Cationic Ir/Me-BIPAM-catalyzed asymmetric intramolecular direct hydroarylation of α-ketoamides

Article abstract of DOI:10.1002/anie.201400147

Asymmetric intramolecular direct hydroarylation of α-ketoamides gives various types of optically active 3-substituted 3-hydroxy-2-oxindoles in high yields with complete regioselectivity and high enantioselectivities (84-98 % ee). This is realized by the use of the cationic iridium complex [Ir(cod) ₂](BAr^F₄) and the chiral O-linked bidentate phosphoramidite (R,R)-Me-BIPAM. Carbon's got a brand new bond: Asymmetric intramolecular direct hydroarylation of α-ketoamides gives various optically active 3-substituted 3-hydroxy-2-oxindoles in high yields with complete regioselectivity and high enantioselectivities. This is realized by the use of an asymmetric cationic iridium complex formed in situ (see Scheme).

Full text of DOI:10.1002/anie.201400147

.
Angewandte
Communications
DOI: 10.1002/anie.201400147
À
Asymmetric C H Functionalization
Cationic Ir/Me-BIPAM-Catalyzed Asymmetric Intramolecular Direct
Hydroarylation of a-Ketoamides**
Tomohiko Shirai, Hajime Ito, and Yasunori Yamamoto*
Abstract: Asymmetric intramolecular direct hydroarylation of
a-ketoamides gives various types of optically active 3-substi-
tuted 3-hydroxy-2-oxindoles in high yields with complete
regioselectivity and high enantioselectivities (84–98% ee).
This is realized by the use of the cationic iridium complex
there is still room for improvement in terms of the enantio-
selectivity. In the course of our study on bidentate phosphor-
amidites as chiral ligands for enantioselective bond-forming
reactions, we achieved direct synthesis of chiral 3-substituted
3-hydroxy-2-oxindoles through
a highly enantioselective
[Ir(cod)2](BArF ) and the chiral O-linked bidentate phosphor-
intramolecular hydroarylation reaction of a-ketoamides by
the use of a cationic iridium and chiral O-linked bidentate
phosphoramidite ((R,R)-Me-BIPAM).
4
amidite (R,R)-Me-BIPAM.
O
xindoles containing a chiral tetrasubstituted carbon at the
We first examined the use of an a-ketoamide (1) for the
asymmetric intramolecular direct hydroarylation reaction in
the presence of a cationic iridium complex with (R,R)-Me-
BIPAM as the catalyst (Table 1). Our initial screening of
counteranions for the cationic iridium complex indicated that
3-position are common structural motifs in many biologically
active compounds.[1] Among them, 3-substituted 3-hydroxy-2-
oxindoles are particularly noteworthy, and various methods
for the synthesis of these chiral compounds have been
developed in the past decade.[2–6] Transition-metal-catalyzed
asymmetric nucleophilic addition reactions of organoboronic
acid derivatives to isatins are powerful and straightforward
approaches.[7] In this field, we have already reported that
a chiral bidentate phosphoramidite ligand (Me-BIPAM),
previously developed for the enantioselective 1,4-addition
of arylbonic acids to enones,[8] the arylation of imines[9] and
carbonyl compounds,[10] and the hydrogenation of a-dehy-
droaminoesters,[11] was efficient for ruthenium-catalyzed
addition reactions of arylboronic acids to isatins.[10d] Transi-
the [Ir(cod)2](BArF ) (cod = cyclooctadiene, BArF
=
4
4
