Synthesis, structure and catalytic activity of a bimacrocylic NHC palladium allyl complex

Article abstract of DOI:10.1016/j.jorganchem.2008.05.034

The preparation of a bimacrocyclic NHC palladium allyl complex 4 is described. The complex was obtained by transmetalation with allyl palladium chloride dimer from the NHC silver complex 2 in 85% yield. Complex 4 was fully characterized by spectroscopic methods and by single-crystal X-ray analysis. In a preliminary catalytic study, complex 4 showed high activity in the Suzuki-Miyaura cross-coupling of unactivated aryl chlorides and bromides with 1-naphthalene-boronic acid at low catalyst loading. Good results were also obtained in the Mizoroki-Heck reaction of aryl bromides with styrene, but a decrease in yield was observed when aryl chlorides were used.

Full text of DOI:10.1016/j.jorganchem.2008.05.034

Journal of Organometallic Chemistry 693 (2008) 2784–2788
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structure and catalytic activity of a bimacrocylic NHC palladium
q
allyl complex
a
b
a,*
Ole Winkelmann , Christian Näther , Ulrich Lüning
a
Otto-Diels-Institut für Organische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany
Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, D-24098 Kiel, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation of a bimacrocyclic NHC palladium allyl complex 4 is described. The complex was
obtained by transmetalation with allyl palladium chloride dimer from the NHC silver complex 2 in
Received 11 March 2008
Received in revised form 6 May 2008
Accepted 26 May 2008
85% yield. Complex 4 was fully characterized by spectroscopic methods and by single-crystal X-ray anal-
ysis. In a preliminary catalytic study, complex 4 showed high activity in the Suzuki–Miyaura cross-cou-
pling of unactivated aryl chlorides and bromides with 1-naphthalene-boronic acid at low catalyst
loading. Good results were also obtained in the Mizoroki–Heck reaction of aryl bromides with styrene,
but a decrease in yield was observed when aryl chlorides were used.
Available online 3 July 2008
Keywords:
Carbenes
Bimacrocycles
N-Heterocyclic carbenes
Palladium
Ó 2008 Elsevier B.V. All rights reserved.
Cross-coupling
1
. Introduction
2. Results and discussion
N-Heterocyclic carbenes (NHCs) have received great attention
since the isolation of a crystalline carbene in 1991 by Arduengo
1] and they are nowadays frequently used in either metalfree
2.1. Synthesis and characterization of palladium complex 4
[
organocatalysis [2] or as beneficial ligands in organometallic
chemistry [3]. As ligands, the nucleophilic NHCs are strong two-
Cl
X
X
X
electron
r-donors, displaying similar properties as trialkylphos-
O
O
O
O
Ag O
N
N
2
N
OX
N
phines [4], and both species are successfully used in palladium
catalyzed cross-coupling reactions [5]. In most cases of phos-
phine-containing catalytic systems, phosphines are used as free
ligands in conjunction with a palladium source. In an analogous
fashion, catalytically active NHC palladium complexes can be
formed in situ from a palladium source and an azolium salt as
the NHC precursor [6], but recently also a number of well-defined,
air- and moisture-stable NHC-bearing palladium complexes with
excellent catalytic activities have been reported [7]. In our contin-
uing investigation of the catalytic properties of bimacrocyclic NHCs
O
O
O
Ag
Cl
1
2
N
Cl
Pd
Pd
Cl
Cl
PdCl2, K CO
2
3
X
O
O
N
[
8], accessible from the respective azolium precursors [9], we
N
OX
X
explored the accessibility of bimacrocyclic NHC palladium
complexes and their catalytic activity in cross-coupling reactions.
O
O
O
N
OX
N
Cl Pd Cl
O
N
Pd
Cl
Cl
3
X = (CH2)8
4
q
Concave reagents, Part 56. For part 55, see Ref. [8].
We have recently reported [8] the synthesis of a number of
NHC metal complexes derived from the concave bimacrocyclic
*
Corresponding author. Tel.: +49 431 880 2450; fax: +49 431 880 1558.
E-mail address: luening@oc.uni-kiel.de (U. Lüning).
