Highly enantioselective organocatalytic -amination reactions of aryl oxindoles: Developing designer multifunctional alkaloid catalysts

Article abstract of DOI:10.1021/ol102493q

An enantioselective -amination of aryl oxindoles catalyzed by a dimeric quinidine has been developed. The reaction is general, broad in substrate scope, and affords the desired products in good yields with good to excellent enantioselectivities. This stud

Full text of DOI:10.1021/ol102493q

ORGANIC
LETTERS
2010
Vol. 12, No. 24
5696-5699
Highly Enantioselective Organocatalytic
r-Amination Reactions of Aryl
Oxindoles: Developing Designer
Multifunctional Alkaloid Catalysts
Tommy Bui, Gloria Herna´ndez-Torres, Ciro Milite, and Carlos F. Barbas III*
The Skaggs Institute for Chemical Biology and the Departments of Chemistry and
Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road,
La Jolla, California 92037, United States
carlos@scripps.edu
Received October 18, 2010
ABSTRACT
An enantioselective r-amination of aryl oxindoles catalyzed by a dimeric quinidine has been developed. The reaction is general, broad in substrate
scope, and affords the desired products in good yields with good to excellent enantioselectivities. This study provides the first examples of a general
organocatalytic method for the creation of nitrogen-containing, tetrasubstituted chiral centers at C3 of various aryl oxindoles. Furthermore, new
catalysts and insights into structural elements of the catalysts that significantly influence enantioselectivities are disclosed.
Catalyst design is instrumental to the development of new
organic transformations, an endeavor that is important for the
advancement of synthetic methodology. Although organocata-
lysts have been designed and developed, a detailed understand-
ing of how they work often lags behind their extensive
applications.1 Recently, we disclosed a novel, dimeric quinidine
catalyst and demonstrated its effectiveness in enantioselective
aminooxygenation of oxindoles.2 This multifunctional catalyst,
with its seemingly flexible 1,3-dibenzyl tether, was far more
Figure 1. (a) Dimeric quinidine catalyst and (b) aryl oxindole
effective than other structurally rigid cinchona alkaloid dimers
tested (Figure 1a).2 These observations were counterintuitive
from a catalyst design perspective and prompted us to explore
the use of this catalyst in other asymmetric transformations,
particularly in enantioselective R-amination reactions of aryl
oxindoles. These reactions would provide access to optically
enolates.
active 3-amino aryl oxindoles, common structural motifs present
in a variety of bioactive molecules, including NITD609 and
SSR-149415, which are drug candidates for the treatment of
malaria and stress-related disorders, respectively.3,4
To date, there is no general catalytic method for the
asymmetric synthesis of 3-amino aryl oxindoles, especially those
with readily cleavable diazo compounds such as di-tert-butyl
(1) Berkessel, A.; Groger, H. In Asymmetric Organocatalysis - From
Biomimetic Concepts to Applications in Asymmetric Synthesis; Wiley-VCH:
Verlag GmbH & Co. KGaA, Weinheim, 2005.
(2) Bui, T.; Candeias, N. R.; Barbas, C. F., III J. Am. Chem. Soc. 2010,
132, 5574.
10.1021/ol102493q  2010 American Chemical Society
Published on Web 11/11/2010

azodicarboxylates.4a,5-8 Several challenges arise when oxin-
doles bear an aryl substituent at the C3 position. First, the
aryl group renders the C3 methine acidic, facilitating a
background reaction to occur. Conversely, it sterically hinders
this position, thereby limiting reactivity. Finally, it is difficult
to differentiate the two enantiotopic faces of the oxindole
enolate at C3 when this position is flanked by two aryl groups
of a similar size, one of which is the aryl oxindole ring
(Figure 1b).
Herein, we report highly enantioselective R-amination reac-
tions of aryl oxindoles with Boc-protected diazo electrophiles.
