Br?nsted acid/rhodium(II) cooperative catalytic asymmetric three-component aldol-type reaction for the synthesis of 3-amino oxindoles

Article abstract of DOI:10.1002/chem.201203993

Cooperation is key! Chiral Br?nsted acid/rhodium(II) cooperative catalysis enabled an enantioselective three-component aldol-type reaction of 3-diazo oxindoles and anilines with glyoxylates to give highly functionalized and structurally diverse 3-amino oxindoles in high stereoselectivity (>20:1 d.r., 99 % ee; see scheme). Copyright

Full text of DOI:10.1002/chem.201203993

COMMUNICATION
DOI: 10.1002/chem.201203993
Brønsted Acid/Rhodium(II) Cooperative Catalytic Asymmetric Three-
Component Aldol-Type Reaction for the Synthesis of 3-Amino Oxindoles
Lei Ren, Xiao-Lei Lian, and Liu-Zhu Gong*[a]
The prevalence of the 3,3’-disubstituted oxindole skeleton
as a core structural element in a large family of natural
products and biologically interesting molecules has attracted
increasing attention from research groups worldwide to de-
velop efficient and stereocontrolled methods of constructing
quaternary stereogenic carbons of this type.[1–4] Of the
known 3,3’-disubstituted oxindoles, those that contain an
amine functionality at C3 have been found in many biologi-
cally active compounds[5] and, therefore, the stereoselective
construction of structures of this type is of significant syn-
thetic importance. The known procedures to construct 3-
amino oxindoles have been established by using the asym-
metric amination of 3-substituted oxindoles,[6] addition reac-
tion to isatin imines,[7] and other methods.[8] Despite these
elegant achievements, stereoselective multicomponent syn-
thetic methods to access highly functionalized 3-amino-2-ox-
indole derivatives are still in great demand.
and anilines (or alcohols) with aldehydes. Herein, we report
a highly enantioselective three-component reaction of 3-di-
azooxindoles and anilines with glyoxylates cooperatively cat-
alyzed by a rhodium complex and chiral phosphoric acid,
which gives highly functionalized 3-amino oxindoles with ex-
cellent enantioselectivities (Scheme 1).
Scheme 1. Brønsted acid/rhodium acetate cooperative catalytic asymmet-
ric three-component aldol-type reaction for the synthesis of 3-amino ox-
indoles.
Previously, Hu and co-workers described a three-compo-
nent aldol-type reaction of diazo esters and anilines with
aryl aldehydes by using rhodium(II) acetate as a catalyst.[9]
However, the resultant products were obtained in low dia-
stereoselectivities. Moreover, an enantioselective version of
this protocol has not been realized yet.[10] Hu and our group
found that a combination of the rhodium complex with
chiral phosphoric acids was able to effectively control the
enantioselectivity of three-component Mannich-type reac-
tions that involved diazo esters, alcohols, and imines.[11] Sub-
sequently, Hu and co-workers successfully applied this strat-
egy to other related transformations.[12] Zhou and co-work-
ers identified the fact that binary catalysts of this type
The reaction basically proceeds via a rhodium-catalyzed
generation of ammonium ylides from 3-diazooxindoles (1)
and anilines (2) followed by a chiral Brønsted acid-catalyzed
enantioselective aldol-type reaction with glyoxylates to give
optically active products 4. In this reaction, the phosphoric
acid presumably activates the formyl group of the glyoxy-
lates through a hydrogen-bonding interaction,[15] and simul-
taneously the phosphoryl oxygen might be able to function
as a Lewis base capable of forming an additional hydrogen
bond with ammonium ylides to stabilize the transition state
(Scheme 2).[2e,16] As such, the key point to control the
stereoselectivity would be the chiral Brønsted acids. Howev-
er, anilines are principally able to form imines 3’ with glyox-
ylates[17] and, therefore, the competitive Mannich reaction
that gives undesired 4’ could occur under the catalysis of the
enable the amide and diazo esters to undergo a highly
[13]
À
stereoselective N H insertion reaction. Terada and Toda
demonstrated that the binary catalyst system was able to
render a relay catalytic cascade carbonyl ylide formation
and enantioselective reduction reaction.[14] However, such a
combined catalyst has not yet been applied to the enantiose-
lective three-component aldol-type reactions of diazo esters
[a] L. Ren, X.-L. Lian, Prof. L.-Z. Gong
Hefei National Laboratory for Physical Sciences at the
Microscale and Department of Chemistry
University of Science and Technology of China
Hefei, 230026 (P.R. China)
Fax : (+86)551-360-6266
E-mail: gonglz@ustc.edu.cn
Supporting information for this article is available on the WWW
under vhttp://dx.doi.org/10.1002/chem.201203993.
Scheme 2. The proposed pathway for the title reaction.
Chem. Eur. J. 2013, 19, 3315 – 3318
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3315

