J = 2.5 Hz, 1 H), 8.54 (dd, J = 1.5 and 2.5 Hz, 1 H),
(m, 3 H), 7.93 (d, J = 6.8 Hz, 2 H), 7.99 (d, J = 6.5 Hz, 2 H), 8.64
(dd, J = 2.1 and 6.0 Hz, 1 H), 8.75 (dd, J = 2.1 and 4.9 Hz, 1 H)
8.83 (d, J = 1.50 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = ppm. 13C NMR (75 MHz, [D6]dmso): δ = 57.5, 72.4, 120.5, 128.2
51.4, 65.3, 127.0, 127.6 (2 C), 127.7, 128.1 (2 C), 128.3 (2 C), 128.7
(2 C), 128.4 (2 C), 128.5, 129.0 (2 C), 129.3, 131.5 (2 C), 134.2,
(2 C), 139.6, 141.2, 142.9, 143.7, 144.0, 157.8 ppm. IR (in KBr): ν
136.2, 155.7, 159.7, 173.9 ppm. IR (in KBr): ν = 3447, 3150, 3073,
˜
˜
= 3298, 3048, 3022, 2919, 2832, 1470, 1450, 1404, 1112, 1061, 1015,
3027, 2925, 2848, 1778, 1588, 1552, 1501, 1450, 1419, 1363, 1255,
835, 753, 697, 620, 589, 538 cm–1. MS (CI, CH4): m/z (%) = 276
1096, 1035, 820, 769, 748, 707, 697, 661, 554 cm–1. MS [ESI,
(100) [M + H]+. HRMS (ESI): calcd. for [M + H]+ 276.1501; found MeCN/MeOH (4:1)]: m/z (%) = 463–454 (100) [M – Cl +MeCN]+,
276.1502.
425–414 (60), 413–406 (65), 387–378 (15) [M
–
Cl
–
Eur. J. Inorg. Chem. 2008, 2739–2745
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2743

V. Terrasson, D. Prim, J. Marrot
FULL PAPER
HCl]+. HRMS (ESI): calcd. for ([M – Cl – HCl]+, 104Pd) 378.0384;
found 378.0381. C18H17Cl2N3Pd (452.67): calcd. C 47.76, H 3.79,
N 9.28; found C 48.01, H 3.82, N 9.29.
= 8.3 Hz, 2 H), 10.07 (s, 1 H) ppm. Data in accordance with pre-
viously reported results.[12]
4-Formyl-2Ј-methyl-1,1Ј-biphenyl (7): 1H NMR (200 MHz, CDCl3):
δ = 2.30 (s, 3 H), 7.25–7.33 (m, 4 H), 7.51 (d, J = 8.6 Hz, 2 H),
7.95 (d, J = 8.6 Hz, 2 H), 10.08 (s, 1 H) ppm. Data in accordance
with previously reported results.[12]
Complex 2c: Yield 105 mg (93%); orange powder; Rf = 0.60
(EtOAc). 1H NMR (300 MHz, [D6]dmso): δ = 3.93 (d, J = 13.3 Hz,
1 H), 4.30 (dd, J = 3.9 and 13.3 Hz, 1 H), 5.58 (s, 1 H), 7.19–7.28
(m, 3 H), 7.46–7.55 (m, 3 H), 7.88–7.95 (m, 4 H), 8.50 (d, J =
2.3 Hz, 1 H), 8.57 (d, J = 3.1 Hz, 1 H), 8.69 (s, 1 H) ppm. 13C
NMR (75 MHz, [D6]dmso): δ = 57.3, 70.8, 128.3 (2 C), 128.4 (2
C), 128.5, 129.1 (2 C), 129.4, 131.2 (2 C), 134.0, 136.3, 141.4, 144.8,
4-Formyl-3Ј-nitro-1,1Ј-biphenyl (8): 1H NMR (200 MHz, CDCl3): δ
= 7.67 (t, J = 8.0 Hz, 1 H), 7.79 (d, J = 8.3 Hz, 2 H), 7.94–8.03
