Chiral bis(oxazolinyl)phenyl RuII catalysts for highly enantioselective cyclopropanation

Article abstract of DOI:10.1002/chem.200903514

chemical equation presantation Good to excellent enantioselectivities (up to 99 % ee) of trans-cyclopropane derivatives were achieved in the cyclopropanation of alkenes with tert-buty1 α-diazoacetate by using 0.5 mol % of a mononuclear chiral ruthenium aqua complex containing the chiral bis(oxazolinyl)phenyl ligand, which was obtained via C-H activation by RuCl ₃·3H₂O in the presence of Mg and cyclooctadiene (cod). Intramolecular cyclopropanation reactions using the new Ru complex also proceed with high yields and excellent enantioselectivities.

Full text of DOI:10.1002/chem.200903514

DOI: 10.1002/chem.200903514
Chiral Bis(oxazolinyl)phenyl RuII Catalysts for Highly Enantioselective
Cyclopropanation
Jun-ichi Ito, Satoshi Ujiie, and Hisao Nishiyama*[a]
Transition-metal complexes with monoanionic meridional
ligands have been widely utilized in stoichiometric and cata-
À
À
À
lytic reactions, such as C H, C C, and C O bond activation
and alkane dehydrogenation, because of their unique reac-
tivity.[1] Novel PCP and NCN pincer ligands have recently
made it possible to design new pincer-type metal complexes
using a combination of two-electron donors and an anionic
center. In this context, introduction of a chiral moiety as
part of the ligand framework to create transition-metal cata-
lysts for asymmetric reactions has been investigated. Re-
cently, several highly reactive Ru and Os complexes with
chiral CNN-type ligands[2] and Fe and Co complexes with
chiral bis(pyridylimino)isoindole ligands[3] were reported as
active chiral catalysts.
During our investigation of transition-metal catalysts with
chiral meridional NCN ligands, we reported the bis(oxazoli-
nyl)phenyl (phebox) ligand as one of a larger family of li-
gands including bis(oxazolinyl)pyridine (pybox).[4] Although
the meridional geometry of the phebox ligand is similar to
that of the pybox ligand, the metal complex with the phebox
ligand possesses a metal–carbon s bond that may induce dif-
ferent reactivities. We anticipated that the simple change
from a benzene substituent to a pyridine ring as the ligand
backbone would lead to a change in the catalytic perfor-
mance of the complex. Indeed, phebox–Rh catalysts were
found to catalyze 1,4-conjugated reductions of a,b-unsatu-
rated carbonyls and related reductive aldol coupling reac-
tions with high selectivity.[4] As part of our effort to develop
Ru catalysts, we recently synthesized new phebox–Ru dinu-
clear complexes 1 and 2, which have been successfully ap-
plied as catalysts for enantioselective hydrogenation and
transfer hydrogenation of aromatic ketones.[5]
It is known that Ru complexes, as well as Cu, Co, and Rh
complexes, have the potential to catalyze the cyclopropana-
tion of alkenes with a-diazoacetate derivatives.[6–10] Al-
though much effort has been devoted to developing highly
enantioselective cyclopropanation reactions, the discovery of
catalytic transformations using a structurally well-defined
catalyst under mild conditions remains an important syn-
thetic target. We previously reported that pybox–Ru com-
plexes are highly active catalysts for asymmetric cyclopropa-
nations.[10] The phebox–Ru complex with an anionic meri-
dional ligand was expected to provide a suitable environ-
ment for the control of the selectivity of the reaction. Re-
cently Iwasa and co-workers reported an enantioselective
cyclopropanation reaction catalyzed by a hypothetical, in
situ generated phebox–Ru complex. Isolation and character-
ization of the Ru complex has not been achieved yet, how-
ever.[11] Here we report a new mononuclear phebox–Ru
aqua complex that catalyzes inter- and intramolecular cyclo-
propanation reactions with enantioselectivities as high as
99% ee.
The binuclear phebox–Ru complexes [(Phebox)Ru(CO)]2-
AHCTUNGRTEG(NNUN ZnCl4) (1, 2), in which two phebox–Ru units were bridged
by a ZnCl4 moiety, were synthesized by reaction of phebox–
H (3) with RuCl3·3H2O in the presence of zinc powder.[5a]
In this reaction, zinc is likely acting as a reducing agent as
well as the source of the bridging fragment. Following this
result, we attempted to prepare other types of Ru com-
plexes containing the phebox ligand. The reaction of the
ligand precursor 3 or 4 with RuCl3·3H2O in EtOH in the
presence of Mg instead of Zn successfully afforded new
[a] Assist. Prof. J.-i. Ito, S. Ujiie, Prof. H. Nishiyama
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University, Chikusa, Nagoya, 464-8603 (Japan)
Fax : (+81)52-789-3209
E-mail: hnishi@apchem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903514.
4986
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Chem. Eur. J. 2010, 16, 4986 – 4990

