Synthesis of peripherally functionalised dendritic binaphthyl (BINAP)-systems and their application as ligands in the copper-catalysed hydrosilylation of acetophenone

Article abstract of DOI:10.1002/adsc.200700586

Two series of peripherally functionalised dendrimers were synthesised based on poly(ethyleneimine) and poly(amidoamine) (PAMAM) dendrimers by reaction of (R)-6-N-[(γ-carboxyl)butanoyl]-aminomethyl-2,2′- bis(diphenylphosphino)-1,1′-binaphthyl ("Glutaroyl-A

Full text of DOI:10.1002/adsc.200700586

FULL PAPERS
DOI: 10.1002/adsc.200700586
Synthesis of Peripherally Functionalised Dendritic Binaphthyl
(
BINAP)-Systems and their Application as Ligands in the
Copper-Catalysed Hydrosilylation of Acetophenone
a
a
a,
Jutta K. Kassube, Hubert Wadepohl, and Lutz H. Gade *
a
Anorganisch Chemisches Institut der Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax : (+49)-6221-545-609; e-mail: lutz.gade@uni-hd.de
Received: December 12, 2007; Published online: February 14, 2008
Dedicated to Professor Andreas Pfaltz on the occasion of his 60th birthday.
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: Two series of peripherally functionalised dendritic ligands and their catalytic properties was
dendrimers were synthesised based on poly(ethylene- established in the asymmetric hydrosilylation of ace-
imine) and poly(amidoamine) (PAMAM) dendri- tophenone. It could be shown that (R)-6-aminometh-
A
C
H
T
R
E
U
N
G
mers by reaction of (R)-6-N-[(g-carboxyl)butanoyl]- yl-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (“H-
aminomethyl-2,2’-bis(diphenylphosphino)-1,1’-bi- Aminap”) can be attached to the dendrimers without
naphthyl (“Glutaroyl-Aminap”) with first to fifth any significant loss of catalytic selectivity compared
generation PPI and zeroth to fourth generation to the monomeric ligand Benzoyl-Aminap. The se-
PAMAM dendrimers using ethyl-N,N-dimethylcarbo- lectivities observed in these reactions were higher
diimide (EDC)/1-hydroxybenzotriazole as coupling than those obtained using unfunctionalised BINAP
reagents. All dendritic ligands were characterised by as ligand.
NMR spectroscopy, elemental analysis and, for gen-
erations G0–G3, MALDI-TOF mass spectrometry. Keywords: asymmetric catalysis; BINAP; copper;
The relationship between the size/generation of the dendrimers; hydrosilylation; immobilisation
Introduction
functional phosphines in the Cu(I)-catalysed hydrosi-
[5]
lylation of acetophenone as reference reaction. In-
In the assessment of multi-site macromolecular cata- troduced to enantioselective catalysis by Noyori et al.
[
1,2]
[6]
lysts, such as dendrimer catalysts,
it is essential to in 1980, the BINAP ligand has found various appli-
establish whether the immobilised catalyst units cations in asymmetric catalysis, resulting the desired
retain their identity and are not altered by the nature chiral products in good to excellent selectivities.
of the dendrimer backbone. The linker and spacer Apart from the most popular BINAP-ruthenium and
units employed in the fixation of the catalysts may be BINAP-rhodium complexes, that catalyse the hydro-
[
7–9]
crucial in this respect as well as the functional groups genation of olefins and ketones,
many other transi-
present in the dendrimer. Regarding the dendrimer tion metal BINAP complexes were discovered to be
core structure itself, the length and conformational ri- active as catalysts in miscellaneous chemical transfor-
[
10,11]
gidity of the branches and spacers are important fac- mations.
tors when evaluating a dendritic catalyst. For immo-
[
3]
Fan et al. have synthesised a variety of functional-
bilised asymmetric catalysts, even subtle conforma- ised BINAP ligands bearing dendritic wedges.
[12–17]
In
tional changes may significantly influence their ste- order to attach the polyaryl ether dendrons to the
reoselectivity. The interplay of all these factors will ligand they introduced amino groups in the 5- or 5,5’-
generally determine detrimental or beneficial dendri- positions. These dendritic ligands have been tested as
[
4]
mer effects on catalyst performance.
catalysts in various hydrogenation reactions, showing
Here we report the immobilisation of the chiral di- similar selectivities as the BINAP ligand itself but im-
phosphine BINAP at the periphery of a series of proved recycling properties.
