Rhodium-catalyzed Michael addition of arylboronic acids to 3-alkylenyloxindoles: Asymmetric synthesis of functionalized oxindoles

Article abstract of DOI:10.1016/j.tetlet.2011.11.071

The rhodium-catalyzed diastereo- and enantioselective Michael addition of arylboronic acids to 3-alkylenyloxindoles has been developed with (R)-binap as a ligand. A wide variety of the desired functionalized oxindoles are smoothly obtained in high yields

Full text of DOI:10.1016/j.tetlet.2011.11.071

Tetrahedron Letters 53 (2012) 438–441
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Rhodium-catalyzed Michael addition of arylboronic acids to
3-alkylenyloxindoles: asymmetric synthesis of functionalized oxindoles
Xiangyang Zhang a,b,c, Guihua Chen a,b,c, Peng Cao a, Jibing Liu a,b,c, Jian Liao a,b,
⇑
a Natural Products Research Center, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, PR China
b Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, PR China
c Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2011
Revised 4 November 2011
Accepted 15 November 2011
Available online 19 November 2011
The rhodium-catalyzed diastereo- and enantioselective Michael addition of arylboronic acids to 3-alkyl-
enyloxindoles has been developed with (R)-binap as a ligand. A wide variety of the desired functionalized
oxindoles are smoothly obtained in high yields (up to 99%) with high enantioselectivities (up to 92% ee)
and good diastereoselectivities (up to 82:18).
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
3-Alkylenyoxindoles
Functionalized oxindoles
Arylboronic acids
Michael addition
Introduction
MeO2C
N
H
I
TBSO
R
O
Pd(OAc)2, P(o-Tol)3
TEA, Bu4NBr, CO
Ar
R
OTBS
3-Substituted oxindoles constitute one of the most important
building blocks in organic synthesis, and this structural unit is
encountered in many biologically active compounds.1 Owing to
its importance, many efficient approaches for their construction
have been developed.2–5 Among these available methods, 3-alkyl-
enyloxindoles as Michael acceptors have been widely used in the
synthesis of functionalized oxindoles in asymmetric organocataly-
sis.4 However, the transition-metal catalyzed conjugated addition
of 3-alkylenyloxindoles has been less studied.5
N
Me
O
2:1 DMA: MeOH
Ar
85 o
C
Me
1
2
Scheme 1. The catalytic synthesis of functionalized oxindoles.
nitroalkenes and other electron deficient olefins have been exten-
sively investigated in rhodium-catalyzed asymmetric conjugate
reaction.9 To the best of our knowledge, the use of 3-alkylenyloxin-
doles as Michael acceptors in Rh-catalyzed 1,4-addition is still
unexplored. As a part of our continuing interests in the construc-
tion of useful chiral building blocks via transition metal-catalyzed
asymmetric reactions,10 and to develop a new strategy to access
oxindole 2 analogues, herein we would like to report a rhodium-
catalyzed diastereo- and enantioselective Michael addition of aryl-
boronic acids to 3-alkylenyloxindoles.
In 2003, Weinreb and co-workers reported the synthesis of
some special functionalized oxindoles 2 via a halogen-selective
tandem intramolecular Heck/carbonylation reaction (Scheme 1).6
These compounds are valuable intermediates in the synthesis of
the marine ascidian metabolite perophoramidine and fungal
metabolite communesin F.6,7 However, racemic oxindoles 2 limited
the opportunity to obtain those optically pure natural products.
Thus, the exploration of efficient approaches to access enantioen-
riched oxindoles 2 and their similar structures is desirable.
Pioneered by Miyaura and Hayashi,8 asymmetric rhodium cata-
lyzed conjugate addition of organometallic reagents to Michael
acceptors has become a fascinating tool for the construction of
many useful heterocycles.9e–i Several Michael acceptors such as
Result and discussion
Initially, (E)-ethyl 2-(1-benzyl-2-oxoindolin -3-ylidene) acetate
3a11 was employed as the starting substrate to examine the reac-
tion conditions. In the presence of KOH at 100 °C, 5 mol % of (R)-
binap-Rh complex could efficiently catalyze the addition of phenyl
boronic acid to 3a. The desired product 5a was obtained in a 92%
a
,b-unsaturated ketones, esters, amides, 1-alkenylphosphonates,
⇑
Corresponding author.
E-mail addresses: jliao10@cioc.ac.cn, jliao@cib.ac.cn (J. Liao).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.11.071