We also provide insight into the role of the dimeric structure
of the catalyst that has informed our design of novel multi-
functional alkaloid catalysts for this reaction. The chemistry
described herein provides the first examples of a general
organocatalytic method for the synthesis of aryl oxindoles
bearing a tetrasubstituted nitrogen-containing center at C3.4a,5,6
Initially, a model reaction using phenyl oxindole 1a and di-
tert-butyl azodicarboxylate was examined in the presence of
cinchona alkaloid catalysts (Figure 2, Table 1). With 10 mol
% of monomeric quinidine-derived catalyst I, II, or III, the
R-amination reaction proceeded smoothly at -20 °C to afford
product 3a in good yields, albeit low enantiomeric excess (ee)
(entries 1-3). Similarly, commercially available bulky hydro-
quinidine dimers IV and V also catalyzed the R-amination and
provided products in moderate yields with low ee (entries 4
Figure 2. Catalyst screen.
Table 1. Optimization Studiesa
(3) (a) Rottmann, M.; McNamara, C.; Yeung, B. K. S.; Lee, M. C. S.;
Zou, B.; Russell, B.; Seitz, P.; Plouffe, D. M.; Dharia, N. V.; Tan, J.; Cohen,
S. B.; Spencer, K. R.; Gonza´lez-Pa´ez, G. E.; Lakshminarayana, S. B.; Goh,
A.; Suwanarusk, R.; Jegla, T.; Schmitt, E. K.; Beck, H.-P.; Brun, R.; Nosten,
F.; Renia, L.; Dartois, V.; Keller, T. H.; Fidock, D. A.; Winzeler, E. A.;
Diagana, T. T. Science 2010, 329, 1175. (b) Griebel, G.; Simiand, J.; Gal,
C. S-L.; Wagnon, J.; Pascal, M.; Scatton, B.; Maffrand, J.-P.; Soubrie, P.
Proc. Natl. Acad. Sci. 2002, 99, 6370. (c) Leugn, C.; Tomaszewski, M.;
Woo, S. (Astrazeneca AB, Swed.). U.S. Pat. Appl. Publ. ( 2007), CODEN:
USXXCO US 2007185179 A1 20070809 Patent written in English.
entry catalyst
solvent
temp (°C) yieldb (%) eec (%)
1
I
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
CH2Cl2
Et2O
toluene
toluene
m-xylene
toluene
-20
-20
-20
-20
-20
-20
-20
-20
-20
-50
-70
-70
-70
-70
-70
-70
-70
86
74
81
77
51
96
94
84
93
87
78
89
72
87
93
73
86
45
27
14
0
35
5
41
35
53
83
88
79
92
95
98
83
83
2
II
3
4
5
III
IV
V
Application: US 2007-671153 20070205
.
6
7
8
9
10
11
12
13
14
15d
16
17d
VI
VII
VIII
IX
IX
IX
IX
IX
IX
IX
IX
X
(4) Recent reviews on oxindoles and their biological activities: (a) Zhou,
F.; Liu, Y.-L.; Zhou, J. AdV. Synth. Catal. 2010, 352, 1381. (b) Peddibhotla,
S. Curr. Bioact. Compd. 2009, 5, 20. (c) Dounay, A. B.; Overman, L. E.
Chem. ReV. 2003, 103, 2945. (d) Marti, C.; Carreira, E. M. Eur. J. Org.
Chem. 2003, 2209
.
(5) Bui, T.; Borregan, M.; Barbas III, C. F. J. Org. Chem. 2009, 74,
8935
.
(6) Organocatalytic R-amination of alkyl oxindoles: (a) Cheng, L.; Liu,
L.; Wang, D.; Chen, Y.-J. Org. Lett. 2009, 11, 3874. (b) Qian, Z.-Q.; Zhou,
F.; Du, T.-P.; Wang, B.-L.; Zhou, J. Chem. Commun. 2009, 6753. In ref
1a, there is one example of R-amination of an aryl oxindole using
di-isopropyl azodicarboxylate to afford product in 74% yield with 63% ee.
For the Lewis acid-based approach: (c) Mouri, S.; Chen, Z.; Mitsunuma,
H.; Furutachi, M.; Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2010,
132, 1255. (d) Yang, Z.; Wang, Z.; Bai, S.; Shen, K.; Chen, D.; Liu, X.;
Lin, L.; Feng, X. Chem.sEur. J. 2010, 16, 6632 (in this reference there is
one example concerning an aryl oxindole). (e) Jia, Y.-X.; Hillgren, J. M.;
Watson, E. L.; Marden, S. P.; Kundig, E. P. Chem. Commun. 2008, 4040.
