Imidazolylpyridine-In(OTf)3 catalyzed enantioselective allylation of ketimines derived from isatins

Article abstract of DOI:10.1039/c6ob00551a

An enantioselective In(OTf)₃-catalyzed allylation of ketimines derived from isatins in the presence of an imidazolylpyridine ligand is described. The reaction proceeded smoothly under mild conditions and resulted in 3-allyl 3-aminooxindoles with good yields and moderate to excellent enantioselectivities (up to 97% ee).

Full text of DOI:10.1039/c6ob00551a

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Imidazolylpyridine-In(OTf)3 catalyzed enantioselective allylation
of ketimines derived from isatins
Tingting Chena, Chun Caia*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An enantioselective In(OTf)3-catalyzed allylation of ketimines
derived from isatins in the presence of imidazolylpyridine ligand is
described. The reaction proceeded smoothly under mild
conditions and resulted the 3-allyl 3-aminooxindoles with good
yields and moderate to excellent enantioselectivities (up to 97%
ee).
Me
HN
F
OMe OH
N
Cl
O
Me
NH
Cl
O
CONMe2
N
N
O
HN
O
N
N
Cl
O
O
S
O
O
N
OMe
O
OEt
3-aminooxindoles bearing a tetrasubstituted carbon stereo-
center at the 3-position were widely found in the core
skeletons of a large family of biologically active natural
MeO
COOH
OEt
AG-041R
Gastrin/CC-B
SSR-149415
Vasopressin Vlb
receptor antagonist
CRTH2 antagonist
receptor antagonist
products and
a series of pharmaceutically important
F
compounds (Figure 1).1 Owing to the usefulness of this
structural motif, the construction of the oxindoles bearing a
nitrogen atom at the C3 quaternary carbon center has
enormously attracted organic synthetic chemists’ attention
and several different strategies with different transition-metal
catalysts and organocatalysts have been reported.2 There are
two major strategies to obtain the tetrasubstituted 3-
aminooxindoles products3 (Scheme 1). One is the catalytic
asymmetric amination of 3-substituted oxindoles.4 For
instance, Feng et al. reported highly efficient enantioselective
α-amination of 3-substituted-2-oxindoles with azodi-
carboxylates5 and nitroso compounds6 catalyzed by a chiral
Sc(OTf)3/N,N’-dioxide complex. Another strategy to construct
3-aminooxindoles could be based on the addition of
nucleophiles to ketimines derived from isatins.7 Thus,
asymmetric Strecker reaction,8 Mannich reaction,9 Nitro-
Mannich Reaction,10 Friedel−Craꢀs reacꢁon11 and Povarov
reaction.12 have been employed using the isatin derivatives as
the electrophiles. Compared with the former, the latter is
more convenient and straightforward with respect to
substrate accessibility.11c
Cl
Cl
Cl
O
HN
NH
Cl
NMe
O
O
N
H
N
H
NITD609
Antimalarial
drug candidate
Anti-aumor agent
Figure 1. Examples of biologically important 3-subtituted 3-amino-2-oxindoles
chemistry. In this context, the allylation of imines, especially
the imines derived from isatins, offers an excellent opportunity
for the development of new C-C bond and results a variety of
homoallylic amines as synthetic intermediates.13 In the
previous work, efficient methods for the construction of 3-allyl
3-aminooxindoles have been widely reported.14 However,
successful asymmetric allyl addition to the ketimines derived
from isatins is far less explored and remains a significant
challenge. In 2009, Silvani et al. 15first described the method
for the asymmetric addition of allylmagnesium bromide to
ketimines derived from isatins, which containing chiral
auxiliary. Unfortunately, only moderate yields as well as
diastereomeric ratio (d.r. up to 89:11) were obtained. Then
subsequently, Xu et al.16 reported the Zn-mediated
diastereoselective allylation of isatin-derived N-sulfinyl
ketimines. This process allowed convenient formation of a
wide range of 3-allyl 3-aminooxindoles in excellent
diastereomeric ratio (d.r. up to 99.5:0.5). Recently, Babu et
al.17 reported a synthetic protocol for the indium-mediated
stereocontrolled addition of various γ-substituted allylic
reagents to the isatin-derived ketimines to obtain the 3-allyl-3-
The development of efficient and stereoselective C-C bond-
forming processes is an importance topic in modern organico
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smaller steric repulsion of the six-membered ring. Indium
complexes based on salen ligands (L3 and L4) were found to
catalyze the reaction in moderate yields with poor
enantiomeric excess (Table 1, entries 5 and 6), whereas other
ligands (L5-L8) resulted in racemic products.
R4
Strategy a
O
R3
N
R2
asymmetric amination of
3-substituted oxindoles
R1
HN
To further explore this reaction, we tested the catalytic
abilities of a series of readily available transition-metal salts.
