Highly stereoselective spirocyclopropanation of various diazooxindoles with olefins catalyzed using Ru(ii)-complex

Article abstract of DOI:10.1039/C8RA09212E

Optically active spirocyclopropyloxindole derivatives were efficiently synthesized from diazooxindoles and olefins in the presence of a Ru(ii)-Pheox catalyst. Among a series of Ru(ii)-Pheox catalysts, Ru(ii)-Pheox 6e was determined to be the best catalyst for spirocyclopropanation reactions of diazooxindoles with various olefins in high yields (up to 98%) with high diastereoselectivities (up to trans:cis = >99:1<) and enantioselectivities (up to 99% ee). Furthermore, as the first catalytic asymmetric synthesis, anti-HIV active candidate 4a and a bioactive compound of AMPK modulator 4c were easily synthesized from the corresponding diazooxindoles 1i and 1b, respectively, in high yields with high enantioselectivities (4a: 82% yield, 95% ee, 4b: 99% yield, 93% ee).

Full text of DOI:10.1039/C8RA09212E

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Highly stereoselective spirocyclopropanation of
various diazooxindoles with olefins catalyzed using
Ru(II)-complex†
Cite this: RSC Adv., 2018, 8, 39865
a
a
Masaya Tone,
Yoko Nakagawa,
Soda Chanthamath,a Ikuhide Fujisawa,a
Naofumi Nakayama,b Hitoshi Goto,
Kazutaka Shibatomi
and Seiji Iwasa
bc
a
a
*
Optically active spirocyclopropyloxindole derivatives were efficiently synthesized from diazooxindoles and
olefins in the presence of a Ru(II)-Pheox catalyst. Among a series of Ru(II)-Pheox catalysts, Ru(II)-Pheox 6e was
determined to be the best catalyst for spirocyclopropanation reactions of diazooxindoles with various olefins
in high yields (up to 98%) with high diastereoselectivities (up to trans:cis ¼ >99:1<) and enantioselectivities (up
to 99% ee). Furthermore, as the first catalytic asymmetric synthesis, anti-HIV active candidate 4a and
a bioactive compound of AMPK modulator 4c were easily synthesized from the corresponding diazooxindoles
1i and 1b, respectively, in high yields with high enantioselectivities (4a: 82% yield, 95% ee, 4b: 99% yield, 93% ee).
Received 7th November 2018
Accepted 20th November 2018
DOI: 10.1039/c8ra09212e
rsc.li/rsc-advances
The discovery of a new class of optically active cyclopropyl oxin- development of effective and powerful methodologies to prepare
doles, as shown below (Fig. 1), has stimulated intensive research the unique structural motif of a spirocyclopropyloxindole
interest in the development of biologically active new organic remains an active research area.
molecules in both academia and industry.1,2 These compounds
We reported a series of Ru(II) phenyloxazoline catalysts (Ru(II)-
were observed to be pharmacologically important substrates with Pheox) in 2010, which effectively promoted the asymmetric intra-
potentially interesting biological activities—e.g., potent HIV and intermolecular carbene transfer reactions to olens, C–H, N–
inhibitors,3 and anti-cancer4 and inotrope agents.5 Furthermore, H, and Si–H bonds in high yields with high enantioselectivities.11
chiral spirocyclopropyl oxindoles are useful key intermediates for Based on this study, we employed Ru(II)-Pheox catalyst to
the ring-expansion reaction of spirocyclopropane rings via synthesize a spirocyclopropanated oxindole from 5-bromo-3-
a concerted mechanism in the presence of a Lewis acid catalyst.6 methylene-indolin-2-one and diazoacetate.12 However, in this
As for the synthetic methodology, the transition-metal-catalyzed study, the reactivities and stereoselectivities were reported with
asymmetric cyclopropanation of diazooxindoles with olens
has been reported, which is one of the most direct and efficient
pathways for the synthesis of chiral spirocyclopropyl oxindoles.