For the chiral auxiliary-based approach: (f) Lesma, G.; Landoni, N.; Pilati,
T.; Sacchetti, A.; Silvani, A. J. Org. Chem. 2009, 74, 4537. (g) Emura, T.;
a Unless otherwise noted, all reactions were run for 24 h. Reactions in
entries 1-8 employed 10 mol % of catalyst; entries 9-17 employed 5 mol
%. b Isolated yields. c Determined by chiral HPLC analysis. d Reactions
run for 48 h.
Esaki, T.; Tachibana, K.; Shimizu, M. J. Org. Chem. 2006, 71, 8559
.
(7) For related amine-catalyzed R-amination of aldehydes and ketones:
(a) Kumaragurbaran, N.; Juhl, K.; Zhuang, W.; Bogevig, A.; Jorgensen,
K. A. J. Am. Chem. Soc. 2002, 124, 6254. (b) Bogevig, A.; Juhl, K.;
Kumaragurbaran, N.; Zhuang, W.; Jorgensen, K. A. Angew. Chem., Int.
Ed. 2002, 41, 1790. (c) List, B. J. Am. Chem. Soc. 2002, 124, 5656. (d)
Chowdari, N. S.; Barbas III, C. F. Org. Lett. 2005, 7, 867. (e) Vogt, H.;
Vanderheiden, S.; Brase, S. Chem. Commun. 2003, 2448. (f) Dahlin, N.;
Bogevig, A.; Adolfson, H. AdV. Synth. Catal 2004, 346, 1101. For
organocatalytic enantioselective R-amination of ꢀ-ketoesters and derivatives
thereof: (g) Pihko, P. M.; Pohjakallio, A. Synlett 2004, 12, 2115. (h) Saaby,
S.; Bella, M.; Jorgensen, K. A. J. Am. Chem. Soc. 2004, 126, 8120. (i) Liu,
X.; Li, H.; Deng, L. Org. Lett. 2005, 7, 167. (j) Terada, M.; Nakano, M.;
and 5). Interestingly, these dimers were not as effective as I
(entry 1 vs entries 4-5). Previously developed dimeric catalysts
VI-VIII from our laboratory were also tested, but they did
not give satisfactory results with respect to ee (entries 6-8).2
These catalysts were synthesized by dimerization of quinidine
at the C9 position using different dibromo benzyl linkers.
Notably, catalyst VII with a free hydroxyl group at C6 provided
product in higher ee than catalyst VI (entry 7 vs entry 6). The
catalyst with the 1,3-dibenzyl linker was more effective than
Ube, H. J. Am. Chem. Soc. 2006, 128, 16044
.
Org. Lett., Vol. 12, No. 24, 2010
5697

that with the 1,2-dibenzyl linker (entry 8 vs entry 6). Not
surprisingly, C-6 quinidine dimer IX (5 mol %) was most
effective among the catalysts examined. These results mirrored
those obtained from previously reported catalytic, enantiose-
lective aminooxygenations of alkyl oxindoles.2
Having established the optimal reaction conditions, we began
to investigate the scope of the R-amination reaction with respect
to oxindole substrates (Table 2). The reaction was general in
The moderate enantioselectivity obtained with catalyst IX
warranted further investigation. We discovered that Boc-
protected aryl oxindole 1a was highly reactive, and a competing
nonselective reaction occurred (due to the inherent reactivity
of aryl oxindoles vida supra), thereby compromising the
enantioselectivity. This problem was solved by performing the
R-amination reaction at low temperature (entry 9 vs entry 10).
However, the ee increased only marginally when the temper-
ature was lowered from -50 to -70 °C (entry 10 vs entry 11).
