Kinetic resolution of planar-chiral (η6-arene)chromium complexes by molybdenum-catalyzed asymmetric ring-closing metathesis

Article abstract of DOI:10.1002/anie.201108292

Molybdenum-catalyzed asymmetric ring-closing metathesis has been used for the kinetic resolution of racemic planar-chiral (η⁶-arene) chromium complexes with excellent enantioselectivity. The products are excellent precursors for the synthesis of various planar-chiral (η⁶-arene) chromium derivatives and can also be applied as chiral ligands in rhodium-catalyzed asymmetric reactions. Copyright

Full text of DOI:10.1002/anie.201108292

Angewandte
Chemie
DOI: 10.1002/anie.201108292
Asymmetric Synthesis
Kinetic Resolution of Planar-Chiral (h6-Arene)Chromium Complexes
by Molybdenum-Catalyzed Asymmetric Ring-Closing Metathesis**
Masamichi Ogasawara,* Wei-Yi Wu, Sachie Arae, Susumu Watanabe, Tomotaka Morita,
Tamotsu Takahashi,* and Ken Kamikawa*
Planar-chiral (h6-arene)chromium complexes are useful chiral
scaffolds in asymmetric synthesis, and have found widespread
application as chiral ligands for asymmetric catalysis, or as
chiral building blocks for natural product syntheses.[1] Typical
methods for the preparation of enantiomerically enriched
planar-chiral (arene)chromium species are based either on
the optical resolution of racemates[2] or on stereoselective
transformations, which include diastereoselective complex-
ation,[3] diastereo- or enantioselective ortho-lithiation by
utilizing a chiral directing group or a chiral base,[4] and
diastereo- or enantioselective nucleophilic addition/hydride
abstraction.[5] Whereas these methods require a stoichiometric
amount of chiral reagents or auxiliaries, asymmetric catalysis
is an attractive and effective alternative for preparing
optically active (h6-arene)chromium complexes. Since the
first report on such a catalytic process by Uemura et al. in
1993,[6] only a handful of examples of the desymmetrization of
prochiral (arene)chromium substrates have been reported by
Gotov and Schmalz,[7] Kꢀndig and co-workers,[8,9] and
phinoarene)chromium species was derived from the (h6-
bromoarene)chromium complex and applied as a chiral
ligand in Rh-catalyzed asymmetric reactions, which achieved
excellent enantioselectivity of up to 99.5%.
The (h6-arene)chromium complexes (rac-1) that were
used in this study contain an h6-(2-substituted alkenylben-
zene) ligand, which constructs the planar-chiral environment
upon coordination to the chromium atom, and an alkenyl-
phosphine ligand. The chiral catalysts were screened by using
racemic
[(h6-2-methylstyrene)Cr(CO)2(methallyldiphenyl-
phosphine)] (1a) as a prototypical substrate. The asymmetric
reactions were carried out in benzene at 408C in the presence
of an appropriate chiral Mo catalyst (10 mol%), which was
=
generated in situ from the Mo precursor [(pyrrolyl)2Mo(
=
CHCMe2Ph)( NC6H3-2,6-iPr2)] and an axially chiral biphe-
nol derivative (Table 1).[16] Under these conditions, the
Mo catalyst that was generated with (R)-L1[17a] gave the
ARCM product 2a in 32% yield with an ee value of 79%, and
unreacted 1a was recovered in 66% yield with an ee value of
50% (entry 1). The krel value ([reaction rate of the fast-
reacting enantiomer]/[reaction rate of the slow-reacting
enantiomer]; selectivity factor) for this reaction is estimated
to be 14.[18] The Mo catalyst that was generated with (R)-L2
gives better enantioselectivity and the krel value improved to
41 (entry 2). Lowering the temperature worsened the selec-
tivity; the krel value was 12 at 238C (entry 3). It was found that
the reaction with the Mo catalyst coordinated with (S)-L3[17b]
shows excellent enantioselectivity; 2a was obtained in 96% ee
and 44% yield, and 1a was recovered in 88% ee and 50%
yield. The krel value for the reaction is 114 (entry 4), and
a nearly perfect kinetic resolution of the two enantiomers of
rac-1a was achieved. Considering the structural similarity
between L1, L2, and L3, these results are quite surprising and
indicate that the slight structural modification to the Mo ca-
talysts affects the enantioselectivity of the asymmetric
reaction.[17a]
Uemura and co-workers[10,11]
.