COMMUNICATION
mononuclear phebox–Ru complexes, [(Phebox)RuCl(CO)-
ACHTUNGTRENNUNG(H2O)] (5: R=H, 6: R=Ph) [Eq. (1)]. Complexes 5 and 6
were isolated by column chromatography on silica gel in 33
and 72% yields, respectively.
shows that the CO and Cl ligands are bonded at axial posi-
tions and the H2O ligand occupies the equatorial position.
Next we checked the reactivity of 6 to obtain information
about its potential catalytic activity. Unfortunately, the reac-
tion of 6 with diazoacetate derivatives afforded complex
mixtures. On the other hand, the reaction of 6 with tert-bu-
tylisocyanide in toluene at room temperature resulted in the
formation of the corresponding isocyanide complex 7 in
78% yield [Eq. (2)]. The 1H NMR spectrum revealed the
signal for the tBu group in the isocyanide ligand at d=
0.79 ppm. In the IR spectrum, the peaks for the C=N and
C=O absorptions were observed at 2154 and 1905 cmÀ1, re-
spectively. The molecular structure of 7 was confirmed by
X-ray analysis (Figure 2).[12] The Ru1 C1 bond length of
À
The new phebox–Ru complexes 5 and 6 were identified
1
on the basis of the H and 13C NMR spectra, in which two
signals for the methyl groups on the benzene ring were ob-
served. This spectral feature indicates that 5 and 6 adopt C1
symmetry. The IR spectra of 5 and 6 exhibited the charac-
teristic absorption for the C=O group at 1927 and
1920 cmÀ1, respectively. The C=O absorption is slightly shift-
ed to a lower wavenumber compared to those measured for
complexes 1 and 2 (1932–1937 cmÀ1).[5a] We presumed that
the source of the CO ligand could be EtOH, which is oxi-
dized to aldehyde followed by decarbonylation.[5] Finally,
the molecular structure of 6 was unambiguously determined
by X-ray analysis (Figure 1).[12] The phebox ligand is meri-
Figure 2. Molecular structure of 7 (ORTEP diagram). Selected bond
À
À
À
lengths [ꢁ]: Ru1 C1 2.015(3), Ru1 C39 2.058(3), Ru1 N1 2.106(2),
À
dionally coordinated to the Ru center. The Ru C1 bond
À
Ru1 N2 2.095(2).
length of 1.953(5) ꢁ is consistent with those of other
phebox–Ru complexes.[5a] On the Ru atom, there are three
different ligands, namely, Cl, CO, and H2O. Figure 1 clearly
2.015(3) ꢁ was slightly longer than that measured for the
similar bond in 6. The isocyanide ligand was coordinated to
Ru in the equatorial position, while the Cl and CO ligands
were coordinated in the same positions as those in 6. The
structural features of 7 suggest that the phebox–Ru complex
6 readily accepts a two-electron donor ligand at the equato-
rial site.
Figure 1. Molecular structure of 6 (ORTEP diagram). Selected bond
À
À
À
lengths [ꢁ]: Ru1 C1 1.953(5), Ru1 C39 1.973(5), Ru1 Cl1 2.3986(14),
À
À
À
Ru1 N1 2.094(4), Ru1 N2 2.102(4), Ru1 O4 2.288(4).
Chem. Eur. J. 2010, 16, 4986 – 4990
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4987