Brønsted acid, as reported previously.[11] Thus, the chemose-
lectivity is another challenge in addition to control of the
stereoselectivity (Scheme 2).
entries 7–11). In particular, toluene provided the best results
(Table 1, entry 11). Increasing the stoichiometry of ethyl
glyoxylate considerably improved the yields and maintained
the stereoselectivity (Table 1, entries 12 and 13). Variation
of the N-substituent of 3-diazo oxindoles 1 led to obvious in-
fluences on the reaction (Table 1, entries 14–17). Of these
substrates, N-isopropyl-3-diazo oxindole 1c gave the highest
stereoselectivity (Table 1, entry 15). Lastly, the reaction was
completely inhibited upon lowering the reaction tempera-
ture to 08C (Table 1, entry 17).
The validation of the hypothesis started with a reaction of
N-methyl-3-diazo oxindole 1a with 2-iodoaniline 2a and
ethyl glyoxylate 3a in dichloromethane by using rhodium
acetate (2 mol%) and binol-based phosphoric acid 5a
(10 mol%) as the combined catalyst. As we expected, the
reaction indeed proceeded to give desired product 4aa in
74% yield, but the stereoselectivity turned out to be unsatis-
factory and the minor diastereomer was obtained in higher
enantioselectivity (Table 1, entry 1). It is worth mentioning
Under the optimized conditions, the generality of the re-
action towards anilines was investigated first (Table 2). A
variety of substituents on the aniline could be tolerated. Ba-
Table 1. Optimization of reaction conditions.[a]
Table 2. The generality of the reaction towards anilines.[a]
Entry
2, R
4
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
2b, 2-Br
2c, 2-Cl
2d, 2-CN
2e, 2-CO2Me
2 f, 2-COPh
2g, 2-COMe
2h, 2-CO2Me, 4-Br
2i, 2-CO2Me, 4-Cl
2j, 2,5-(CO2Me)2
2k, 2-COMe, 4-Me
2l, 2-COMe, 4-Br
2m, 2-COMe, 4-Cl
2n, 2-COMe, 4-F
2o, 2-COMe, 3-F
2p, 2-COMe, 5-F
2q, 2-COMe, 4,5-F2
2r, 3-NO2
4cb
4cc
4 cd
4ce
4cf
4cg
4ch
4ci
4cj
4ck
4cl
4 cm
4cn
4co
4cp
4cq
4cr
4cs
99
98
91
85
63
92
95
88
89
74
96
87
81
85
88
90
63
77
11:1
11:1
16:1
13:1
10:1
22:1
38:1
27:1
23:1
19:1
31:1
22:1
38:1
23:1
21:1
16:1
3:1
80
82
91
93
90
97
99
98
99
99
98
90
97
99
95
98
Entry
5
1
Solvent
4
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
5 f
5 f
5 f
5 f
5 f
5 f
5 f
5 f
5 f
5 f
5 f
5 f
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1c
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CCl4
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4aa
4ba
4ca
4da
4ca
74
82
88
72
86
77
60
60
65
62
62
76
70
80
92
65
trace
1.6:1
2.5:1
1.5:1
1.9:1
1.7:1
3.5:1
3.8:1
4.0:1
4.0:1
3.8:1
4.2:1
4.5:1
4.5:1
5.5:1
10:1
5.0:1
–
34 (80)
35 (62)
68 (45)
21 (83)
16 (37)
73 (62)
66 (42)
75 (70)
73 (57)
71 (35)
82 (51)
82 (53)
82 (50)
77 (0)
PhF
PhCl
10
11
12[e]
13[f]
14[e]
15[e]
16[e]
17[e,g]
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
38 (2)
13 (0)
2s, 4-NO2
5:1
85 (55)
74 (50)
–
[a] Reaction conditions: A mixture of 1c (0.15 mmol) and 2 (0.15 mol) in
toluene (1 mL) was added to a mixture of [Rh2A(OAc)4] (2 mol%), 3a
CTHUNGTRENNUNG
(0.75 mmol), and 5 f (10 mol%) in toluene (1 mL) at 258C for 1 h.
[b] Isolated yield. [c] Determined by 1H NMR spectroscopy of the crude
product. [d] The major diastereomer was determined by using HPLC.
[a] Unless indicated otherwise, the reaction was carried out by addition
of 1 (0.15 mmol) and 2a (0.15 mol) in solvent (1 mL) to a mixture of
[Rh2ACHTUNGTRENNUNG(OAc)4] (2 mol%), 3a (1.5 mmol), and 5 (10 mol%) in solvent
(1.0 mL) at 258C for 1 h. [b] Isolated yield. [c] Determined by 1H NMR
spectroscopy of the crude product. [d] Major (minor) diastereomer, de-
termined by using HPLC. [e] The ratio of 1/2a/3a was 1:1:5. [f] The ratio
of 1/2a/3a was 1:1:2. [g] At 08C.
sically, the presence of an electron-withdrawing group on
the aniline provided high enantioselectivity. For example, in
monosubstituted anilines, higher enantioselectivities were
observed for species with an electron-withdrawing group at
the 2-position (Table 2, entries 1–6). Disubstituted anilines
also participated in the three-component reaction in high
yields that ranged from 74 to 96% and with excellent
stereoselectivities of up to 38:1 d.r. and 99% ee (Table 2, en-
tries 7–15). The reaction conditions were also amenable to
trisubstituted anilines, which gave high conversion rates with
excellent stereoselectivity, as exemplified by the reaction
with 2q (Table 2, entry 16). Anilines bearing either a meta-
or a para- substituent also underwent a consecutive transfor-
that the undesired Mannich reaction was not observed (4’ in
Scheme 2), presumably because the aldol-type reaction is
faster than the corresponding Mannich side-reaction. A
survey of chiral phosphoric acids identified (R)-TRIP (5 f;
TRIP=3,3’-bis(2,4,6-triisopropylphenyl)-1,1’-binaphthyl-2,2’-
diyl hydrogen phosphate) as a promising co-organocatalyst
in terms of the diastereo- and enantioselectivity (Table 1,
entries 2–6). Solvent screening revealed that nonpolar sol-
vents are seemingly the reaction media of choice (Table 1,
3316
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 3315 – 3318

