Application of a planar chiral η5-cyclopentadienylrhenium(I)tricarbonyl complex in asymmetric catalysis: Highly enantioselective phenyl transfer to aldehydes

Article abstract of DOI:10.1002/1521-3773(20010417)40:8<1488::AID-ANIE1488>3.0.CO;2-B

A novel planar chiral cyrhetrene 1 has been identified as an efficient catalyst precursor for the asymmetric synthesis of diarylmethanol compounds 2 by phenylation of aldehydes. Compared to the corresponding ferrocene derivative, the performance of the cyrhetrene is superior or at least equally as good and gives excellent enantioselectivities even with catalyst loadings as low as 2 mol%.

Full text of DOI:10.1002/1521-3773(20010417)40:8<1488::AID-ANIE1488>3.0.CO;2-B

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and application of novel cyrhetrene 3, which was found to be
superior in terms of both enantioselectivity and catalyst
loading in the phenyl transfer to a wide variety of alde-
hydes.[10]
Application of a Planar Chiral
h5-Cyclopentadienylrhenium(i)tricarbonyl
Complex in Asymmetric Catalysis:
Highly Enantioselective Phenyl Transfer to
Aldehydes**
Modifications of the metal ± p-fragment of 2 were expected
to have an impact on the steric and electronic properties of the
catalyst. Assuming that decreased electron density on the
ligand would result in increased Lewis acidity of the catalyst,
we focussed our attention primarily on the synthesis of metal
tricarbonyl complexes.[9] There, the electron-withdrawing
properties of the carbonyl groups would lower the electron
density on the metal center as well as on the remaining
cyclopentadienyl fragment bearing the stereogenic elements.
If these electronic changes are then efficiently transferred to
the catalytically active site and if the simultaneously occurring
steric modifications are not counterproductive, beneficial
effects in the catalysis could result. Due to its expected
chemical stability rhenium(i) complex 3 appeared to be the
complex of choice. To the best of our knowledge, h5-cyclo-
pentadienylrhenium(i)tricarbonyl complexes have not been
utilized in asymmetric catalysis, and only one nonracemic
planar chiral derivative has been described.[11, 12] Our synthesis
of 3 started from cyrhetrenyl carboxylic acid 4, which was
obtained in a straightforward manner from [Re2(CO)10] and
cyclopentadienyl carboxylic acid following an excellent, high-
yielding procedure published by Jaouen et al.[13] The cyrhe-
trenyl oxazoline 6[14] was synthesized in an analogous manner
to the corresponding ferrocene derivatives via amide 5, which
was cyclized using the Appel protocol (Scheme 2).[15] Directed
Carsten Bolm,* Martin Kesselgruber, Nina Hermanns,
Jens P. Hildebrand, and Gerhard Raabe
Dedicated to Professor Günter Helmchen
on the occasion of his 60th birthday
Asymmetric metal catalysis is one of the most active areas
in modern organic chemistry, and the number of novel ligands
for catalytic asymmetric transformations is growing rapidly.[1]
Among these compounds, planar chiral ferrocenes[2] are of
immense importance and some of them have already found
application in industrial processes.[3] Often, ferrocenes are
superior to other metal p complexes,[4] although for some
catalyses it was found that a variation of the p-bound metal
fragment can be of benefit.[5]
Recently, we developed a system for the enantioselective,
catalytic synthesis of diarylmethanol compounds from alde-
hydes by utilizing ferrocene 1[6] and a zinc species generated in
situ from ZnPh2/ZnEt2 (Scheme 1).[7] Further optimization of
the process was expected to be possible by variation of the
metal ± p-fragment of 2.[8, 9] Herein, we describe the synthesis
Scheme 1. Enantioselective phenyl transfer onto aldehydes and catalyti-
cally active metal complexes.