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.05.034

O. Winkelmann et al. / Journal of Organometallic Chemistry 693 (2008) 2784–2788
2785
imidazolinium salt 1. In contrast to other transition metal complexes,
Table 1
Selected bond lengths (Å) for palladium complex 4
palladium complex 3 was only obtained in poor yield via the free car-
bene, and 3 was not accessible by transmetalation from silver com-
plex 2 [8]. The use of NHC silver complexes as carbene transfer
reagents is a popular method for the synthesis of NHC metal com-
plexes by transmetalation, avoiding the handling of the sensitive free
carbenes [10]. However, Nolan and coworkers have reported the syn-
thesis of catalytically active NHC palladium complexes by reaction of
the free carbenes with allyl palladium chloride dimer [7e]. We were
delighted to find that in an analogous fashion the reaction of silver
complex 2 with allyl palladium chloride dimer led to the smooth for-
mation of NHC palladium complex 4 in 85% yield.
Pd1a
Pd2a
Pd–C1/C41
Pd–C32/C72
Pd–C33/C73
Pd–C34/C74
Pd–Cl1/Cl2
2.045(2)
2.105(3)
2.142(3)
2.211(2)
2.3875(6)
2.038(2)
2.102(3)
2.131(3)
2.233(2)
2.3674(6)
a
Pd1 and Pd2 belong to the respective crystallographically independent
complexes.
both kinds of precatalysts are believed to result in the same mono-
In the crystal structure of 4 (see Fig. 1), the asymmetric unit
contains two crystallographically independent molecules. Selected
bond lengths are given in Table 1. The structure of the NHC ligand
in palladium complex 4 is similar to that in complex 3 [8], with
the two phenyl substituents oriented almost perpendicular to the
0
ligated NHC–Pd -species as the active catalyst [3a]. A detailed
reaction pathway for catalyst activation has been proposed for
NHC palladium allyl complexes [7e].
2.2. Catalytic activity of palladium complex 4
heterocycle, and the torsion angles C1–N1–C4–C5 and C1–N2–
3
C23–C22 are 81.42(1)° and 70.89(1)°, respectively. The
g
coordi-
NHC palladium complexes are known to catalyze a number of
nation of the allyl moiety to the palladium center is comparable
cross-coupling reactions [3,7]. We decided to investigate the reac-
tivity of complex 4 in Mizoroki–Heck and Suzuki–Miyaura cross-
coupling reactions. Complex 3 was not tested in respective reactions
due to the small amount of 3 obtained in its synthesis [8]. While the
use of aryl chlorides as coupling partners is nowadays established in
the latter case [7a,7b,7c,7e], the Mizoroki–Heck olefination of unac-
tivated aryl chlorides is not trivial and mainly bromides and
activated chlorides are successfully used in the reaction [7c,12].
The results obtained with catalyst 4 are summarized in Table 2.
The olefinations were performed in tetrabutylammonium bromide
as an ionic liquid at 140 °C (16 h reaction time), following a protocol
by Beller and coworkers [12c]. Using 1 mol% of catalyst 4, good
results were obtained in the reaction of activated and electronically
neutral aryl bromides with styrene, and also hindered 2-bromo-tol-
uene afforded 72% of the coupling product. Unfortunately, a
decrease in yield was observed when analogous aryl chlorides were
used as the coupling partners (Table 2, entries 1 and 2). While acti-
vated 4-chloro-acetophenone still afforded 37% of the desired prod-
uct, only 10% yield of stilbene was obtained with chlorobenzene. The
low yields are due to incomplete conversion and could not be raised
significantly with higher catalyst loadings (Table 2, entry 1).
to that reported for the respective complex using the SIPr NHC
ligand
(SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-yli-
dene) [7e]. The angles C1–Pd1–C32, C1–Pd1–C34 and C34–Pd1–
Cl1 are 92.95(10)°, 161.34(10)° and 93.67(9)° in 4, respectively,
C1–Pd1–Cl1 is 104.4(6)° and C32–Pd1–C34 is 68.72(12)°. The
internal angle of the heterocycle N1–C1–N2 is 108.03(18)°.
