Synthesis of Forms of a Chiral Ruthenium Complex Containing a Ru-Colefin(sp2) Bond and Their Application to Catalytic Asymmetric Cyclopropanation Reactions

Article abstract of DOI:10.1021/acs.orglett.0c00050

Organometallic complexes [Ru-C_olefin(sp²)-Ru(II)-Pheox 2a-2d] containing a Ru-C_olefin(sp²) bond have been prepared from unsaturated chiral oxazoline derivatives and evaluated for asymmetric cyclopropanation reactions. The corresponding optically active cyclopropanes were obtained with high yields and high stereoselectivities (≥99/<1 trans/cis, 99% trans ee). The enantioselectivities were found to be affected by the geminal substituent on the Ru-C(sp²) bond. In particular, Ru(II)-Prox catalyst 2c, in which there was no geminal substituent on the metal, was shown to have the highest enantioselectivities.

Full text of DOI:10.1021/acs.orglett.0c00050

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Letter
Synthesis of Forms of a Chiral Ruthenium Complex Containing a
2
Ru−C
(sp ) Bond and Their Application to Catalytic Asymmetric
olefin
Cyclopropanation Reactions
Hayato Inoue, Nga Phan Thi Thanh, Ikuhide Fujisawa, and Seiji Iwasa*
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sı Supporting Information
2
ABSTRACT: Organometallic complexes [Ru−C (sp )−Ru(II)−
olefin
2
Pheox 2a−2d] containing a Ru−C (sp ) bond have been prepared
olefin
from unsaturated chiral oxazoline derivatives and evaluated for
asymmetric cyclopropanation reactions. The corresponding optically
active cyclopropanes were obtained with high yields and high
stereoselectivities (≥99/<1 trans/cis, 99% trans ee). The enantiose-
lectivities were found to be affected by the geminal substituent on the
2
Ru−C(sp ) bond. In particular, Ru(II)−Prox catalyst 2c, in which
there was no geminal substituent on the metal, was shown to have the
highest enantioselectivities.
omplexes containing a carbon−metal (C−M) bond,
Recently, catalytic asymmetric synthesis of multifunctionalized
cyclopropanes from disubstituted diazo compounds and
C
known as pincer−metal complexes, have received
4
considerable attention because of their high reactivity, stability,
internal olefins using chiral dirhodium complexes, iron
1
5
6
and tunability in both synthetic and organometallic chemistry.
complexes, and copper−BOX complexes has been reported.
Moreover, an efficient catalytic system for the asymmetric
cyclopropanation of functionalized diazo compounds with
olefins in the presence of chemobiocatalysts has been reported
to afford the corresponding cyclopropanes in high yields, with
As applications of C−M bond-containing complexes, we
reported highly efficient inter- and intramolecular carbene
transfer reactions such as catalytic asymmetric cyclopropana-
tions of alkenes and allenes, as well as N−H and Si−H
insertion reactions of diazo compounds, using chiral ruthenium
7
excellent enantioselectivities. Despite the high stereoselectiv-
catalysts as a series of Ru(II)−Pheox complexes with a
ities, these reactions require special conditions because of the
use of biocatalysts.
2
C (sp )−Ru bond in a single chiral environment (Scheme
Ar
2
1
a). Their catalytic activity and stereoselectivity are indeed
During the course of our continuous studies of the
development of asymmetric catalysts based on Ru(II)−Pheox
complexes, we attempted to tune the reactivity and selectivity
of the metal center in the Ru(II)−Pheox complex by changing
Scheme 1. Previous Studies and This Work
2
2
8−17
the C ligand from Ru−C (sp ) to Ru−C (sp ).
This is
Ar
olefin
because of the limited availability of Ru and the poor
compatibility of Ru(II)−Pheox 1 with electron-donating and
electron-withdrawing groups on the aromatic ring. Accord-
ingly, we evaluated the catalytic efficiency and stereoselectivity
2
of Ru−C (sp ) complexes for cyclopropanation (Scheme
olefin
1
b). Initially, alkenyl oxazoline ligands were readily synthesized
in three steps by chlorination of a commercially available α,β-
unsaturated carboxylic acid, condensation with (S)-phenyl-
glycinol, and oxazoline formation with methanesulfonyl
chloride in the presence of a base such as triethylamine and
highly efficient and modulable. Moreover, various structural
combinations have been demonstrated by taking into account
structural features such as the nature of the coordinating Ar
groups bearing electron-withdrawing and electron-donating
Received: January 6, 2020
2
groups and the effectiveness of a central metal of a C (sp )−
Ar
M bond.
Transition metal-catalyzed asymmetric cyclopropanation of
3
diazo compounds with olefins is among the best developed.
