Synthesis of chiral ferrocenyl-substituted β-amino cyclopentadienes and their complexation to transition metals

Article abstract of DOI:10.1021/om970850w

CBS reduction of the (chloroacyl)ferrocenes 3 provides the chloro alcohols 5 with ≥98% ee, which upon treatment with cyclopentadienides yield the chiral β- or γ-hydroxy cyclopentadienes 6 and 7. The hydroxyl function of 6 and 7 can be substituted with full retention of configuration by a range of N- or S-nucleophiles, giving efficient access to the optically active linked amino-cyclopentadienyl ligands 11-13. These were complexed to an iron center, yielding a ferrocenyl diamine with a large bite angle between the nitrogen donor atoms. Furthermore the first chelating complexation reaction to titanium is presented.

Full text of DOI:10.1021/om970850w

Organometallics 1998, 17, 7-9
7
Syn th esis of Ch ir a l F er r ocen yl-Su bstitu ted â-Am in o
Cyclop en ta d ien es a n d Th eir Com p lexa tion to Tr a n sition
Meta ls
Lothar Schwink and Paul Knochel*,†
Fachbereich Chemie der Philipps-Universita¨t Marburg, Hans-Meerwein-Strasse,
D-35032 Marburg, Germany
Thomas Eberle and J un Okuda*
Institut fu¨r Anorganische Chemie und Analytische Chemie der J ohannes
Gutenberg-Universita¨t Mainz, J .-J .-Becher-Weg 24, D-55099 Mainz, Germany
Received September 29, 1997X
Summary: CBS reduction of the (chloroacyl)ferrocenes
3 provides the chloro alcohols 5 with g98% ee, which
upon treatment with cyclopentadienides yield the chiral
â- or γ-hydroxy cyclopentadienes 6 and 7. The hydroxyl
function of 6 and 7 can be substituted with full retention
of configuration by a range of N- or S-nucleophiles,
giving efficient access to the optically active linked
amino-cyclopentadienyl ligands 11-13. These were
complexed to an iron center, yielding a ferrocenyl di-
amine with a large bite angle between the nitrogen donor
atoms. Furthermore the first chelating complexation
reaction to titanium is presented.
solved problem, since the complexation of the second
cyclopentadienyl moiety of a bis(cyclopentadienyl) ligand
with only one of its diastereotopic faces to the central
metal is difficult to control.3 Even if this problem can
be adequately addressed, giving the diastereomerically
pure rac-metallocene, a cumbersome resolution proce-
dure is still needed to access the enantiomerically pure
complex.4 To circumvent these difficulties, several
authors developed stereospecific syntheses for ansa-
metallocenes starting with an elaborated optically active
ligand system, which makes a diastereo- and enantio-
selective complexation possible.5 However, simpler
solutions are still being sought.
Since the initial report by Brintzinger in 1979,
bridged-cyclopentadienyl complexes of structure 1 have
gained a great deal of attention.1 These chiral ansa-
The replacement of one cyclopentadienyl ligand in
ansa-metallocenes by an amido ligand, as depicted in
the structures 2, on the one hand removes the stereo-
chemical problems in the complexation process and on
the other hand creates an electronically more unsatur-
ated and sterically more accessible metal center.6 Re-
cently, such linked amido-cyclopentadienyl complexes
have shown their potential as novel catalysts for copo-
lymerizations with ethene.7 Chiral derivatives have
been prepared by utilizing planar chirality on the
cyclopentadienyl ring8 or by introducing the chiral
subtituent R′ on the nitrogen atom.9 Herein we describe
the efficient and flexible synthesis of optically active
metallocenes have found many applications as catalysts
in the stereoregular polymerization of R-olefins as well
as in asymmetric hydrogenation and carbomagnesation
reactions of double bonds.2 For all these reactions it is
necessary to have diastereomerically and enantiomeri-
cally pure complexes available. This is not an easily
(3) (a) Diamond, G. M.; Rodewald, S.; J ordan, R. E. Organometallics
1995, 14, 5-7. (b) Huttenloch, M. E.; Diebold, J .; Rief, U.; Brintzinger,
H. H.; Gibert, A. M.; Katz, T. J . Organometallics 1992, 11, 3600-3607.
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J . Organomet. Chem. 1982, 232, 233-247. (b) Schaefer, A.; Karl, E.;
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54, 4154-4158.
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Ostrander, R.; Rheingold, A. L.; Bosnich, B. Organometallics 1993, 12,
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J . Am. Chem. Soc. 1992, 114, 2276-2277.
† E-mail: Knochel@ps1515.chemie.uni-marburg.de.
X Abstract published in Advance ACS Abstracts, December 15, 1997.
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Brintzinger, H. H. Angew. Chem. 1979, 91, 837-838; Angew. Chem.,
Int. Ed. Engl. 1979, 18, 777-778.
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5985-5986. (e) Broene, R. D.; Buchwald, S. L. J . Am. Chem. Soc. 1993,
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S0276-7333(97)00850-9 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/05/1998