[*] Prof. Dr. C. Bolm, Dipl.-Ing. M. Kesselgruber,
Dipl.-Chem. N. Hermanns, Dr. J. P. Hildebrand,[‡]
Priv.-Doz. Dr. G. Raabe
Institut für Organische Chemie der RWTH Aachen
Professor-Pirlet-Strasse 1, 52056 Aachen (Germany)
Fax : (‡49)241-8888-391
E-mail: carsten.bolm@oc.rwth-aachen.de
Scheme 2. Synthesis of complex 3.
‡
[ ] Current address: Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
ortho-metalation[16] of 6 using nBuLi in diethyl ether[17] and
subsequent quenching of the resulting lithiated species
with benzophenone led to diastereomeric products in
a ratio of 9:1 in favor of the product with S,Rp configuration.[18]
All transformations give good to excellent yields, and the
compounds are crystalline and stable to air, light, and
moisture.[14]
To confirm the relative configuration of 3, the solid-state
structure of the cyrhetrenyl complex was determined by
single-crystal X-ray structure analysis.[14, 19] As depicted in
Figure 1 one phenyl group of 1 is equatorial, while the other
occupies an axial position with regard to the cyclopentadienyl
[**] We are grateful to the Deutsche Forschungsgemeinschaft (DFG)
within the Collaborative Research Center (SFB) 380 ªAsymmetric
Synthesis by Chemical and Biological Methodsº and the Fonds der
Chemischen Industrie for financial support and to Dr. C. W. Lehmann,
MPI für Kohlenforschung, Mülheim, for collecting the X-ray diffrac-
tion data. M.K. acknowledges the DFG for a predoctoral fellowship
(Graduiertenkolleg). We thank Degussa and Witco for donations of
chemicals and Professor Dr. A. Salzer as well as Dipl.-Ing. M. Treu for
inspiring discussions. We also appreciate the stimulus of Dr. K. MunÄiz
for the investigation of the influence of the metal ± p-fragment on
catalyses of this type.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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Table 1. Asymmetric phenyl transfer to various aldehydes.
Entry[a]
Substrate
ee of product [%][b,c]
Absolute
config.
of alcohol[d]
2 mol%
10 mol%
of 3
of 3
1
96
98 (97)
R
2
3
4
83
95
85
96 (91)
95 (95)
99 (98)
R
R
R
Figure 1. Structure of
probability).
3 (ORTEP plot; ellipsoids plotted with 50%
5
6
76
74
83 (75)
78 (78)
S
S
backbone. The relative location of the tert-butyl group at the
oxazoline group is trans with respect to the axial phenyl group.
Overall, the structural features of cyrhetrene 3 are very
similar to those of ferrocene 1, which suggested its potential
use in catalysis.
The catalytic properties of 3 were explored in the asym-
metric phenyl transfer from organozinc compounds to alde-
hydes. Initial approaches by ourselves[7a] and others[10] using
diphenylzinc as phenyl source, encountered the difficulty that
the uncatalyzed background reaction with the substrate is
comparatively rapid, leading to a diminished enantioselectiv-
ity.[20] The use of diethylzinc as an additive had been shown to
be the key to significantly raise the enantiomeric exces-
ses.[7b, 10b] The replacement of the CpFe moiety by the
Re(CO)3 fragment now led to a further improvement of this
transformation (Table 1).