©
XXXX American Chemical Society
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A
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1
8
the X-ray ORTEP diagrams showed clear Ru−Colefin(sp2)
bonds. The four acetonitrile moieties coordinated to the
ruthenium center and were stabilized by the 18-electron rule.
Complexes 2a−d remained stable during purification by silica
gel column chromatography with elution by an acetonitrile/
organic solvent system; they could be stored for several
months under an argon atmosphere, while Ru(II)−Pheox 1 is
somewhat unstable under high-vacuum conditions or in
coordinating organic solvents such as THF and ether. The
DBU (Scheme 2). The ruthenium complexes were prepared
2a
by C−H bond activation with alkenyl oxazoline ligands.
Scheme 2. Synthesis of Novel Ru(II) Complexes
1
H NMR spectrum of 2c showed a characteristic chemical shift
of the olefinic protons, and the corresponding doublet was
10a,b
shifted downfield to ∼9.8 ppm.
We also synthesized a
complex having an ethylene-oxazoline ligand; however, the
desired complex could not be purified because of its unstable
structure, even though the characteristic signals were observed
in the NMR spectrum of the reaction mixture.
The new entries of ruthenium complexes 2a−d were then
examined to evaluate their catalytic activities and chiral
induction under the standard reaction conditions for the
catalytic cyclopropanation of ethyl diazoacetate with olefins as
well as the Ru(II)−Pheox 1-catalyzed reaction. The results
are listed in Table 1. The catalytic asymmetric intermolecular
Thus, treatment of the oxazoline ligand with [RuCl2-
(
benzene)] , 1 N NaOH, and KPF in an acetonitrile solution
2
6
at 85 °C gave novel ruthenium complexes 2a−d in 62−92%
isolated yields. These complexes were fully characterized by X-
ray diffraction (Figure 1; see the Supporting Information), and
19
Table 1. Optimization of Catalytic Intermolecular
a
Cyclopropanation Reactions
e
b
c
d
entry catalyst solvent
temp (°C) yield (%) trans/cis ee (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
CH Cl2
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
0
40
−10
−30
86
75
79
97
91
38
82
96
93
98
93
73
96
92
88/12
88/12
89/11
90/10
88/12
92/8
88/12
92/8
92/8
97/3
96/4
93/7
96/4
96
78
90
97
97
97
96
96
97
97
97
95
98
99
2
2a
2b
2c
2d
2c
2c
2c
2c
2c
2c
2c
2c
2c
CH Cl2
2
CH Cl2
2
CH Cl2
2
CH Cl2
2
toluene
CH CN
3
Et O
2
THF
0
1
2
3
4
MTBE
MTBE
MTBE
MTBE
MTBE
97/3
a
To a mixture of catalyst (3 mol %) and styrene 3a (5.0 equiv, 1.5
mmol) in the solvent (3.5 mL) was added 4a (0.3 mmol) in CH Cl
Figure 1. ORTEP diagrams of the complexes. PF and solvent were
6
2
2
omitted for the sake of clarity. Crystal structure of 2a. Selected bond
distances (angstroms) and angles (degrees): Ru1−C5, 2.005(3);
Ru1−N1, 2.083(2); Ru1−N2, 2.142(3); Ru1−N3, 2.026(3); Ru1−
N4, 2.021(3); Ru1−N5, 2.020(3); C5−Ru1−N1, 78.1(1); Ru1−C5−
C4, 117.31(2); Ru1−N1−C1, 113.3(2); Ru1−C5−C6, 134.3(2).
Crystal structure of 2b. Selected bond distances (angstroms) and
angles (degrees): Ru1−C5, 2.035(4); Ru1−N1, 2.074(3); Ru1−N2,
b
c
(
1.5 mL) over 4 h by a syringe pump. Isolated yield. Determined by
1
d e
crude H NMR. Determined by chiral HPLC analysis. Abbrevia-
tions: THF, tetrahydrofuran; MTBE, methyl tert-butyl ether; PMP, p-
methoxyphenyl.
carbene transfer reaction of a diazoester with 3a proceeded
smoothly in the presence of 3 mol % catalysts 2a−d to give the
optically active cyclopropylester 5a (Table 1, entries 1−5).
2
2
.156(3); Ru1−N3, 2.024(3); Ru1−N4, 2.015(3); Ru1−N5,
.020(4); C5−Ru1−N1, 79.4(1); Ru1−C5−C4, 114.6(2); Ru1−
N1−C1, 112.2(2); Ru1−C5−C6, 124.3(3). Crystal structure of 2c.