8
Organometallics, Vol. 17, No. 1, 1998
Communications
Ta ble 1. Su bstitu tion of th e
Cyclop en ta d ien yl-Su bstitu ted F er r ocen yl Aceta tes
9 a n d 10 by Nitr ogen a n d Su lfu r Nu cleop h iles
Sch em e 1
Sch em e 2
entry no.
11-13
n
R
Nua
yield/%
1
2
3
4
5
6
7
8
9
11a
11b
11c
11d
12a
12b
12c
13a
13b
13c
13d
1
1
1
1
1
1
1
2
2
2
2
H
H
H
H
Me
Me
Me
H
H
H
N(H)Bn
N(H)-t-Bu
SMe
64
77
32
78
42
45
33
73
66
64
49
SAc
N(H)Bn
N(H)-i-Pr
N(H)-t-Bu
N(H)Me
N(H)Bn
N(H)-i-Pr
N(H)-t-Bu
10
11
H
a
MeOH/water was used as solvent for the substitution with
N-nucleophiles, while acetic acid was used for S-nucleophiles.
linked amino-cyclopentadienyl ligands with an asym-
metric center in the bridge and our initial attempts to
coordinate them to transition-metal centers.
The synthesis of the chiral ligands started with a CBS
reduction of the chloro-functionalized ferrocenyl ketones
3, which in turn are easily available by Friedel-Crafts
acylation (Cl(CH2)nCOCl, AlCl3, CH2Cl2) of ferrocene.10
The resulting alcohols 5 were obtained in good yields
with selectivities exceeding 98% ee using 30 mol % of
the oxazaborolidine 4 as catalyst (Scheme 1).11
The alkyl chloride function in 5a was substituted by
an excess of cyclopentadienyl (NaCp, THF, room tem-
perature, 2 days, 57%) and tetramethylcyclopentadienyl
anions (KMeCp, THF, 65 °C, 4 h, 59%), respectively. The
substitution is not a direct process but involves the
initial formation of the corresponding epoxide by HCl
elimination. The intermediate epoxide is then opened
by the nucleophile, affording the â-hydroxy cyclopenta-
dienes 6 (Scheme 2).12 In a similar way, direct substi-
tution of the chloride in 5b could be easily effected
(NaCp, THF, room temperature, 0.5 h, 80%), yielding
the γ-hydroxy cyclopentadiene 7 with a three-carbon
spacing between the potential coordination sites.
The cyclopentadienyl alcohols 6 and 7 may themselves
act as ligands for transition metals.13 However, they
proved to be valuable starting materials for further
elaboration because the hydroxyl group in a position R
to the ferrocenyl moiety can be easily exchanged for
other donor functionalities.14 These derivatization reac-
tions proceed under very mild solvolytic conditions and
do not affect the sensitive diene part of the molecule.
Thus, substitution by a methoxy group could be achieved
by simply stirring 6a in methanol with some acetic acid
as catalyst, affording ligand 8 in 72% yield (eq 1).
For most purposes it is preferable to activate the
hydroxyl function by conversion to the corresponding
acetate (Ac2O, pyridine, room temperature, 12 h, then
removal of the volatiles) prior to substitution. The
acetates 9 and 10, which are obtained in quantitative
yields, can then be treated with an appropriate nucleo-
phile in the same flask. For example, the acetates 9
and 10 were subjected to reaction with several primary
amines (Table 1; entries 1, 2, and 5-11), NaSMe (entry
3), and KSAc (entry 4). Further possible options include
(9) (a) Okuda, J .; Verch, S.; Spaniol, T. P.; Stu¨rmer, R. Chem. Ber.
1996, 129, 1429-1431. (b) Ciruelos, S.; Cuenca, T.; Go´mez, R.; Go´mez-
Sal, P.; Manzanero, A.; Royo, P. Organometallics 1996, 15, 5577-5585.
(c) McKnight, A. L.; Masood, M. A.; Waymouth, R. M.; Straus, D. A.
Organometallics 1997, 16, 2879-2885.
(10) Schlo¨gl, K.; Egger, H. Monatsh. Chem. 1962, 95, 376-392.
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Chem. Commun. 1983, 469-470. (b) Corey, E. J .; Bakshi, R. K.;
Shibata, S.; Chen, C.-P.; Singh, V. K. J . Am. Chem. Soc. 1987, 109,
7925-7926. (c) Corey, E. J .; Bakshi, R. K.; Shibata, S. J . Am. Chem.
Soc. 1987, 109, 5551-5553. (d) Singh, V. K. Synthesis 1992, 605-617.
(e) Wright, J .; Frambes, L.; Reeves, P. J . Organomet. Chem. 1994, 476,
215-217. (f) Ohno, A.; Yamane, M.; Hayashi, T.; Oguni, N.; Hayashi,
M. Tetrahedron: Asymmetry 1995, 6, 2495-2502. (g) Woltersdorf, M.;
Kranich, R.; Schmalz, H.-G. Tetrahedron 1997, 53, 7219-7230. (h)
Schwink, L.; Knochel, P. Tetrahedron Lett. 1996, 37, 25-28. (i) Locke,
A. J .; Richards, C. J . Tetrahedron Lett. 1996, 37, 7861-7864.
(12) Two distinguishable H-shift isomers were found for all com-
pounds with an unsubstituted cyclopentadienyl moiety. Up to four
isomers could be detected by NMR for the tetramethyl-substituted
cyclopentadienes.
(13) (a) For the preparation and complexation of chiral R-hydroxy
cyclopentadienes: Schwink, L.; Vettel, S.; Knochel, P. Organometallics
1995, 14, 5000-5001. For â-hydroxy cyclopentadienes: (b) Christoffers,
J .; Bergman, R. G. Angew. Chem. 1995, 107, 2423-2425; Angew.
Chem., Int. Ed. Engl. 1995, 34, 2266-2268. (c) Hermann, W. A.;
Morawietz, M. J . A.; Priermeier, T. Angew. Chem. 1994, 106, 2025-
2028; Angew. Chem., Int. Ed. Engl. 1994, 33, 1946-1949. (d) Rieger,
B.; Steiman, M.; Fawzi, R. Chem. Ber. 1992, 125, 2373-2377. (e)
Rieger, B.; J any, G.; Fawzi, R.; Steiman, M. Organometallics 1994,
13, 647-653.
(14) (a) Gokel, G. W.; Marquarding, D.; Ugi, I. K. J . Org. Chem.
1972, 37, 3052-3058. (b) Gokel, G.; Hoffmann, P.; Klusacek, H.;
Marquarding, D.; Ruch, E.; Ugi, I. Angew. Chem. 1970, 82, 77-78;
Angew. Chem., Int. Ed. Engl. 1970, 9, 64-65.