1
3
The C resonance for the carbene carbon atom of 4 was ob-
served at 211.4 ppm which is significantly more downfield than
in palladium complex 3 (183.5 ppm) [8]. A comparison of related
palladium complexes with the SIPr NHC ligand shows a very sim-
ilar trend: the carbene carbon atom resonates at 215.4 ppm when
SIPr is incorporated into an allyl palladium complex [7e], but in the
3
-chloro-pyridine containing complex this resonance is observed
at 184.9 ppm [11]. This drastic difference in chemical shifts (
Dd
is 30.5 ppm for SIPr and 27.9 ppm for our bimacrocyclic NHC li-
gand) must be due to a different electronic situation at the palla-
dium center, caused by the varying ancillary ligands. However,
In contrast to the low conversion of aryl chlorides observed
with catalyst 4 in the olefinations, high yields were obtained in
the reaction of aryl chlorides with 1-naphthalene-boronic acid at
low catalyst loading in short reaction times (0.2 mol% 4, 2 h at
6
0 °C). As we are currently working on an axially chiral modifica-
tion of the bimacroclic NHC ligand [9b], we were interested in
the ability of complex 4 to produce axially chiral biaryls (however,
0
in racemic form). 1,1 -Binaphthyl (Table 2, entry 8) could be pro-
duced in 91% yield with 1-bromo-naphthalene, but due to its low
barrier of racemization [13] it might not be an appropriate candi-
date for asymmetric catalysis. When 1-bromo-2-methoxy-naph-
thalene was reacted with 1-naphthalene-boronic acid, only 21%
yield of the desired product was obtained (Table 2, entry 9), along
0
with unreacted bromide and 1,1 -binaphthyl (68%, based on boro-
nic acid), resulting from the homocoupling of the boronic acid. A
suppression of this side-reaction might be possible [14], but no
efforts were taken yet to improve this yield.
3
. Conclusion
The accessibility of the NHC palladium complex 4 by transmeta-
Fig. 1. Crystal structure of palladium complex 4 with displacement ellipsoids
drawn at the 50% probability level. Only one of the two crystallographically
independent molecules is represented. The carbon atom C33 of the allyl moiety is
disordered in two positions and only the atom of higher occupancy is shown for
clarity.
lation from silver complex 2 is a great improvement compared to
the synthesis of palladium complex 3 that could only be obtained
in poor yield [8]. In the Suzuki–Miyaura cross-coupling reaction of
aryl chlorides, the activity of the bimacrocyclic complex 4 is

2
786
O. Winkelmann et al. / Journal of Organometallic Chemistry 693 (2008) 2784–2788
Table 2
Catalytic activity of complex 4 in Mizoroki–Heck and Suzuki–Miyaura cross-coupling reactions
a
b
Entry
Substrates
Product
Yieldc
O
O
1
X = Br: 86%
X = CI: 37% (43%)
X
X
d
2
3
X = Br: 73/%
X = Cl: 10%
72%
Br
Br
4
5
79%
83%
Cl
B(OH)
2
6
7
8
91%
92%
91%
Cl
B(OH)
2
Cl
B(OH)
2
B(OH)
2
Br
9
21%
Br
B(OH)
2
MeO
Me O
a
Conditions: Palladium complex 4 (10
l
mol), aryl halide (1 mmol), styrene (1.5 mmol), NaOAc (1.2 mmol), KOtBu (0.09 mmol), 16 h at 140 °C in tetrabutylammonium
bromide (2 g).
b
Conditions: Palladium complex 4 (2
Isolated yield.
20 lmol of palladium complex 4.
lmol), aryl halide (1 mmol), boronic acid (1.2 mmol), KOtBu (1.4 mmol), 2 h at 60 °C in 2-propanol (2 mL).
c
d
comparable to related non-macrocyclic NHC palladium allyl com-
plexes [7e], but the formation of a tri-ortho-substituted biaryl
proved to be difficult with catalyst 4 in this preliminary study. In
the Mizoroki–Heck reaction, high yields of the desired coupling
products were obtained with aryl bromides, while a decrease in
yield was observed with the use of analogous aryl chlorides. As
the catalytic activity of complex 4 in cross-coupling reactions
shows promise, we are currently focusing on a chiral version of
the bimacrocyclic NHC ligand for respective asymmetric reactions
4. Experimental
4.1. General
The NHC silver complex 2 [8] and allyl palladium chloride dimer
1
13
[7b] were prepared according to the literature. H and C NMR
spectra were recorded with Bruker DRX 500 or ARX 300 instru-
ments at room temperature and are referenced to tetramethylsil-
ane. NMR multiplicities are abbreviated as follows: s = singlet,
d = doublet, t = triplet, m(c) = multiplet (centered), br = broad
[
9b].