Selected bond distances (angstroms) and angles (degrees): Ru1−
C37, 2.030(1); Ru1−N5, 2.060(8); Ru1−N1, 2.022(9); Ru1−N6,
The yield and enantioselectivity depended on the geminal
2
substituents on the Ru−C (sp ) bond of the catalysts.
olefin
These results suggested that the enantioselectivity was affected
2
7
.143(9); Ru1−N7, 1.990(6); Ru1−N8, 2.023(6); C37−Ru1−N5,
8.2(3); Ru1−C37−C10, 118.0(6); Ru1−N5−C24, 112.1(5); Ru1−
2
by the type of geminal substituents on the Ru−C (sp )
olefin
bond. That is, Ru(II)−Prox catalysts 2c and 2d, with no
C37−H25, 136.0(5). Crystal structure of 2d. Selected bond distances
2
substituent or bearing a plane such as C(sp ) or a lone pair at
(
2
2
angstroms) and angles (degrees): Ru1−C5, 1.999(6); Ru1−N1,
the geminal position, gave the highest yields and enantiose-
lectivities (Table 1, entries 1, 4, and 5). This result could be
attributed to the steric effect between the metal−carbene
.075(6); Ru1−N2, 2.134(6); Ru1−N3, 2.024(5); Ru1−N4,
.020(5); Ru1−N5, 2.011(5); C5−Ru1−N1, 78.6(2); Ru1−C5−
C4, 117.7(4); Ru1−N1−C1, 112.3(4); Ru1−C5−O2, 122.3(4).
B
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Letter
complexes as the reactive intermediate and olefins. Because 2c
gave the highest yield and enantioselectivity, further
optimization of the solvent, and the reaction temperature
effects was examined. The results are shown in entries 6−10 in
Table 1. MTBE containing an ether group was found to be the
best solvent for 2c. Furthermore, a temperature in the range of
90% yield, with 96/4 trans/cis and 98% trans ee. The obtained
heteroatom-substituted optically active cyclopropane deriva-
2
0
tives are useful candidates for medicinal development.
Furthermore, an electron-deficient olefin such as an α,β-
unsaturated carbonyl compound also showed significant
stereoselectivity in terms of both enantio- and diastereose-
lectivity (Table 2, entry 10). We attempted the cyclo-
propanation of aliphatic olefins and internal olefins such as
1-hexene and β-methylstyrene and cyclopropanation of acyl
diazo acetate with styrene. Unfortunately, 2c-catalyzed cyclo-
propanation was also not observed because the dimer from the
diazo compound was obtained as the main product. Cyclo-
propanation of these substrates is limited to using the original
Ru(II)−Pheox catalyst 1. The absolute configuration of the
cyclopropane carboxylate was determined to be (1R,2R) by
−10 to −30 °C gave improved results with the highest
enantioselectivity and diastereoselectivity. Consequently, 2c
led to a highly stereocontrolled process and was found to be a
superior catalyst for the intermolecular carbene transfer
reaction as compared to our previously developed 1 [Ru-
(
2
II)−Pheox]. Under the optimal reaction conditions, catalyst
c was applied for the reaction of various olefin derivatives.
The results are summarized in Table 2. Various styrene
2a,19
Table 2. Substrate Scope for Asymmetric Intermolecular
comparison with the literature data.
a
Cyclopropanation Reactions
Next, we examined the catalytic asymmetric intramolecular
cyclopropanations of trans-allylic diazoacetates using the newly
prepared catalysts 2a−d. Among these catalysts, 2c gave the
best yield and high enantioselectivity in MTBE solvent, similar
to the case of the intermolecular cyclopropanations (Table 3,
Table 3. Optimization of Catalytic Intramolecular
a
Cyclopropanation Reactions
b
c
entry
catalyst
solvent
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
1
CH Cl2
96
87
95
96
91
69
87
95
98
96
97
90
99
96
98
98
97
98
2
2a
2b
2c
2d
2c
2c
2c
2c
CH Cl2
2
CH Cl2
2
CH Cl2
2
CH Cl2
2
toluene
Et O
2
THF
MTBE
a
To a solution of the catalyst (1 mol %) in the solvent (1.0 mL) was
added 6a (0.2 mmol) in CH Cl (1.0 mL) at room temperature.
Isolated yield. Determined by chiral HPLC analysis.
2
2
b
c
21
entries 1−5). Ether was found to be a suitable solvent for the
carbene transfer reactions catalyzed by 2c. To explore the
scope and limitation of the intramolecular cyclopropanation
using 2c as the catalyst, a diverse range of trans-allylic
diazoacetates, which were derived from the corresponding
a
To a mixture of 2c (3 mol %) and styrene 3a−j (5.0 equiv, 1.5
mmol) in MTBE (3.5 mL) was added 4a (0.3 mmol) in CH Cl (1.5
mL) over 4 h by a syringe pump. Isolated yield. Determined by
crude H NMR. Determined by chiral HPLC analysis. 0.46 g of 5i
was synthesized on a 1.96 mmol scale in 90% yield, 96/4 trans/cis,
2
2
b
c
22
allylic alcohols by the Fukuyama method, were investigated
1
d
e
(
Table 4). Intramolecular cyclopropanation of all the diazo
substrates proceeded rapidly (<1 min) to give the correspond-
ing products in high yields, with high enantioselectivities. The
observed enantioselectivity was slightly higher than that
f
and 98% trans ee. Slow addition for 12 h.
