Synthesis of chlorinated biphenyls by Suzuki cross-coupling using diamine or diimine-palladium complexes

Article abstract of DOI:10.1002/ejoc.200800119

Several novel diimines (Salen-type ligands) 2a-2i and their reduced diamine counterparts 3b,3d-3g and 3i form complexes 4a-4i, 5b,5d-5g, and 5i with PdCl₂ in DMF or methanol. Using 1 mol-% of the isolated complexes 4e and 5f many polychlorinated biphenyls (PCBs) can be prepared in moderate to excellent yields according to the Suzuki cross-coupling protocol with contact to air. Several 4-acetylbiphenyls prepared by this method can be converted in moderate yields into the corresponding biphenylcarboxylic acids (BCAs) by alkaline cleavage. An X-ray crystal structure determination confirms the structure of complex 5f. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800119

FULL PAPER
DOI: 10.1002/ejoc.200800119
Synthesis of Chlorinated Biphenyls by Suzuki Cross-Coupling Using Diamine
or Diimine-Palladium Complexes
Tuula Kylmälä,[a] Noora Kuuloja,[a] Youjun Xu,[b] Kari Rissanen,[c][‡] and
Robert Franzén*[a]
Keywords: Diimine ligands / Diamine ligands / Palladium complexes / Suzuki cross-coupling / Biaryl compounds / Polychlori-
nated biphenyls (PCBs) / Biphenylcarboxylic acids (BCAs) / Suzuki reaction
Several novel diimines (Salen-type ligands) 2a–2i and their
reduced diamine counterparts 3b,3d–3g and 3i form com-
plexes 4a–4i, 5b,5d–5g, and 5i with PdCl2 in DMF or meth-
anol. Using 1 mol-% of the isolated complexes 4e and 5f
many polychlorinated biphenyls (PCBs) can be prepared in
moderate to excellent yields according to the Suzuki cross-
yls prepared by this method can be converted in moderate
yields into the corresponding biphenylcarboxylic acids
(BCAs) by alkaline cleavage. An X-ray crystal structure de-
termination confirms the structure of complex 5f.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
coupling protocol with contact to air. Several 4-acetylbiphen- Germany, 2008)
lished, Salen-ligands coordinate to a large variety of metals
Introduction
(Pd, Co, Cu, Ni, Mn, V, Zn, Cr, Al, Ru).[8–15] Due to this
complexation-ability these ligands are currently most inten-
sively studied classes of chiral Schiff bases. Moreover,
Salen-type complexes often promote the reaction under
very mild conditions[16] and are excellent catalysts for a
large group of different reactions such as epoxidations of
alkenes and styrenes,[9,15] alkylations,[16] oxidations,[13] cy-
clopropanations,[15] Michel reactions,[11] Diels–Alder reac-
tions,[10] Strecker reactions,[17] and addition of Et2Zn to
benzaldehyde.[15] However, according to the best of our
knowledge, only some complexes of this type have been
tested for Suzuki cross-coupling reactions. Styring et al.[18]
and Sarkar et al.[6] have obtained good results from studies
on the use of Salen-type ligands in some reactions, but
modifications to the structure have not further been per-
formed. Spectroscopic data for these types of ligands can-
not also be found in papers. Pd-Salen complexes have also
not yet been used for the preparation of polychlorinated
biphenyls (PCBs) or chlorinated biphenylcarboxylic acids
(BCAs). PCBs incur a number of human diseases. PCBs are
well known industrial organochlorine chemicals that caused
big environmental concern in the 1980s and 1990s.[19] More-
over, it is well known that PCBs are important environmen-
tal contaminants that have been implicated in a number of
human diseases, such as cancer and cardiovascular disease.
However, possible mechanisms of PCB toxicity are still po-
orly understand, mainly because there still are not efficient
and easy method to prepare PCB compounds. Many litera-
ture reviews on synthesis and analyses of PCBs have pre-
viously been reported.[20–22] It is well known that for many
BCAs, such as the environmental pollutants 3-chloro-4-
For many years cross-coupling reactions catalyzed by
transition metals have been most powerful and convenient
tools in modern organic synthesis.[1] Although many dif-
ferent types of ligands have been discovered for these types
of reactions, there is still a need for new creative innova-
tions. For example, the development of catalysts not includ-
ing phosphane ligands is of special interest due to environ-
mental concerns. As reported in several scientific papers, it
is very difficult to separate the remaining phosphane moiety
during the work-up process.[2–7] Moreover, many ligands are
often air-sensitive or expensive, which places significant lim-
its on their synthetic applications.