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate)
complex
À
would be more favorable than other counteranions (BF4
,
SbF6À, TfOÀ, ClO4À, and ClÀ), although the yield and
enantioselectivity were moderate (62%, 71% ee) (entries 1–
6). All reactions selectively gave 4-acetyl-3-hydroxy-3-
phenyl-2-oxindole (2) with complete regioselectivity by
À
enantioselective direct addition at the C H bond in the
more hindered ortho position to a carbonyl group. Further
optimization of reaction conditions was performed using
tion-metal-catalyzed C H functionalization has emerged in
[Ir(cod)2](BArF ). As a result of screening several solvents,
À
4
recent years as a powerful tool for the formation of carbon–
carbon bonds from simple starting materials.[12,13] Very
recently, it was shown that direct nucleophilic addition to
the highest efficiency with regard to the reaction was
Table 1: Optimization of reaction conditions.[a]
À
imines or carbonyls through transition-metal-catalyzed C H
bond activation provides a concise and highly efficient
pathway to synthesize amines and alcohols.[14–17] Intramolec-
À
ular cyclizations by C H bond activation have been reported
for the synthesis of oxindoles.[18] Iridium complexes have also
À
been shown to be efficient catalysts for C H bond functio-
nalization.[13e,19,20] In 2009, Shibata and co-workers reported
cationic Ir/(S)-H8-BINAP-catalyzed enantioselective synthe-
sis of a chiral 4-acetyl-3-hydroxy-3-methyl-2-oxindole with
À
Entry Catalyst (mol%)
Solvent Yield of 2/3 [%] ee of 2/3 [%]
72% ee using the method of direct C H bond functionaliza-
tion.[20a] Although this reaction is the first example of
1
[Ir(cod)2](BArF ) (5) C6H5Cl
62/trace
37/trace
12/trace
15/trace
3/trace
n.r.
71/–
53/–
38/–
29/–
29/–
–
4
À
enantioselective direct addition of a C H bond to ketones,
2
[Ir(cod)2](BF4) (5)
C6H5Cl
3
4
5
[Ir(cod)2](SbF6) (5) C6H5Cl
[Ir(cod)2](OTf) (5) C6H5Cl
[Ir(cod)2](ClO4) (5) C6H5Cl
[*] T. Shirai, Prof. Dr. H. Ito, Prof. Dr. Y. Yamamoto
Division of Chemical Process Engineering and Frontier Chemistry
Center (FCC), Faculty of Engineering, Hokkaido University
Kita 13 Nishi 8 Kita-ku, Sapporo, Hokkaido 060-8628 (Japan)
E-mail: yasuyama@eng.hokudai.ac.jp
6
7
8
9
[Ir(cod)2]Cl (5)
C6H5Cl
[Ir(cod)2](BArF ) (5) THF
94/trace
90/trace
66/–
88/–
77/–
58/–
57/20
4
[Ir(cod)2](BArF ) (5) DME
4
[Ir(cod)2](BArF ) (5) dioxane 28/trace
4
10
[Ir(cod)2](BArF ) (5) diglyme 19/trace
4
[**] This work was financially supported by the MEXT (Japan) program
“Strategic Molecular and Materials Chemistry through Innovative
Coupling Reactions” of Hokkaido University.
11[b]
[Ir(cod)2](BArF ) (5) diglyme 21/27
4
[a] Reaction conditions: a-ketoamides (0.25 mmol), iridium catalyst
(5 mol%), and (R,R)-Me-BIPAM (1.1 equiv to Ir) in solvent (1 mL),
stirred for 24 h at 1358C. [b] Run at 1508C. n.r.=no reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400147.
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Table 2: Optimization of reaction conditions.[a]
Entry DG
Ir precatalyst
(mol%)
Ligand
Yield
[%]
ee[b]
[%]
1
Ac
[Ir(cod)2](BArF )
(R,R)-Me-
BIPAM
(R,R)-Me-
BIPAM
(R,R)-Me-
BIPAM
(R,R)-Me-
BIPAM
90
90
88
4
(5)
2
Bz
[Ir(cod)2](BArF )
88
4
(5)
Figure 1. ORTEP diagram of 11a, with thermal ellipsoids set at 50%.