Communications
Organometallics, Vol. 17, No. 1, 1998
9
Sch em e 3
substitution with ammonia, secondary amines, phos-
phines (e.g. R2PH), and even carbon nucleophiles.15
In conclusion, we have developed an efficient enan-
tioselective route to chiral cyclopentadienes with a
linked functionality in a â- or γ-position capable of
acting as a second coordination site for a given central
metal.16 The synthetic approach is highly flexible
because four parameters can be independently varied:
(a) the length of the linking carbon chain, (b) the
substitution pattern of the cyclopentadienyl moiety, (c)
the nature of the donor atom in the bridge, and (d) the
substituents on the donor atom.
With the new ligands 11-13 in hand, we investigated
their suitability for complexation to transition metals.
For a simple test doubly deprotonated 11a (2 equiv of
LDA, THF, -78 °C, 2 h) was allowed to react with
iron(II) chloride (-78 to 0 °C, 1 h), yielding the trifer-
rocene 14, which has a diamine ligand with a large bite
angle between the donor atoms (Scheme 3).17
The complexation of 12b to a titanium center was
achieved by following a well-established protocol.18
Thus, the dianion (2 equiv of n-BuLi, THF, room
temperature, 2 h) of the isopropylamino-substituted
ligand 12b was subsequently treated with TiCl3‚3THF
and lead(II) chloride, providing the desired bridged
complex 15 in 72% yield (Scheme 3).
Studies with respect to the catalytic activity of 15 in
polymerization and asymmetric hydrogenation reactions
are currently underway.
(15) (a) Almena Perea, J . J .; Ireland, T.; Knochel, P. Tetrahedron
Lett. 1997, 38, 5961-5964. (b) Togni, A.; Rihs, G.; Blumer, R. E.
Organometallics 1992, 11, 613-621. (c) Togni, A.; Ha¨usel, R. Synlett
1990, 633-635. (d) Hayashi, T.; Mise, T.; Fukushima, M.; Kagotani,
M.; Nagashima, N.; Hamada, Y.; Matsumoto, A.; Kawakami, S.;
Konishi, M.; Yamamoto, K.; Kumada, M. Bull. Chem. Soc. J pn. 1980,
53, 1138-1151.
(16) For preparation of chiral â-sulfonamido and achiral γ-amino
cyclopentadienes: (a) Lensik, C. Tetrahedron: Asymmetry 1995, 6,
2033-2038. (b) Hughes, A. K.; Meetsma, A.; Teuben, J . H. Organo-
metallics 1993, 12, 1936-1945.
Ack n ow led gm en t. We thank the DFG (Grant No.
SFB 260) and the Fonds der Chemischen Industrie for
generous financial support. We thank BASF AG, Sipsy
SA, and Witco AG for the generous gift of chemicals.
(17) (a) Sawamura, M.; Hamashima, H.; Shinoto, H.; Ito, Y. Tetra-
hedron Lett. 1995, 36, 6479-6482. (b) Kuwano, R.; Sawamura, M.;
Shirai, J .; Takahashi, M.; Ito, Y. Tetrahedron Lett. 1995, 36, 5239-
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J . Organometallics 1995, 14, 3129-3131. (b) Okuda, J .; Eberle, T.;
Spaniol, T. P. Chem. Ber./ Recl. 1997, 130, 209-215.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures, analytical data, and copies of the NMR
spectra for the compounds 3 and 5-15 (12 pages). Ordering
information is given on any current masthead page.
OM970850W