Asymmetric organocatalytic direct C(sp2)-H/C(sp3)-H oxidative cross-coupling by chiral iodine reagents

Article abstract of DOI:10.1002/anie.201309967

An asymmetric organocatalytic direct C-H/C-H oxidative coupling reaction of N¹,N³-diphenylmalonamides has been well established by using chiral organoiodine compounds as catalysts, wherein four C-H bonds were stereoselectively functionalized to give structurally diverse spirooxindoles with high levels of enantioselectivity. More importantly, the findings indicated that chiral hypervalent organoiodine reagents can serve as alternative catalysts for the creation of enantioselective functionalization of inactive C-H bonds. 'I' on the ball: The title oxidative coupling reaction of N ¹,N³-diphenylmalonamides has been established by using chiral organoiodine compounds as catalysts. Four C-H bonds were stereoselectively functionalized to give spirooxindoles with high levels of enantioselectivity.

Full text of DOI:10.1002/anie.201309967

.
Angewandte
Communications
DOI: 10.1002/anie.201309967
À
C H Functionalization
2
3
À
À
Asymmetric Organocatalytic Direct C(sp ) H/C(sp ) H Oxidative
Cross-Coupling by Chiral Iodine Reagents**
Hua Wu, Yu-Ping He, Lue Xu, Dong-Yang Zhang, and Liu-Zhu Gong*
À
À
Abstract: An asymmetric organocatalytic direct C H/C H
oxidative coupling reaction of N1,N3-diphenylmalonamides
has been well established by using chiral organoiodine
À
compounds as catalysts, wherein four C H bonds were
stereoselectively functionalized to give structurally diverse
spirooxindoles with high levels of enantioselectivity. More
importantly, the findings indicated that chiral hypervalent
organoiodine reagents can serve as alternative catalysts for the
À
creation of enantioselective functionalization of inactive C H
bonds.
Scheme 1. Oxidative cross-coupling reaction catalyzed by an organo-
catalyst.
T
he direct oxidative cross-coupling reaction of two individ-
À
ual C H bonds has been recognized as an ultimately ideal
goal for the formation of carbon–carbon bonds.[1] Over the
past few years the transition-metal-catalyzed oxidative cross-
reactions, promoted by chiral organoiodine reagents, occured
between C H and C X bonds (Scheme 1a).
contrast, enantioselective C H/C H oxidative coupling reac-
tions, catalyzed by chiral hypervalent iodine (Scheme 1b),
have been described to a lesser extent.
[6,7]
À
À
À
coupling reactions between two C H bonds have received
In sharp
À
À
a great deal of attention and led to the explosive emergence of
new protocols.[2] However, the field has still met with many
formidable issues which continue to challenge the chemistry
À
community. In particular, asymmetric catalytic C H bond-
Recently, Du, Zhao, and co-workers found that the use of
2.2 equivalents of PIFA [PhI(TFA)2] oxidized the anilide
derivatives 1 into racemic spirooxindoles (2) (Scheme 2a).[8]
Previous studies have revealed that aryl iodides are able to
undergo oxidation reactions in the presence of oxidants to
generate hypervalent iodine compounds, which actually
participated in the oxidation reaction (Scheme 2b). As such,
the asymmetric catalytic oxidative coupling reaction for the
transformation of 1 into optically active 2 would be princi-
pally accessed by using chiral iodine reagents as organo-
catalysts in the presence of oxidants (Scheme 2b).
coupling reactions represent the most formidable challenge.
So far, there are only a limited number of successful reports
describing the use of a chiral transition-metal complex to
À
afford asymmetric coupling reactions between two C H
bonds.[3] Although asymmetric organocatalysis has achieved
significant advances in the last decade, asymmetric organo-
À
catalytic asymmetric oxidative coupling reactions of C H
bonds has met with even much less success.[4] Herein, we
report a direct asymmetric organocatalytic oxidative cross-
À
coupling reaction to functionalize four C H bonds by using
chiral iodide compounds.
In recent decades, hypervalent-iodine-catalysis has ena-
bled a variety of transformations.[5] However, enantioselec-
tive oxidative coupling reactions catalyzed by chiral hyper-
valent iodine remain to be developed. Thus far, there are only
a few elegent examples describing chiral-organoiodine-cata-
lyzed oxidative coupling reactions with mCPBA or H2O2 as
oxidants.[6] Moreover, all these asymmetric oxidative coupling
[*] H. Wu, Y. P. He, L. Xu, D. Y. Zhang, 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 (China)
E-mail: gonglz@ustc.edu.cn
[**] We are grateful for financial support from NSFC (21232007,
21072181), MOST (973 project 2010CB833300), and the Ministry of
Education. We thank Shi-Ming Zhou for the determination of
absolute configuration.
À
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309967.
Scheme 2. Asymmetric catalytic intramolecular C H/C H oxidative
cross-coupling reaction. TFA=trifluoroacetate.
