Dual Catalysis for the Redox Annulation of Nitroalkynes with Indoles: Enantioselective Construction of Indolin-3-ones Bearing Quaternary Stereocenters

Article abstract of DOI:10.1002/anie.201504697

The enantioselective redox annulation of nitroalkynes with indoles is enabled by gold/chiral phosphoric acid dual catalysis. A range of indolin-3-one derivatives bearing quaternary stereocenters at the C2 position were afforded in good yields and excellen

Full text of DOI:10.1002/anie.201504697

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504697
German Edition: DOI: 10.1002/ange.201504697
Asymmetric Catalysis
Dual Catalysis for the Redox Annulation of Nitroalkynes with Indoles:
Enantioselective Construction of Indolin-3-ones Bearing Quaternary
Stereocenters
Ren-Rong Liu, Shi-Chun Ye, Chuan-Jun Lu, Gui-Lin Zhuang, Jian-Rong Gao, and Yi-Xia Jia*
Abstract: The enantioselective redox annulation of nitro-
alkynes with indoles is enabled by gold/chiral phosphoric acid
dual catalysis. A range of indolin-3-one derivatives bearing
quaternary stereocenters at the C2 position were afforded in
good yields and excellent enantioselectivities (up to 96% ee)
from readily available starting materials.
cient methods to chiral indolin-3-ones, in particular from
readily available starting materials, is highly desirable.
Recently, dual catalysis employing transition metals and
chiral phosphoric acids (CPAs) has appeared as an attractive
[
5]
strategy for asymmetric catalysis. Based on this strategy,
enantioselective transformations of metal carbenoids gener-
ated from diazo compounds have been well developed,
including multicomponent reactions of nucleophiles, electro-
I
ndoles and oxindoles are unique substructures that fre-
[
6,7]
quently occur in natural alkaloids and biologically active
philes, and diazoacetates via zwitterionic intermediates,
[1]
molecules. Numerous methods have been developed for the
enantioselective NÀH or SÀH insertion reactions with diazo
[8]
construction of optically active 2-oxindoles (indolin-2-
compounds, and sequential processes combining carbonyl
[
2]
[9]
ones). In sharp contrast, reliable approaches towards the
asymmetric synthesis of structurally similar indolin-3-ones are
ylide formation and reduction. In comparison, asymmetric
transformations of a-oxo metal carbenoids, which can be
[3]
very limited. Indeed, indolin-3-one derivatives that bear
quaternary stereocenters at the C2 position are very intrigu-
ing for their widespread occurrence in natural products, such
as (À)-isatisine A, (À)-isatisine A acetonide, strobilanthosi-
de A, (À)-trigonoliimine C, (+)-austamide, and halichro-
generated by transition-metal-catalyzed redox reactions of
[10]
alkynes, have remained much less exploited. We noticed
that intramolecular redox annulations of nitroalkynes are
involved in several attractive reactions, including isatogen
[11]
formation and dipolar cycloaddition cascade reactions. The
formation of an a-oxo metal carbenoid was generally
proposed as the initial step in these reactions. We thus
envisaged that an enantioselective redox annulation of nitro-
alkynes with indoles might be realized by gold/CPA dual
[
4]
me A (Figure 1). A few elegant procedures have been
established for their synthesis, but most of them rely on
[3]
transformations of preexisting indolin-3-one ring systems.
Consequently, the development of straightforward and effi-
[12]
catalysis. As proposed in Scheme 1, the desired product
Scheme 1. Proposed redox annulation reaction of nitroalkynes with
indoles.
Figure 1. Selected natural products with an indol-3-one framework.
[
*] Dr. R.-R. Liu, S.-C. Ye, Dr. C.-J. Lu, Dr. G.-L. Zhuang, Prof. Dr. J.-R. Gao,
Prof. Dr. Y.-X. Jia
College of Chemical Engineering
Zhejiang University of Technology
Chaowang Road 18#, Hangzhou 310014 (China)
E-mail: yxjia@zjut.edu.cn
could be afforded either via zwitterionic intermediate II
(
path a; formed by the intermolecular interception of a-oxo
gold carbenoid I with the indole) or via chiral ion pair IV
path b; generated by intramolecular trapping of the nitroso
(
group in I followed by protonation). Herein, we report gold/
CPA-catalyzed enantioselective redox annulation reactions of
nitroalkynes with indoles. A range of indolin-3-ones with
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504697.