For most examples, complex 3 shows higher enantioselec-
tivity than ferrocene 1. A significant increase was observed in
reactions with ortho-substituted aldehydes (Table 1, entries 2
and 7) which are now among the best substrates for this
transformation. Especially noteworthy is the result obtained
with 2,4,6-trimethylbenzaldehyde (Table 1, entry 7; 98% ee),
which demonstrates that even ortho-disubstitution at the
aromatic aldehyde is well tolerated. The reactions with other
substrates follow trends that have previously been observed in
catalyses with 1: cinnamyl aldehyde displays lower levels of
enantioselection (Table 1, entry 8), and aliphatic aldehydes
are still somewhat problematic with respect to asymmetric
induction (Table 1, entries 5 and 6). In several cases, even a
catalyst loading of only 2 mol% of 3 was sufficient to achieve
high ee values similar to the ones obtained with 10 mol% of 1
(Table 1, entries 1, 3, 5, 8).[21]
In summary, we have described the synthesis of the h5-
cyclopentadienylrhenium(i)tricarbonyl complex 3 and dem-
onstrated its use in asymmetric catalysis. Compared to its
analogous ferrocene derivative, cyrehetrene 3 shows signifi-
cantly higher enantioselectivities in the catalyzed phenyl
transfer from a phenylzinc species to aromatic and aliphatic
aldehydes, leading to the currently most efficient approach for
this reaction. Further studies are directed towards the use of
7
8
9
80
88
93
98 (92)
92 (90)
98 (96)
R
R
R
[a] All reactions gave good to quantitative yields (>80% on a 0.25 mmol
scale). [b] Determined by HPLC using a chiral stationary phase. For exact
separation conditions see Supporting Information. [c] Values in parenthe-
ses represent ee values obtained with 10 mol% of ferrocene ligand 1.
[d] Determined by comparison of the order of peak elution during HPLC
with literature values, or tentatively assigned by assumption of an identical
reaction pathway (entries 4, 6, 7, 9).
other h5-cyclopentadienylrhenium(i)tricarbonyl complexes in
asymmetric catalysis.
Received: November 10, 2000
Revised: February 6, 2001 [Z16073]
[1] a) R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley, New
York, 1994; b) Catalytic Asymmetric Synthesis (Ed.: I. Ojima),
2nd ed., Wiley-VCH, New York, 2000; c) Comprehensive Asymmetric
Catalysis (Eds.: E. N. Jacobsen, A. Pfaltz, H. Yamamoto), Springer,
Berlin, 1999; d) Transition Metals for Organic Synthesis (Eds.: M.
Ä
Beller, C. Bolm), Wiley-VCH, Weinheim, 1998; e) C. Bolm, K. Muniz,
Chem. Soc. Rev. 1999, 28, 51.
[2] a) Ferrocenes (Eds.: T. Hayashi, A. Togni), VCH, Weinheim, 1995;
b) Metallocenes (Eds.: A. Togni, R. L. Haltermann), Wiley-VCH,
Weinheim, 1998.
[3] For accounts on the importance of ferrocene-type ligands for
industrial processes see: a) H.-U. Blaser, F. Spindler, Chimia 1997,
51, 297; b) R. Imwinkelried, Chimia 1997, 51, 300; c) H.-U. Blaser, F.
Spindler in Comprehensive Asymmetric Catalysis, Vol. 3 (Eds.: E. N.
Jacobsen, A. Pfaltz, H. Yamamoto), Springer, Berlin, 1999, p. 1427.
[4] a) H. C. L. Abbenhuis, U. Burckhardt, V. Gramlich, A. Martelletti, J.
Spencer, I. Steiner, A. Togni, Organometallics 1996, 15, 1614; b) L.
Schwink, P. Knochel, Chem. Eur. J. 1998, 4, 950.
[5] a) T. Hayashi, A. Ohno, S.-j. Lu, Y. Matsumoto, E. Fukuyo, K. Yanagi,
J. Am. Chem. Soc. 1994, 116, 4221; b) C. E. Garrett, M. M.-C. Lo, G. C.
Fu, J. Am. Chem. Soc. 1998, 120, 7479.
Angew. Chem. Int. Ed. 2001, 40, No. 8
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Ä
[6] For other catalytic applications of ligand 1 see: a) C. Bolm, K. Muniz-
Fernandez, A. Seger, G. Raabe, Synlett 1997, 1051; b) C. Bolm, K.
MunÄiz-Fernandez, A. Seger, G. Raabe, K. Günther, J. Org. Chem.
Asymmetric Catalysis, Vol. 2 (Eds.: E. N. Jacobsen, A. Pfaltz, H.
Yamamoto), Springer, Berlin, 1999, p.911.