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To validate the proposed strategy (Scheme 2b), the initial
investigation was performed on an asymmetric catalytic
selectivity (entry 1). Previous observations have suggested
that the addition of either Brønsted or Lewis acids enhanced
the catalytic activity of hypervalent organoiodines.[7e,9] Thus,
various strong Brønsted acids and Lewis acids were evaluated
to identify the best additive (entries 2–5). Among them,
CF3COOH turned out to be the preeminent acid and was able
to provide 2a with 45% yield and 31% ee (entry 2). With
these relatively encouraging results in hand, a series of chiral
iodoarenes was next examined for the enantioselective
oxidative intramolecular coupling reaction of 1a (entries 6–
16). Although chiral organoiodines (3a-e) have successfully
been applied to various protocols, as described previously,[6d,7]
they showed poor enantioselectivity (entries 1 and 6–9).
Inspired by conformationally-flexible design introduced by
Ishihara and co-workers,[6d] modification of the chiral organo-
iodines by introduction of an additional stereogenic center to
the original organoiodines was carried out to improve the
catalytic performance, thus leading to a new family of chiral
organoiodine catalysts (3 f–i). Indeed, the organoiodines 3 f–
h, synthesized from 3a and (S)-proline derivatives, exhibited
much higher enantioselectivity (entries 10–12). In particular,
both 3 f and 3g were able to deliever high enantioselectivities
of 81 and 83% ee, respectively (entries 10 and 11). The
comparison of the chiral iodine 3i, which was incorporated
with (R)-proline ester, with 3g suggested that the (S)-proline
ester was seemingly a matched chiral auxiliary and turned out
to be slightly benificial to the stereochemical control (entry 11
versus 13). Interestingly, the installation of achiral pyrrolidine
and piperidine in the catalyst, as shown in 3j[6d] and 3k, also
gave signigficantly higher enantioselectivities (79 and 77% ee,
respectively; entries 14 and 15) than the structually similar
amides 3c–e (up to 33% ee, entries 7–9). More interestingly,
when the acyclic tertiary amide 3l was employed for the
reaction, high enantioselectivity (80%) was also obtained
(entry 16). These results demonstrated that the tertiary amide
(3 f–l), rather than secondary amide (3c–e) or carboxylic acid
(3a), played key role in inducing the high enantioselectivity.
Additionally, the stereochemistry of the product was con-
trolled by the stereogenic center close to the iodide, whereas
the introduction of romote chirality exerts minor impact on
the stereochemical control. Additional optimization of the
reaction conditions was fouced on the oxidants. A variety of
commonly used oxidants were examined and it was found that
neither H2O2 nor TBHP was a good oxidant (entries 17 and
18).[10] Notably, the enantioselectivity could be enhanced to
86% ee and a good yield was also obtained by conducting the
reaction using ethaneperoxoic acid (MeCO3H) as the oxidant
(entry 19).[11] However, the use of excess amounts of trifluoro-
acetic acid (4.0 equivalents) led to a slightly diminished
enantiomeric excess (entry 20).
À
À
intramolecular C H/C H oxidative cross-coupling reaction
of N1-benzyl-N3-methyl-N1,N3-diphenylmalonamide (1a) in
the presence of 15 mol% of the chiral organoiodine 3a and
2.6 equivalents of mCPBA in CH3NO2 at room temperature
(Table 1). The transformation indeed proceeded, but fur-
nished the desired 1-benzyl-1’-methyl-3,3’-spirobi(indoline)-
2,2’-dione (2a) in only 23% yield and with a poor enantio-
Table 1: Optimization of the transformation catalyzed by chiral organo-
iodine compounds.[a]
Entry
3
Oxidant
Acid
Yield
[%][b]
ee
[%][c]
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3g
3g
3g
3g
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
mCPBA
H2O2
free
CF3CO2H
p-TSA
23
45
52
12
–
44
53
38
47
52
58
57
45
47
34
36
–
7
31
13
0
TMSOTf
BF3·Et2O
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
CF3CO2H
[d]
–
22
33
7
9
5
10
11
12
13
14
15
16
17
18
19
20
81
83
71
78
79
77
80
[d]
–
[d]
TBHP
MeCO3H
MeCO3H
–
62
61
–
86[e]
82[f]
To explore the generality and the substrate scope of this
À
À
direct C H/C H oxidative coupling procedure, a range of
N1,N3-diphenylmalonamides (1) was oxidized with MeCO3H
(4.0 equiv) in the presence of 15 mol% 3g under the
optimized reaction conditions (Table 2). The N1,N3-diphenyl-
malonamides 1b–f, having different substituents at both
nitrogen atoms, underwent the asymmetric oxidative coupling
reaction to give the corresponding spirooxindoles 2b–f in
moderate to good yields with high levels of enantioselectivity
[a] Unless indicated otherwise, the reaction of 1a (0.1 mmol) was carried
out in CH3NO2 (1.0 mL) at room temperature for 16 h in the presence of
the chiral catalyst 3 (15 mol%), a co-oxidant (2.6 equiv), and an acid
(2.0 equiv). [b] Yield of the isolated product. [c] The ee value was
determined by HPLC analysis. [d] No desired product was obtained.