A
C
H
T
R
E
U
N
G
poly(propyleneimine) (PPI) and poly(amidoamine)
BINAP-copper(I) complexes have found use as cat-
(
PAMAM) dendrimers in order to employ these poly- alysts, inter alia, in the conjugate reduction of 2-pyrid-
Adv. Synth. Catal. 2008, 350, 1155 – 1162
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Jutta K. Kassube et al.
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yl vinyl sulfones, affording the chiral sulfones with aminomethyl-2,2’-bis(diphenylphosphino)-1,1’-bi-
[
18]
yields and enantioselectivities of 90% and higher.
naphthyl (H-Aminap) 1 in a yield of 94%. Following
A more established reaction is the hydrosilylation of the strategy that was developed in our group to syn-
[
22]
aryl ketones. Using acetophenone as substrate the thesise dendritic pyrphos-systems, glutaric acid an-
BINAP ligand was found to give a relatively low hydride was used to attach an appropriate carboxylate
enantioselectivity of 75% compared to other diphos- linking unit yielding the Glutaroyl-Aminap ligand 2
[
19]
phine ligands tested by Lipshutz et al. By variation (Scheme 1).
of the reaction conditions the selectivity could be im- The performance of chiral catalysts containing the
proved to over 90%. We report the results of den- BINAP ligand may be dependent on the presence or
[
20]
drimer immobilisation for this reaction in this work.
absence of heterosubstituents in the inaphthyl back-
bone since they influence the conformational sub-
space of the ligand skeleton. In order to be able to
evaluate the dendrimer catalysts in comparison with
an appropriately functionalised mononuclear BINAP-
[
23]
Results and Discussion
Synthesis and Characterisation of the BINAP Ligand- Cu catalyst we synthesised a derivative with similar
Linker Unit as well as a BINAP Reference System
local steric and electronic properties as 1, 2 and the
functionalised dendrimers. To this end the amino
A fixation of the BINAP ligand to the periphery of group in H-Aminap 1 was protected by reaction with
dendrimers has not been reported to date although it benzoyl chloride, giving the highly crystalline Benzo-
is interesting to investigate the catalytic results ob- yl-Aminap 3 in 75% yield (Scheme 2).
tained with the monomeric ligand and compare them
Crystals of 3 suitable for a single crystal X-ray
with the catalyst performance in the more densely structure analysis were obtained from dichlorome-
packed “surface” of a dendritic molecular system. thane/methanol. Its molecular structure is depicted in
The aim was therefore to obtain peripherally func- Figure 1 along with the principal bond lengths and
tionalised dendritic catalysts based on a functionalised angles.
BINAP ligand and to assess the way the catalyst char-
The principal structural features are similar to the
acteristics depend on the dendrimer size and thus the crystallographically determined molecular structure of
[
24]
number and density of the reactive centres.
the unfunctionalised BINAP.
However, the influ-
In order to attach BINAP to the peripheral amino ence of the substituent in the backbone is manifested
end groups of the PPI and PAMAM dendrimers, we in a different torsion angle of the binaphthyl unit.
decided to introduce a methylenamino function in the Whereas for BINAP a torsion angle of 87.848 be-
6-position of the binaphthyl backbone of the ligand, a tween the two naphthyl fragments is observed, the
position which was thought to interfere least with the corresponding angle in 3 is 78.7(4)8. Moreover, a
diphosphine binding site of the metal. This was weak (dipole-quadrupole) interaction between the p-
A
C
H
T
R
E
U
N
G
achieved by reduction of the previously reported system of the protection group and a CÀH unit of a
BINAP derivative (R)-6-cyano-2,2’-bis(diphenylphos- phenyl ring bound directly to a P atom is observed
[
21]
phino)-1,1’-binaphthyl
with LiAlH , giving (R)-6- [distance(phenyl-centroid)···H=2.66 Š]. The other
4
Scheme 1. Synthesis of the Glutaroyl-Aminap 2.
Scheme 2. Synthesis of the Benzoyl-Aminap 3.
