Organometallics 2009, 28, 2001–2004
2001
Diastereoselective Bis-Cyclopalladation of Ferrocene-1,1′-diyl Bis-Imidazolines: Translation of Central via Axial into Planar Chirality Sascha Jautze,† Stefan Diethelm,‡ Wolfgang Frey,† and Rene´ Peters*,† Institut fu¨r Organische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany, and Laboratorium fu¨r Organische Chemie, ETH Zu¨rich, Wolfgang-Pauli-Strasse 10, CH-8093 Zu¨rich, Switzerland ReceiVed February 20, 2009 Summary: An axially chiral reactiVe precursorsa trans-configured Pd(II) chelate complexstoward the planar chiral bis-palladacycle FBIP-Cl, which was recently shown to be an excellent enantioselectiVe catalyst for both aza-Claisen rearrangements and direct Michael additions, has been identified and allows us to explain the stereochemical outcome of the first reported direct diastereoselectiVe bis-cyclopalladation. In contrast to the trans-configured chelate complex, its cis-configured counterpart turned out to be a dead end under the initial cyclopalladation conditions and required harsher conditions for C-H actiVation. The development of readily accessible catalysts for the production of chiral functionalized building blocks in a highly enantioselective, atom-economic, and cost-efficient fashion is still one of the major challenges for organic chemists.1 In this context we have recently described the planar chiral ferrocene bis-imidazoline bis-palladacycle 2 (FBIP-Cl) (Table 1, entry 1), which was readily prepared in diastereomerically pure form in four steps from ferrocene.2 This complex provides exceptionally efficient catalysts, after activation with different silver sulfonate salts for counterion exchange, for both the aza-Claisen rearrangement of Z-configured allylic trifluoroacetimidates2,3 and direct Michael additions of R-cyano acetates to enones4 despite the fundamental differences with regard to the involved transition-state geometries. Both reactions have various operational advantages, as they proceed with unusually low catalyst loadings at high concentrations, do not require inert gas handling, and typically provide excellent product yields for a broad range of substrates. The key step in the formation of 2 is the first example of a direct diastereoselective bis-cyclopalladation.2,5,6 While the 1H NMR spectra of the crude product of this reaction were too complex to directly determine the diastereoselectivity, a short * To whom correspondence should be addressed. E-mail: rene.peters@ oc.uni-stuttgart.de. † Universita¨t Stuttgart. ‡ ETH Zu¨rich. (1) (a) Sheldon, R. A.; Arends, I.; Hanefeld, U. Green Chemistry and Catalysis; Wiley-VCH: Weinheim, Germany, 2007. (b) Blaser, H. U.; Schmidt, E. Asymmetric Catalysis on Industrial Scale; Wiley-VCH: Weinheim, Germany, 2004. (2) (a) Jautze, S.; Seiler, P.; Peters, R. Angew. Chem., Int. Ed. 2007, 46, 1260. (b) Jautze, S.; Seiler, P.; Peters, R. Chem. Eur. J. 2008, 14, 1430. (3) Xin, Z.-q.; Fischer, D. F.; Peters, R Synlett 2008, 1495. (4) Jautze, S.; Peters, R. Angew. Chem., Int. Ed. 2008, 47, 9284. (5) Only few direct diastereoselective monocyclopalladations of chiral ferrocenes are known: (a) Kuz’min, L. G.; Struchkov, Y. T.; Troitskaya, L. L.; Sokolov, V. I.; Reutov, O. A. IzV. Akad. Nauk SSSR, Ser. Khim. 1979, 1528. (b) Lo´pez, C.; Bosque, R.; Solans, X.; Font-Bardia, M. Tetrahedron: Asymmetry 1996, 7, 2527. (c) Zhao, G.; Xue, F.; Zhang, Z.Y.; Mak, T. C. W. Organometallics 1997, 16, 4023. (d) Zhao, G.; Wang, Q.-G.; Mak, T. C. W. Tetrahedron: Asymmetry 1998, 9, 1557. (dd) Zhao, G.; Wang, Q.-G.; Mak, T. C. W. Organometallics 1998, 17, 3437. (e) Peters, R.; Xin, Z.-q.; Fischer, D. F.; Schweizer, W. B. Organometallics 2006, 25, 2917. (f) Weiss, M. E.; Fischer, D. F.; Xin, Z.-q.; Jautze, S.; Schweizer, W. B.; Peters, R. Angew. Chem., Int. Ed. 2006, 45, 5694.