A survey of solvents resulted in conditions that provided
excellent yield and ee: the optimal results were obtained when
the R-amination was performed in toluene at -70 °C for 48 h
(entry 15). The long reaction time was required to ensure good
yield and ee of the desired product (24 h, entry 14 vs 48 h,
entry 15). As previously observed, the free hydroxyl groups in
catalyst IX were important for high yield and enantioselectivity
(entry 15 vs entry 17). These results suggest that these hydroxyl
groups might direct or orient the incoming azadicarboxylate
electrophile via weak hydrogen bonding before C-N bond
formation takes place.2,9
Table 2. Enantioselective R-Aminations of Aryl Oxindoles
entry
X
Y
product
yielda (%)
eeb (%)
1
2
3
4
5
6
7
8
H
OMe
H
H
H
H
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
93
96
71
95
76
87
94
92
91
93
87
98
95
96
97
96
96
98
97
73
93
87
4′-Me
3′-OMe
4′-tBu
4′-F
4′-Me
3′-OMe
4′-F
H
OMe
OMe
OMe
OCF3
F
9
10
11
3′-OMe
3′-OMe
3j
3k
a Isolated yields. b Determined by chiral HPLC analysis.
(8) For selected related C-C bond-forming reactions involving oxindoles
in organocatalysis :(a) Bui, T.; Syed, S.; Barbas, C. F., III J. Am. Chem.
Soc. 2009, 131, 8756. (b) He, R.; Ding, C.; Maruoka, K. Angew. Chem.,
Int. Ed. 2009, 48, 4559. (c) Galzerano, P.; Bencivenni, G.; Pesciaioli, F.;
Mazzanti, A.; Giannichi, B.; Sambri, L.; Bartoli, G.; Melchiorre, P.
Chem.sEur. J. 2009, 15, 7846. (d) Duffey, T. A.; Shaw, S. A.; Vedejs, E.
J. Am. Chem. Soc. 2009, 131, 14. (e) Shaw, S. A.; Aleman, P.; Christy, J.;
Kampf, J. W.; Va, P.; Vedejs, E. J. Am. Chem. Soc. 2006, 128, 925. (f)
Ogawa, S.; Shibata, N.; Inagaki, J.; Nakamura, S.; Toru, T.; Shiro, M.
Angew. Chem., Int. Ed. 2007, 46, 8666. (g) Tian, X.; Jiang, K.; Peng, J.;
Du, W.; Chen, Y.-C. Org. Lett. 2008, 10, 3583. (h) Chen, X.-H.; Wei, Q.;
Luo, S.-W.; Xao, H.; Gong, L.-Z. J. Am. Chem. Soc. 2009, 131, 13819. (i)
Wei, Q.; Gong, L.-Z. Org. Lett. 2010, 12, 1008. (j) Bencivenni, G.; Wu,
L.; Mazzanti, A.; Giannichi, B.; Pesciaioli, F.; Song, M.; Bartoli, G.;
Melchiorre, P. Angew. Chem., Int. Ed. 2009, 48, 7200. (k) Cucinotta, C. S.;
Kosa, M.; Melchiorre, P.; Cavalli, A.; Gervasio, F. L. Chem.sEur. J. 2009,
15, 7913. (l) Deng, J.; Zhang, S.; Ding, P.; Jiang, H.; Wang, W.; Li, J.
AdV. Syn. Catal. 2010, 352, 833. (m) Zhu, Q.; Lu, Y. Angew. Chem., Int.
Ed. 2010, DOI: 10.1002/anie.201003837. Selected references for C-C bond-
forming reactions of oxindoles involving transition metal catalysis: (n) Kato,
Y.; Furutachi, M.; Chen, Z.; Mitsunuma, H.; Matsunaga, S.; Shibasaki, M.
J. Am. Chem. Soc. 2009, 131, 9168. (o) Doumay, A. B.; Hatanaka, K.;
Kodanko, J. J.; Oestreich, M.; Overman, L. E.; Pfeifer, L. A.; Weiss, M. M.
J. Am. Chem. Soc. 2003, 125, 6261. (p) Trost, B. M.; Zhang, Y. J. Am.
Chem. Soc. 2006, 128, 4590. (q) Trost, B. M.; Zhang, Y. J. Am. Chem.