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Synthesis of Peripherally Functionalised Dendritic Binaphthyl (BINAP)-Systems
royl-Aminap)64
FULL PAPERS
and
{G0}-PAMAM-(Glutaroyl-
Aminap) to {G4}-PAMAM-(Glutaroyl-Aminap) , re-
4
64
spectively, depicted in Figure 2, is reflected in their
1
3
NMR spectra, in particular, the C NMR spectra of
the functionalised PPI dendrimers of the generations
1–5. In all spectra the resonance patterns are virtually
superimposible due to the coincidence of the core sig-
nals upon going to higher generations. The assignment
of the signals in the aromatic region of the spectra as
listed in the Experimental Section was achieved with
the aid of HMBC and HMQC experiments.
The complete functionalisation of the dendrimers
can be assessed with reasonable accuracy by
C NMR spectroscopy because the signal of the
Figure 1. Molecular structure of Benzoyl-Aminap 3. Hydro-
gen atoms omitted for clarity. Selected bond lengths [Š] and
angles [8]: C(1)ÀC(1’) 1.499(5), C(2)ÀP(1) 1.834(4), C(2’)À
1
3
2
carbon atoms of the (unfunctionalised) CH NH2
ÀC(2) 1.378(5), C(1’)ÀC(2’) 1.393(5); groups of the PPI dendrimers at about 40 ppm as well
P(2) 1.833(4), C(1)
C(1)ÀC(2)ÀP(1) 118.8(3), C(1’)ÀC(2’)ÀP(2) 120.1(3), NÀ as the signal at around 30 ppm, assigned to the
C(23)ÀC(6) 110.5(3); torsion angle C(2’)ÀC(1’)ÀC(1)ÀC(9) CH CH NH carbon atoms are two characteristic
2
2
2
7
8.7(4); ring centroid [C(25) C(26) C(27) C(28) C(29) probes. Upon complete functionalisation both disap-
C(30)]···H
ACHTREUNG
(13’) 2.66 Š (2.60 Š with CH distance normalised
peared.
Compared to the spectra of the functionalised PPI
to 1.08 Š).
dendrimers the resolution of the signals in the spectra
of the functionalised PAMAM dendrimers is lower as
3
1
metric parameters are very similar to those of the can be easily seen in Figure 3 where the P NMR
parent compound.
spectra of {G5}-PPI-(Glutaroyl-Aminap)64 and {G4}-
PAMAM-(Glutaroyl-Aminap)64 are shown. Whereas
the spectrum of the PAMAM dendrimer shows a
broadened resonance at À15.76 ppm, in the P NMR
spectrum of the PPI dendrimer all signals of the AB
system, resulting from the unsymmetrical BINAP-
3
1
Synthesis and Characterisation of the BINAP-
Functionalised PPI and PAMAM Dendrimers
The synthesis of the functionalised dendrimers re- functionalisation, can be detected at À15.63 ppm and
5
quired a quantitative coupling reaction of Glutaroyl- À15.73 ppm, with a J coupling of 11.2 Hz. This dif-
P,Pꢁ
Aminap 2 with the amino end groups of the dendri- ferent NMR spectroscopic behaviour is attributed to
mer. This was achieved by using ethyl-N,N-dimethyl- the differences in internal mobility of the respective
carbodiimide (EDC)/1-hydroxybenzotriazole as cou- molecules and thus relaxation behaviour, the confor-
pling reagents, a strategy well known in polypeptide mationally more flexible PPI giving the sharper sig-
[25,26]
synthesis.
Using this method, Aminap-functional- nals.
ised PPI and PAMAM dendrimers bearing 4 to 64 di-
phosphine ligands in their periphery were obtained in
good yields (Scheme 3).
The Enantioselective Cu-Catalysed Hydrosilylation of
Acetophenone
1
13
All dendritic ligands were characterised by H,
C
31
and P NMR spectroscopy, and elemental analysis.
The lower generations (up to G3) have been also In order to assess the relationship between the size/
characterised by MALDI-TOF mass spectrometry generation as well as the type of the dendritic struc-
whilst the higher generations defied an unfragmented ture (PPI or PAMAM) and the catalytic properties,
transfer into the gas phase.
the respective functionalised dendrimers, the phos-
The high topological symmetry of the dendrimers phine functionalised systems were employed as li-
G1}-PPI-(Glutaroyl-Aminap)4 to {G5}-PPI-(Gluta-
{
Scheme 3. Coupling of Glutaroyl-Aminap 2 with the amino end groups of the PPI and PAMAM dendrimers.