O. Winkelmann et al. / Journal of Organometallic Chemistry 693 (2008) 2784–2788
2787
signal. The following abbreviations are used for assignments:
Table 3
Crystallographic data for palladium complex 4
Ar = aromatic, Im = imidazolidin. Mass spectra were recorded with
a Finnigan MAT 8200 or MAT 8230. IR spectra were recorded with a
Perkin–Elmer Paragon 1000 spectrometer. Elemental analyses
were carried out with an EuroEA 3000 Elemental Analyzer from
Euro Vector.
Formula
2 4
C34H47ClN O Pd
689.59
yellow/block
Formula weight (g/mol)
Color/habit
Crystal size (mm)
Crystal system
Space group
a (Å)
0.13 ꢁ 0.11 ꢁ 0.09
monoclinic
1
P2 /c (no. 14)
3
4.2. Synthesis of
g
-allyl-chloro-(2,11,13,22-tetraoxa-1,12(1,3,2)-
28.5370(15)
9.0128(5)
26.3644(12)
90
105.489(6)
90
6534.6(6)
8
170(2)
2
b (Å)
c (Å)
dibenzena-23-(1,3)-imidazolidina-bicyclo[10.10.1]-tricosaphane-23 -
ylidene)palladium(II) (4)
a
(°)
b (°)
(°)
To a stirred solution of allyl palladium chloride dimer (24 mg,
c
3
6
5
lmol) in dichloromethane (1 mL) was added silver complex 2
V (Å )
Z
(
90 mg, 0.14 mmol), dissolved in dichloromethane (5 mL). After
T (K)
stirring for 1 h at room temperature, the mixture was passed
through a short pad of silica gel, and the silica gel was rinsed with
dichloromethane. The filtrate was concentrated in vacuo and the
product was precipitated by addition of n-pentane. The precipitate
was washed with n-pentane and dried in vacuo. A yellow solid was
ꢀ1
l
(mm
)
0.689
h Range (°)
1.94–26.00
±35, ±11, ±31
49934
12515
0.0340
10898
762
0.0318/0.0841
0.0376/0.0872
1.044
+0.826 and ꢀ0.775
Index ranges (h,k,l)
Measured reflections
Independent reflections
R
int
obtained. Yield: 75 mg (0.11 mmol, 85%).
Reflections with [I >2
Parameters
r
(I)]
1
M.p. 115 °C (decomp.). H NMR (500 MHz, CDCl
3
): d (ppm) =
7
4
.17 (t, J = 8.4 Hz, 2H, Ar-H-4), 6.52 (d, 3J = 8.4 Hz, 4H, Ar-H-3,5),
.68 (m , 1H, CHallyl), 4.14 (m , 4H, OCH ), 4.05–3.90 (m, 8H,
, Im-H-4,5), 3.67 (d, J = 7.3 Hz, 1H, HCHallyl), 3.24 (br s, 1H,
3
R
R
1
wR
wR
2
[[I > 2
r
(I)]
a
a
1
2
(all data)
c
c
2
2
a
3
GOF (on F )
OCH
HCHallyl), 2.61 (d, J = 13.4 Hz, 1H, HCHallyl), 1.94 (m
2
Largest difference in peak and hole (e Å-3)
3
c
, 4H, CH
2
),
3
C
P
P
= {P
P
GOF = {P
[w
a
[w(F2 ꢀ F ) ]/ w(F ) ]}1/2;
2
2
2 2
1
R1
=
(jjF j ꢀ jF jj)/ jFoj;
wR2
1
.80–1.60 (m, 9H, HCHallyl, CH
2
), 1.55–1.45 (m, 12H, CH
2
).
o
c
1/2
o
c
o
2
o
2
2
(F
ꢀ F
c
) ]/(n ꢀ p)}
.
3
NMR (125 MHz, CDCl ): d (ppm) = 211.4 (Im-C-2), 157.2 (Ar-C-
2
,6), 128.9 (Ar-C-4), 118.5 (Ar-C-1), 113.9 (CHallyl), 104.0 (Ar-C-
3
,5), 70.2 (CH2allyl), 68.2 (OCH
2
), 68.0 (OCH
2
), 50.7 (Im-C-4,5), 48.7
ꢀ
1
(CH2allyl), 28.7, 28.5, 26.4, 23.6 (CH
2
). IR (KBr):
m
~ (cm ) = 3069,
0
1
1
,1 -binaphthyl [17] was confirmed by H NMR and mass spectro-
2
934, 2855, 1594, 1503, 1461, 1387, 1297, 1102, 776, 731. Anal.