Recently, we reported the preparation of phosphine-
chelate (h6-arene)chromium complexes by Ru-catalyzed ring-
closing metathesis.[12] We also demonstrated that Mo-cata-
lyzed asymmetric ring-closing metathesis (ARCM) was highly
effective for the asymmetric synthesis of the various planar-
chiral ferrocenes.[13] Thus, we are interested in controlling the
planar chirality of (h6-arene)chromium complexes by
ARCM.[14,15] Indeed, the kinetic resolution of racemic (h6-
1,2-disubstituted benzene)chromium complexes proceeds
efficiently in the presence of a chiral Mo-alkylidene species
to give the planar-chiral chromium complexes with excellent
enantiomeric purity. Furthermore, a highly enantiomerically
enriched (h6-bromoarene)chromium complex that was pre-
pared by using the present method is an excellent precursor to
various planar-chiral (arene)chromium derivatives. A (phos-
After the optimization studies, the Mo/(S)-L3 species was
applied to the other substrates. The substrate rac-1b, which
has an ethyl substituent in place of the ortho-methyl group in
1a, was also resolved as above. The enantioselectivity and the
reaction efficiency (krel = 75 (entry 5)) of the kinetic resolu-
tion of rac-1b were still high. Substrate 1c contains an h6-
bromostyrene ligand. As the bromo substituents in 1c/2c can
be easily replaced by other functional groups by standard
organic transformations (see below),[19] they serve as versatile
precursors to various planar-chiral compounds. The reaction
of rac-1c under the optimized conditions afforded complex 2c
in 97% ee and 47% yield. Unreacted 1c was also recovered in
89% ee and 50% yield. The krel value for this reaction is 198
[*] Prof. M. Ogasawara, S. Arae, Dr. S. Watanabe, Prof. T. Takahashi
Catalysis Research Center and Graduate School of Life Science
Hokkaido University, Kita-ku, Sapporo 001-0021 (Japan)
E-mail: ogasawar@cat.hokudai.ac.jp
Dr. W.-Y. Wu, T. Morita, Prof. K. Kamikawa
Department of Chemistry, Graduate School of Science
Osaka Prefecture University, Sakai, Osaka 599-8531 (Japan)
E-mail: kamikawa@c.s.osakafu-u.ac.jp
[**] This work was supported by the Cooperative Research Program of
the Catalysis Research Center, Hokkaido University (Grant no.
10A0002 and no. 11A0003) and a Grant-in-Aid for Scientific
Research on Innovative Areas to M.O. and K.K. from MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108292.
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Table 1: Mo-catalyzed ARCM kinetic resolution of planar-chiral (h6-arene)chromium complexes.[a]
Temp. [8C][b]
Time [h]
ee (yield) of
ee (yield) of 2 [%][d,e]
Config./optical
rotation sign of 2
krel
[f]
Entry
1
L
recovered 1 [%][c,d,e]
1
2
3
4
1a
1a
1a
1a
1b
1c
1d
1e
1 f
(R)-L1
(R)-L2
(R)-L2
(S)-L3
(S)-L3
(S)-L3
(R)-L3
(R)-L3
(R)-L3
40
40
23
40
40
40
40
23
40
12
12
12
18
12
12
8
50 (66)
72 (48)
48 (61)
88 (50)
63 (55)
89 (50)
98 (42)
3 (69)
79 (32)
90 (42)
76 (30)
96 (44)
95 (42)
97 (47)
90 (49)
14 (14)
45 (34)
(R)-(ꢀ)
(R)-(ꢀ)
(R)-(ꢀ)
(S)-(+)
(S)-(ꢀ)
(S)-(ꢀ)
(ꢀ)
14
41
12
114
75
198
87
1.4
3.0
5[g]
6
7
8[h]
9
1
12
(+)
12 (55)
(ꢀ)
[a] The reaction was carried out in benzene in the presence of an appropriate metathesis catalyst that was generated in situ (10 mol%) unless
otherwise noted. [b] Oil bath temperature. [c] The ee value of recovered 1 was determined after conversion into 2 by treatment with the 2nd-generation
Grubbs’ catalyst. [d] The ee value was determined by HPLC on a chiral stationary phase (see the Supporting Information for details). [e] Yield of
isolated product is given after purification by chromatography on a silica gel column. [f] Calculated based on a first-order equation (Ref. [18]). [g] With
20 mol% catalyst loading. [h] With 5 mol% catalyst loading.