Adv. Synth. Catal. 2008, 350, 1155 – 1162
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Jutta K. Kassube et al.
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Figure 2. {G4}-PPI-(Glutaroyl-Aminap)32 7.
gands in the Cu(I)-based asymmetric hydrosilylation employed as the reference ligand, diphenylsilane was
of acetophenone (Scheme 4). used as a hydrid source and a substrate:catalyst (cata-
The reaction conditions for this reference reaction lytic site) ratio of 33:1 was chosen in order to allow
were optimised using those published by Lipshutz for short reaction times even at very low tempera-
[19]
et al.
as a starting point. Benzoyl-Aminap 3 was tures. However, it should be pointed out that the cata-
lyst loading could be reduced below 0.1 mol% leading
to turnover numbers of greater than 1000.
A significant increase in selectivity was generally
observed on decreasing the reaction temperature, and
therefore all subsequent test reactions were carried
out at À788C. Upon changing the solvent from pure
Scheme 4. Cu-catalysed hydrosilylation of acetophenone.
toluene (83% ee) to a toluene:THF mixture an im-
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Synthesis of Peripherally Functionalised Dendritic Binaphthyl (BINAP)-Systems
FULL PAPERS
Aminap, resulting in dendritic ligands that catalyse
this reaction with a selectivity of around 93% ee, a se-
lectivity which is slightly higher than that of the un-
functionalised BINAP ligand. This absence of a “den-
dritic” effect indicates that the catalytic conversion is
entirely controlled by the first coordination sphere of
the copper. This is in part due to the marked coordi-
nation preference of Cu(I) for the soft diphosphine
compared to the chemically hard functionalities in the
dendrimer backbone.
Similar results have been observed using the den-
Figure 3. Comparison of the 243 MHz P NMR spectra of dritic PAMAM-ligands as catalysts as it is shown in
3
1
{
G5}-DAB-dendr-(Glutaroyl-Aminap)64 8 (left) and {G4}- Figure 5. The lack of influence of the dendrimer struc-
PAMAM-(Glutaroyl-Aminap)64 13 (right).
ture upon the catalyst performance is another indica-
tion that the selectivity of the transformation is due
to the shape of the first coordination sphere of the Cu
proved selectivity was observed reaching its optimum centres.
for a solvent ratio of 8:3 (94% ee). However, in con-
trast to the results obtained under similar reaction
[
20]
conditions by Bellemin–Laponnaz et al., the selec- Conclusions
tivity remained unchanged upon variation of the
silane from diphenylsilane (ee=94%) to phenylsilane In summary, two series of immobilised poly(Aminap)
A
H
R
U
G
(
ee=94%) or phenylmethylsilane (ee=93%). Testing ligands have been synthesised, based on PPI and
the unfunctionalised BINAP as ligand under these op- PAMAM dendrimers as backbones. All dendritic li-
timised reaction conditions gave a selectivity of 91% gands were obtained in good yields, resulting in sys-
ee indicating a slight improvement in the enantiose- tems containing up to 64 diphosphine ligands at their
lectivity of the hydrosilylation of acetophenone (3% periphery.
ee) due to the functionalisation of the BINAP in the
It has been demonstrated that they can be used as
6-position with an aminomethyl group. In Figure 4 the ligands in the copper-catalysed hydrosilylation of ace-
results of the catalytic tests carried out with the func- tophenone without any loss of selectivity resulting
tionalised PPI dendrimers are shown in comparison from their immobilisation onto the dendrimer com-
to the monomeric systems BINAP and Benzoyl- pared to the monomeric ligand Benzoyl-Aminap. No-
Aminap 3.
tably, the observed selectivities are even higher than
On the whole, it has been found that the selectivity those obtained when using the unfunctionalised
remained almost unchanged by the immobilisation of BINAP as ligand. This indicates that the Cu(I)-cata-
Figure 4. Enantioselectivity of the asymmetric hydrosilylation of acetophenone for the different catalyst generations.
Adv. Synth. Catal. 2008, 350, 1155 – 1162
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Jutta K. Kassube et al.