In an effort to find an efficient metal complex that could
promote the Suzuki reaction, we first decided to investigate
the scope and limitations of several novel diimines (Salen-
type ligands) complexed to Pd. As previously been pub-
[a] Department of Chemistry and Bioengineering, Tampere Uni-
versity of Technology,
Hermiankatu 7D, P. O. Box 541, Tampere 33720, Finland
Fax: +358-3-3115-2108
E-mail: robert.franzen@tut.fi
[b] School of Pharmaceutical Engineering, Shenyang Pharmaceuti-
cal University,
103 Wenhua Road, Shenyang 110016, China
Fax: +86-24-23986411
E-mail: xuyouju@mail.sy.ln.cn
[c] Nanoscience Center, Department of Chemistry, University of
Jyväskylä,
Survontie 9, P. O. Box 35, 40014 JYU, Finland
Fax: +358-14-2602651
E-mail: Kari.Rissanen@jyu.fi
[‡] X-ray crystallography
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
Eur. J. Org. Chem. 2008, 4019–4024
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. Kylmälä, N. Kuuloja, Y. Xu, K. Rissanen, R. Franzén
FULL PAPER
(2,4,5-trichlorophenyl)benzoic acid and 2-chloro-4-(2,4,6- crease the enantioselectivity of the catalyst in the alkylation
trichlorophenyl)benzoic acid, there are no synthetic meth- reaction.[25] This difference in catalytic ability between the
ods available until today.[23]
Salen-ligand analogs prompted us to prepare first the novel
Belokon and North reported previously that the intro- Salen-type ligands 2a–2i and their reduced counterparts 3b,
duction of substituents onto the aromatic rings of the Salen 3d–3g and 3i (Scheme 1). Secondly, the corresponding Pd
ligand decreases the activity of the copper catalyst[24] but complexes 4a–4i, 5b, 5d–5g,5i were prepared in an effort to
electron-donating groups, on the other hand (such as meth- find an efficient catalyst for the Suzuki reaction. We re-
oxy) and electron-withdrawing groups (like nitro and tri- cently developed a ligandless version of Suzuki reaction in
fluoromethyl) in para position to the phenol oxygen de- water,[4] but this protocol was limited to the synthesis of
biaryl phenols or carboxylic acids. With the new approach
presented in this paper we try to overcome this problem by
preparing biaryls using easily synthesized palladium-di-
imine or -diamine complexes instead.
The present initial study shows that our novel li-
gands[8–14,26] 2a–2i and 3b, 3d–3g,3i can easily be prepared
in good yield and with high purity and that their Pd com-
plexes 4a–4i and 5b, 5d–5g,5i (Scheme 1) promote the Su-
zuki reaction in the preparation of chlorinated biphenyls in
moderate to excellent yields.
Results and Discussion
Ligands 2a–2i and 3b, 3d–3g and 3i form complexes 4a–
4i and 5b, 5d–5g and 5i with Pd (see Scheme 1). Complexes
4e, 5e, 5f and 5i could be isolated as crystals but only those
of complex 5f were obtained with sufficient quality for a
crystal structure analysis (Figure 1).
Figure 1. A plot of the X-ray structure of 5f with atom labels. The
thermal displacement parameters are shown at 50% probability
level.
In the solid state 5f shows an unsymmetrical twisted
structure with the expected square-planar Pd coordination
sphere. The Pd–N and Pd–Cl bond lengths are normal for
this kind of Pd complex: Pd1–N8 2.068(2), Pd1–N15
2.030(2), Pd1–Cl1 2.3309(7) and Pd1–Cl2 2.3265(6) Å. Re-
markably, 5f forms a centrosymmetric dimer through hy-
drogen bonds from N8 and N15 to the Cl1 and Cl2 of the
adjacent complex (the contact distance being 3.365 Å for
N8···Cl2 and 3.279 Å for N15···Cl1).