Aer the rst report by Arai7 and Zhou8 independently with Rh
and Hg catalysts, Ding and co-workers reported the asymmetric
cyclopropanation of diazooxindoles with alkenes using a C2-
symmetric spiroketal bisphosphine/Au(I) complex with good
enantioselectivity.9 More recently, Xu and co-workers reported
efficient dirhodium catalysts, which exhibited good to excellent
enantioselectivity control with alkyl alkenes.10 Although effective
catalysts and catalytic systems have been reported for the asym-
Fig. 1 Bioactive spirocyclopropyl oxindoles.
metric cyclopropanation of diazooxindoles with olens, the
aDepartment of Environmental and Life Sciences, Toyohashi University of Technology,
Tempaku-cho, Toyohashi 441-8580, Japan. E-mail: iwasa@ens.tut.ac.jp
bCONFLEX Corporation, Shinagawa Center Building, 6F, 3-23-17 Takanawa Minato-
ku, Tokyo 108-0074, Japan
cDepartment of Computer Science and Engineering, Toyohashi University of
Technology, Tempaku-cho, Toyohashi 441-8580, Japan
† Electronic supplementary information (ESI) available. CCDC 1874398. For ESI
and crystallographic data in CIF or other electronic format see DOI:
Fig. 2 Synthetic strategies of optically active spirocyclopropyl oxindole.
10.1039/c8ra09212e
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Table 1 Screening of various catalystsa
The reaction of diazooxindole 1a proceeded smoothly at room
temperature to afford the desired cyclopropane product 3a in high
yield with moderate enantioselectivity (Table 1, entry 3).
To improve the enantioselectivity, the Ru(II)-Pheox 6a cata-
lyst was modied in terms of electron density on the aromatic
ring connecting with Ru(II) and a chiral environment (6b–e).
Catalyst screening of various Ru(II)-Pheox 6 catalysts is shown in
Table 1 (entries 3–7). Consequently, we determined that the
reactivity- and enantioselectivity-related electronic effect on the
aromatic ring was slightly effective and increased the enantio-
selectivity to 61% ee with the use of Ru(II)-Pheox 6d having an
electron-withdrawing group such as NO2 (Table 1, entry 6).
However, a much more effective catalyst, Ru(II)-Pheox 6e15
having an indane-derived chiral environment, was observed to
have higher activity and enantioselectivity than the other Ru(II)-
Pheox 6a catalysts, resulting in the corresponding product in
high yield (98%) with excellent diastereoselectivity (94:6) and
high enantioselectivity (92% ee) (Table 1, entry 7).
To improve the diastereoselectivity and enantioselectivity of
this catalytic system further, we examined the cyclopropanation
in various solvents (Table 2, entries 1–5). For all tested solvents,
spirocyclopropanation proceeded smoothly in high yield but
with differing diastereoselectivities and enantioselectivities.
Toluene led to an improvement of trans-selectivity (trans/cis ¼
97:3) and enantioselectivity (94% ee). Toluene and dichloro-
methane were observed to be suitable solvents for the spi-
rocyclopropanation of diazooxindole and olens catalyzed by
Ru(II)-Pheox 6e. The effect of temperature on the reaction was
also investigated (Table 2, entries 6–9), showing that enantio-
selectivity could be improved to 96% ee at a temperature of 0 ꢀC
(Table 2, entry 7). As such, 0 ꢀC was determined to be the
optimal temperature for this catalytic system.
Entry
Catalyst
Time [h]
Yieldb [%]
trans/cisc
eed [%]
1
2
3
4
5
6
7
4
5
10
4 days
6
3 day
24
5
90
75
95
95
93
91
91
69:31
81:19
95:5
97:3
95:5
95:5
94:6
ꢁ92
48
50
46
49
6a
6b
6c
6d
6e
61
92
6
a
Reaction condition: catalyst (1 mol%) and styrene 2a (5.0 equiv., 1
mmol) were dissolved in CH2Cl2 (2.0 mL), and diazooxindole 1a (0.2
b
c
mmol) in CH2Cl2 (2 mL) was added. Isolated yield. Determined by
NMR. d Determined by chiral HPLC analysis.
moderate, or even high, diastereoselectivities. To improve the
yield and enantioselectivity, we initially used Ru(II)-Pheox for the
reaction of a diazooxindole of an olen (Fig. 2).
First, we attempted a catalytic asymmetric cyclopropanation of
diazooxindole 1a with styrene 2a catalyzed by authorized
complexes such as 413 and 514 and a series of Ru(II)-Pheox catalysts.
The results are summarized in Table 1. The cyclopropanation with
the commonly used catalysts 4 and 5 proceeded with low stereo-
selectivities (Table 1, entries 1 and 2). A series of Ru(II)-Pheox
catalysts was also tested for the spirocyclopropanation reaction.