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Figure 5. Comparison of the selectivity using functionalised PPI- or PAMAM-dendrimers as ligands.
solution (1M, 3 mL). The resulting suspension was filtered
and the solvent was removed under vacuum. The residue
lysed hydrosilylation of ketones is particularly suited
for the development of immobilised catalysts, and we was disolved in dichloromethane and extracted with brine.
are beginning to turn our attention to cheaper support The organic phase was dried over MgSO and the solvent
4
materials possessing less regular branched structures. removed under vacuum to afford 1 as a pale yellow solid.
Yield: 2.14 g (94%).
Moreover, extending the use of these new dendritic
systems as ligands to other catalytic reactions is the
objective of current and future research in our labora-
tory.
As has been emphasised previously, the kinetically
controlled stereoselection depends on very small in-
Preparation of (R)-6-N-[(g-Carboxyl)butanoyl]-
aminomethyl-2,2’-bis(diphenylphosphino)-1,1’-
binaphthyl (Glutaroyl-Aminap) (2)
(
R)-6-Aminomethyl-2,2’-bis(diphenylphosphino)-1,1’-bi-
naphthyl 1 (1.02 g, 1.57 mmol) and glutaric anhydride
0.20 g, 1.72 mmol) were dissolved in dichloromethane and
crements of free activation enthalpy. It is therefore an
excellent sensitive probe for “dendrimer effects” and
will continue to be studied in this fundamental con-
(
stirred for 18 h at room temperature. Subsequently, aqueous
HCl (1M, 10 mL) was added. After stirring for 10 min,
aqueous KOH (1M) was added, the aqueous phase removed
and the organic layer washed twice more with water (2”
[27]
text.
As monodispersed macromolecules, chiral
dendrimer catalysts provide ideal model systems for
less regularly structured but commercially more
[
28]
viable supports such as hyperbranched polymers.
1
0 mL). The solvent was removed under vacuum to afford 2
as a pale yellow solid. Yield: 1.12 g (93%).
Experimental Section
Preparation of (R)-6-N-Benzoylaminomethyl-2,2’-
bis(diphenylphosphino)-1,1’-binaphthyl (Benzoyl-
Aminap) (3)
The general information about equipment and methods em-
ployed in this work along with the complete spectroscopic
and analytical data of all new compounds is provided in the
Supporting Information.
(
R)-6-Aminomethyl-2,2’-bis(diphenylphosphino)-1,1’-bi-
naphthyl 1 (1.05 g, 1.61 mmol) was dissolved in pyridine
5 mL) and cooled to 08C. Benzoyl chloride (206 mL, 0.25 g,
.77 mmol) was subsequently added dropwise. After stirring
(
1
Preparation of (R)-6-Aminomethyl-2,2’-
bis(diphenylphosphino)-1,1’-binaphthyl (H-Aminap)
for 2 h at room temperature dichloromethane (5 mL) was
added and the solution was extracted with H O (1”10 mL),
2
(
1)
aqueous HCl (1M, 1”10 mL), aqueous KOH (1M, 1”
(
(
(
R)-6-Cyano-2,2’-bis(diphenylphosphino)-1,1’-binaphthyl
2.27 g, 3.50 mmol) was added to a suspension of LiAlH4
1.01 g, 2.66 mmol) in THF (50 mL) and toluene (100 mL).
10 mL) and then again H O (2”10 mL). The organic phase
2
was dried over MgSO4 and the solvent removed under
vacuum. The residue was recrystallised from dichlorome-
thane/methanol to afford 3 as colourless crystals. Yield:
0.91 g (75%).
After stirring for 6 h at 1008C, the mixture was cooled to
8C and hydrolysed by addition of H O (3 mL) and KOH
0
2
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Synthesis of Peripherally Functionalised Dendritic Binaphthyl (BINAP)-Systems
FULL PAPERS
General Procedure for the Synthesis of the
Functionalised Dendrimers
reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Center: CCDC 669722. These
data can be obtained free of charge http://www.ccdc.cam.
ac.uk/cgi-bin/catreq.cgi? [or from CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK; Phone: (+44)-1223-336408, Fax:
A mixture of Glutaroyl-Aminap 2 (1.15 equiv.), EDC·HCl
(
1.27 equiv.), 1-HOBT (1.73 equiv.) and triethylamine (1.85
equiv.) was stirred in DMF at 08C for 40 min. To this sus-
pension was added the respective amino-terminated poly(-
propyleneimine) (PPI) or poly(amidoamine) (PAMAM)
dendrimer dissolved in DMF. The resulting solution was
warmed to room temperature and stirred for 48 to 90 h. All
volatiles were completely removed under vacuum. The resi-
due was taken up in 20 mL CH Cl and thoroughly extracted
(
+44)-1223-336033; E-mail: deposit@ccdc.cam.ac.uk].
