Palladium/copper-catalyzed oxidative C-H alkenylation/N-dealkylative carbonylation of tertiary anilines

Article abstract of DOI:10.1002/anie.201303911

C-H/C-N activation: The first palladium/copper-catalyzed aerobic oxidative C-H alkenylation/N-dealkylative carbonylation of tertiary anilines has been developed. Various functional groups were tolerated and acrylic ester could also be suitable substrates. This transformation provided efficient and straightforward synthesis of biologically active 3-methyleneindolin-2-one derivatives from cheap and simple substrates. Copyright

Full text of DOI:10.1002/anie.201303911

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Angewandte
Communications
DOI: 10.1002/anie.201303911
Synthetic Methods
À
Palladium/Copper-Catalyzed Oxidative C H Alkenylation/N-
Dealkylative Carbonylation of Tertiary Anilines**
Renyi Shi, Lijun Lu, Hua Zhang, Borui Chen, Yuchen Sha, Chao Liu, and Aiwen Lei*
Dedicated to Professor Xiyan Lu on the occasion of his 85th birthday
Anilines are valuable commodity chemicals and useful
synthetic building blocks for agrochemicals, active pharma-
ceutical ingredients, and process chemicals. Although anilines
have been widely utilized in chemical synthesis, the ortho-
exhibit extraordinary biological and pharmaceutical proper-
ties (for example protein kinase inhibitors, phosphodiesterase
inhibitors, and anti-rheumatic agents).[4] For example, Suni-
tinib was commercialized by Pfizer, Inc. in 2006 to treat renal
cell carcinoma and gastrointestinal stromal tumors.[5] Tenidap
(commercialized in 1993; Pfizer, Inc.) is an anti-inflammatory
medicine for the therapy of arthritis and other ailments.[6]
Moreover, 3-methyleneindolin-2-ones could be easily utilized
to synthesize important indolin-2-one derivatives.[7] Conse-
quently, considerable efforts have been made to develop
efficient methods for the synthesis of oxindole derivatives.
However, most of these methods involve multistep proce-
dures under harsh reaction conditions.[8] Herein, we have
developed an efficient and straightforward approach for the
synthesis of 3-methyleneindolin-2-one derivatives by using
commercial and simple tertiary anilines, olefins, and CO gas
as the substrates.
À
selective C H bond functionalization of anilines still remains
an outstanding challenge.[1] During the past decade, Pd-
À
À
catalyzed aromatic C H functionalization to form C C and
[2]
À
C X bonds has been intensively investigated. However, to
our best knowledge, the oxidative alkenylation of anilines
À
through the ortho-selective direct C H bond functionaliza-
tion have never been achieved. On the other hand, activation
À
of the C N bond has become a hot topic in organic synthesis
recently, in which most efforts were focused on the amination
transformation.[3] As a typical example of tertiary amines,
À
tertiary anilines have also been utilized in C N bond cleavage
transformations.[3g,i] However, the utilization of tertiary ani-
lines as nitrogen nucleophiles with alkyl as leaving group in
oxidative carbonylation has remained undeveloped. Herein,
Our experiment was initiated by treating 4-methylstyrene
(2a) with N,N-dimethylaniline (1a) in the presence of 1 atm
CO/O2 (Table 1). By optimizing various reaction parameters,
the combination of a catalytic amount of [PdCl2(PPh3)2],
Cu(OAc)2·H2O, dppp, and AcOH in a solvent mixture of
toluene and DMF using O2 as the terminal oxidant at 1008C
was found to be the best reaction conditions for this trans-
formation (Table 1, entries 1, 13, and 14). No product was
detected without the addition of Cu(OAc)2·H2O (entry 2).
Removing dppp or replacing dppp with PPh3 both decreased
the yield dramatically (entries 3 and 11). The carbonylation
reaction afforded lower yield when AcOH was removed
(entry 4). The variation of the mixed solvent also led to less
efficiency in terms of chemical yields (entries 5 and 10). When
[PdCl2(PPh3)2] was replaced by Pd(OAc)2 or PdCl2, a rela-
tively low yield of corresponding product was obtained
(entries 6 and 7). The use of a copper salt is essential for the
reaction. No product was detected with CuCl2 as the co-
catalyst, while Cu(OAc)2 showed less efficiency (entries 8 and
9). Decreasing the reaction temperature slowed the reaction,
affording 46% yield of 3a (entry 12). With lower loading of
[PdCl2(PPh3)2], 71% yield of 3a was obtained by lowering the
amount of Cu(OAc)2·H2O, dppp, and solvent accordingly
(entry 13). When the carbonylation was conducted under
mixed gas outside the CO/O2 explosion limits (CO/O2 = 1:7),
74% yield of desired product was obtained (entry 14).
With the above optimized conditions in hand, the carbon-
ylation of a variety of styrene derivatives with N,N-dimethyl-
aniline were tested (Scheme 2). The products obtained are
mixtures of Z- and E-isomer because of the isomerization
between them.[9] In general, both electron-donating and
À
we present the first palladium/copper-catalyzed oxidative C
H alkenylation/N-dealkylative carbonylation of tertiary ani-
lines towards the synthesis of 3-methyleneindolin-2-ones
derivatives (Scheme 1).
