Resolution of planar chiral cationic (η6-arene) tricarbonylmanganese complexes

Article abstract of DOI:10.1021/ja905712k

(Chemical Equation Presented) This contribution describes the first resolution of cationic (η⁶-arene)Mn(CO)₃ complexes through addition of D-(+)-camphor enolate, isomerization of the stereogenic center, and separation and recrystalli

Full text of DOI:10.1021/ja905712k

Published on Web 09/17/2009
Resolution of Planar Chiral Cationic (η6-Arene)tricarbonylmanganese
Complexes
Antoine Eloi, Franc¸oise Rose-Munch,* and Eric Rose*
UPMC UniV Paris 06, IPCM, CNRS UMR 7201, Equipe Chimie Organique et Organome´tallique,
Case 181, 4 Place Jussieu, F-75005 Paris, France
Received July 10, 2009; E-mail: francoise.rose@upmc.fr; eric.rose@upmc.fr
Planar chiral transition-metal π complexes of ortho- and meta-
disubstituted arenes, such as (η6-arene)tricarbonylchromium com-
plexes, have been widely applied as stoichiometric auxiliaries and
suitable starting materials for asymmetric preparation of biologically
active substances as well as ligands for asymmetric catalysis.1 In
contrast, very little is known about the planar chirality of the
isoelectronic cationic (η6-arene)tricarbonylmanganese complexes,
in spite of their potential usefulness.2 Indeed, there have been no
reports of their resolution, and to date the synthesis of only two
nonracemic planar chiral cationic η6-arene-Mn complexes starting
from (p-cresol)Mn(CO)3+ has been described.3 In this context, the
discovery of a general methodology for preparing enantiomerically
pure planar chiral cationic η6-arene-Mn complexes may be deemed
as an exciting challenge. We describe herein the first strategy for
the resolution of ortho- and meta-disubstituted planar chiral racemic
η6-arene-Mn complexes using D-(+)-camphor enolate as the chiral
auxiliary.4 A potential application is illustrated by the enantiose-
lective synthesis of 4-substituted cyclohexenones.
and purification by chromatography afforded two diastereoisomers
in 44 and 48% yield with >80 and 84% de, respectively, as
1
measured by H NMR spectroscopy. After recrystallization, the
complexes (R, 2pR)-2a and (R, 2pS)-2a6 were isolated, each with
>98% de (Scheme 2).
Scheme 2. Separation of the Diastereoisomers 2a and
Rearomatization
Our approach relies on the following observation. Stabilized
carbanions add easily to the arene ring of η6-arene-Mn complexes,
giving the corresponding η5 derivatives.2 These neutral η5-
cyclohexadienyl-Mn complexes give back the starting material,
but this requires harsh conditions such as a mixture of CF3CO2H
and HPF6 used to provide the counteranion.5 To take advantage of
this drawback, we tried to find an appropriate chiral stabilized
carbanion that could be used in such an addition/elimination
(η6-η5)/(η5-η6) “round trip” sequence (Scheme 1).
Complexes (R,2pR)-2a and (R,2pS)-2a were stirred in CH2Cl2
at room temperature in the presence of AgBF4/SiMe3Cl. Under these
conditions, the presence of a silicium derivative favors enolate
-
trapping by formation of a silyl enol ether, while the BF4 anion
plays the role of the counteranion, stabilizing the cationic species
after the chiral auxiliary elimination. Thus, complexes (1pR)-1a
and (1pS)-1a were successfully isolated in 94 and 96% yield,
respectively, each with >98% ee (Scheme 2). We succeeded in
extending the present methodology to achieve the resolution of
m-chloro compound (rac)-1b, m-bromo compound (rac)-1c, and
o-trimethylsilylanisole complexes (rac)-1d, and we obtained the
corresponding complexes (1pR)-1b and (1pS)-1b, (1pR)-1c and
(1pS)-1c (Scheme 1), and (1pR)-1d and (1pS)-1d (Scheme 3) in
yields ranging from 91 to 99%, each with >98% ee, after
rearomatization of the parent enantiopure η5 complexes.