H. Nishiyama et al.
The cyclopropanation reaction of styrene 8a with tert-
butyl a-diazoacetate 9 using a catalytic amount of phebox–
Ru complex was examined (Table 1). Initially we investigat-
in high yields of 85% with excellent enantioselectivities of
98–99% ee (Table 2, entries 1–7). A similar result was ob-
tained for vinylnaphthalene 8i (Table 2, entry 8). Use of
Table 1. Enantioselective cyclopropanation of styrene with N2CHCO2-
Table 2. Enantioselective cyclopropanation of alkenes with N2CHCO2-
ACHTUNGTRENNUNG
(tBu).[a]
AHCTUNGTRENNUNG
(tBu) catalyzed by 6.[a]
Entry
Cat.
Temp
[8C]
Yield
[%][b]
trans:cis[c]
ee [%][d]
of trans
Entry
Alkene
Yield
[%][b]
trans:cis[c]
ee [%][d]
of trans
1
2
3
4
1
2
5
6
6
6
0
0
0
0
0
25
60
62
83
90
83
73
88:12
95:5
95:5
92:8
93:7
88:12
98
94
1
2
3
4
5
6
8b
8c
8d
8e
8 f
8g
87
88
92
89
85
87
92:8
94:6
95:5
96:4
96:4
95:5
99
99
99
98
99
98
98
98[e]
99
5[f]
6
90
[a] Styrene 8a (2.0 mmol), diazoacetate 9 (1.0 mmol), catalyst [2.5 mmol
(1 and 2), 5.0 mmol (5 and 6)], solvent: CH2Cl2, 2 h for addition of diazo-
acetate then stirring for 1 h. [b] Isolated yield. [c] Determined by
1H NMR spectroscopy. [d] Determined by HPLC using Daicel CHIRAL-
CELL OJ-H (hexane). [e] Absolute configuration (1R,2R) was deter-
mined by comparison with the reported specific rotation. [f] 8a
(1.0 mmol), 9 (1.0 mmol), 6 (5.0 mmol).
7
8h
86
92:8
98
8
8i
8j
88
91
93:7
99
98
ed the use of dinuclear complex 1 as a catalyst precursor
(0.5 mol% Ru), which was anticipated to generate a coordi-
natively unsaturated intermediate via elimination of zinc
chloride. Slow addition of a solution of 9 in CH2Cl2 to a so-
lution of styrene 8a in the presence of 1 at 08C afforded a
mixture of 10a and 11a in 60% yield with excellent enantio-
selectivity of 98% ee (Table 1, entry 1). As found for the
pybox–Ru catalyst,[10] trans-cyclopropane 10a with the same
absolute configuration (1R,2R) was the predominant diaste-
reomer. The diastereoselectivity of the trans and cis isomers
was improved by the use of 2, which possesses a phenyl
group at the 4-position of the oxazoline rings instead of an
isopropyl group as in 1 (Table 1, entry 2). Only moderate
yields of cyclopropane were obtained with complexes 1 and
2. As expected, the use of 5 or 6 improved the yields (83%
for 5, 90% for 6) with good trans-selectivity and excellent
enantioselectivity (up to 98% ee, Table 1, entries 3 and 4).
Although cyclopropanation reactions often suffer from com-
petitive formation of fumaric and maleic esters via diazo
coupling if an excess amount of alkene is not used, the reac-
tion of one equivalent of 8a with 9 proceeded to give the cy-
clopropane in 83% yield with an excellent ee of 99%
(Table 1, entry 5). Increasing the temperature to 258C, how-
ever, led to a decrease in both the yield and enantioselectiv-
ity (Table 1, entry 6).
9[e]
82:18
10[f]
11[g]
8k
8l
80
59
35:65
92:8
93
87
[a] Alkene 8 (2.0 mmol), 9 (1.0 mmol), 6 (5.0 mmol), solvent: CH2Cl2,
08C, 2 h for addition of diazoacetate then stirring for 1 h. [b] Combined
yield of trans and cis isomer. [c] Determined by 1H NMR spectroscopy.
[d] Determined by HPLC. [e] tert-Butyl 2-styrylcyclopropane-1-carboxyl-
ate (10j) was formed. [f] 98% ee for the cis isomer 11k. [g] 8l
(5.0 mmol), 9 (1.0 mmol), 6 (0.020 mmol).
phenylbutadiene 8j proceeded in high yield with excellent
enantioselectivity, but only moderate diastereoselectivity
(Table 2, entry 9). In the case of a-methylstyrene 8k, the cis
isomer was the predominant product, with a trans:cis ratio
of 35:65 (Table 2, entry 10). The catalyst 6 was also utilized
for the cyclopropanation of non-activated alkene 4-phenyl-
1-butene (8l) to give the desired product with good enantio-
selectivity (Table 2, entry 11). Catalysis of the cyclopropana-
tion of trans-b-methylstyrene and indene with the phebox–
Ru catalyst, however, did not occur.
In addition to intermolecular cyclopropanations, the
phebox–Ru complexes are suitable catalysts for intramolec-
ular reactions.[13] Cinnamyl diazoacetate 12, in the presence
of 1 mol% of 6 or 0.5 mol% of 2, resulted in the clean for-
mation of the corresponding bicyclic compound 13 in high
yield with excellent enantioselectivity of 99% ee
(Scheme 1).
Cyclopropanation reactions of various styrene derivatives
8b–h containing electron-donating and -withdrawing groups
with a-diazo acetate 9 were then performed in the presence
of 0.5 mol% of 6 at 08C. The catalytic reaction proceeded
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Chem. Eur. J. 2010, 16, 4986 – 4990