Scheme 1. Prepared Salen-type 2a–2i and diamine-type ligands 3b,
3d–3g,3i. Structures of Pd complexes 4a–4i, and 5b, 5d–5g,5i.
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4019–4024

Synthesis of Chlorinated Biphenyls
Using all ligands 2a–2i complexed to palladium we then
2-Chlorobiphenyl (9a) could thereafter be synthesized
initially studied the Suzuki reaction between phenylboronic using 2a, 2d, 2e and 2f when complexed in situ with PdCl2.
acid 6a and 4-bromonitrobenzene (7a) as the first model Compound 9a was synthesized starting from 1 equiv. of 1-
reaction in an effort to understand how our diimine-type chloro-2-iodobenzene (7b) and 1.3 equiv. of phenylboronic
ligands would promote this coupling reaction most ef- acid (6a) in DMF at 120 °C for 24 h under argon using
ficiently. Results are collected in Table 1. Reactions pro- different bases. The highest yield (88%) was obtained when
ceeded in situ and we noticed that a ligand/Pd ratio of 5:1 30 mol-% of complex 4e was used. (Table 2). Reaction to
should be used to completely form the palladium species. prepare 9a was successful, so we concluded that it would
With large excess of ligand, we could conclude that all pal- perhaps also be possible to synthesize other polychlorinated
ladium was complexed. Our aim was then to further investi- PCBs by this protocol.
gate the best complex alternatives and to study the scope
Table 2. In situ synthesis of 2-chlorobiphenyl (9a) using 10 mol-%
of Pd-Salen complexes.
and limitations for these in more demanding reactions.
Table 1. Pd-Salen-catalyzed Suzuki reaction of boronic acid 6a with
bromide 7a.
Entry
Catalyst
Base [equiv.]
Yield (%)
46[b]
40
59
26
36
[a]
1
2
3
4
5
6
7
8
PdCl2
K2CO3 (3)
NaOH (3)
K2CO3 (3)
K2CO3 (3)
NaOH (3)
K2CO3 (3)
K2CO3 (3)
K2CO3 (3)
4a
4d
4e
74
88[c]
30[d]
4i
Entry Catalyst Base
[mol-equiv.]
Yield
Yield
(inert gas: Ar) (with contact to the air)
[a] Ligandless PdCl2 as catalyst. [b] Pd amount 20 mol-%. [c] Pd
amount 30 mol-%. [d] With contact to the air. Yields determined
by quantitative NMR analyses.[27]
[a]
1
2
3
4
5
6
7
8
9
PdCl2
K2CO3(2.0)
K2CO3 (3.0)
K2CO3 (2.0)
K2CO3 (3.0)
K2CO3 (2.0)
K2CO3 (3.0)
K2CO3 (2.0)
K2CO3 (3.0)
K2CO3 (2.0)
K2CO3 (3.0)
K2CO3 (3.0)
K2CO3 (2.0)
K2CO3 (3.0)
NaHCO3 (3.0)
K2CO3 (3.0)
NaHCO3 (3.0)
K2CO3 (3.0)
53
–
57
–
62
86
25
–
4a
57
85
39
58
63
98
54
89
67
–
97
79
70
87
72
4c
Ligands 2e, 2f and 2i were reduced as previously shown
in Scheme 1. The catalytic activity of reduced and non-re-
duced ligands complexed with PdCl2 (to produce 4e, 4f, 4i,
5e, 5f, and 5i, respectively) were further examined in the
syntheses of 4-acetylbiphenyls 10a–10c (Table 3). Overall,
reduced ligands gave better yields than their non-reduced
counterparts. The most active complexes 4e, 5e, 5f and 5i
were isolated resulting in same yields than in situ reactions.
Isolated catalysts 4e and 5f were chosen for more specific
studies for preparing PCBs. As summarized in Table 4,
1 mol-% of complex 4e and 5f catalyzed the reaction be-
tween the corresponding chlorinated aryl halide and phen-
ylboronic acid using 3 equiv. of K2CO3 in DMF giving
moderate yields. When using the complex 5f for the synthe-
sis of pentachlorinated PCBs, the yields decreased [for ex-
ample, 2,3,5,2Ј,3Ј-PCB (9r) (10%, X = Br); 2,4,5,2Ј,3Ј-PCB
(9s) (9%, X = I). These results are not reported in Table 4.]