+
Scheme 1. Resolution of (η6-Arene)Mn(CO)3 Complexes Using a
Chiral Nucleophile “Round Trip”
Scheme 3. Resolution of o-Trimethylsilylanisole Complex (rac)-1d
Camphor enolate, obtained by treatment of D-(+)-camphor with
n-BuLi in THF at -78 °C, appeared to be the best chiral enolate
for efficient separation of the diastereoisomers by TLC plates. Thus,
we settled on the following protocol: a solution of D-(+)-camphor
enolate was added to complex 1a, giving a mixture of four
diastereoisomers because of the planar chirality of the Mn system
and the new C9 stereogenic center formed on the camphyl group
once linked to the Mn complex. The axial hydrogen H9 was
epimerized, giving the more thermodynamically stable complex
upon transfer of a saturated solution of K2CO3 in MeOH to the
crude mixture. On TLC plates, two spots were clearly distinguished,
The assignment of the absolute configuration of the planar chiral
η5-cyclohexadienyl moiety was possible through X-ray analysis of
suitable crystals of the diastereoisomer (R,2pR)-2a (Figure 1). On
this basis, the configuration of the corresponding cationic η6-
arene-Mn complex (1pR)-1a was deduced and then unambiguously
9
14178 J. AM. CHEM. SOC. 2009, 131, 14178–14179
10.1021/ja905712k CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
confirmed by X-ray analysis (Figure 1). Thus, the planar chirality
was preserved during the rearomatization process.
conservation of stereochemical information. This general methodol-
ogy corresponds to an addition/elimination (η6-η5)/(η5-η6) “round
trip” sequence using a cheap, commercialy available chiral nucleo-
phile. As it is compatible with the presence of a halide or an alkoxy
group, a huge field of applications involving functionalizations by
Pd cross-coupling reactions2b as well as by lithiation/electrophilic
quench2f,g,6 can be envisaged. This finding opens novel perspectives
for the development of (arene)tricarbonylmanganese complexes in
organometallic and organic enantioselective syntheses, as illustrated
in the present work by dearomatization of enantiopure meta-
disubstituted (arene)Mn(CO)3+ complexes into enantiopure 4-sub-
stituted and 3,4-disubstituted cyclohexenones.
Acknowledgment. We thank P. Herson (UMR 7201, UPMC,
Univ Paris 6) for the X-ray structure analysis, the Ecole Normale
Supe´rieure (Paris) for financial support to A.E., and CNRS for
financial support.
Figure 1. ORTEP views of complexes (R,2pR)-2a and (1pR)-1a.
We next investigated the potential of these enantiopure η6
complexes in the enantioselective synthesis of substituted cyclo-
hexenones 47 using a strategy of manganese-mediated double
nucleophilic addition (Scheme 4).2d,8 Regioselective meta addition
of LiAlH4 to the enantiopure m-chloro- and m-bromoanisole
complexes (-)-1b and (-)-1c yielded complexes (+)-3b and (+)-
3c, respectively. Dearomatization9 by addition of a second nucleo-
phile, LiC(CH3)2CN, to the C5 carbons of complexes (+)-3b and
(+)-3c, followed by FeCl3 oxidative demetalation of the anionic
η4-Mn intermediate and acidic hydrolysis, generated the corre-
sponding enantiopure 3-chloro- and 3-bromo-substituted 2-cyclo-
hexen-1-ones (+)-4b and (+)-4c in 76% yield. They could be easily
transformed by nucleophilic substitution of the halogeno groups
into a wide panel of chiral organic compounds. Treatment of (+)-
3c with 1 equiv of BuLi followed by hydrolysis gave the
enantiopure (η5-2-methoxycyclohexadienyl)Mn(CO)3 complex (-)-
3d, a key organometallic chiral synthon that cannot be obtained
through resolution of the (η6-anisole)Mn(CO)3 parent complex
because this monosubstituted complex has no planar chirality. By
the same procedure, the enantiopure 2-cyclohexen-1-one (+)-4d
was generated in 75% yield, as was the enone (+)-4e in 80% yield
by addition of LiCHPh2 as the second nucleophile (Scheme 4).
Supporting Information Available: Crystallographic data for
compounds (R, 2pR)-2a and (1pR)-1a (CIF) and full experimental
procedures with analytical data. This material is available free of charge
via the Internet at http://pubs.acs.org.
References
(1) (a) Bolm, C.; Muniz, K. Chem. Soc. ReV. 1999, 28, 51. (b) Salzer, A. Coord.
Chem. ReV. 2003, 242, 59. (c) Semmelhack, M. F.; Chlenov, A. Top.