(m, 3 H), 8.22–8.28 (m, 1 H), 8.47 (t, J = 2.0 Hz, 1 H), 10.08 (s, 1
H) ppm. Data in accordance with previously reported results.[13]
145.6, 160.3 ppm. IR (in KBr): ν = 3437, 3088, 3063, 3022, 2914,
˜
4-Phenylacetophenone (9): 1H NMR (200 MHz, CDCl3): δ = 2.64
(s, 3 H), 7.37–7.53 (m, 3 H), 7.60–7.72 (m, 4 H), 8.05 (d, J = 8.8 Hz,
2 H) ppm. Data in accordance with previously reported results.[14]
1783, 1588, 1491, 1445, 1404, 1255, 1148, 1055, 748, 692, 554, 482
cm–1. MS [ESI, MeCN/MeOH (4:1)]: m/z (%) = 496–487 (50) [M –
Cl + MeCN + MeOH]+, 463–454 (70) [M – Cl + MeCN]+, 425–
414 (100), 413–406 (90), 387–378 (30) [M – Cl – HCl]+. HRMS
(ESI): calcd. for ([M – Cl – HCl]+, 104Pd) 378.0384; found 378.0378.
C18H17Cl2N3Pd (452.67): calcd. C 47.76, H 3.79, N 9.28; found C
47.48, H 3.61, N 9.12.
4-Phenylphenol (10): 1H NMR (200 MHz, CDCl3): δ = 4.99 (s, 1
H), 6.92 (d, J = 8.8 Hz, 2 H), 7.28–7.59 (m, 7 H) ppm. Data in
accordance with previously reported results.[15]
4-Methoxy-1,1Ј-biphenyl (11): 1H NMR (200 MHz, CDCl3): δ =
3.91 (s, 3 H), 7.06 (d, J = 8.8 Hz, 2 H), 7.34–7.54 (m, 3 H), 7.58–
7.69 (m, 4 H) ppm. Data in accordance with previously reported
results.[14]
Complex 2d: Yield 115 mg (99%); yellow powder; Rf = 0.40
1
(EtOAc). H NMR (300 MHz, [D6]dmso): δ = 2.33 (s, 3 H), 4.01
(d, J = 12.7 Hz, 1 H), 4.31 (d, J = 12.7 Hz, 1 H), 5.45 (s, 1 H),
7.24–7.56 (m, 10 H), 7.86 (td, J = 1.5 and 7.7 Hz, 1 H), 8.03 (dd,
J = 1.9 and 7.3 Hz, 2 H), 8.66 (d, J = 5.5 Hz, 1 H) ppm. 13C NMR
(75 MHz, [D6]dmso): δ = 48.5, 68.5, 80.2, 123.3, 123.8, 128.1 (2 C),
128.8, 129.4 (4 C), 129.7, 132.6 (2 C), 132.7, 135.7, 140.3, 148.6,
2-Cyano-2Ј-methyl-1,1Ј-biphenyl (12): 1H NMR (200 MHz,
CDCl3): δ = 2.22 (s, 3 H), 7.21–7.42 (m, 5 H), 7.47 (td, J = 1.4 and
7.6 Hz, 1 H), 7.65 (td, J = 1.4 and 7.6 Hz, 1 H), 7.77 (dd, J = 1.4
and 7.7 Hz, 1 H) ppm. Data in accordance with previously reported
results.[16]
163.9 ppm. IR (in KBr): ν = 3447, 3058, 3032, 2925, 1726, 1609,
˜
1491, 1475, 1450, 1373, 1286, 1240, 1153, 1045, 912, 892, 784, 753,
702, 559 cm–1. MS [ESI, MeCN/MeOH (4:1)]: m/z (%) = 474–465
(10) [M – Cl + MeCN]+, 440–431 (50) [M – Cl – HCl + MeCN]+,
428–419 (100), 400–391 (70) [M – Cl – HCl]+. HRMS (ESI): calcd.