filtration over silica gel using CH2Cl2 as eluent furnished the diastereomerically pure product (dr > 100:1) with Sp,S′p,S*p,S*′p configuration,7 despite the fact that seven diastereomers, in principle, could have been formed.8 Herein we report a mechanistic explanation of the configurational outcome. To understand why the product was formed in only moderate yield (56%), the polar major side product was isolated by eluting the filter cake with CHCl3. 1H and 13C NMR as well as MS spectroscopy indicated the formation of a C1-symmetric chelate complex9 of ferrocene10 bis-imidazoline 1 (FBI) rather than a cyclopalladation product. This was confirmed by X-ray crystal structure analysis (Figure 1).11 The imidazolines bind in a cis mode to the square-planar Pd center (N-Pd-N′ angle 91.44°). While the upper imidazoline (as depicted) in 3 adopts an orientation (Cp-CdN torsion angle 34.3°) which is relatively close to the one found in 2, where the Cp’s and the corresponding CdN bonds are almost in plane, the lower imidazoline strongly deviates from coplanarity (Cp′-CdN′ torsion angle -58.4°) and the coordinating N atom points more in the direction of the pro-(Rp)-o-C′ atom: i.e. opposite to the configuration found in 2 in terms of planar chirality and opposite to the arrangement of the upper imidazoline in 3, which points to the pro-(Sp)-o-C atom.12 As in 1 and 2, the sulfonylated N atom of the upper imidazoline moiety in 3 is pyramidalized as a result of steric repulsion between the Ph residue at the imidazoline 5-position and the sulfonyl group. This effects a transfer of stereoinformation to the sulfonylated N atom, resulting in a preferred orientation in which the sulfonyl group avoids the ferrocene core. In contrast, the sulfonylated N atom of the lower imidazoline unit is almost planar,13 to minimize repulsive steric interactions (6) Previous syntheses of nonracemic ferrocenyl bis-palladacycles relied on double ortho-lithiation, iodination, and subsequent oxidative addition of Pd(0) to the corresponding bis-iodoferrocenes: Kang, J.; Yew, K. H.; Kim, T. H.; Choi, D. H. Tetrahedron Lett. 2002, 43, 9509. (7) The stereodescriptors with regard to the planar chirality are used according to: Schlo¨gl, K. Top. Stereochem. 1967, 1, 39. We employ different superscripts to distinguish the four Cp ligands. S′p indicates the configuration of a bottom Cp,while an asterisk indicates a second ferrocene unit. (8) The seven diastereomers would have the following configurations: Sp,S′p,S*p,S*′p, Rp,S′p,S*p,S*′p, Rp,R′p,S*p,S*′p, Rp,S′p,R*p,S*′p, Rp,S′p,S*p,R*′p, Rp,R′p,R*p,S*′p, and Rp,R′p,R*p,R*′p. (9) Application of Pd chelate complexes with ferrocene bis-oxazolines to Suzuki and Heck couplings: Lee, S. J. Organomet. Chem. 2006, 691, 1347. (10) Recent review about applications of ferrocenes in asymmetric catalysis: Arraya´s, R. G.; Adrio, J.; Carretero, J. C. Angew. Chem., Int. Ed. 2006, 45, 7674. (11) Crystallographic data for 3 have been deposited as Supporting Information and with the Cambridge Crystallographic Data Centre as deposition number 721052; the material is available free of charge via the Internet at http://www.ccdc.cam.ac.uk/products/csd/request/. (12) Note that 3 is not only axially chiral but also pseudoplanar chiral due to the rigid conformation.
10.1021/om900137c CCC: $40.75 2009 American Chemical Society Publication on Web 03/12/2009
2002 Organometallics, Vol. 28, No. 7, 2009
Communications
Table 1. Screening of Reaction Conditions for Pd Complexation of 1
yielda (%) entry
Pd source (x)
additive (amt (equiv))
1b 2c 3 4c 5d 6 7 8 9 10b 11 12 13b 14e 15e
Na2[PdCl4] (2.35) Na2[PdCl4] (2.35) Na2[PdCl4] (2.35) Na2[PdCl4] (1.5) Na2[PdCl4] (1.0) [PdCl2(NCCH3)2] (1.0) [PdCl2(NCCH3)2] (1.0) [PdCl2(NCCH3)2] (1.0) [PdCl2(NCCH3)2] (1.0) [PdCl2(NCCH3)2] (2.35) [PdCl2(NCCH3)2] (2.35) [PdCl2(NCCH3)2] (2.35) Na2[PdCl4] (2.35) Na2[PdCl4] (2.35) Na2[PdCl4] (2.35)
NaOAc (2.8) NaOAc (1.5)
T (°C) room room room room room -18 -18 -18 room room room 80 room 80 80
NaOAc (4.0) PS (2.35) PS (2.35) NaOAc (4.0)
a Isolated yields. b In DCM/MeOH (1:1). c In THF/MeOH (1:1). contains impurities which could not be removed.
d
temp temp temp temp
In THF.
Figure 1. X-ray crystal structure of 3 (ORTEP plot, ellipsoids at the 50% probability level, H atoms and solvent molecules omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd-N, 2.044(2); Pd-N′, 2.044(3); Pd-Cl, 2.2858(9); Pd-Cl′, 2.2873(9); N-Pd-N′, 91.44(11); N-Pd-Cl, 91.03(8); Cl-Pd-Cl′, 89.58(4); Cl′-Pd-N′, 88.09(8); N-Pd-Cl′, 177.71(8); N′-Pd-Cl, 175.50(8).
between the sulfonyl group and the ferrocene core. For the same reason, the nonbonding N p orbital slightly deviates by ca. 9° (13) The pyramidality at a given atom can be expressed by the difference between 360° and the sum of the three bond angles at that atom. For the present analysis, corresponding values at the sulfonylated N atoms are 9° (upper Cp in 3 as depicted in Figure 1) and 0° (lower Cp).
t (h)
temp temp temp temp temp
e
18 17 62 0.5 2 0.75 2 4 168 18 47 6 21 6 7.5
1