[e] Used MeCO3H (4.0 equiv). [f] Used 4.0 equiv of CF3CO2H.
mCPBA=meta-chloroperbenzoic acid, p-TSA=para-toluenesulfonic
acid, TBHP=tert-butylhydroperoxide, Tf=trifluoromethanesulfonyl,
TMS=trimethylsilyl.
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Angewandte
Communications
Table 2: Scope of the intramolecular oxidative coupling reaction.[a]
R1
R2
R3
R4
Yield
[%][b]
ee
Scheme 3. X-ray structure of (S)-2b.
Entry
2
[%][c]
1
2
3
4
5
6
7
8
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
H
H
H
H
H
H
H
H
H
H
H
H
5’-Me
5’-Me
5’-Me
H
H
H
H
Me
Bn
Bn
–
Me
Bn
Et
72
50
45
52
40
45
57
26
52
53
36
41
78
57
51
85
90
86
86
86
88
82
84
81
82
26
85
82
85[f]
81
[d]
–
–
[e]
H
–
5-Ph
5-Cl
5-F
5-CO2Me
5-tBu
5-OMe
7-Me
5-Me
5-Me
5-Ph
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Me
Bn
Me
Bn
Me
Bn
Me
Bn
Me
Me
Me
Bn
Bn
9
10
11
12
13
14
15
[a] Unless indicated otherwise, the reaction of 1 (0.1 mmol) was carried
out in CH3NO2 (1.0 mL) at room temperature for 16 h in the presence of
3g (15 mol%), MeCO3H (4.0 equiv), and CF3CO2H (2.0 equiv). [b] Yield
of isolated product. [c] The ee value was determined by HPLC analysis.
[d] N1,N3-bis(2-methylbenzyl)-N1,N3-diphenylmalonamide was used as
a substrate. [e] N1,N3-bis(naphthalen-1-ylmethyl)-N1,N3-diphenylmalo-
namide was used as a substrate. [f] A scale-up reaction of 1o (1.0 mmol)
was examined and afforded 2o with 53% yield and 84% ee.
Scheme 4. The plausible intermediates to account for the stereochem-
istry.
intermediate Aa is remote from the bulky substituent the
Si face of the 2-indololate moiety is open to nucleophilic
attack by the benzene ring (syn addition-elimination), thus
favorably giving (S)-2b. In contrast, the Re face of the 2-
indololate moiety is shielded by the bulky cyclic amide in Ab,
thus making the syn addition-elimination disfavored and
leading to the generation of the enantiomer in a minor
amount.
(up to 90% ee, entries 1–5). Moreover, the presence of
a substituent including either a halogen, electron-withdraw-
ing, or electron-donating group at the 4-postion of the N1,N3-
diphenylmalonamides (1g–k) was well tolerated and pro-
vided the corresponding products (2g–k) in fairly good yields
and high stereochemical outcomes (entries 6–10). However,
the presence of a strong electron-donating substituent (OMe)
at the 4-position of the substrate (1l) led to a significantly
diminished enantioselectivity (entry 11). In addition, a high
enantioselectivity was aslo obtained for a substrate bearing
a substituent at the 2-position of the benzene ring (1m), albeit
with a lower yield (entry 12). The disubstituted N1,N3-
diphenylmalonamides 1n–p also participated in the oxidative
coupling reaction to furnish the corresponding 2n–p in good
yields and high optical purity (entries 13–15). The absolute
configuration of 2b was determined to be S by X-ray
crystallographic analysis (Scheme 3).[12] The configuration of
other products was assigned by analogy.
Electron-spin resonance (ESR) studies on the reaction
suggested that the radical process could be excluded.[12]
Alternatively, the reaction might proceed by a previously
proposed pathway.[8] Actually, the intramolecular Friedel–
Crafts addition of anilide to the iodo-enol intermediate A, as
shown in Scheme 4, creates the stereogenic center. Princi-
pally, the present oxidative cyclization reaction should
proceed in an SN2’ manner (syn addition–elimination) for
associative intermediates. Because the aniline moiety of the
In summary, we have established the first asymmetric
À
À
catalytic direct intramolecular C H/C H oxidative cross-
coupling of N1,N3-diphenylmalonamides by using chiral
À
organoiodine compound as an organocatalyst. Four C H
bonds are stereoselectively functionalized in the reaction.
This transformation provides a straitforward method to access
optically active spirooxindoles. More importantly, the find-
ings imply that chiral hypervalent organoiodine reagents can
serve as alternative catalysts for the creation of enantiose-
À
lective functionalization of inactive C H bonds.
Received: November 16, 2013
Revised: December 16, 2013
Published online: February 19, 2014
À
Keywords: asymmetric catalysis · C H functionalization ·
chirality · cross-coupling · organoiodine
.
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[11] The low yield resulted from a side-product as described in the
Supporting Information, and it was aslo reported in Ref. [8]. In
addition, the opmization of conditions was unable to improve
the results. For details, see the Supporting Information.
[12] The details of X-ray analytical data and ESR studies were
described in the Supporting Information.
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