X. Zhang et al. / Tetrahedron Letters 53 (2012) 438–441
439
Table 1
Screening of various reaction conditionsa
ROOC
ROOC
[RhCl(C2H4)2]2/L
Ph
O
*
(5.0 mol % Rh)
+
O
PhB(OH)2
*
base (50 mol %)
solvent/H2O
N
R'
N
4a
R'
3a: R=Et, R'=Bn
3b: R=Me, R'=Bn
3c: R=tBu, R'=Bn
5a-d
3d: R=tBu, R'=CH3
O
S
S
O
O
S
PPh2
PPh2
PPh2
PPh2
O
O
O
O
O
P
NEt2
PPh2
OMOM
O
(R)-L1
Ligand
(R)-L2
(R)-L3
Base
(R)-L4
(R,R)-L5
Entry
Substrate
Solvent
Temp (°C)
Yieldb (%)
drc
eed (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21e
3a
3a
3a
3a
3a
3b
3c
3d
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
L1
L2
L3
L4
L5
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
Dioxane
Dioxane
Dioxane
CH2Cl2
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
K2CO3
KF
100
100
100
40
92
89
90
40
Trace
95
95
92
Trace
40
10
15
80
83
95
90
94
56
77/23
77/23
77/23
53/47
n.d.
77/23
71/29
66/34
n.d.
87/13
68/32
66/34
74/26
89/11
71/29
70/30
73/27
79/21
76/24
75/25
75/25
78/83
77/77
78/83
37/80
n.d.
76/77
83/82
80/76
n.d.
83/82
70/74
78/85
72/76
80/74
82/82
81/81
83/82
81/86
88/89
88/89
88/89
CH2Cl2
40
Dioxane
Dioxane
Dioxane
Toluene
THF
100
100
100
100
65
75
75
75
75
100
100
100
100
40
DMF
ClCH2CH2Cl
tBuOH
EtOH
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
K3PO4
Et3N
KOH
KOH
KOH
98
93
96
25
40
a
b
c
Reaction conditions: 3 (0.2 mmol), PhB(OH)2 (0.6 mmol), [Rh(C2H4)2Cl]2 (5.0 mol %), ligand (6.0 mol %), and base (1.0 M, 0.1 mL) in 1.0 mL solvent, 10 h.
Isolated yield.
Determined by chiral HPLC or 1H NMR of the crude material.
The ee values were determined by chiral HPLC.
2.5 mol % of Rh was used.
d
e
yield with a 77:23 diastereomeric ratio, as well as 78% and 83% ee
for each isomer (Table 1, entry 1). Other chiral phosphorus ligands
like (R)-segphos L2 or (R)-phosphoramidite L3 could get the similar
results, but there was no improvement of the yield or stereoselec-
tivities (Table 1, entry 2 and 3). Considering chiral sulfoxides were
promising ligands in Rh-catalyzed 1,4-addition reactions, chiral
tert-butanesulfinylphosphine (R)-L410a and (R,R)-1,2-bis(tert-
butylsulfinyl)benzene (R,R)-L510b were also evaluated, but turned
out to be the great loss of the activities (Table 1, entries 4 and 5).
As a whole, (R)-binap was the best ligand we tested for this reac-
tion. In the following, the influence of substituent R on the ester
group of 3 was investigated. Methyl ester 3b afforded the corre-
sponding product in a 95% yield with a 77: 23 diastereomeric ratio,
and a slightly decreased ee of 76% and 77%, respectively (Table 1,
entry 6). Meanwhile, for the tert-butyl ester 3c, the corresponding
adduct was obtained with a 95% yield, 71:29 dr, and 83% ee of the
major diastereomer (Table 1, entry 7). Furthermore, when the N-
protecting group was changed to methyl from Bn group, the diaste-
reoselectivity and the ees were slightly decreased (Table 1, entry 8,
a 92% yield, 66:34 dr, 80% ee for the major diastereomer and 76% ee
for minor diastereomer). Various solvents were surveyed (Table 1,
entries 9–14) and dioxane was the best solvent for this reaction.
Inorganic bases such as K2CO3, KF and K3PO4 showed no significant
difference compared with KOH (Table 1, entries 14–16), and obvi-
ously, organic base (triethylamine) is harmful to the reaction (Ta-
ble 1, entry 18). We found that enantioselectivities can be
promoted to 88% and 89% ee when the temperature was decreased
to 40 °C, and no additional advantage under lower temperature at
25 °C. In addition, reducing the catalyst loading to 2.5% of Rh
showed no influence on ee and the yield decreased slightly (Table
1, entries 19–21).
With the optimal reaction conditions in hand, the substrate
scope was next investigated and the results are summarized in
Table 2.12 Arylboronic acids with different substituents, including
electron-rich and electron-poor, as well as meta- and para-groups,
reacted smoothly with 3-alkylenyloxindoles 3c to generate prod-
ucts 5c–5k with high yields (96–99%), high enantioselectivities
(83–92%) and moderate dr values (up to 76: 24 dr, Table 2, entries
1–8). Disappointingly, the ortho-substituted arylboronic acids
(such as 1-methyl- and 1-methoxy-phenylboronic acid) failed to
react with 3c. On the other hand, 3-alkylenyloxindoles bearing
substituents on 5-position could also be phenylated efficiently un-
der these conditions, indicating traceless electronic influences of
oxindole block (Table 2, entries 9-10). However, 4-Br- 3-alkyleny-
loxindole demonstrated moderate dr and ee (Table 2, entry 11). In
the end, when the substituent R3 on 3 was not ester but methyl