(entry 6). The kinetic resolution of substrate 1d, which
contains a diisopropylmethallylphosphine, catalyzed by Mo/
(R)-L3 is also highly efficient and shows a krel value of 87.
Both the recovered substrate (1d) and the ARCM product
(2d) were obtained in greater than 90% ee (entry 7). The
enantioselectivity of the ARCM kinetic resolution was
strongly dependent on the structures of the alkenyl groups
in both the h6-arene ligand and the phosphine ligand
(entries 8 and 9).[13a] Introduction of a Ph2P(allyl) ligand in
place of the Ph2P(methallyl) ligand in rac-1e diminishes the
enantioselectivity of the asymmetric reaction. With the two
monosubstituted olefin moieties in the substrate, the ARCM
of 1e was extremely rapid and was complete within 1 h at
408C. For this reason, the kinetic resolution of rac-1e was
conducted at 238C for 1 h with a lower loading of the catalyst.
The reaction with Mo/(R)-L3 gave the ARCM product 2e in
14% ee and the ee value for recovered 1e was 3%. The
selectivity factor for the reaction is only 1.4 (entry 8).
Likewise, the selectivity of the kinetic resolution reaction
was decreased when the vinyl group in the h6-arene ligand was
replaced by an allyl group, as in 1 f (entry 9).
25
The absolute configuration of (ꢀ)-2c (½aꢁD ¼ꢀ61.4 (c =
0.40 in EtOAc for 97% ee)), which is the enantiomer that is
preferentially cyclized by Mo/(S)-L3, was determined as the
S configuration by single-crystal X-ray crystallography (see
the Supporting Information for details).[20] The absolute
configurations of (+)-2a and (ꢀ)-2b were also determined
as the S configurations by stereoretentive derivatization from
(S)-(ꢀ)-2c (see Scheme 2). The configurations of the other
ARCM products were assigned by analogy.
By the combination of the two successive ARCM
operations, (R)-(+)-2c could be obtained in almost enantio-
merically pure form. After the first kinetic resolution of rac-
1c catalyzed by Mo/(S)-L3 (Table 1, entry 6), the recovered
substrate, (R)-1c with an ee value of 89%, was subjected to
a second ARCM kinetic resolution with Mo/(R)-L3 to give
(R)-(+)-2c in greater than 99% ee (Scheme 1).
Enantiomerically pure (S)-2c was also obtained by the
recrystallization of (S)-2c (97% ee), which was produced by
Mo-catalyzed ARCM, from hexane/EtOAc. The bromo
substituent in 2c is lithiated under standard conditions, and
subsequent reactions with an appropriate electrophile give
the corresponding planar-chiral (arene)chromium complexes
in good yields with complete retention of optical purity in (S)-
2c (Scheme 2). The absolute configurations of the planar-
Scheme 1. Two successive ARCM kinetic resolutions of 1c give
(R)-(+)-2c in over 99% ee.
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phenylboroxine in the presence of Rh/(R)-4 gave the
asymmetric addition product in 94% yield with 88% ee
(Scheme 3, bottom). Applications of (R)-4 in other reactions
that are catalyzed by transition metals are now under
investigation and will be reported in due course.[24,25]
In summary, we have developed an effective method for
the kinetic resolution of racemic planar-chiral (h6-arene)-
chromium complexes by Mo-catalyzed asymmetric ring-
closing metathesis. After the appropriate derivatization, the
obtained optically active (diphenylphosphinoarene)chro-
mium complex can be utilized as an excellent chiral ligand
in Rh-catalyzed asymmetric reactions.