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quaternary stereocenters at the C2 position were afforded in
good yields and excellent enantioselectivities (up to 96% ee)
from readily available starting materials.
failed to give a satisfying result (entry 15). However, to our
delight, the reactions were favorably influenced by the
addition of molecular sieves (entries 16–18), and the enantio-
selectivity of 3a was remarkably improved to 95% ee when
At the outset, we chose 1-nitro-2-(phenylethynyl)benzene
[13]
(
1a) and indole (2a) as the model substrates to optimize the
3 Š molecular sieves were added (entry 18).
reaction conditions. An initial reaction in the presence of
With the optimized reaction conditions in hand, we then
investigated the nitroalkyne and indole scope. First, indole
derivatives with various substituents were subjected to the
reaction with 1a (Table 2, entries 1–11). Both electron-with-
AuCl (7 mol%) and trifluoroacetic acid (TFA) furnished the
3
desired product 3a in 95% yield (Table 1, entry 1), but the use
[
a]
Table 1: Optimization of the reaction conditions.
[a]
Table 2: Nitroalkyne and indole scope.
1
2
3
[b]
[c]
Entry
R /R (1)
R (2)
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/Ph (1a)
H/4-nPrC H (1b)
H (2a)
94 (3a)
86 (3b)
80 (3c)
79 (3d)
65 (3e)
92 (3 f)
96 (3g)
81 (3h)
71 (3i)
70 (3j)
96 (3k)
73 (3l)
83 (3m)
65 (3n)
80 (3o)
75 (3p)
76 (3q)
70 (3r)
70 (3s)
75 (3t)
95
91
95
86
96
86
87
89
91
90
93
90
93
55
85
93
95
94
96
95
5-F (2b)
5-Cl (2c)
5-Br (2d)
[
d]
[
b]
[c]
Entry
[Au]
Acid
T [8C]
Yield [%]
ee [%]
5-CO Me (2e)
2
[
d]
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
TFA
TFA
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4j
4j
4j
4j
4j
4j
4j
25
25
25
25
25
25
25
25
25
25
25
25
0
95
60
80
NR
10
30
87
24
35
89
–
–
13
–
[d]
[d]
5-OMe (2 f)
5-Me (2g)
6-F (2h)
6-Cl (2i)
6-Br (2j)
7-Me (2k)
H (2a)
H (2a)
H (2a)
H (2a)
6-Cl (2i)
6-Br (2j)
H (2a)
5-F (2b)
6-Cl (2i)
3
33
57
40
33
58
55
68
71
–
45
87
78
95
[d]
6
4
H/3-MeC H (1c)
6
4
H/cyclopropyl (1d)
4-F/Ph (1e)
4-F/Ph (1e)
1
1
1
1
1
1
1
1
1
78
90
88
4-F/Ph (1e)
AuCl3
5-Cl/Ph (1 f)
5-Cl/Ph (1 f)
5-Cl/Ph (1 f)
[PPh AuNTf ]
AuBr3
0
0
0
0
trace
54
3
2
[
[
[
e]
f]
AuCl3
AuCl3
AuCl3
85
87
94
[a] Reaction conditions: 1 (0.3 mmol), 2 (0.6 mmol), AuCl (7 mol%), 4j
3
g]
(10 mol%), and 3 Š molecular sieves (150 mg) in toluene (0.1m) at 08C
for 15–20 h. [b] Yield of isolated product. [c] Determined by HPLC
analysis on a chiral stationary phase. [d] At À208C. M.S.=molecular
sieves.
0
[
a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), [Au] (7 mol%),
and acid (10 mol%) in toluene (0.1m). [b] Yield of isolated product.
c] Determined by HPLC analysis on a chiral stationary phase. [d] TFA
1.0 equiv). [e] With 5 Š molecular sieves (100 mg). [f] With 4 Š
[
(
molecular sieves (100 mg). [g] With 3 Š molecular sieves (100 mg).
drawing and electron-donating substituents on the indole ring
were well tolerated, mostly affording the desired products in
excellent enantioselectivities. Electron-withdrawing substitu-
ents generally led to higher enantioselectivities but lower
yields than electron-donating groups (3b, 3c, 3e, and 3h–3j
vs. 3 f and 3g). However, a lower enantioselectivity was
observed for the reaction of 5-bromo-substituted indole 2d,
even at À208C (entry 4). A few substituted nitroalkynes were
then reacted with indole. Substrates with aryl substituents
of a catalytic amount of TFA led to a decreased yield
(
entry 2). We then moved our attention to chiral Brønsted
acids to develop an enantioselective version. A range of
BINOL-based phosphoric acids 4 were thus tested in
combination with AuCl . As expected, the reactions pro-
3
ceeded smoothly to afford the desired products; the use of
CPAs with fluorine substituents led to better yields and
enantioselectivities, probably owing to their higher acidity
2
(R ) attached to the alkyne moiety afforded the products in
(
entries 7 and 10–12). In particular, the desired product was
obtained in 90% yield and 68% ee when CPA 4j was used
entry 12). Lowering the temperature to 08C slightly
good yields and enantioselectivities (entries 12 and 13),
whereas a cyclopropyl group resulted in poor yield and
enantioselectivity (entry 14). Furthermore, the halogenated
nitroalkynes 1e and 1 f reacted well with various indoles,
furnishing the corresponding products in good yields and
excellent enantioselectivities (entries 15–20).