Soc. 2007, 129, 14548. (r) Lee, S.; Hartwig, J. F. J. Org. Chem. 2001, 66,
3402. (s) Hills, I. D.; Fu, G. C. Angew. Chem., Int. Ed. 2003, 42, 3921. (t)
Kozlowski, M. C.; Linton, E. C. J. Am. Chem. Soc. 2008, 130, 16162.
Synthesis of oxindoles via organocatalytic additions of carbonyl compounds
to isatins: (u) Luppi, G.; Monari, M.; Correa, R. J.; Violante, F. A.; Pinto,
A. C.; Kaptein, B.; Broxterman, Q. B.; Garden, S. J.; Tomasini, C.
Tetrahedron 2006, 62, 12017. (v) Chen, J.-R.; Liu, X.-P.; Zhu, X.-Y.; Li,
L.; Qiao, Y.-F.; Zhang, J.-M.; Xiao, W.-J. Tetrahedron 2007, 63, 14037.
(w) Itoh, T.; Ishikawa, H.; Hayashi, Y. Org. Lett. 2009, 11, 3854.
Asymmetric synthesis of oxindoles via nucleophilic additions to isatins using
metal catalysis: (x) Toullec, P. Y.; Jagt, R. B.; de Vries, J. G.; Feringa,
B. L.; Minnaard, A. L. Org. Lett. 2006, 8, 2715. (y) Shintani, R.; Inoue,
M.; Hayashi, T. Angew. Chem., Int. Ed. 2006, 45, 3353. (z) Tomita, D.;
Yamatsugu, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc. 2009, 131,
6946. (aa) Grant, C. D.; Krische, M. J. Org. Lett. 2009, 11, 4485. (bb)
Itoh, J.; Han, S. B.; Krische, M. J. Angew. Chem., Int. Ed. 2009, 48, 6313.
(cc) Lai, H.; Huang, Z.; Wu, Q.; Qin, Y. J. Org. Chem. 2009, 74, 283. (dd)
Hanhan, N. V.; Sahin, A. H.; Chang, T. W.; Fettinger, J. C.; Franz, A. K.
Angew. Chem., Int. Ed. 2010, 49, 744.
scope, tolerating aryl oxindoles of different electronic natures
and with different aromatic substitution patterns. For example,
oxindoles bearing electron-neutral and electron-rich substituents
at C5 afforded the desired products in excellent yields and ee’s
(entries 1-2). Moreover, reactions with substrates with various
aryl groups at the C3 position also provided products in moderate
to good yields with excellent ee’s (entries 3-6). It is noteworthy
that oxindole 3f, which contains a fluorine atom, was obtained
in good yield and high ee (entry 6). Enantiomerically enriched
syntheses of fluorine-containing molecules are of importance
in drug discovery and development.10 Under our optimized
conditions, oxindoles with various substituents at C5 and
different substitution patterns on the C3-substituted aryl ring
were all viable substrates (entries 7-11). The yields and ee’s
of the desired products were high for most of these substrates.
The absolute configuration at the newly created center was
determined to be R by comparison with a known oxindole
derivative of 3a.11 Finally, we demonstrated that product 3a
could be converted into the corresponding, free amino aryl
oxindole in good yield with excellent optical purity.11
To determine the role of the dimeric structure and the quinidine
units in catalysis by IX, analogues were synthesized and examined
in the enantioselective R-amination of 1a (Table 3). Catalysts XI
and XII had only one quinuclidine moiety within the catalyst
(9) Hydroxy-directing effects of cinchona alkaloid catalysts: (a) Hiem-
stra, H.; Wynberg, H. J. Am. Chem. Soc. 1981, 103, 417. (b) Li, H.; Wang,
Y.; Tang, L.; Deng, L. J. Am. Chem. Soc. 2004, 126, 9906.
(10) Banks, R. E.; Smart, B. E.; Tatlow, J. C. Organofluorine Chemistry:
Principles and Commercial Applications; Plenum Press: NewYork, 1994.
(11) See Supporting Information.