Finally we decided to use our newly designed diamine-
4d
95
–
4e
41
55
75
97
–
–
53
–
4f
4g
4h
4i
61
[a] Ligandless PdCl2 was used for the reaction. Complex 4b gave
yields less than 10% and are not reported in the Table. Yields deter-
mined by quantitative NMR analyses.[27]
As shown in Table 1, the catalysts formed in situ indeed
catalyze the reactions because ligandless PdCl2 promotes type complex 5f for the synthesis of chlorinated 4-acetyl-
the reaction with only 49–57% yield. The reactions pro- biaryls. Reactions proceeded in moderate to excellent yields
ceeded with moderate to excellent yields when performed 23%–99% (Table 5). Alkaline cleavage of the produced
[28]
ˇ
under argon atmosphere. However, with the catalysts 4a, ketones according Zabjek and Petricˇ was also utilized in
4d, and 4f good yields could also be obtained if the reac- order to obtain BCAs in different yields.
tions were performed in the air. During ligand tuning it was
Although the overall yields in the synthesis of com-
rewarding to discover that catalysts containing both elec- pounds 11a, 11b, 11d–11j remained low, the high purity and
tron-donating (4d 98%, 4g 97%) or -withdrawing (4e 89%, easy separation of the compounds can be seen as an advan-
4f 75%) substituents gave excellent results.
tage of the method.
Eur. J. Org. Chem. 2008, 4019–4024
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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T. Kylmälä, N. Kuuloja, Y. Xu, K. Rissanen, R. Franzén
Table 4. Synthesis of PCBs using isolated catalysts 4e and 5f.
FULL PAPER
Table 3. Preparation of 4-acetylbiphenyls. Reactions with 4f and 4i
as catalysts are performed in situ. 4e, 5e, 5f and 5i are used as
isolated complex.
Entry
9
PCB
X
Catalyst Yield (%)
1
b
c
d
e
f
4
I
4e
5f
4e
4e
5f
5f
5f
5f
5f
5f
4e
5f
5f
4e
5f
5f
95
51
32
87
35
10
30
23
45
40
68
29
35
52
20
13
2
2,3
Br
Br
I
3
2Ј,4Ј
4
3,4
5
3,5
Br
Br
I
6
g
h
i
2,3,5
7
2,4,5
8
2,2Ј,3Ј
2,2Ј,4Ј
2,3,4Ј
2,4,4Ј
2,3,2Ј,4Ј
2,3,3Ј4Ј
2,4,3Ј4Ј
3,5,2Ј,3Ј
3,5,2Ј,4Ј
I
Product yield[a] (%)
9
j
k
l
m
n
o
p
q
I
X
4e
4f
4i
5e
5f
5i
10
11
12
13
14
15
16
I
I
10a
10b
10c
H
Cl
F
19
11
12
30
1
2
26
1
4
92
94
91
98
98
95
99
46
99
Br
I
I
[a] All yields determined by quantitative NMR analyses.[27]
Br
Br
Table 5. Preparation of BCAs by alkaline cleavage of ketones synthesized via Suzuki cross-coupling using complex 5f[a]
.
[a] All yields were determined by quantitative NMR analyses.[27]
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4019–4024

Synthesis of Chlorinated Biphenyls
(10 mL), and ethanol (25 mL) was added and solution was heated
to 80 °C. The aldehyde (15.00 mmol, 1.98 equiv.) was dissolved in
Conclusions
Diimine-type ligands prepared form diaminocyclohexane ethanol (varied amounts, 25–70 mL, depending on the aldehyde’s
solubility in ethanol) and added to the reaction mixture during
30 min in a slow stream. After addition, the mixture was refluxed
for 2 h, then water (10 mL) was added. The cooled reaction mixture
was stirred with further cooling (below +5 °C) for 2 h. The product
was extracted with dichloromethane and washed twice with dis-
tilled water and twice with brine and dried with MgSO4. The prod-
uct was filtered, evaporated and dried under vacuum.