Subsequently, we briey examined the effect of N-substituted
group under these optimized conditions. The result is summa-
rized in Table 3. N-Alkyl diazooxindoles were subjected to these
conditions with 2a. N-Benzyl group had some effects on the yield
and resulted in the highest stereoselectivity in terms of both
Table 2 Optimization of reaction conditionsa
Entry
Solvent
T [ꢀC]
Time [h]
Yieldb [%]
trans/cisc
eed [%]
1
2
3
4
5e
6
7
8
9
THF
CH2Cl2
Acetone
CH3CN
Toluene : CH2Cl2
Toluene
Toluene
Toluene
Toluene
rt
rt
rt
rt
rt
rt
0
7
6
6
42
6
24
24
24
24
77
91
87
53
97
95
94
92
92
96:4
94:6
94:6
77:23
91:9
97:3
94:6
98:2
92:8
94
92
91
76
95
94
96
92
91
ꢁ10
ꢁ20
a
Reaction condition: catalyst (1 mol%) and styrene 2a (5.0 equiv., 1 mmol) were dissolved in CH2Cl2 (2.0 mL), and diazooxindole 1a (0.2 mmol) in
CH2Cl2 (2 mL) was added. b Isolated yield. c Determined by NMR. d Determined by chiral HPLC analysis. Toluene : CH2Cl2 ¼ 1 : 1.
e
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Table 3 Substituent effect on nitrogen of diazooxindole 1a–1da
entry 7). However, low enantioselectivity of 24% ee was observed
in the case of cyclopropanation of 4-Me2N-substituted-styrene,
which might be unstable during the purication following
cyclopropane ring-opening reaction assisted by an electron-
donating group (Table 4, entry 12).
Furthermore, various oxindoles examined under similar
conditions resulted in high yields and stereoselectivities, except
5-Br group (Table 4, entries 13–15). The cyclopropanation of
aliphatic alkenes such as 1-hexane, multi-substituted olens,
inner alkene derivatives and a,b-unsaturated carbonyl
compounds was also examined; however, no cyclopropane
product was observed owing to rapid dimerization from the
diazo compound.
Furthermore, hetero-atom-substituted olens were exam-
ined for spirocyclopropanation reactions. The results are
summarized in Table 5. Various N-alkyl diazooxindoles were
subjected to these conditions with 2a, and all the tested diaz-
ooxindoles provided the cyclopropane product in good yields
with high diastereoselectivity and enantioselectivity.
When the substrate has low solubility in toluene, a mixed
solvent system such as CH2Cl2 : toluene ¼ 1 : 1 is efficient to
improve the reactivity without decreasing the stereoselectivity
(Table 5, entries 1, 2, and 4). Proton nuclear magnetic resonance
(H-NMR) analysis suggested the quantitative yield for the spi-
rocyclopropanation reaction of vinyl phthalimide 2o although
the isolated yield was 37%. The product 3w from vinyl phthali-
mide was unstable during the purication through column
chromatography on both silica gel and alumina (Table 5, entry 4).
Furthermore, we investigated the estimation of the transi-
tion state related to the reaction mechanism using
Entry
R1
Product
Yieldb [%]
trans/cisc
eed [%]
1e
2
3
H
Et
iPr
Bn
3b
3c
3d
3e
98
94
86
97
93:7
97:3
96:4
>99:1<
92
97
95
98
4
a
Reaction condition: catalyst (1 mol%) and styrene 2 (5.0 equiv., 1
mmol) were dissolved in CH2Cl2 (2.0 mL), and diazooxindole 1 (0.2
b
c
mmol) in CH2Cl2 (2 mL) was added. Isolated yield. Determined by
NMR. Determined by chiral HPLC analysis. e CH2Cl2 was used as the
d
solvent.
diastereoselectivity and enantioselectivity even far from the reac-
tive site (Table 3, entry 4). When i-Pr group was used to substitute
nitrogen, the yield was slightly decreased (Table 3, entry 3).