4-Cyano-2Ј-methyl-1,1Ј-biphenyl (13): 1H NMR (200 MHz,
CDCl3): δ = 2.29 (s, 3 H), 7.19–7.35 (m, 4 H), 7.45 (d, J = 8.7 Hz,
2 H), 7.72 (d, J = 8.7 Hz, 2 H) ppm. Data in accordance with
previously reported results.[12]
for ([M
– Cl –
HCl]+, 104Pd) 391.0588; found 391.0585.
C20H20Cl2N2Pd (465.71): calcd. C 51.58, H 4.33, N 6.02; found C
51.31, H 4.28, N 5.91.
2Ј-Formyl-4-methoxy-1,1Ј-biphenyl (14): 1H NMR (200 MHz,
CDCl3): δ = 3.86 (s, 3 H), 7.00 (d, J = 8.8 Hz, 2 H), 7.30 (d, J =
8.8 Hz, 2 H), 7.47 (m, 2 H), 7.61 (td, J = 1.6 and 7.3 Hz, 1 H),
8.01 (m, 1 H), 10.00 (s, 1 H) ppm. Data in accordance with pre-
viously reported results.[17]
General Procedure for the Suzuki Reaction: To a stirred solution of
aromatic halide (0.5 mmol), boronic acid (0.6 mmol), and Cs2CO3
(407 mg, 1.25 mmol) in dmf/H2O (95:5, 1 mL) was added the palla-
dium complex (33 µL, 1.5ϫ10–3 , 5ϫ10–5 mmol, 0.01% or 67 µL,
7.5ϫ10–3 , 5ϫ10–4 mmol, 0.1%). The mixture was stirred at
100 °C until the reaction was complete. EtOAc (10 mL) and water
(10 mL) were then added, and the aqueous phase was extracted
with EtOAc (3ϫ5 mL). The combined organic layers were dried
(MgSO4), filtered, and concentrated under vacuum. The crude
product was purified by flash chromatography on silica gel to give
the biaryl product.
4-Methoxy-2Ј-nitro-1,1Ј-biphenyl (15): 1H NMR (200 MHz,
CDCl3): δ = 3.84 (s, 3 H), 6.96 (d, J = 8.8 Hz, 2 H), 7.27 (d, J =
8.8 Hz, 2 H), 7.40–7.48 (m, 2 H), 7.59 (m, 1 H), 7.80 (m, 1 H) ppm.
Data in accordance with previously reported results.[18]
Acknowledgments
We are grateful to the Ministère de l’Education Nationale de la
Recherche et de la Technologie (MENRT)-France for a grant
(V. T.) and to Centre National de la Recherche Scientifique
(CNRS) and the Université de Versailles-Saint-Quentin-en-Yvel-
ines for financial support.
For reactions conducted in water, water (1 mL) and K2CO3
(173 mg, 1.25 mmol) were used instead of dmf/H2O (95:5) and
Cs2CO3, respectively.
4-Methyl-4Ј-nitro-1,1Ј-biphenyl (3): 1H NMR (200 MHz, CDCl3): δ
= 2.44 (s, 3 H), 7.32 (d, J = 8.0 Hz, 2 H), 7.54 (d, J = 8.0 Hz, 2
H), 7.72 (d, J = 9.0 Hz, 2 H), 8.28 (d, J = 9.0 Hz, 2 H) ppm. Data
in accordance with previously reported results.[9]
[1] a) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457–2483 and
the references cited therein; b) A. Suzuki, J. Organomet. Chem.
1999, 576, 147; c) F. Alonso, I. P. Beletskaya, M. Yus, Tetrahe-
dron 2008, 64, 3047–3101; d) B. Puget, J.-P. Roblin, D. Prim,
Y. Troin, Tetrahedron Lett. 2008, 49, 1706–1709.
[2] For recent review see: J.-P. Corbet, G. Mignani, Chem. Rev.
