A highly efficient palladium-catalyzed decarboxylative ortho-acylation of azobenzenes with α-oxocarboxylic acids: Direct access to acylated azo compounds

Article abstract of DOI:10.1002/chem.201301818

Avoiding additives: A highly efficient and mild Pd-catalyzed decarboxylative ortho-acylation of azobenzenes with α-oxocarboxylic acids was developed that provides an alternative route to acylated azo compounds. This decarboxylative acylation process was completed in the absence of any additives at ambient temperature, to afford the acylated azobenzenes in moderate to good yields (see scheme). Copyright

Full text of DOI:10.1002/chem.201301818

COMMUNICATION
DOI: 10.1002/chem.201301818
A Highly Efficient Palladium-Catalyzed Decarboxylative ortho-Acylation of
Azobenzenes with a-Oxocarboxylic Acids: Direct Access to Acylated Azo
Compounds
Hongji Li,[a] Pinhua Li,[a] Hui Tan,[a] and Lei Wang*[a, b]
Recently,
transition-metal-catalyzed
decarboxylative
at low temperature,[11b] and the aerobic oxidative dehydro-
genation of aryl amines in the presence of a transition-metal
catalyst.[11c–d] Although the established methodologies have
been successfully applied in the synthesis of azo compounds,
the restricted reaction conditions, limited substrate scope,
and the formation of complex by-products are undesira-
ble.[11a–b] It is worth noting that the synthesis of sterically
hindered ortho-substituted azo compounds is also a big chal-
lenge, and is a reaction that has been little studied thus far.
We have recently developed a Pd-catalyzed acylation of azo-
benzenes with aldehydes in the presence of tert-butylhydro-
peroxide (TBHP), leading to a series of acylated azoben-
zenes.[12] In our ongoing synthesis of ortho-substituted azo-
benzenes, we report herein a highly efficient and mild decar-
boxylative[13] ortho-acylation of azobenzenes with a-oxocar-
boxylic acids catalyzed by Pd at room temperature in the
absence of an additive (Scheme 1).
cross-coupling reactions involving the use of readily avail-
able a, b-unsaturated and aryl carboxylic acids as potential
coupling partners, in place of aryl halides or organometallic
reagents, has attracted much attention and has provided
a powerful tool for the construction of chemical bonds in or-
ganic synthesis.[1] Early in 2002, Myers reported a palladi-
um-catalyzed decarboxylative Heck-type olefination of ben-
zoic acids.[2] Since then, significant progress has been made
on decarboxylative cross-coupling reactions.[3] For example,
Goossen,[3a–d] Forgione,[3e] Li,[3f] Liu,[3g,h] Miura,[3i] Lee,[3j] and
Tunge[3k,l] have studied decarboxylative biaryl coupling, ole-
fination, alkynylation, aryl ketone formation and so forth.
Very recently, Ge,[4] Duan,[5] Kim,[6] Tan,[7] Zhu,[8] and
Saxena[9] have further demonstrated the feasiblity of decar-
boxylative acylation of arenes and other substrates by Pd-
À
catalyzed group-directed C H activation. Despite the ach-
ievements that have been made with respect to decarboxyla-
tion reactions, high reaction temperatures, and additives re-
quired for the decarboxylative process partially limit their
potential applications. Therefore, it is desirable to develop
a mild decarboxylative cross-coupling reaction that proceeds
in the absence of any additives.
It is well known that azo compounds are important mate-
rials and have been broadly applied in many fields due to
their special properties.[10] For instance, they can be used as
light triggered switches in surface-modified materials,[10a]
molecular machines,[10b] protein probes,[10c] and so forth.[10d–f]
As a result of their importance, some typical methodologies
have been established for the preparation of azobenzenes.[11]
For example, the reduction of nitro compounds in the pres-