We subsequently explored the scope and generality of the
catalytic system (Table 4). Various vinyl arenes were reacted with
diazooxindole 1a in the presence of Ru(II)-Pheox 6e. Most of the
vinyl arenes provided the corresponding spirocyclopropanation
products in high yields (up to 98% yields) with high diaster-
eoselectivities and enantioselectivities 93–99% ee (Table 4,
entries 1–11). Germinal disubstituted olen, a-methylstyrene
2g, also provided the product 3k in good yield with excellent
diastereoselectivity and enantioselectivity (97% ee) (Table 4,
Table 4 Spirocyclopropanation of diazooxindole 1a with styrene derivatives 2a
Entry
R1
R2
R3
Product
Yieldb [%]
trans/cisc
eed [%]
1
2
3
4
Ph
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
5-Br
6-Cl
6-OMe
3a
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
94
92
80
96
97
83
93
85
98
98
79
74
93
98
93
94:6
92:8
96
95
99
96
95
96
97
94
94
93
97
24
87
99
95
o-MeC6H4
m-MeC6H4
p-MeC6H4
p-tBuC6H4
Np
>99:1<
>99:1<
>99:1<
>99:1<
98:2
96:4
96:4
96:4
>99:1<
>99:1<
89:11
96:4
5
6e
7f
8e
9
Ph
p-NO2C6H4
p-BrC6H4
p-ClC6H4
p-OMeC6H4
p-NMe2C6H4
Ph
10
11
12
13
14
15e
Ph
Ph
3s
98:2
a
Reaction condition: catalyst (1 mol%) and styrene 2 (5.0 equiv., 1 mmol) were dissolved in CH2Cl2 (2.0 mL), and diazooxindole 1 (0.2 mmol) in
CH2Cl2 (2 mL) was added. b Isolated yield. c Determined by NMR. d Determined by chiral HPLC analysis. e Toluene : CH2Cl2 was used as the solvent.
f
Slow addition for 4 h, and stirring for 20 h.
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Table 5 Cyclopropanation of diazooxindole 1a with vinyl ester and
vinyl aminesa
Entry
R1
Product
Yieldb [%]
trans/cisc
eed [%]
1e,f
2e,f
3t
84
85
98:2
90
92
3u
>99:1<
Scheme 1 Synthesis of bioactive compounds.
3
3v
92
37
92:8
78
93
In summary, the catalytic asymmetric cyclopropanation
reaction of diazooxindoles and olens by a series of Ru(II)-Pheox
catalysts is very effective for the synthesis of optically active
spirocyclopropyloxindole derivatives. The stereoselectivity is
good to excellent in most cases and optically active spi-
rocyclopropyloxindole derivatives can be synthesized under
a mild reaction condition in a short step. Furthermore, this
strategy could be applied for the rst synthesis of optically
active bioactive compounds such as anti-HIV candidates and an
AMPK modulator.
4e,f,g
3w
86:14
a
Reaction condition: catalyst (1 mol%) and styrene 2 (5.0 equiv., 1
mmol) were dissolved in CH2Cl2 (2.0 mL), and diazooxindole 1 (0.2
b
c
mmol) in CH2Cl2 (2 mL) was added. Isolated yield. Determined by
NMR. Determined by chiral HPLC analysis. Toluene : CH2Cl2 was
d
e
f
used as the solvent. Slow addition for 4 h, and stirring for 20 h.
g
98% conversion.
Conflicts of interest
There are no conicts to declare.
computational chemical analysis for the metal–carbene
complex derived from diazooxindole and Ru(II)-Pheox catalyst
(Table S1, Fig. 3 and S1–S3†).17–19 The computational chemical
analysis shows the Ru-oxindole carbene complex, which
suggests that p–p interaction between oxindole and the indane
aromatic ring controls the chiral environment to induce high
enantioselectivity. Thus, an olen may approach from only one
side to provide high stereoselectivity as shown in Fig. 3. The
absolute stereochemistry was determined using X-ray analysis
(Scheme 1).
To demonstrate the utility of our direct enantioselective
cyclopropyloxindole synthesis, we demonstrated the rst cata-
lytic asymmetric synthesis of bioactive compounds such as anti-
HIV activity candidate 4a and AMPK modulator 4c from diazo-
oxindoles 1i and 1b, respectively. Both spirocyclopropanations
proceeded smoothly, resulting in high yield with high enan-
tioselectivity (4a: 82% yield, 95% ee, 4b: 99% yield, 93% ee as it
is methyl ester).2e,16
Acknowledgements
S. I. thank Ikeda Bussan Co. Ltd. for partial nancial support for
this study.
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