47ClN Pd (689.64): C, 59.22; H, 6.87; N, 4.06. Found:
C, 59.42; H, 7.01; N, 4.18%.
metric analysis.
Calc. for C34
H
2 4
O
4
.5. Single-crystal X-ray structure determination of complex 4
4.3. General procedure for Mizoroki–Heck cross-coupling reactions
General: Crystal data and details of the structure determination
are given in Table 3. Suitable single crystals were grown by slow
diffusion of n-pentane into a solution of 4 in chloroform. Data col-
lection was performed using an STOE Imaging Plate Diffraction
A flask was charged with styrene (156 mg, 1.50 mmol), tetrabu-
tylammonium bromide (2 g), sodium acetate (99 mg, 1.2 mmol),
potassium tert-butoxide (10 mg, 89 mol), palladium complex 4
7 mg, 0.01 mmol) and the aryl halide (1.0 mmol). The flask was
l
System (IPDS-1) with graphite-monochromated Mo K
a radiation
(
(k = 0.71073 Å). Structure solutions were performed with direct
flushed with nitrogen, sealed with a rubber septum and heated
to 140 °C for 16 h. After cooling to room temperature, the mixture
was triturated in water (10 mL) and extracted with diethyl ether
methods using SHELXS-97. Structure refinements were performed
2
against F with SHELXL-97 [18]. One of the allylic carbon atoms is
disordered in two positions and was refined using a split model.
All non-hydrogen atoms except the disordered carbon atom of
lower occupancy were refined anisotropically. The hydrogen atoms
were placed in ideal positions and refined using a riding model.
The asymmetric unit contains two crystallographically indepen-
dent molecules.
(
3 ꢁ 15 mL). The organic layer was dried with magnesium sulfate,
the solvent was evaporated in vacuo and the product was purified
by column chromatography on silica gel. The purity and identity of
previously described E-stilbene [12c], E-p-acetyl-stilbene [12c],
E-o-methyl-stilbene [12b] and 1-E-styryl-naphthalene [15] was
1
confirmed by H NMR and mass spectrometric analysis.
5
. Supplementary material
4.4. General procedure for Suzuki–Miyaura cross-coupling reactions
CCDC 680552 contains the supplementary crystallographic data
A flask was charged with potassium tert-butoxide (154 mg,
.37 mmol), boronic acid (1.2 mmol) and palladium complex 4
for 4. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
1
(
1.4 mg, 2.0 mol). The flask was flushed with nitrogen and sealed
l
with a rubber septum. Via syringe 2-propanol (1 mL) was added
and the mixture was stirred at room temperature. After 5 min,
the aryl halide (1.0 mmol), dissolved in 2-propanol (1 mL), was in-
jected via syringe and the mixture was heated to 60 °C for 2 h. After
cooling to room temperature, water (10 mL) was added to the mix-
ture and it was extracted with dichloromethane (3 ꢁ 15 mL). The
organic layer was dried with magnesium sulfate, the solvent was
evaporated in vacuo and the product was purified by column chro-
matography on silica gel. The purity and identity of previously
References
[1] A.J. Arduengo III, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361.
[2] (a) N. Marion, S. Diez-Gonzáles, S.P. Nolan, Angew. Chem., Int. Ed. 46 (2007)
2988;
(
(
b) V. Nair, S. Bindu, V. Sreekumar, Angew. Chem., Int. Ed. 43 (2004) 5130;
c) D. Enders, T. Balensiefer, Acc. Chem. Res. 37 (2004) 534.
[3] Recent reviews: (a) E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, Angew. Chem.,
Int. Ed. 46 (2007) 2768;
(
(
b) N.M. Scott, S.P. Nolan, Eur. J. Inorg. Chem. (2005) 1815;
c) V. César, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33 (2004) 619;
described 1-phenyl-naphthalene [7b], 1-o-tolyl-naphthalene [7a],
0
1
-p-tolyl-naphthalene [16], 1,1 -binaphthyl [7b] and 2-methoxy-
(d) W.A. Herrmann, Angew. Chem., Int. Ed. 41 (2002) 1290;

2
788
O. Winkelmann et al. / Journal of Organometallic Chemistry 693 (2008) 2784–2788
(
3
(
(
e) D. Bourissou, O. Guerret, F.P. Gabbai, G. Bertrand, Chem. Rev. 100 (2000)
9;
f) T. Weskamp, V.P.W. Böhm, W.A. Herrmann, J. Organomet. Chem. 600
2000) 12.