In(OTf)3 was the most efficient catalyst to provide the desired
product 3-allyl-3-((4-methoxyphenyl)amino)indolin-2-one 2a in
93% yield with 77% ee, whereas all other transition-metal salts
investigated afforded only low yields and enantioselectivities
(Table 1, entries 2, 6-9). With In(OTf)3 as the catalyst, the
reaction medium was then evaluated and was found to have a
profound effect on the efficiency and stereocontrol of the
product. Among the tested solvents, the best enantio-
selectivity was achieved for MeOH at room temperature (93%
yield, 77% ee, Table 1, entry 2). Low yields and enantio-
selectivities were observed in THF, DMF and EtOH (Table 1,
entries 10-12). Only trace amount of 2a was obtained with
isopropanol as the solvent. The reaction conducted in CH2Cl2
and toluene both resulted in racemic products.
R4
O
R3
N
R1
N
R2
Strategy b
O
R3
asymmetric addition of nucleophiles
to ketimines derived from isatins
N
R2
Scheme 1. Two major strategies for the construction of the tetrasubstituted 3-
aminooxindoles.
aminooxindoles with high diastereomeric ratio (d.r. up to
95:5). In 2013, Nakamura et al.18 demonstrated the first
example of enantioselective allylation of ketimines derived
from isatins with allyltrimethoxysilane in the presence of
palladium pincer complexes with chiral bis(imidazoline)s and
silver fluoride. However, even high yields (84-96%) and
enantioselectivities (82-95%) of the homoallylic amines have
been obtained, this transformation still suffer from some
drawbacks, such as the facts that it generally required a long
reaction time (24-72h) and conducted under strict reaction
Variation in reaction temperature is known to affect both yield
and enantioselectivity of the product significantly, therefore
we investigated the allylation reaction at different
temperatures. Unfortunately, a slight decrease of enantio-
selectivity was observed by varying the reaction temperature
(Table 1, entries 13 and 14), indicating that room temperature
was an important feature of this catalytic system. To further
explore this reaction, we then turned our attention to examine
catalyst loading. When the catalyst loading was decreased
from 10 to 2.5 mol %, high yield (91%) was still obtained and
enantiomeric excess remarkably increased (77 vs. 94% ee),
although a long reaction time was necessary (Table 1, entries 2
and 15-17).
condition (-30o
C). What’s more, only racemic products were
obtained when the ketimines derived from isatins without
protecting groups on the N1-atom were utilized. In 2014,
Wang et al.19 also reported the allylations of the Boc-imines
catalysed by the chiral Pd-pincer-complexes, resulting
moderate enantiomeric excess (72%). As a consequence, the
development of an efficient synthetic route under mild
conditions for accessing chiral 3-allyl 3-aminooxindoles is still
needed.
Herein, we reported a highly practical approach for the
synthesis of a wide range of 3-allyl 3-aminooxindoles via
indium-catalyzed asymmetric allylation of ketimines derived
from isatins in the presence of imidazolylpyridine ligand under
mild reaction conditions.
R2
N
R2
N
H
O
O
N
N
N
R1
N
N
N
R3
OH HO
R3
Ph
Ph
R1
Our model reaction was performed with ketamine 1a derived
from isatin and p-methoxyaniline as the substrate and
R3
R3
allyltributyltin as an allyl reagent. Firstly,
a preliminary
L3, R1=-(CH2)4- R2=H
L4, R1=-(CH2)4- R2=tBu
L5, R =Ph R = Bu
L6
L1, R1=Ph
L2, R1=-(CH2)4-
screening of different chiral ligands (Figure 2), such as
imidazolylpyridine (L1-L2), salen (L3-L5), bis(oxazoline) (L6),
BINAP (L7) and BINOL (L8), was carried out with In(OTf)3 using
methanol as solvent. Imidazolylpyridine ligand L1 and L2 were
synthesized from 2-cyanopyridine and corresponding 1,2-
diamines. Contrast to the reaction without ligand, higher yield
and faster reaction rate were observed when L1 and L2 was
used (Table 1, entries 1-3). L2 gave lower ee than L1 for its
t
2
1
OH
OH
PPh2
PPh2
L7
L8
Figure 2. Selected examples of chiral ligands examined.
