Organometallics 2006, 25, 2917-2920
2917
Synthesis and Diastereoselective ortho-Lithiation/Cyclopalladation of
Enantiopure [2-Imidazolyl]-1′,2′,3′,4′,5′-pentamethylferrocenes and
-1′,2′,3′,4′,5′-pentaphenylferrocenes
Rene´ Peters,* Zhuo-qun Xin, Daniel F. Fischer, and W. Bernd Schweizer
Laboratorium fu¨r Organische Chemie, ETH Zu¨rich, Ho¨nggerberg HCI E 111, Wolfgang-Pauli-Strasse 10,
CH-8093 Zu¨rich, Switzerland
ReceiVed February 26, 2006
Summary: ConVenient synthetic routes to enantiomerically pure
1′,2′,3′,4′,5′-pentamethyl and -pentaphenylferrocenyl imidazo-
lines are described. While the former complexes were diaste-
reoselectiVely ortho-lithiated and subsequently functionalized
by trapping with Various electrophiles, the latter complexes
could be diastereoselectiVely cyclopalladated, allowing the
preparation of the first enantiomerically pure ferrocenyl pal-
ladacycles bearing C5Me5 or C5Ph5 spectator ligands. The
planar chiral palladacycles haVe been shown to be promising
catalysts for aza-Claisen rearrangement reactions of allylic
trifluoroacetimidates.
of which have been successfully used as chiral ligands in
asymmetric catalysis,3 to the best of our knowledge, stereose-
lective ortho-metalations of 1′,2′,3′,4′,5′-pentamethylferrocene
derivatives have been limited so far to Ugi type amines.11 In
addition, during the preparation of this communication the
diastereoselective lithiations of enantiomerically pure 1′,2′,3′,4′,5′-
pentamethylferrocenyl oxazolines12 and of one acetal13 have
been described. The fact that 1′,2′,3′,4′,5′-pentamethylferrocenyl
oxazolines have only very recently been reported might be due
in part to the previously difficult accessibility of the carboxylic
acid 3.14 The carboxylation of pentamethylferrocene 1 has been
described to proceed by metalation with n-BuLi/t-BuOK and
subsequent trapping with dry ice to furnish acid 3 in 86% yield
based on 35% conversion.15 In our hands this procedure led to
inseparable mixtures of carboxylic acids due to a competing
deprotonation of the methyl groups of the Cp* ligand. The
Recently, we have described the first preparation of optically
pure 2-ferrocenyl imidazolines and their diastereoselective ortho-
metalation.1 These new heterocyclic systems are currently
investigated in our laboratory as chiral catalysts,2 taking
advantage of the strongly electron donating character of the
ferrocene moiety, which significantly increases the basicity of
the amidine functional group. To amplify this effect, we are
interested in the corresponding 1′,2′,3′,4′,5′-pentamethylfer-
rocene complexes, because a Cp* spectator ligand further
magnifies the electron-donating properties of the metallocene
backbone. Furthermore, the Cp* ring significantly increases the
steric bulk of the metallocene moiety, which should have a
conformational influence. To amplify the latter effect, we are
also attracted by 1′,2′,3′,4′,5′-pentaphenylferrocenyl imidazo-
lines.
In comparison to the rich and elaborate chemistry of enan-
tiomerically pure ferrocene derivatives,3 methods that permit
entry to optically active 1′,2′,3′,4′,5′-pentamethyl- or 1′,2′,3′,4′,5′-
pentaphenylferrocenes by asymmetric synthesis are relatively
underdeveloped. Despite several complementary diastereo- or
enantioselective ortho-metalation procedures for Cp′-unsubsti-
tuted ferrocenyl derivatives, which make use of various ortho-
directing groups such as amines,4 oxazolines,5 sulfoxides,6
acetals,7 hydrazones,8 oxazaphospholidine-oxides,9 or amides,10
providing access to numerous planar chiral derivatives, many
1
published H NMR data15 are not consistent with pure acid 3
(four different Cp signals instead of two), and the chemical shifts
1
are not in accordance with our H NMR data.