3-Methyleneindolin-2-ones are recognized as a particu-
larly useful class of compounds in medicinal chemistry that
Scheme 1. Strategies towards syntheses of indolin-2-ones.
[*] R. Shi, L. Lu, H. Zhang, B. Chen, Y. Sha, C. Liu, Prof. A. Lei
College of Chemistry and Molecular Sciences, Wuhan University
Wuhan 430072, Hubei (P. R. China)
E-mail: aiwenlei@whu.edu.cn
Prof. A. Lei
Laboratory of Chemical Genomics
School of Chemical Biology and Biotechnology
Peking University Shenzhen Graduate School
Shenzhen 518055 (P. R. China)
[**] This work was supported by the 973 Program (2012CB725302), the
National Natural Science Foundation of China (21025206 and
21272180), and the Research Fund for the Doctoral Program of
Higher Education of China (20120141130002). We are also grateful
for support from the Program for Changjiang Scholars and
Innovative Research Team in University (IRT1030).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303911.
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Table 1: Palladium/copper-catalyzed oxidative C H alkenylation/N-dealkylative carbonylation of 1a and
electron-withdrawing substituents
2a.[a]
on the benzene ring of substituted
styrenes were well-tolerated under
the current conditions (3b, 3d, 3e,
3 f, and 3j). The position of sub-
stituent on benzene ring seems to
have little influence on the product
yield (3c). Styrenes substituted with
the halogens F, Cl, and Br afforded
the corresponding 3-methylenein-
dolin-2-ones in moderate to good
yields (3g, 3h, and 3i). To our
delight, a 35% yield was obtained
by employing butyl acrylate as the
reactant (3k), in which only E-
isomer was detected.
Entry
[Pd]
[Cu]
Ligand
Solvent
Yield [%]
1
2
3
4
5
6
7
8
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
Pd(OAc)2
Cu(OAc)2·H2O
–
dppp
dppp
–
Tol/DMF
Tol/DMF
Tol/DMF
Tol/DMF
Toluene
Tol/DMF
Tol/DMF
Tol/DMF
Tol/DMF
Tol/DMSO
Tol/DMF
Tol/DMF
Tol/DMF
Tol/DMF
74
0
14
28
6
26
13
65
0
32
26
46
71
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2
dppp
dppp
dppp
dppp
dppp
dppp
dppp
PPh3
dppp
dppp
dppp
PdCl2
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
9
CuCl2
Furthermore, we investigated
the scope of N,N-dialkylanlines,
and the results are listed in
10
11
12[b]
13[c]
14[d]
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Scheme 3.
N,N-dimethylaniline
substituted with a methyl group
underwent smooth reactions with
2a, giving the desired products in
75% and 52% yields, respectively
74 (66)
[a] The yield was determined by HPLC, calibrated using biphenyl as the internal standard. Standard
reaction conditions: 2a (0.2 mmol), 1a (0.4 mmol), [PdCl2(PPh3)2] (10 mol%), Cu(OAc)2·H2O
(30 mol%), dppp (10 mol%), AcOH (50 mol%), CO/O2 =3:1, toluene/DMF=3.0:0.3 mL, 1008C, 24 h.
[b] 808C. [c] Cu(OAc)2·H2O (20 mol%), dppp (5 mol%), toluene/DMF=2.0:0.2 mL, 36 h. [d] CO/
O2 =1:7. Yield of isolated product; E/Z=4.1:1, which was determined by 1H NMR spectroscopy.
À
À
Scheme 2. Palladium/copper-catalyzed oxidative C H alkenylation/N-
dealkylative carbonylation between various olefins 2 and 1a. Standard
reaction conditions: 2 (0.2 mmol), 1a (0.4 mmol), [PdCl2(PPh3)2]
(10 mol%), Cu(OAc)2·H2O (30 mol%), dppp (10 mol%), AcOH
(50 mol%), CO/O2 =1:7, toluene/DMF=3.0:0.3 mL, 1008C, 24 h.
Yield of isolated product. The ratio of E/Z was determined by 1H NMR
spectroscopy.
Scheme 3. Palladium/copper-catalyzed oxidative C H alkenylation/N-
dealkylative carbonylation between various N,N-dialkylanilines 1 and
2a. Standard reaction conditions: 2a (0.2 mmol), 1 (0.4 mmol), [PdCl2-
(PPh3)2] (10 mol%), Cu(OAc)2·H2O (30 mol%), dppp (10 mol%),
AcOH (50 mol%), CO/O2 =1:7, toluene/DMF=3.0:0.3, 1008C, 24 h.
Yield of isolated product. The ratio of E/Z was determined by 1H NMR
spectroscopy.
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(3l and 3m). N,N-dimethylanilines bearing halogens and
electron-withdrawing substituents were also allowed to react
with 2a to afford the corresponding 3-methyleneindolin-2-
ones in moderate yields (3n, 3o, 3p, 3q, 3r, and 3s). When
N,N-diethylaniline was employed as a substrate, the reaction
also took place smoothly to furnish the desired product (3t).