2a
derivatives with electron-donating and electron-withdrawing
group substituents (Table 2, entries 1−4 and 6) provided the
corresponding cyclopropanes in high yields, with excellent
stereoselectivities. In the case of α-methylstyrene, the desired
product was obtained in moderate yield, with high
enantioselectivity but decreased diastereoselectivity (Table 2,
entry 5). Heteroatom-substituted olefins also gave impressive
results for the intermolecular cyclopropanation (Table 2,
entries 7−9). The 1.96 mmol scale reaction gave 5i (0.46 g) in
obtained with Ru catalyst 1 by 3−7% ee. trans-Cinnamyl
diazoacetate bearing an electron-withdrawing group at the para
position of the benzene ring gave much higher enantiose-
lectivity as compared to the substrate bearing an electron-
donating group (Table 4, entries 3 and 4). Because the
intramolecular cyclopropanations were extremely fast, the
reactivity and efficiency were examined in terms of turnover
number (TON) and turnover frequency (TOF). The results
are summarized in Table 5. TON and TOF values of 1600 and
C
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Organic Letters
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Letter
Table 4. Substrate Scope for Asymmetric Intramolecular
Cyclopropanation Reactions
ASSOCIATED CONTENT
sı Supporting Information
■
a
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00050.
Experimental procedures and characterization data
(
PDF)
b
c
entry
R1
R2
R3
yield (%)
ee (%)
99
Accession Codes
1
2
3
4
5
6
Ph
Ph
H
H
H
H
Me
H
H
Me
H
H
H
96
99
98
d
CCDC 1906275−1906277 and 1963802 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
93
90
4-MeO-Ph
d
4-NO -Ph
96 (93)
99 (97)
2
Me
H
99
91
98
99
H
a
To a solution of 2c (1 mol %) in CH Cl (1.0 mL) were added 6a−f
2
2
b
(
0.2 mmol) in CH Cl (1.0 mL) at room temperature. Isolated yield.
2 2
d
AUTHOR INFORMATION
Corresponding Author
c
■
Determined by chiral HPLC analysis. 204 mg of 7d was synthesized
on a 1.0 mmol scale in 93% yield and 97% ee.
Seiji Iwasa − Department of Applied Chemistry and Life Science,
Toyohashi University of Technology, Toyohashi 441-8580,
Japan; orcid.org/0000-0002-5012-1420; Email: iwasa@
chem.tut.ac.jp
1
96, respectively, were obtained, and a high enantioselectivity
of ∼96−99% was maintained.
Table 5. Catalytic Efficiencies of Ru(II)−Prox (2c)
Authors
Hayato Inoue − Department of Applied Chemistry and Life
Science, Toyohashi University of Technology, Toyohashi 441-
8
580, Japan
Nga Phan Thi Thanh − Department of Applied Chemistry and
X
time
TOF
yield
c
ee
Life Science, Toyohashi University of Technology, Toyohashi
a
−1
b
d
entry (mol %)
(min)
TON
(min
)
(%)
(%)
4
41-8580, Japan
1
2
3
4
5
1
1
1
5
60
60
96
196
980
1180
1600
96
196
196
20
96
98
98
59
16
99
97
96
95
94
Ikuhide Fujisawa − Department of Applied Chemistry and Life
Science, Toyohashi University of Technology, Toyohashi 441-
0.5
0.1
0.05
0.01
8
580, Japan
e
e
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00050
26
a
b
TON, number of moles of the desired product (6a)/catalyst. TOF
TON/reaction time (minutes). Isolated yield. Determined by
chiral HPLC analysis. Reaction did not lead to full conversion.
c
d
=
e
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a Grant-in-Aid for Scientfic
Research (B) (26288087) and Ikeda Bussan Co. Ltd.
■
In summary, we developed a new series of Ru−Colefin(sp2)
bond-containing organometallic complexes and successfully
applied them to the catalytic asymmetric inter- and intra-
molecular cyclopropanations, which are carbene transfer
reactions. It is noteworthy that high yields and stereoselectivity
were achieved for trans-cyclopropane carboxylates even with a
low catalyst loading. Because alkenyl oxazoline ligands can be
easily prepared and resolved, 2c is a practically applicable
catalyst for synthesizing a variety of chiral cyclopropane
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1
(
E
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Org. Lett. XXXX, XXX, XXX−XXX