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Table 1. Optimization of the reaction conditions. a
Table 2. Scope of substrates.a
R1
N
PMP
N
R1
PMP
HN
HN
L1, In(OTf)3
MeOH, RT
L, metal salt
solvent,RT
O
R3
SnBu3
O
R3
O
SnBu3
O
N
N
N
N
R2
1a-1o
H
R2
2a-2o
H
2a
1a
entry
R1
R2
R3
Product
yieldb
/%
91
eec
/%
94
84
72
94
91
/
entry
metal salt
(x mol %)
ligand
(y mol %)
/
solvent
time
(h)
12
1
yieldb
/%
76
eec
/%
/
1
2
4-MeOC6H4
Ph
H
H
H
H
H
H
H
H
H
H
2a
2b
2c
2d
2e
2f
1
2
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(10)
Cu(OTf)2(10)
Sc(OTf)3(10)
Zn(OTf)2(10)
PdCl2(10)
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
THF
87
L1(10)
L2(10)
L3(10)
L4(10)
L1(10)
L1(10)
L1(10)
L1(10)
L1(10)
L1(10)
L1(10)
L1(10)
L1(10)
L1(7.5)
L1(5.0)
L1(2.5)
93
77
66
26
69
/
3
4-MeC6H4
4-ClC6H4
H
89
3
1.5
4
87
4
H
87
4
83
5
4-BrC6H4
H
90
5
4
86
6
4-NO2C6H4
3-MeC6H4
2-MeC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
H
trace
88
6
24
24
24
24
24
24
24
1
trace
43
7
H
2g
2h
2i
97
83
43
63d
74
60
71
63d
94
96
7
5
8
H
83
8
48
13
17
22
34
58
67
66
80
92
94
9
CH3
Ph
Bn
H
H
85
9
27
10
11
12
13
14
15
16
H
2j
84
10
11
12
13d
14e
15
16
17
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(10)
In(OTf)3(7.5)
In(OTf)3(5.0)
In(OTf)3(2.5)
56
H
2k
2l
81
DMF
68
5-Me
5-MeO
5-Cl
6-Cl
7-F
88
EtOH
77
H
2m
2n
2o
2p
86
MeOH
MeOH
MeOH
MeOH
MeOH
95
H
83
4
79
H
91
3
90
H
87
4
89
6
91
a
Reactions was performed with isatinimine (0.2 mmol) and allyltributyltin (0.3
a
mmol), In(OTf)3 (2.5 mol %), L1 (2.5 mol %) and MeOH (2 mL) at room
Reactions was performed with 1a (0.2 mmol), allyltributyltin (0.3 mmol), metal
b
c
b
temperature. Isolated yield. The ee was checked by chiral HPLC by using a
salt (x mol %), ligand (y mol %) and solvent (2 mL) at room temperature.
Isolated yield. c The ee was checked by chiral HPLC by using a Venusil CA column.
d The reaction was carried out at 50oC. e The reaction was carried out at 0 oC.
d
Venusil CA column. The ee was checked by chiral HPLC by using a Venusil CO
column.
1l-1n. Contrast to 1a, the ketimines with the electron-donating
groups (-Me and -OMe) and electron-withdrawing group (-Cl)
at 5-position gave the corresponding products in good yields
but only moderate enantiomeric excess (60%, 71% and 63%
respectively). Moreover, 6-bromo-substituted and 7-fluoro-
Having established the optimal reaction conditions to carry out
substituted allylation reaction, we then studied the
applicability of the reaction to a wide variety of ketimines
derived from isatins and anilines (Table 2). Initially, the
ketimines derived from the anilines 1a-1h were evaluated.
Gratifyingly, in this protocol, almost all of the para-, meta- or
ortho-substituted anilines, including electron-rich and
electron-deficient ones, were suitable substrates and
substituted isatin derivatives
(1o, 1p) could react with
allyltributyltinas efficiently to give compounds 2o and 2p
(Table 2, entries 15 and 16) in good yields and enantiomeric
excesses above 94%.
produced the corresponding homoallylic amines 2a-e, 2g-h in
The absolute configuration of 2a was assigned as S by X-ray
crystallographic analysis (Figure 3), and the configuration of
other products was tentatively assumed by analogy.
good yields and up to 97% ee (Table 2, entries 1-5, entries 7-8).
Due to the steric hindrance effect, ketimine derived from o-
methylaniline resulted in the lower yield than the one derived
from p-methylaniline while maintaining high enantioselectivity
(Table 2, entry 3 vs. entry 8). Unfortunately, when 3-((4-
nitrophenyl)imino)indolin-2-one 1f was employed in the
allylation reaction, the yield of 2f dramatically decreased and
only trace amount of product was obtained (Table 2, entry 6).
This may be due to the strong electron-withdrawing ability of
the nitro-group.
Next, the protocol was applied to the ketimines derived from
substituted isatins. In the previous reported procedures,18
a
protecting group at the N1-atom, which is difficultly to
deprotect, is essential to obtain the enantioselective target
product. Surprisingly, in our method, the introduction of R2 at
the N1-atom resulted lower yields and enantioselectivities
(Table 2, entries 9-11). We then turned our attention to the
reactivity of the ketimines derived from 5-substituted-isatins
Figure 3. X-ray crystallographic analysis of 2a.
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Based on the stereochemical outcome of the reaction, a
possible mechanism for the addition of allyltributyltin to the
ketimines derived from isatins is proposed (Figure 4). Initially,
Imidazolylpyridine-In(OTf)3 reacts with allyltributyltin to give
an allyl coordinated complex. On the other hand, through
coordination, the metal Indium holds it in close proximity with
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