Experimental Section
General procedure for the Mo-catalyzed ARCM kinetic resolution of
=
=
1: [(pyrrolyl)2Mo( CHCMe2Ph)( NC6H3-2,6-iPr2)] (5.4 mg, 10 mmol)
and 3,3’-tBu2-5,5’,6,6’-tetramethyl-2,2’-biphenol (L3, 3.5 mg, 10 mmol)
were dissolved in dry benzene (1 mL) in a test tube with a Teflon-
sealed screw cap in a glovebox under prepurified argon. After stirring
the mixture for 15 min at room temperature, benzene (3 mL) and the
(h6-arene)chromium complex 1 (100 mmol) were added. The test tube
was sealed tightly and taken out of the glovebox. The test tube was
immersed in an oil bath that was maintained at 408C, and the mixture
was stirred for 12 h. After quenching the reaction by the addition of
acetone (ca. 100 mL), the reaction mixture was evaporated to dryness
under reduced pressure. The crude product was purified by chroma-
tography on a silica gel (hexane/benzene = 1:2) under nitrogen to give
the ARCM product 2 and the recovered substrate 1. Recovered 1 was
converted into the corresponding 2 by treatment with the second-
generation Grubbsꢁ catalyst (2 mol%) in dichloromethane for
analysis by HPLC on a chiral stationary phase. The characterization
data of the ARCM products and the conditions for the HPLC analysis
are described in the Supporting Information.
Scheme 2. Stereoretentive conversion of (S)-(ꢀ)-2c into various
planar-chiral (h6-arene)chromium complexes.
chiral chromium complexes are retained during the lithiation/
substitution sequence.[19] The levorotatory enantiomer (S)-
(ꢀ)-2c affords (+)-2a and (ꢀ)-2b, after the lithiation/
alkylation sequence, respectively. With these analyses, both
(+)-2a and (ꢀ)-2b, which are the enantiomers that are
preferentially cyclized in the Mo/(S)-L3-catalyzed ARCM,
were identified as the S enantiomers. In the same way, the h6-
(diphenylphosphino)arene species (R)-(ꢀ)-4 and the h6-
benzoate derivative (R)-(ꢀ)-5 were obtained from (S)-2c in
optically pure forms (Scheme 2). The structure of complex 4
was confirmed by X-ray crystal-structure analysis (see the
Supporting Information for details).[20]
Received: November 24, 2011
Revised: January 2, 2012
Published online: February 3, 2012
The synthetic utility of (R)-(ꢀ)-4 as a chiral ligand[21] was
examined in two Rh(I)-catalyzed asymmetric reactions. It was
found that (R)-(ꢀ)-4 is an extremely effective chiral ligand in
the asymmetric 1,4-addition of phenylboronic acid to cyclo-
hexenone,[22] and (R)-3-phenylcyclohexanone was obtained
with a 99.5% ee in 98% yield (Scheme 3, top). The usefulness
of (R)-(ꢀ)-4 was also demonstrated in the Rh-catalyzed
addition of arylboroxine to an N-tosyl aldimine.[23] The
reaction of the N-tosyl imine of p-chlorobenzaldehyde with
Keywords: asymmetric synthesis · chirality · chromium ·
kinetic resolution · metathesis
.
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Chemie
[25] The complex 1c, which is a precursor to 4, is prepared from [(h6–
2-bromobenzaldehyde)Cr(CO)3] (SM2c; see the Supporting
Information). The use of enantiomerically pure SM2c, which
can be prepared as reported, allows direct access to enantio-
merically pure 4 on a practical scale without using the ARCM
protocol. Details of this alternative synthetic route to (S)- or (R)-
4 as well as further application of 4 in asymmetric reactions will
be reported in due course. For the preparation of enantiomer-
ically pure SM2c, see: a) Ref. [4e]; b) N. Taniguchi, M. Uemura,
Tetrahedron 1998, 54, 12775.
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