3
CO2Me
[Ir(cod)2](BArF )
37
95
4
(5)
4
CONMe2 [Ir(cod)2](BArF )
>99
>99
96
98 (S)
98 (S)
97 (S)
82
to a variety of a-ketoamides. In most cases, complete
regioselectivity, high yield, and excellent ee values were
obtained. Both electron-donating and electron-withdrawing
substituents on the aromatic ring at the ketone side of the
substrates were tolerated. In some cases (entries 9 and 12),
the enantioselectivity decreased slightly. However, the enan-
tioselectivity was improved by using preformed [Ir(cod)-
4
(5)
5[c]
6
CONMe2 [Ir(cod)2](BArF ) (5) (R,R)-Me-
4
BIPAM
(R,R)-Me-
BIPAM
(R,R)-Me-
BIPAM
(R,R)-Me-
BIPAM
CONMe2 [Ir(cod)2](BArF )
4
(3)
7
NHAc
H
[Ir(cod)2](BArF )
63
4
(5)
((R,R)-Me-bipam)](BArF ). These phenomena indicated that
8
[Ir(cod)2](BArF )
n.r.
91
–
4
4
the cationic iridium catalyst precursor, [Ir(cod)2](BArF ),
(5)
4
9
CONMe2 [Ir(cod)2](BArF )
(R)-BINAP
49
4
sometimes slightly catalyzed this chemical transformation
under our standard conditions.
(5)
10
CONMe2 [Ir(cod)2](BArF )
(R)-MONO-
32
45
4
(5)
PHOS
Table 3: Asymmetric intramolecular direct hydroarylation of a-ketoami-
[a] Reaction conditions: a-ketoamides (0.25 mmol), [Ir(cod)2](BArF )
des.[a]
4
(5 mol%), and (R,R)-Me-BIPAM (1.1 equiv to Ir) in DME (1 mL), stirred
for 24 h at 1358C. [b] The absolute configuration of the chiral center
within the product is given in parentheses. [c] Reaction mixture was
stirred at 1358C, 16 h. Bz=benzoyl.
observed in 1,2-dimethoxyethane (DME) at 1358C (90%,
88% ee; entry 8). When the reaction carried out at 1508C in
diglyme, the corresponding 3-hydroxy-3-phenyl-2-oxindole 2
and 3 were obtained as a mixture of two regioisomers
(entry 11).
Entry
R
R’
Yield [%]
ee[b] [%]
1
2
3
4
5
6
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CF3
H
H
H
C6H5 (6a)
4-CF3C6H4 (6b)
4-FC6H4 (6c)
>99 (11a)
98 (11b)
99 (11c)
80 (11d)
94 (11e)
73 (11 f)
97 (11 f)
97 (11g)
98 (11h)
90 (11h)
87 (11i)
91 (11j)
88 (11j)
97 (11k)
69 (11l)
85 (11l)
96 (11m)
96 (11n)
95 (11o)
93 (11p)
98 (S)
98
90
98
91
84
84
97
86
92
97
88
94
98
84
80
91
94
92
90
4-BrC6H4 (6d)
4-PhC6H4 (6e)
4-PhOC6H4 (6 f)
4-PhOC6H4 (6 f)
3-CF3C6H4 (6g)
3-CH3C6H4 (6h)
3-CH3C6H4 (6h)
2-FC6H4 (6i)
2-naphthyl (6j)
2-naphthyl (6j)
3,5-(CF3)2C6H3 (6k)
3,4-(CH2O2)C6H3 (6l)
3,4-(CH2O2)C6H3 (6l)
C6H5 (6m)
We next examined the directing group attached to the
aromatic ring of the aniline side (Table 2). The dimethyl
amino carbonyl group was most effective and the enantiose-
lectivity additionally improved to 98% ee (entry 4). The
reaction with substrate 8 gave no desired product, suggesting
that a directing group is indispensable in this reaction. When
the reaction time was 16 h, there was no change in yield or
selectivity (entry 5). Moreover, it is notable that catalyst
loading can be reduced to 3 mol% without loss of enantio-
selectivity (entry 6). Among the chiral ligands screened, the
use of analogous C2-symmetric (R)-BINAP (entry 9) and
monodentate phosphoramidite, (R)-MONOPHOS (entry 10)
resulted in lower selectivities, 49% ee and 45% ee, respec-
tively. The absolute configuration of the product was assigned
as the S enantiomer based on X-ray crystallographic analysis
of the compound of 11a (Figure 1).[21]
7[c]
8
9
10[c]
11
12
13[c]
14
15
16[c]
17
18
19
20
CH3 (6n)
CH2CH3 (6o)
CH(CH3)2 (6p)
[a] Reaction conditions: a-ketoamides (0.25 mmol), [Ir(cod)2](BArF )
4
(5 mol%), and (R,R)-Me-BIPAM (1.1 equiv to Ir) in DME (1 mL) was
stirred for 16 h at 1358C. [b] The absolute configuration of the chiral
center within the product is given in parentheses. [c] 5 mol% of
With these optimized conditions established, we next
investigated the substrate scope of this reaction using 5 mol%
of catalyst. As shown in Table 3, the reactions are applicable
[Ir(cod)((R,R)-Me-bipam)](BArF ) was used as the catalyst.