Interestingly, when N-ethyl-N-methylaniline was treated with
2a, two types of products 3a and 3t were obtained in 13% and
48% yields respectively, indicating that the less sterically
hindered alkyl group is much more facile for cleavage
(Scheme 4).
To gain some preliminary understanding of the reaction
mechanism, several experiments were carried out under the
standard conditions (Scheme 5). First, the reaction of N,N-
dibenzyl-4-methylaniline with 2a afforded the desired prod-
uct in 37% yield as well as 0.12 mmol of benzaldehyde, which
indicated that the benzyl group was converted into benzalde-
À
hyde by C N bond cleavage (Scheme 5, Eq. 1). N-Methyl-N-
phenylcinnamamide 4 was reported to undergo an intra-
molecular Heck reaction to form 3a.[8e] Therefore, 4 was
tested as the substrate in our reaction. To our surprise, no
product was detected under the standard conditions, which
rules out the intermediacy of amide 4 (Scheme 5, Eq. 2).
Under the standard conditions, 5 underwent an intramolec-
ular carbonylation smoothly to afford 3a in 88% yield
(Scheme 5, Eq. 3). Surprisingly, the reaction of 2a and 1a did
not take place in the absence of CO (Scheme 5, Eq. 4). From
the above results, we assume that 5 might probably be the
reaction intermediate and that CO plays an important role in
À
the oxidative C H alkenylation step. Both intramolecular and
intermolecular KIE experiments were carried out, affording
KIE values in 3.17 and 3.00, respectively (Scheme 5, Eq. 5 and
Eq. 6). Furthermore, we conducted the experiments measur-
ing the reaction rates of 1a and [D5]-1a by using in situ IR
spectroscopy, in which a KIE of 3.0 was observed (see the
Supporting Information). All these results indicated that the
À
C H cleavage might be involved in the rate-determining step.
Huang et al. had recently investigated the copper-cata-
lyzed oxidative amination of benzoxazoles using tertiary
amines as nitrogen group sources.[3a] They proposed that C N
bond activation was promoted by copper and oxygen. The
above experimental results induced us to propose a mecha-
À
À
Scheme 4. Palladium/copper-catalyzed oxidative C H alkenylation/N-
dealkylative carbonylation between 2a and 1u.
À
nism for this novel palladium/copper-catalyzed oxidative C
H
alkenylation/N-dealkylative carbonylation (Scheme 6).
Taking 4-methylstyrene 2a with N,N-dimethylaniline 1a as
an example, the electrophilic palladation of 1a afforded the
intermediate II, and the following olefin insertion and b-H
À
elimination produced 5. Then, 5 underwent C N bond
cleavage in the presence of copper salts and O2, affording
the intermediate IV,[3a,h] which further underwent transmeta-
lation and CO insertion to form the intermediate VI. The
Scheme 5. Preliminary mechanistic studies.
Scheme 6. Proposed mechanism.
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subsequent intramolecular Heck reaction gave 3a and
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In conclusion, we have developed the first aerobic
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palladium/copper-catalyzed oxidative C H alkenylation/N-
dealkylative carbonylation of tertiary anilines to form 3-
methyleneindolin-2-ones derivatives. Moderate to good
yields were obtained and a variety of functional groups
were tolerated. This transformation provides an effective and
straightforward method towards the synthesis of biologically
and medicinally useful 3-methyleneindolin-2-ones from com-
mercial and simple substrates. Preliminary mechanism studies
revealed that the intermolecular selective ortho-alkenylation
of N,N-dialkylanilines is the first and rate-determining step,
and the alkyl leaving group was converted into the corre-
sponding aldehyde. Further studies on substrate scope and
mechanism are currently underway and will be reported in
due course.
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Experimental Section
General procedure: [PdCl2(PPh3)2] (14.0 mg, 10 mol%) and Cu-
(OAc)2·H2O (12.0 mg, 30 mol%) were combined in an oven-dried
Schlenk tube equipped with a stir-bar. In a glove box, dppp (8.2 mg,
10 mol%) was added in the tube. A balloon filled with CO and O2
(the ratio is 1:7) was connected to the Schlenk tube via the side tube
and purged 3 times. Then DMF (0.3 mL) and toluene (3.0 mL) were
added to the tube via a syringe. Styrene (0.2 mmol), anilines
(0.4 mmol), and AcOH (6 mg, 50 mol%) were then added to the
tube. The Schlenk tube was heated at 1008C for 24 h and then cooled
to room temperature. After the balloon gas was released carefully, the
reaction was quenched by water and extracted with CH2Cl2 three
times. The combined organic layers were dried over anhydrous
Na2SO4 and evaporated under vacuum. The desired products were
obtained in the corresponding yields after purification by flash
chromatography on silica gel with hexane/ethyl acetate.
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Received: May 7, 2013
Revised: July 8, 2013
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À
À
Keywords: aminocarbonylation · C H activation · C
N bond activation · indolin-2-ones · palladium
.
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