[21] In a separate study (ref. [6c]), we had shown that the use of
diastereomeric mixtures of 1 can lead to high enantioselectivity. As
a consequence we proposed that this phenomenon could be exploited
in cases where the directed ortho-metalation was not completely
diastereoselective. Here we have now found that using 10 mol% of a
7:1 mixture of (S,Rp)-3 and diastereomeric (S,Sp)-3, the phenylation of
p-chlorobenzaldehyde proceeded with 97% ee. Thus, it appears that
this mixture as such can be used in catalysis. The overall process
applying 3 in the aryl transfer is thereby simplified even further.
Ä
1998, 63, 7860; c) C. Bolm, K. Muniz, J. P. Hildebrand, Org. Lett. 1999,
Ä
1, 491; d) K. Muniz, PhD thesis, RWTH Aachen, 1998; e) see also in:
Ä
K. Muniz, C. Bolm, Chem. Eur. J. 2000, 6, 2309.
[7] a) C. Bolm, K. MunÄiz, Chem. Commun. 1999, 1295; b) C. Bolm, N.
Ä
Hermanns, J. P. Hildebrand, K. Muniz, Angew. Chem. 2000, 112, 3607;
Angew. Chem. Int. Ed. 2000, 39, 3465.
[8] Use of 1-[o-(diphenylhydroxymethyl)-phenyl]-3-tert-butyloxazoline
in the addition of the phenylzinc species to 4-chlorobenzaldehyde
resulted in an ee of 62% compared with 97% obtained with ferrocene
1.
[9] A positive effect upon the introduction of a metal fragment on the
enantioselectivity in the Pd-catalyzed asymmetric allylations was
found by Helmchen et al. There, a P,N-chelating ligand with a
cymantrene unit was used. a) G. Helmchen, S. Kudis, P. Sennhenn,
H. Steinhagen, Pure Appl. Chem. 1997, 69, 513; b) S. Kudis, G.
Helmchen, Angew. Chem. 1998, 110, 3210; Angew. Chem. Int. Ed.
1998, 37, 3047. Studies on the use of the manganese tricarbonyl
complex of 2 (M ˆ [Mn(CO)3]) in the catalyzed phenyl transfer
revealed a slightly lower enantioselectivity compared to that of 3
(97% ee in the addition onto 4-chlorobenzaldehyde). The complete
results of our screening study of metal complexes will be reported in
due course.
[10] For other catalyzed diphenylzinc additions to aldehydes, see: a) P. I.
Dosa, J. C. Ruble, G. C. Fu, J. Org. Chem. 1997, 62, 444; b) W.-S.
Huang, Q.-S. Hu, L. Pu, J. Org. Chem. 1999, 64, 7940; c) W.-S. Huang,
L. Pu, Tetrahedron Lett. 2000, 41, 145, and references therein.
[11] W. H. Bosch, U. Englert, B. Pfister, R. Stauber, A. Salzer, J.
Organomet. Chem. 1996, 506, 273.
Individual Alumina Nanotubes**
Lin Pu,* Ximao Bao, Jianping Zou, and Duan Feng
Avid attention has been given to the preparation, proper-
ties, and applications of nanotubes of different materials.
Nanotubes composed of carbon,[1] tungsten disulfide (WS2),[2]
boron nitride (BN),[3] vanadium oxide [VO2.40(C16H33NH2)],[4]
titanium dioxide (TiO2),[5] and others, were studied during the
last decade. However, the reproducible usage of nanotubes in
electrical devices is complicated by the fact that the tubes exist
in different chiralities and diameters.[6] Moreover, the raw
materials consist of dense networks of closely connected
nanotubes, and individual tubes are often obtained by ultra-
sonic agitation, which may introduce defects into the tubes.[7]
Here we report on two easy and controlled electrochemical-
anodizing routes for the synthesis of individual alumina
nanotubes (ANTs) in a single fabricating step. The structure
of ANTs provides clues to unraveling the mechanism of
nanotube growth and gives valuable hints on solving the long-
standing problem of the self-organization mechanism in the
porous anodization of aluminum.[8±15]
[12] Some compounds containing this moiety have been used for labeling
of biologically active substances because of the importance of
radioactive rhenium isotopes for medicinal diagnosis and treatment.