Alternatively, 3 has been prepared in moderate yield by
hydrolysis of the corresponding methyl ester, which was
obtained by coordination of [(C5H4CO2CH3)Na] to [FeCp*-
(NCCH3)3PF6].16 Since the latter complex is highly sensitive
and because its preparation occurs photolytically, which sig-
nificantly reduces the practicality of this procedure, acid 3 was
prepared by us via a simple two-step sequence, which has
previously been applied to the preparation of ferrocene car-
boxylic acid17 and 1′,2′,3′,4′,5-pentaphenylferrocene carboxylic
(6) (a) Rebie`re, F.; Riant, O.; Ricard, L.; Kagan, H. B. Angew. Chem.
Int. Ed. Engl. 1993, 32, 568. (b) Riant, O.; Argouarch, G.; Guillaneux, D.;
Samuel, O.; Kagan, H. B. J. Org. Chem. 1998, 63, 3511.
(7) Riant, O.; Samuel, O.; Flessner, T.; Taudien, S.; Kagan, H. B. J.
Org. Chem. 1997, 62, 6733.
(8) (a) Enders, D.; Peters, R.; Lochtman, R.; Raabe, G. Angew. Chem.
Int. Ed. 1999, 38, 2421. (b) Enders, D.; Peters, R.; Lochtman, R.; Raabe,
G.; Runsink, J.; Bats, J. W. Eur. J. Org. Chem. 2000, 3399. (c) Enders, D.;
Peters, R.; Lochtman, R.; Runsink, J. Eur. J. Org. Chem. 2000, 2839.
(9) Vinci, D.; Mateus, N.; Wu, X.; Hancock, F.; Steiner, A.; Xiao, J.
Org. Lett. 2006, 8, 215.
* To whom correspondence should be addressed. E-mail:
(1) Peters, R.; Fischer, D. F. Org. Lett. 2005, 7, 4137.
(10) Tsukazaki, M.; Tinkl, M.; Roglans, A.; Taylor, N. J.; Snieckus, J.
Am. Chem. Soc. 1996, 118, 685.
(2) For applications of imidazolines in catalysis see, for example: (a)
Bastero, A.; Ruiz, A.; Claver, C.; Milani, B.; Zangrando, E. Organometallics
2002, 21, 5820. (b) Menges, F.; Neuburger, M.; Pfaltz, A. Org. Lett. 2002,
4, 4713. (c) Busacca, C. A.; Grossbach, D.; So, R. C.; O’Brien E. M.;
Spinelli, E. M. Org. Lett. 2003, 5, 595.
(3) (a) Ferrocenes; Hayashi, T., Togni, A., Eds.; VCH: Weinheim,
Germany, 1995. (b) Richards, C. J.; Locke, A. J. Tetrahedron: Asymmetry
1998, 9, 2377.
(11) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Togni, A.;
Albinati, A.; Mu¨ller, B. Organometallics 1994, 13, 4481.
(12) Geisler, F. M.; Helmchen, G. J. Org. Chem. 2006, 71, 2486.
(13) Wo¨lfle, H.; Kopacka, H.; Wurst, K.; Ongania, K.-H.; Go¨rtz, H.-H.;
Preishuber-Pflu¨gl, P.; Bildstein, B. J. Organomet. Chem. 2006, 691, 1197.
(14) Witte, P.; Lal, T. K.; Waymouth, R. M. Organometallics 1999, 18,
4147.
(4) (a) Marquarding, D.; Klusacek, H.; Gokel, G.; Hoffmann, P.; Ugi, I.
J. Am. Chem. Soc. 1970, 92, 5389. (b) Ganter, C.; Wagner, T. Chem. Ber.
1995, 128, 1157.
(5) (a) Richards, C. J.; Damalidis, T.; Hibbs, D. E.; Hursthouse, M. B.
Synlett 1995, 74. (b) Sammakia, T.; Latham, H. A.; Schaad, D. R. J. Org.
Chem. 1995, 60, 10. (c) Nishibayashi, Y.; Uemura, S. Synlett 1995, 79.
(15) Bildstein, B.; Hradsky, A.; Kopacka, H.; Malleier, R.; Ongania, K.-
H. J. Organomet. Chem. 1997, 540, 127.
(16) Labande, A.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2002, 124,
1782. Note that Geisler and Helmchen used FeCl2 to prepare the ester in
59% yield (see ref 12).
(17) Reeves, P. C. Org. Synth. 1977, 56, 28.
10.1021/om060187o CCC: $33.50 © 2006 American Chemical Society
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