440
X. Zhang et al. / Tetrahedron Letters 53 (2012) 438–441
Table 2
Table 2 (continued)
Asymmetric conjugate addition of arylboronic acids to 3-alkylenyloxindolesa
Entry
10
Product
Yieldb (%)
drc
eed (%)
78/84
O
R3
R3
O
O
[RhCl(C2H4)2]2/(R)-BINAP
R1
Ar
O
*
*
R1
(5.0 mol % Rh)
Cl
5m
96
82/18
+
ArB(OH)2
*
*
O
dioxane/H2O
O
N
KOH (50 mol %)
R2
N
N
4
40 oC, 10 h
R2
3
5
Bn
Entry
1
Product
Yieldb (%)
drc
eed (%)
88/89
O
*
Br
O
11e
5n
5o
86
66/34
50/52
*
O
*
O
N
Bn
5c
5e
98
76/24
70/30
*
O
N
Bn
*
*
O
12
Trace
n.d.
n.d.
OMe
O
O
O
*
N
Bn
2
3
98
97
88/87
86/83
*
O
a
N
Bn
Reaction conditions: substrate
[Rh(C2H4)2Cl]2 (5.0 mol % Rh), (R)-BINAP(6.0 mol %), and KOH (1.0 M, 0.1 mL) in
3
(0.2 mmol), ArB(OH)2 (0.6 mmol),
1.0 mL dioxane.
O
b
Isolated yield.
Me
Determined by chiral HPLC or 1H NMR of the crude material.
The ee values were determined by chiral HPLC.
The reaction time is 20 h.
c
*
d
5f
5g
5h
5i
70/30
67/33
75/25
71/29
69/31
75/25
78/22
*
e
O
N
Bn
O
group, only a trace amount of the desired product was obtained
(Table 2, entry 12).
O
O
O
O
O
*
4
5
6
7
8
9
98
98
99
97
96
99
86/83
88/89
88/88
90/92
91/91
78/81
*
O
O
O
O
O
N
Conclusion
Bn
O
In summary, we have disclosed the rhodium-catalyzed diaste-
reo- and enantioselective Michael addition of arylboronic acids to
3-alkylenyloxindoles. The corresponding adducts are obtained in
high yields (up to 99%) with high enantioselectivities (up to 92%
ee) and good diastereoselectivities (up to 82:18). The present reac-
tion provides a simple approach to synthesize optically active func-
tionalized oxindole derivatives. The further transformations and
applications in the key synthetic scaffolds in medicinal chemistry
are underway in our laboratory.
Cl
*
*
N
Bn
O
F
*
*
N
Acknowledgments
Bn
O
We are grateful for support from the NSFC (21072186 and
20872139), the West Light Foundation of Chinese Academy of Sci-
ences, and the National Basic Research Program of China (973 Pro-
gram, 2010CB833300) and Chengdu Institute of Biology, Chinese
Academy of Sciences (Y0B1051100).
*
5j
OMe
*
N
Bn
O
Supplementary data
*
5k
Cl
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.11.071.
*
N
Bn
References and notes
O
O
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*
*
Br
5l
O
N
Bn

X. Zhang et al. / Tetrahedron Letters 53 (2012) 438–441
441
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(R)-binap (7.5 mg, 6.0 mol %), [RhCl(C2H4)2]2 (1.9 mg, 5.0 mol % Rh) and 1.0 mL
dioxane. The mixture was stirred at 25 °C for 40 min, and then arylboronic
acids (0.6 mmol, 3.0 equiv), substrate 3 (0.2 mmol) and degassed aq. KOH
(1.0 M, 0.1 mL, 0.1 mmol) was added sequentially. The mixture was stirred for
appropriate time at 40 °C and directly charged onto a column (silica gel) for
flash chromatography with a mixture of Petroleum ether/EtOAc to afford
product 5.
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