Organomet. Chem. 2004, 7, 21. (d) McGlinchey, M. J.; Ortin, Y.; Seward,
C. M. In ComprehensiVe Organometallic Chemistry III; Crabtree, R. H.,
Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2006; Vol. 5, pp 201-289.
(e) Rosillo, M.; Dominguez, G.; Pe´rez-Castells, J. Chem. Soc. ReV. 2007,
36, 1589. (f) Astruc, D. Organometallic Complexes and Catalysis; Springer:
Berlin, 2007; pp 243-249.
(2) (a) Rose-Munch, F.; Rose, E. Eur. J. Inorg. Chem. 2002, 1269. (b) Auffrant,
A.; Prim, D.; Rose-Munch, F.; Rose, E.; Schouteeten, S.; Vaissermann, J.
Organometallics 2003, 22, 1898. (c) Prim, D.; Andrioletti, B.; Rose-Munch,
F.; Rose, E.; Couty, F. Tetrahedron 2004, 60, 3325. (d) Ku¨ndig, E. P.; Pape,
A. Top. Organomet. Chem. 2004, 7, 71. (e) Sweigart, D. A.; Reingold, J. A.;
Son, S. U. In ComprehensiVe Organometallic Chemistry III; Crabtree, R. H.,
Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2006; Vol. 5, Chapter 10,
pp 761-814. (f) Jacques, B.; Chavarot, M.; Rose-Munch, F.; Rose, E. Angew.
Chem., Int. Ed. 2006, 45, 3481. (g) Jacques, B.; Chanaewa, A.; Chavarot-
Kerlidou, M.; Rose-Munch, F.; Rose, E.; Ge´rard, H. Organometallics 2008,
27, 626.
(3) Son, S. U.; Park, K. H.; Lee, S. J.; Seo, H.; Chung, Y. K. Chem. Commun.
2002, 1230.
(4) The camphyl group has been shown to provide an interesting asymmetric
environment in Cr complexes. See: Paramahamsan, H.; Pearson, A. J.;
Pinkerton, A. A.; Zhurova, E. A. Organometallics 2008, 27, 900.
(5) Balssa, F.; Gagliardini, V.; Rose-Munch, F.; Rose, E. Organometallics 1996,
15, 4373.
Scheme 4. Preparation of Enantiopure 2-Cyclohexen-1-ones
(6) R is the absolute configuration of the C9 carbon atom of the camphyl group.
For the stereochemical description of the η5 complexes, see: Jacques, B.;
Eloi, A.; Chavarot-Kerlidou, M.; Rose-Munch, F.; Rose, E.; Ge´rard, H.;
Herson, P. Organometallics 2008, 27, 2505.
(7) Enantiopure 4-substituted cyclohexenones are useful building blocks for a
wide variety of natural products and for a number of synthetic transforma-
tions. See, for example: Evarts, J.; Torres, E.; Fuchs, P. L. J. Am. Chem.
Soc. 2002, 124, 11093.
(8) Roell, B. C.; McDaniel, K. F.; Vaughan, W. S.; Macy, T. S. Organometallics
1993, 12, 224.
(9) (a) Semmelhack, M. F.; Yamashita, A. J. Am. Chem. Soc. 1980, 102, 5924.
(b) Miles, W. H.; Brinkman, H. R. Tetrahedron Lett. 1992, 33, 589. (c)
Lee, T.-Y.; Kang, Y. K.; Chung, Y. K.; Pike, R. D.; Sweigart, D. A. Inorg.
Chim. Acta 1993, 214, 125. (d) Pearson, A. J.; Vickerman, R. J. Tetrahedron
Lett. 1998, 39, 5931. (e) Dudones, J. D.; Pearson, A. J. Tetrahedron Lett.
2000, 41, 8037. (f) Pape, A. R.; Kaliappan, K. P.; Ku¨ndig, E. P. Chem. ReV.
2000, 100, 2917.
In conclusion, our preliminary results describe the first resolution
of cationic ortho- and meta-disubstituted (η6-arene)Mn(CO)3 com-
plexes through D-(+)-camphor enolate addition, isomerization of
the C9 stereogenic center, separation of the corresponding η5-
cyclohexadienyl diastereoisomers, and then elimination of the chiral
auxiliary by a quantitative method of rearomatization with total
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