(
improved the enantioselectivity (entry 13). An examination
of various gold catalysts revealed that [PPh AuCl]/AgNTf2
(
3
Tf = trifluoromethanesulfonyl) was much inferior and gave
only a trace amount of the product (entry 14). AuBr also
3
1
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Scheme 3. Reaction scale-up and further transformation. Reaction
Scheme 2. Control reaction. Reaction conditions: a) AuCl (7 mol%),
conditions: a) 1a (4 mmol), 2a (8 mmol), AuCl (7 mol%), 4j
3
3
2
58C, 3 h; b) 4j (10 mol%), 3 Š molecular sieves (150 mg), 08C, 15 h.
(10 mol%), and 3 Š molecular sieves (1.0 g) in toluene (30 mL) at 08C
for 18 h; b) 3a (0.2 mmol), N H ·H O (1.6 mmol) in MeOH (6.0 mL)
2
4
2
at 808C for 6 h.
To understand the possible reaction mechanism, a control
reaction was carried out. As shown in Scheme 2, isatogen 5a
was detected as the sole product in the absence of indole and
to liberate the free amine was conducted using N H ·H O as
2 4 2
chiral phosphoric acid with 7 mol% of AuCl as the catalyst,
a reducing agent, which furnished indolin-3-one 6 in 81%
yield and without any reduction in enantiopurity.
3
[11a]
[16]
which is consistent with literature precedents.
Subsequent
introduction of indole 2a, CPA 4j (10 mol%), and 3 Š
molecular sieves to the reaction system at 08C afforded 3a in
In summary, we have successfully developed an efficient
redox annulation reaction of nitroalkynes and indoles by
gold/chiral phosphoric acid dual catalysis, which provides
facile access to indolin-3-ones bearing C2 quaternary stereo-
centers in high yields and excellent enantioselectivities. This
method is practical and straightforward as it can be easily
scaled up and readily available starting materials are used.
Moreover, the first asymmetric Friedel–Crafts reaction that
employs nitrones as the alkylating reagents was developed.
9
0% yield and 95% ee, which is comparable to the results
obtained under the optimized reaction conditions (Table 2,
entry 1). These observations suggest that the reaction might
proceed according to dual-catalysis path b (Scheme 1),
although path a cannot be ruled out at this stage. As implied
by the results of the control reaction, we then tested the direct
addition of indoles to a series of isatogens 5, which constitutes
the first example of asymmetric Friedel–Crafts reactions
[14,15]
using nitrones as alkylating reagents.
As shown in
Table 3, the corresponding N-hydroxy indolin-3-ones 3 were Acknowledgements
generally afforded in excellent yields and enantioselectivities.
We are grateful for generous support from the National
Natural Science Foundation of P. R. China (21372202,
21402175), the New Century Excellent Talents in University
Table 3: Substrate scope of the Friedel–Crafts reaction between isato-
gens 5 and indoles.
[
a]
(NCET-12-1086), and the Zhejiang Natural Science Fund for
Distinguished Young Scholars (R14B020005).
Keywords: asymmetric catalysis · carbenoids ·
chiral phosphoric acids · gold · indolinones
3
[b]
[c]
Entry
R
R
Yield [%]
ee [%]
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11205–11208
Angew. Chem. 2015, 127, 11357–11360
1
2
3
4
5
6
7
8
9
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
Ph (5a)
5-F (2b)
5-Cl (2c)
5-Br (2d)
5-Me (2g)
6-F (2h)
6-Cl (2i)
6-Br (2j)
7-Me (2k)
H (2a)
92 (3b)
93 (3c)
90 (3d)
91 (3g)
93 (3h)
92 (3i)
91 (3j)
92 (3k)
95 (3u)
94 (3v)
95 (3w)
93
94
89
91
90
93
95
90
91
95
53
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5
Received: May 27, 2015
Published online: August 5, 2015
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