[11] No signal was reported in the literature for the carbene carbon atom of the
respective NHC palladium complex. Following the synthetic protocol from the
literature, the aforementioned complex (‘‘PEPPSI-SIPr”) was obtained and the
respective resonance could be observed undoubtedly at 184.9 ppm (150 MHz,
[
4] K. Öfele, W.A. Herrmann, D. Mihalios, M. Elison, E. Herdtweck, W. Scherer, J.
Mink, J. Organomet. Chem. 459 (1993) 177.
5] G.C. Fu, A.F. Littke, Angew. Chem., Int. Ed. 41 (2002) 4176.
6] C. Zhang, J. Huang, M.L. Trudell, S.P. Nolan, J. Org. Chem. 64 (1999) 3804.
7] (a) C.J. O’Brien, E.A.B. Kantchev, C. Valente, N. Hadei, G.A. Chass, A. Lough, A.C.
Hopkinson, M.G. Organ, Chem. Eur. J. 12 (2006) 4743;
3
CDCl ). Except for the missing resonance of the carbene carbon atom, all
spectroscopic data are in perfect agreement with our findings: M.G. Organ, M.
Abdel-Hadi, S. Avola, N. Hadei, J. Nasielski, C.J. O’Brien, C. Valente, Chem. Eur. J.
13 (2007) 150.
[
[
[
[12] (a) W.A. Herrmann, M. Elison, J. Fischer, C. Köcher, G.R.J. Artus, Angew. Chem.,
Int. Ed. Engl. 34 (1995) 2371;
(
b) N. Marion, O. Navarro, J. Mei, E.D. Stevens, N.M. Scott, S.P. Nolan, J. Am.
(b) C. Yang, S.P. Nolan, Synlett 10 (2001) 1539;
Chem. Soc. 128 (2006) 4101;
(c) K. Selvakumar, A. Zapf, M. Beller, Org. Lett. 4 (2002) 3031;
(d) G. Zou, W. Huang, Y. Xiao, J. Tang, New. J. Chem. 30 (2006) 803;
(e) C. Wang, Y. Liu, S. Peng, J. Chen, S. Liu, J. Organomet. Chem. 692 (2007)
3976.
(
(
(
(
c) G.D. Frey, J. Schütz, E. Herdtweck, W.A. Herrmann, Organometallics 24
2005) 4416;
d) O. Navarro, R.A. Kelly III, S.P. Nolan, J. Am. Chem. Soc. 125 (2003) 16194;
e) M.S. Viciu, R.F. Germaneau, O. Navarro-Fernandez, E.D. Stevens, S.P. Nolan,
[13] A.S. Cooke, M.M. Harris, J. Chem. Soc. (1963) 2365.
[14] W.D. Miller, A.H. Fray, J.T. Quatroche, C.D. Sturgill, Org. Process Res. Dev. 11
(2007) 359.
[15] S.R. Dubbaka, P. Vogel, Chem. Eur. J. 11 (2005) 2633.
[16] C. Cho, H. Yun, K. Park, J. Org. Chem. 68 (2003) 3017.
[17] H. Li, Y. Wu, W. Yan, J. Organomet. Chem. 691 (2006) 5688.
[18] G.M. Sheldrick, SHELXL-97, Universität Göttingen, Göttingen, Germany, 1998.
Organometallics 21 (2002) 5470.
8] O. Winkelmann, C. Näther, U. Lüning, J. Organomet. Chem. 693 (2008) 923.
9] (a) O. Winkelmann, C. Näther, U. Lüning, Eur. J. Org. Chem. (2007) 981;
[
[
(
(
b) O. Winkelmann, D. Linder, J. Lacour, C. Näther, U. Lüning, Eur. J. Org. Chem.
2007) 3687.
[
10] I.J.B. Lin, C.S. Vasam, Coord. Chem. Rev. 251 (2007) 642.