Á
For an example see: S. Top, H. El Hafa, A. Vessieres, J. Quivy, J.
Vaissermann, D. W. Hughes, M. J. McGlinchey, J.-P. Mornon, E.
Thoreau, G. Jaouen, J. Am. Chem. Soc. 1995, 117, 8372.
[13] S. Top, J.-S. Lehn, P. Morel, G. Jaouen, J. Organomet. Chem. 1999, 583,
63.
[14] New compounds have been fully characterized by spectroscopic
methods and elemental composition established by combustion
analysis or HR-MS (see Supporting Information). A concise compar-
ison between complexes 1 and 3 will be discussed elsewhere.
[15] R. Appel, Angew. Chem. 1975, 87, 863; Angew. Chem. Int. Ed. Engl.
1975, 14, 801.
Two different preparation methods (Figure 1), designated
normal stepwise anodization (NSA) and lateral stepwise
anodization (LSA), were used to make ANTs. The major
difference between these two arrangements is the position on
the sample (Al/Si) to which the potential difference U is
applied. For NSA, it is the bottom surface of the Si substrate,
and for LSA, the top surface of the Al metal film. This results
in completely different current paths for the two methods.
Note, however, that the orientation of the sample is not
important.
[16] a) V. Snieckus, Chem. Rev. 1990, 90, 879; concerning the analogous
lithiation of ferrocenyl oxazolines, see: b) T. Sammakia, H. A.
Latham, D. R. Schaad, J. Org. Chem. 1995, 60, 10; c) Y. Nishibayashi,
S. Uemura, Synlett 1995, 79; d) C. J. Richards, A. W. Mulvaney,
Tetrahedron: Asymmetry 1996, 7, 1419.
[17] Attempted ortho-metalation of 6 with sBuLi resulted in the formation
of products which presumably stem from nucleophilic addition. Those
reactions are well-known from the chemistry of chromium(tricarbon-
yl) arene complexes. For examples, see: E. P. Kündig, D. Amurrio, R.
Liu, A. Ripa, Synlett 1991, 657.
[18] For the nomenclature, see: K. Schlögl, Top. Stereochem. 1967, 1, 39.
[19] Crystal data for 3: trigonal, a ˆ 11.1669(3), c ˆ 35.5114(14) Š, Z ˆ 2 Â
3 (two symmetrically independent molecules), Vˆ 3835.0(3) Š3, space
group P32, colorless crystals obtained by recrystallization from
MTBE, measured on a SMART Bruker diffractometer at 100 K,
R ˆ 0.046, Rw ˆ 0.033, GOF ˆ 1.471. Crystallographic data (excluding
structure factors) for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-147702. Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
[20] In the analogous dialkylzinc additions to aldehydes this background
reaction takes place only to a minor extent. For reviews on this
reaction, see: a) R. Noyori, M. Kitamura, Angew. Chem. 1991, 103, 34;
Angew. Chem. Int. Ed. Engl. 1991, 30, 49; b) K. Soai, S. Niwa, Chem.
Rev. 1992, 92, 833; c) see also: K. Soai, T. Shibata in Comprehensive
The transmission electron microscope (TEM) images in
Figure 2 show a general view of the ANTs. They are attached
to the anodic porous alumina (APA) mother film. In the TEM
[*] Dr. L. Pu, Prof. Dr. X. Bao, Dr. J. Zou, Prof. Dr. D. Feng
National Laboratory of Solid State Microstructures
and
Department of Physics, Nanjing University
Nanjing 210093 (China)
Fax : (‡86)25-359-5535
E-mail: xmbao@nju.edu.cn
[**] This work was supported by the National Natural Science Foundation
under the contract No. 59832100.
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