2006, 106, 2651–2710 and the references cited therein.
[3] P. W. van Leuuwen, P. Kamer, J. Reek, P. Dierkes, Chem. Rev.
2000, 100, 2741–2770.
[4] For recent phosphane-free catalytic systems, see: a) Z.-X.
Wang, Z.-Y. Chai, Eur. J. Inorg. Chem. 2007, 4492–4499; b) M.
Bröring, C. Kleeberg, E. C. Tejero, Eur. J. Inorg. Chem. 2007,
3208–3216; c) S. Haneda, Z. Gan, K. Eda, M. Hayashi, Orga-
2,4-Dimethoxy-4Ј-nitro-1,1Ј-biphenyl (4): 1H NMR (200 MHz,
CDCl3): δ = 3.84 (s, 3 H), 3.88 (s, 3 H), 6.61 (m, 2 H), 7.28 (d, J
= 8.8 Hz, 1 H), 7.67 (d, J = 8.3 Hz, 2 H), 8.23 (d, J = 8.3 Hz, 2 H)
ppm. Data in accordance with previously reported results.[10]
4-Nitro-4Ј-trifluoromethyl-1,1Ј-biphenyl (5): 1H NMR (200 MHz,
CDCl3): δ = 7.73–7.80 (m, 6 H), 8.32 (d, J = 9.0 Hz, 2 H) ppm.
Data in accordance with previously reported results.[11]
1
4-Phenylbenzaldehyde (6): H NMR (200 MHz, CDCl3): δ = 7.42–
7.55 (m, 3 H), 7.65 (m, 2 H), 7.76 (d, J = 8.3 Hz, 2 H), 7.97 (d, J
2744
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2739–2745

N-Heterocyclic Benzhydrylamines as N,N-Bidentate Ligands in Pd Complexes
nometallics 2007, 26, 6551–6555; d) S. Haneda, C. Ueba, K.
Eda, M. Hayashi, Adv. Synth. Catal. 2007, 349, 833–835; e) C.
Bianchini, G. Lenoble, W. Oberhauser, S. Parisel, F. Zanobini,
Eur. J. Inorg. Chem. 2005, 4794–4800 and the references cited
therein.
10.4772(11) Å, b = 12.8641(13) Å, c = 14.3890(13) Å, β =
103.478(4)°, V = 1885.9(3) Å3; Z = 4; µ = 1.272 mm–1; 102328
reflections measured at 100 K; independent reflections: 5569
[5205 Fo Ͼ 4σ (Fo)]; data were collected up to a 2Θmax value
of 60.64° (98.3% coverage). Number of variables: 227; R1
=
[5] a) V. Terrasson, S. Marque, M. Georgy, J.-M. Campagne, D.
Prim, Adv. Synth. Catal. 2006, 348, 2063–2067; b) J. Michaux,
V. Terrasson, S. Marque, J. Wehbe, D. Prim, J.-M. Campagne,
Eur. J. Org. Chem. 2007, 2601–2603.
[6] V. Terrasson, S. Marque, A. Scarpacci, D. Prim, Synthesis
2006, 1858–1862.
[7] a) L. Keller, M. Vargas-Sanchez, D. Prim, F. Couty, G. Evano,
J. Marrot, J. Organomet. Chem. 2005, 690, 2306–2311; b) V. V.
Dunina, L. G. KuzЈmina, M. Y. Kazakova, O. N. Gorunova,
Y. K. Grishin, E. I. Kazakova, Eur. J. Inorg. Chem. 1999, 1029–
1039.