Synthesis of 3-Amino Oxindoles
COMMUNICATION
mation to afford 3-amino-2-oxindole derivatives in moderate
yields, but the enantioselectivities were much lower, which
indicates that the stereoselectivity depends greatly on the
substituent position on the aniline (Table 2, entries 17 and
18). The configuration of 4ca was assigned by using X-ray
analysis (see the Supporting Information).
The optimized reaction conditions were next applied to 3-
diazo oxindoles with different substituents at the benzene
ring (Table 3). In general, 3-diazo oxindoles with either elec-
Experimental Section
Typical experimental procedure for the catalytic asymmetric synthesis of
3-amino oxindoles: A mixture of [Rh
acid 5 f (0.015 mmol), and 3 (0.75 mmol) was suspended in toluene
(1 mL) at 258C and stirred for 10 min, then (0.15 mmol) and
2ACHTUNGERTN(NUNG OAc)4] (0.003 mmol), phosphoric
1
2
(0.15 mmol) in toluene (1 mL) was added by syringe over 1 h. The reac-
tion mixture was stirred at this temperature until the reaction was com-
plete (monitored by TLC, 10–24 h). The reaction mixture was directly
subjected to flash column chromatography on silica gel (eluent: petrole-
um ether/ethyl acetate 3:1) to give product 4. The diastereoselectivity
was determined by 1H NMR spectroscopy of the crude product and the
enantiomeric excess was determined by HPLC by using a chiral column.
Table 3. The generality of the reaction towards 3-diazo oxindoles.[a]
Acknowledgements
We are grateful for financial support from NSFC (21232007), MOST (973
project 2010CB833300), BASF, and the Ministry of Education.
Entry 1, R
3, R’
4
Yield [%][b] d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
1e, 4-Cl
1 f, 5-F
3a, OEt 4eh 50
3a, OEt 4 fh 95
3a, OEt 4gh 81
8:1 99
13:1 87
34:1 98
21:1 96
24:1 91
17:1 91
>20:1 97
16:1 82
1g, 5-Me
1h, 5-OMe 3a, OEt 4hh 86
1i, 6-Br
1j, 7-Br
1a
Keywords: aldol reaction · asymmetric catalysis · Brønsted
acid · cooperative effects · indoles
3a, OEt 4ih
3a, OEt 4jh
3b, Me
3c, Ph
88
92
4kh 69
4lh 78
1a
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CTHUNGTRENNUNG
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component reaction smoothly to give the desired products
in excellent enantioselectivities of up to 99% ee. Seemingly,
the position of the substituent has little effect on the enan-
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either 6-bromo- or 7-bromo-3-diazo oxindole (1i or 1j) pro-
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In summary, we have realized a highly stereoselective
three-component aldol-type reaction of 3-diazo oxindoles
and anilines with glyoxylates by using rhodium/chiral
Brønsted acid cooperative catalysis. In this reaction, the am-
monium ylide generated from diazo esters and anilines cata-
lyzed by rhodium acetate acts as an active nucleophile to
participate in an enantioselective aldol-type reaction with
glyoxylates under the catalysis of the chiral phosphoric acid.
The protocol provides a straightforward and efficient route
to access highly functionalized and structurally diverse 3-
amino oxindoles in high optical purity. More importantly,
these findings offer a platform for the rhodium/chiral phos-
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enantioselective versions of other aldol-type transformations
that involve either ammonium or oxonium ylide intermedi-
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Received: November 8, 2012
Published online: February 10, 2013
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