4
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Furthermore, in the reaction of a-ketoamides having
a disubstituted aromatic ring at the ketone side, the corre-
sponding 3-aryl-3-hydroxy-2-oxindoles were also obtained
with high enantioselectivities (entries 12–16). High enantio-
selectivity was maintained even when the aromatic ring of the
aniline side has a substituent, such as a CF3 group (91% ee;
entry 17). The method was easily extended to a variety of
aliphatic a-ketoamides and afforded the corresponding 3-
alkyl-3-hyroxy-2-oxindoles in high enantioselectivities (90–
94% ee; entries 18–20).
species 17. Finally, reductive elimination to product 11 from
17 occurs and regenerates the active iridium catalyst 14. On
the basis of a previous report,[9a] the enantioselective insertion
can be rationalized by intermediate 16-favored, which
exhibits less steric congestion (Figure 3), and thus explains
why the carbonyl group is attacked preferentially on its si face
to produce the (S)-enantiomer.
In conclusion, we have developed a cationic iridium/
(R,R)-Me-BIPAM-catalyzed highly enantioselective intramo-
À
lecular direct hydroarylation of a-ketoamides through C H
Finally, a plausible catalytic cycle and enantioselection
mechanisms are proposed in Figures 2 and 3, based on the X-
ray structure of 11a (Figure 1) and previous reports.[3c,20a]
functionalization. (R,R)-Me-BIPAM gave various types of
optically active 3-substituted 3-hydroxy-2-oxindoles in high
yields with complete regioselectivity and excellent enantio-
First, [Ir((R,R)-Me-bipam)](BArF ) ([Ir], 14) is formed by
selectivities by enantioselective direct addition at the C H
À
4
an iridium catalyst precursor, [Ir(cod)2](BArF ), and (R,R)-
bond in the more hindered ortho position to a carbonyl group.
Further studies of this method toward expansion of the
substrate scope are in progress.
4
Me-BIPAM in situ. Subsequently, 14 reacts with substrate 6 to
afford aryl iridium complex 15, which is coordinated with the
two carbonyl groups of the amide. Asymmetric hydroaryla-
tion of the ketone carbonyl group would proceed from 16,
thus producing enantiomerically enriched iridium alkoxide Experimental Section
General procedure: To
a
flame-dried flask, [Ir(cod)2](BArF )
4
(0.0125 mmol, 5 mol%) and (R,R)-Me-BIPAM (0.0138 mmol,
5.5 mol%) and dry dimethoxyethane (1.0 mL) were added under an
N2 atmosphere. The solution was stirred at room temperature for
30 min, followed by the addition of a-ketoamide (0.25 mmol). The
reaction mixture was then heated at 1358C. After being stirred for
16 h, the mixture was purified with silica gel column chromatography
(eluent: n-hexane/ethyl acetate) to afford pure 3-substituted-3-
hydroxy-2-oxindole.
Received: January 7, 2014
Keywords: asymmetric catalysis · C-H functionalization ·
.
iridium · oxindoles · phosphoramidites
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