Acknowledgements
2
2
We thank the Deutsche Forschungsgemeinschaft (SFB 623,
C8) and the Fonds der Chemischen Industrie (Ph.D. scholar-
ship to J.K.) for support. We are also grateful to Dr. Jürgen
Gross (Heidelberg) and Drs. Volker Sauerland and Franz-
Josef Mayer-Posner (Bruker Daltonik GmbH, Bremen) for
recording the mass spectra.
with KOH (0.2M, 2”15 mL), H O (2”15 mL), hydrochloric
2
acid (0.2M, 2”15 mL), KOH (0.2M, 2”15 mL) and again
H O (2”15 mL). Finally, the solvent was removed under
2
vacuum. The resulting white solid was washed several times
with n-pentane and dried under vacuum yielding the
Aminap-functionalised dendrimers as off-white powders.
Yields: 4: 467 mg (79%); 5: 446 mg (85%); 6: 442 mg
(
(
(
87%); 7: 414 mg (81%); 8: 525 mg (88%); 9: 437 mg
84%); 10: 452 mg (82%); 11: 485 mg (86%); 12: 501 mg References
88%); 13: 487 mg (84%).
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General Procedure for the Hydrosilylation
Experiments
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CuCl and 2.9 mg (0.03 mmol) of t-BuONa were dissolved in
2
mL of toluene/THF (8/3) and the resulting mixture was
stirred for 30 min at room temperature. Diphenylsilane
210 mL, 1.1 mmol) was then added and the solution cooled
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(
met. Chem., Springer, Heidelberg, 2006, 20.
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none dissolved in 1 mL toluene/THF (8/3), the reaction mix-
ture was stirred for 24 h at this temperature.
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Shreiner, C. N. Moorefield, G. R. Newkome, New J.
A solution of K CO (3 mL, 1% in methanol) was added
2
3
to hydrolyse the reaction mixture, then the solvent was re-
moved under vacuum and the residue purified by column
chromatography on silica (pentane/diethyl ether, 85/15).
Enantioselectivities were determined by GC {Chiraldex B-
À1
PM (50 m”0.25 mm); 408C, 58Cmin to 1208C, 14 min; tR
[
(R)-sec-phenylethyl alcohol]=25.59 min, t [(S)-sec-phenyl-
R
ethyl alcohol]=26.34 min}.
X-Ray Crystal Structure Analysis of Compound 3
Crystal data: C H NOP , orthorhombic, space group
52
39
2
P2 2 2 , a=9.1301(11), b=17.959(2), c=24.074(3) Š, V=
1
1 1
3
À1
3
947.4(8) Š , Z=4, m=0.151 mm , F =1584. Reflections
000
measd.: 65346, indep.: 6966 [Rint =0.129], index ranges
À10<=h<=10, 0<=k<=21, 0<=l<=28, q range 2.0 to
258. Intensity data were collected at 100 K with a Bruker
AXS Smart 1000 CCD diffractometer (Mo-Ka radiation,
graphite monochromator, l=0.71073 Š). Data were correct-
ed for Lorentz, polarisation and absorption effects (semiem-
pirical, SADABS, transmission factors: max. 0.7464, min.
[30]
[29]
0.6510).
The structure was solved by direct methods
and refined by full-matrix least squares methods based on
2
[31]
F .
All non-hydrogen atoms were given anisotropic dis-
placement parameters. The positions of the hydrogen atoms
were taken from difference Fourier syntheses and refined.
Final R values [I>2s(I)]: R(F)=0.0587, wR(F2)=0.1335,
GoF=0.977, absolute structure parameter 0.02(12). Crystal-
lographic data (excluding structure factors) for the structure
Adv. Synth. Catal. 2008, 350, 1155 – 1162
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1161

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