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Org. Lett., Vol. 12, No. 24, 2010

Table 3. Insight into the Role of the Dimeric Structure
entry
catalyst
product
yielda (%)
eeb
1
2
3
4
5
IX
XI
XII
XIII
I
3a
3a
3a
3a
3a
93
94
89
89
85
98
98
98
94
79
a Isolated yield. b Determined by chiral HPLC analysis.
molecule. In XI, the 1,3-dibenzyl linker was attached to an electron-
rich π-quinoline ring, and in XII, it was attached to a simple phenyl
ring. If π-π interactions between the two quinoline units in IX
and XI were critical for catalysis, one would expect that XII would
be a less effective catalyst.12 In catalyst XIII, the linker was a single
benzyl group. The molecular weight and complexity of catalyst
IX is greater than those of analogues XI, XII, and XIII.
Significantly, catalysts IX, XI, and XII provided the desired
product in similar yields with excellent ee’s (Table 3, entries 1-3).
Interestingly, when monomeric catalyst XIII was used, the ee was
lower (entry 4 vs entries 1-3). The drop in ee was even more
significant when the 1,3-dibenzyl linker was replaced by a methyl
group in catalyst I (entry 5). The effects of the C6-alcohol protecting
group on ee were striking. This effect was not expected as this
group is distal to the quinuclidine nitrogen atom where the reactive
oxindole enolate is presumably generated. These observations
provide valuable information that will aid future catalyst design
and development. Our hypothesis is that the hydroxy protecting
group at C6 affects the conformation of quinidine; this conformation
impacts the stereochemistry-determining step and ultimately the
enantioselectivity of the R-amination reaction (Figure 3, A-C).12,13
Our proposed transition state structures are consistent with the
following experimental observations: (a) the importance of a π-rich
moiety that protects the C6 hydroxyl group in the catalyst to
maintain high selectivity (Figure 3B), (b) the free hydroxyl group
at C9 to ensure high yield and ee, and (c) the same sense of
asymmetric induction is observed for both aminooxygenation and
R-amination of oxindoles.2,11
Figure 3. Proposed active conformer (A) and proposed transition
state structures with catalyst XII (B and C).
In summary, we have developed dimeric and designer quinidine-
catalyzed enantioselective R-aminations of aryl oxindoles to afford
the desired products in good yields with good to excellent
enantioselectivities. These reactions are general and provide entry
to a broad range of aryl oxindoles including those that contain
fluorine atoms. The organocatalytic method employs readily
cleavable Boc-protected diazo electrophiles for the R-amination
and allows access to optically active 3-amino aryl oxindoles with
minimal functional group manipulation under mild conditions. The
reactions reported here provide the first examples of a general
organocatalytic approach to the generation of tetrasubstituted
nitrogen-containing centers at the C3 position of a variety of aryl
oxindoles. Our experiments revealed important and somewhat
unexpected structural elements of the catalyst that are crucial for
high enantioselectivities. These insights allowed for the design and
development of practical, multifunctional catalysts that possess
Bronsted acidic, Lewis basic, and π-stacking sites, that together
enable effective catalysis of the R-amination reaction. These
catalysts will be investigated in other asymmetric transformations,
and the results of these studies will be reported in due course.
Acknowledgment. We thank the Skaggs Institute for
Chemical Biology for funding.
Supporting Information Available: General experimental,
general procedures, spectroscopic data for catalysts IX and
XI-XII, synthetic route to 3l [(R)-3-amino-3-phenyloxindole],
determination of the absolute configuration, HPLC chromato-
grams, and NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
(12) We believe that the electron-rich π-systems of the two quinoline
moieties in IX are likely to participate in π-π interactions with the oxindole
substrate. See: (a) Dolling, U.-H.; Davis, P.; Grabowski, E. J. J. J. Am.
Chem. Soc. 1984, 106, 446. (b) Knowles, R. R.; Lin, S.; Jacobsen, E. N.
J. Am. Chem. Soc. 2010, 132, 5030.
(13) (a) Studies of the active conformation of cinchona alkaloid catalysts:
Li, H.; Liu, X.; Wu, F.; Tang, L.; Deng, L. Proc. Natl. Acad. Sci. 2010,
DOI: 10.1073/pnas.1004439107, Early Edition. (b) Calculation of the lowest-
energy conformation of cinchona alkaloids: Caner, H.; Biedermann, P. U.;
Agranat, I. Chirality 2003, 15, 637
.
OL102493Q
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5699