2a–2i and their reduced analogs (diamines) 3b, 3d–3g,3i
form metal complexes 4a–4i and 5b, 5d–5g,5i with PdCl2.
When using the isolated complexes 4e and 5f several PCBs
can be prepared in moderate to excellent yields. The novel
ligands 2d (including an electron-donating methoxy group
at C-2Ј), 2e and 3f (including the electron-withdrawing sub-
stituents CF3 and F at C-2Ј) give promising results in the
Suzuki–Miyaura reaction when complexed to palladium.
The isolated diamine complex 5f appears to be a promising
candidate for the synthesis of PCBs with a catalyst-loading
of 1 mol-%. Using this complex several 4-acetylbiaryls can
be synthesized. These products can conveniently be con-
verted into the corresponding carboxylic acids.
General Procedure of Preparing Ligands 3b, 3d–3 g, 3i: To the li-
gands (10 mmol) 2b, 2d–2g, 2i dissolved to methanol was added
NaBH4 (950 mg, 25 mmol) in small portions with vigorous stirring.
Stirring was continued at room temp. overnight. Then water was
added to the reaction mixture and the product was extracted with
dichloromethane and dried with MgSO4. The product was filtered,
evaporated and dried under vacuum.
General Procedure of Complex Formation with PdCl2 To Obtain 4e,
5f and 5i: Ligand 3f or 3i (2.00 mmol) and PdCl2 (1.00 mmol) were
dissolved in methanol and refluxed overnight. Mixtures were co-
oled to room temperature and filtered through glass sinter (type
3G), methanol was used for washing. The solid crude product was
dissolved in dichloromethane and filtered through the sinter, the
solvents were evaporated. Recrystallisation from dichloromethane/
methanol (1:1) gave the complexes 5f and 5i. Complexes could be
obtained as orange-yellow crystals; ligand 2e (1 g, 2.35 mmol) and
PdCl2 (415 mg, 2.34 mmol) were refluxed under argon in acetoni-
trile for 14 h. The solvent was evaporated and the residue was redis-
solved in DCM. Filtration through a 3G glass sinter and slow evap-
oration gave little yellow crystals of 4e.
Experimental Section
General: Solvents and reagents were purchased from Sigma–
Aldrich or Merck and were used without further purification.
Thin-layer chromatography (TLC) was performed on precoated
(Kieselgel 60 F254) aluminium plates and detected by UV light.
Silica gel 100, particle size 0.063–0.2 mm, was used for column
1
chromatography. H and 13C NMR spectra were measured with a
Varian Mercury 300 MHz spectrometer using CDCl3 or DMSO as
solvent. Chemical shifts are reported as δ values (ppm) relative to
tetramethylsilane which was used as internal standard except for
NMR analyses that were performed in DMSO. In these cases
chemical shifts are reported relative to DMSO the shift value.
Yields for the Suzuki reactions were determined by quantitative
NMR analyses[27] and TCE (1,1,1-trichloroethane) or nitromethane
was used as standard. IR spectra of ligands were analyzed in the
450–4000 cm–1 range using KBr discs (with 1 wt-% ligand) with a
Perkin–Elmer Spectrum One FT-IR spectrometer.
General Procedure of Suzuki Reaction with the Isolated Crystalline
Catalyst: Catalyst (0.0056 mmol), aryl bromide (0.56 mmol), phen-
ylboronic acid (0.728 mmol), base (232 mg, 1.68 mmol, 3 equiv.)
and DMF (1.5 mL) was added, either under argon or with contact
to the air. Thereafter, the reaction mixture was stirred for 24 h at
120 °C (argon or air conditions). The mixture was cooled to room
temperature. Ethyl acetate (ca. 20 mL) was added and the suspen-
sion filtered through a 3G glass sinter in order to remove most of
the used catalyst. The organic layer was washed with distilled water
(2ϫ15 mL) and brine (2ϫ15 mL) and then dried with MgSO4.
Evaporation of the solvents and purification of the residue by col-
umn chromatography (n-hexane as eluent for PCBs and hexane/
EtOAc, 13:1 including 1% methanol as eluent for acetylbiphenyls
and nitrobiphenyls). Thereafter, all yields were determined by
quantitative NMR analyses[27] with either TCE or nitromethane as
standards.