Chiral Bis(oxazolinyl)phenyl RuII Catalysts for Enantioselective Cyclopropanation
COMMUNICATION
added
a solution of tert-butyl diazoacetate 9 (142 mg, 1.0 mmol) in
CH2Cl2 (0.5 mL) at 08C over the course of 2 h by using a syringe pump.
After completion of the addition, the mixture was stirred at 08C for an
additional 1 h. The mixture was then concentrated under reduced pres-
sure. The ratio of the trans and cis isomers of the crude product was de-
termined by 1H NMR spectroscopy (trans:cis=92:8). The crude product
was purified by silica gel column chromatography with hexane/ethyl ace-
tate (50:1) as eluent to give the trans-cyclopropane 10a (182 mg) and the
corresponding cis-cyclopropane 11a (15 mg) as colorless oils. Combined
yield (197 mg, 0.90 mmol, 90%). The enantiomeric purities of the cyclo-
propanes 10a and 11a were determined by HPLC. For 10a: [a]D22
=
À257.9 (c=1.06 in CHCl3) [Lit.[7e] 98% ee (1R,2R)-isomer, [a]2D0 =À247.6
(c=0.20 in CHCl3)]; 1H NMR (300 MHz, CDCl3, rt): d =1.21–1.28 (m,
1H), 1.48 (s, 9H), 1.51–1.58 (m, 1H), 1.82–1.88 (m, 1H), 2.41–2.48 (m,
1H), 7.08–7.11 (m, 2H), 7.16–7.22 (m, 1H), 7.24–7.31 ppm (m, 2H);
13C NMR (75 MHz, CDCl3, rt): d =17.2, 25.4, 25.9, 28.3, 80.5, 125.8,
126.1, 128.2, 140.2, 172.2 ppm; chiral HPLC (Daicel CHIRALCEL OJ-H,
hexane, 0.7 mLminÀ1) showed 98% ee (tmajor =14.8 min, tminor =17.8 min).
For 11a: [a]2D2 =À19.6 (c=0.60, CHCl3) [Lit.[7e] 98% ee (1R,2S)-isomer,
[a]2D0 =À18.4 (c=0.34, CHCl3)]; 1H NMR (300 MHz, CDCl3, rt): d=1.14
(s, 9H), 1.21–1.28 (m, 1H), 1.62–1.69 (m, 1H), 1.95–2.03 (m, 1H), 2.49–
2.58 (m, 1H), 7.15–7.30 ppm (m, 5H); chiral HPLC (Daicel CHIRAL-
CEL OJ-H x 2, hexane, 0.5 mLminÀ1) showed 98% ee (tmajor =44.8 min,
Scheme 1. Intramolecular cyclopropanation of 12.
A possible explanation of the selectivity achieved with the
phebox–Ru complex is shown in Scheme 2. Replacement of
the H2O ligand with the diazoacetate group results in the
formation of the corresponding Ru carbene intermediate
t
minor =41.3 min).
Acknowledgements
Scheme 2. Proposed transition-state model for cyclopropanation.
This research was partly supported by a Grant-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science, and
Technology, Japan (Concerto Catalysis; 460:18065011) and the Japan So-
ciety for the Promotion of Science (Nos. 18350049, 20750073).
A.[14] This hypothesis is based on the behavior of similar
pybox–Ru catalysts, for which a carbene complex was de-
tected and isolated.[10] In the case of the pybox–Ru system,
such a carbene intermediate adopts a C2-symmetric struc-
ture, which provides a suitable environment for asymmetric
reactions. Although the phebox–Ru system can be described
as having C1 symmetry due to the coordination of the Cl
and CO ligands at the apical positions, it is possible that a
pseudo C2-symmetric environment could be involved when
the carbene group is coordinated in the equatorial position.
Considering the absolute configuration of the cyclopropane
10a, we conclude that the styrene attacks the Re face of the
carbenoid to minimize the steric repulsion between the tert-
butyl group of the diazo compounds and the phenyl group
of the styrene, resulting in predominant formation of the
trans isomer with the (1R,2R) configuration.
Keywords: alkenes
cyclopropanation · ruthenium · tridentate ligands
·
asymmetric
catalysis
·
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0.0050 mmol) and styrene 8a (208 mg, 2.0 mmol) in CH2Cl2 (1.5 mL) was
Chem. Eur. J. 2010, 16, 4986 – 4990
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www.chemeurj.org
4989

H. Nishiyama et al.
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Received: December 22, 2009
Published online: March 25, 2010
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Chem. Eur. J. 2010, 16, 4986 – 4990