[8] Crystal data and structure refinements for 2a, 2c, and 2d:
Analyses were carried out by using a Siemens SMART three-
circle diffractometer equipped with a CCD bidimensional de-
0.0297, wR2 = 0.0810, S = 1.226; highest residual electron den-
sity 0.336 eÅ–3 (all data R1 = 0.0346, wR2 = 0.0945). Data
reduction was performed with the SAINT software. The ab-
sorption correction was based on multiple and symmetry-
equivalent reflections in the data set by using the SADABS
program based on the method of Blessing. The structures were
solved by direct methods and refined by full-matrix least-
squares with use of the SHELX-TL package. CCDC-677262
(for 2a), -677263 (for 2c), and -677264 (for 2d) contain the sup-
plementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
K. Shudo, T. Ohta, T. Okamoto, J. Am. Chem. Soc. 1981, 103,
645–653.
S. Yanagisawa, T. Sudo, R. Noyori, K. Itami, J. Am. Chem.
Soc. 2006, 128, 11748–11749.
L. Liu, Y. Zhang, Y. Wang, J. Org. Chem. 2005, 70, 6122–6125.
N. Kataoka, Q. Shelby, J. P. Stambuli, J. F. Hartwig, J. Org.
Chem. 2002, 67, 5553–5566.
B. Tao, D. W. Boykin, J. Org. Chem. 2004, 69, 4330–4335.
T. Mino, Y. Shirae, T. Saito, M. Sakamoto, T. Fujita, J. Org.
Chem. 2006, 71, 9499–9502.
I. Ryu, H. Matsubara, S. Yasuda, H. Nakamura, D. P. Curran,
J. Am. Chem. Soc. 2002, 124, 12946–12947.
[9]
tector with Mo-Kα monochromatized radiation (λ
=
[10]
0.71073 Å). Crystal data 2a: C19H18Cl2N2Pd, Mw = 451.65,
monoclinic, space group Cc; dimensions: a = 8.5312(1) Å, b =
[11]
[12]
18.0606(3) Å,
c = 12.2962(2) Å, β = 95.703(1)°, V =
1885.20(5) Å3; Z = 4; µ = 1.270 mm–1; 19418 reflections mea-
sured at room temperature; independent reflections: 5422 [5098
Fo Ͼ 4σ (Fo)]; data were collected up to a 2Θmax value of 60.12°
(99.6% coverage). Number of variables: 217; R1 = 0.0202, wR2
[13]
[14]
=
0.0432, S = 1.034; highest residual electron density:
[15]
[16]
[17]
[18]
0.336 eÅ–3 (all data R1 = 0.0230, wR2 = 0.0442). Crystal data
2c: C18H17Cl2N3Pd, Mw = 452.65, monoclinic, space group
P21/c; dimensions: a = 16.9212(15) Å, b = 15.2833(12) Å, c =
16.4784(14) Å, β = 118.744(3)°, V = 3736.4(5) Å3; Z = 8; µ =
1.283 mm–1; 54638 reflections measured at room temperature;
independent reflections: 10941 [9053 Fo Ͼ 4σ (Fo)]; data were
collected up to a 2Θmax value of 60.24° (99.3% coverage).
Number of variables: 434; R1 = 0.0860, wR2 = 0.2192, S =
1.206; highest residual electron density 2.423 eÅ–3 (all data R1
= 0.0989, wR2 = 0.2263). Crystal data 2d: C20H20Cl2N2Pd, Mw
= 465.68, monoclinic, space group P21/n; dimensions: a =
S. Y. Liu, M. J. Choi, G. C. Fu, Chem. Commun. 2001, 2408–
2409.
H. J. Wu, C. F. Lin, J. L. Lee, W. D. Lu, C. Y. Lee, C. C. Chen,
M. J. Wu, Tetrahedron 2004, 60, 3927–3934.
A. N. Cammidge, V. H. M. Goddard, H. Gopee, N. L. Har-
rison, D. L. Hughes, C. J. Schubert, B. M. Sutton, G. L. Watts,
A. J. Whitehead, Org. Lett. 2006, 8, 4071–4074.
Received: February 11, 2008
Published Online: May 6, 2008
Eur. J. Inorg. Chem. 2008, 2739–2745
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2745