X-ray Crystallography:[29] Crystals of 5f for the single-crystal X-ray
diffraction analysis were grown in the mixture of dichloromethane
and methanol 1:1 by slow solvent evaporation. Suitable crystals
were selected and analysis were performed using Bruker Kappa
Apex II diffractometer with graphite-monochromatized Mo-Kα (λ
= 0.71073 Å) radiation. Collect software[30] was used for the data
measurement and DENZO-SMN[31] for the processing. The struc-
tures were solved by direct methods with SIR97[32] and refined by
full-matrix least-squares methods with WinGX software,[33] which
utilizes the SHELXL-97 module.[34] All CH hydrogen positions
were calculated using a riding-atom model with UH = 1.2ϫUN or
1.2ϫUC.
General Procedure of Suzuki Reaction with the in-situ-Generated
Catalyst: The ligand (0.0028 mmol) and PdCl2 (1 mg,
0.0056 mmol) were first dissolved in 1 mL of DMF and then stirred
for 2 h under argon or air at 120 °C. Thereafter all reactants were
added in one portion. The next synthetic steps were executed as
described above.
Crystal Data for 5f: C20H24Cl2F2N2Pd·CH2Cl2, M = 342.30,
orange prism, 0.20ϫ0.30ϫ0.40 mm , triclinic, space group P1, a
3
¯
= 9.4943(3) Å, b = 9.9187(3) Å, c = 12.8329(3) Å, α = 85.007(2), β
= 79.830(2), γ = 86.151(2)°, V = 1183.34(6) Å3, Z = 2, Dc
=
=
General Procedure of Alkaline Cleavage of Ketones: According to
1.663 gcm–3, F000 = 596, µ = 1.263 mm–1, T = 123.0(1) K, 2θmax
[28]
ˇ
Zabjek and Petricˇ the 4-acetylbiphenyl (0.2 mmol, 1 equiv.) and
50.0°, 4163 unique reflections, 3964 with IϾ2σ(I), Rint = 0.0208,
271 parameters, 0 restraints, GoF = 1.097, R1 = 0.0361, wR2
KOH (62 mg, 1.11 mmol) were dissolved in THF (10 mL) and re-
fluxed at 65 °C using a CaCl2 tube. After stirring for further 4 h
more KOH (62 mg, 1.11 mmol) was added, then the mixture was
stirred at 65 °C for 90 h. Mixture was then cooled to room tempera-
ture, water (ca. 20 mL) was added and by-products were extracted
with chloroform. After addition of aqueous HCl (3 ) followed by
extraction to chloroform, the organic phase was dried with MgSO4,
filtered and evaporated to yield the carboxylic acid.
=
0.0562 [IϾ2σ(I)], R1 = 0.0383, wR2 = 0.0575 (all data), 0.370 Ͻ
∆ρ Ͻ –0.355 eÅ–3.
General Procedure of Preparing Ligands 2a–2i: Ligands were syn-
thesized by the modified Jacobsen method.[26] (R,R)-Cyclohexane-
1,2-diammonium mono-(+)-tartrate (2.00 g, 7.54 mmol) and
K2CO3 (2.1 g, 15.19 mmol) were dissolved in distilled water
Eur. J. Org. Chem. 2008, 4019–4024
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4023

T. Kylmälä, N. Kuuloja, Y. Xu, K. Rissanen, R. Franzén
FULL PAPER
(R,R)-Cyclohexane-1,2-diammonium mono-(+)-tartrate[12] used for
the synthesis of all ligands was prepared by a method developed
by Jacobsen[26]; instead of drying the product at 40–45 °C it was
dried under vacuum.
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Supporting Information (see also the footnote on the first page of
this article): Spectroscopic data of synthesized compounds;
refs.[35–43] refer to data in the Supporting Information.
[22]
[23]
[24]
[25]
Acknowledgments
We thank the Academy of Finland for financial support (No.
110043). We are also grateful to Liaoning Natural Science Founda-
tion for the Talent Doctors Fund (110043/RF and 122350/KR).
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CCDC-674638 (for 5f) contains the supplementary crystallo-
graphic data. These data can be obtained free of charge from
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Published Online: July 4, 2008
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 4019–4024