ence of an excessive amount of reducing agent,[11a] the clas-
sic coupling reaction of diazo salts with aromatic compounds
Scheme 1. Pd-catalyzed decarboxylative acylation of azobenzenes with a-
oxocarboxylic acids.
We initiated our studies with the acylation of azobenzene
(1a) with 2-oxo-2-phenylacetic acid (2a) in 1,2-dichloro-
ethane (DCE) using Pd
shown in Table 1. Although no reaction occurred in the
presence of Cu(OAc)2, 24% of acylated product 3a was iso-
ACHTUNGERTN(NUNG OAc)2 as a catalyst. The results are
AHCTUNGTRENNUNG
lated when the oxidant was switched to (NH4)2S2O8 at room
temperature for 36 h (Table 1, entries 1 and 2). Gratifyingly,
the use of K2S2O8 could enhance the acylation of 1a with
2a, leading to the formation of 3a in 76% yield (Table 1,
[a] Dr. H. Li, Prof. P. Li, H. Tan, Prof. Dr. L. Wang
Department of Chemistry
entry 3). Unfortunately, other oxidants, such as MnACHTUNGTRENNUNG(OAc)3
Huaibei Normal University
and DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) were
ineffective in the model reaction (Table 1, entries 4 and 5).
When the model reaction was carried out under an oxygen
atmosphere (1.0 atm) at room temperature for 48 h, 3a was
obtained in 14% yield, and most of the starting material
was recovered (Table 1, entry 6). The palladium source was
found to be crucial for this transformation and several com-
mercially available PdII salts were tested in the acylation of
Huaibei, Anhui 235000 (P. R. China)
Fax : (+86)561-3090518
E-mail: leiwang@chnu.edu.cn
[b] Prof. Dr. L. Wang
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Shanghai 200032 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301818.
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Table 1. Optimization of the acylation of azobenzene (1a) with 2-oxo-2-
phenylacetic acid (2a).[a]
process smoothly and generated the desired products (3b–h)
in 73–85% yield. It should be noted that when 2-([1,1’-bi-
phenyl]-4-yl)-2-oxoacetic acid and 2-(naphthalen-1-yl)-2-
oxoacetic acid were reacted with 1a, the corresponding
products 3i and 3j were obtained in 62 and 70% yield, re-
spectively. Furthermore, it was found that the meta-substi-
tuted 2-oxo-2-phenylacetic acids 2k and 2l also reacted with
1a and afforded 3k and 3l in good yields. An ortho-position
effect was found in the reaction of 1a with 2-oxo-2-phenyl-
acetic acids possessing the bulky groups CF3 or CH3 on the
phenyl ring (3m and 3p). Meanwhile, no evident ortho-posi-
tion effect was observed in the reaction of 1a with 2-oxo-2-
phenylacetic acids containing Cl or F on the phenyl ring (3n
and 3o). To our delight, when disubstituted 2-oxo-2-phenyl-
acetic acids 2q and 2r were reacted with 1a, the desired
products 3q and 3r were formed in 76 and 70% yield, re-
spectively. However, a-oxocarboxylic acid 2s exhibited less
reactivity and provided 3s with a yield of only 53%, even
after a prolonged reaction time. Next, the performance of
some representative 4,4’-disubstitued azobenzenes was ex-
amined under the optimized reaction conditions. When
MeO, Me, and Cl, were introduced into the phenyl rings of
azobenzenes, the decarboxylative acylations of 4,4’-dimethyl
azobenzene, 4,4’-dimethoxy azobenzene, and 4,4’-dichloro
azobenzene with 2a proceeded well and the corresponding
products 3t–v were isolated in good yields (80, 85, and
79%, respectively). However, 2-oxopropanoic acid (2w)
failed to react with 1a under the present reaction conditions,
and none of the desired product 3w was detected.
Entry Pd source
Oxidant
Cu(OAc)2
(NH4)2S2O8
K2S2O8
Mn(OAc)3
DDQ
O2 (1.0 atm)
K2S2O8
Additive
Yield [%][b]
1
2
3
4
5
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
G
–
–
–
–
–
–
–
–
0
24
76
0
G
R
0
6
7
E
14
70
42
32
40
53
8
PdCl2
K2S2O8
9
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
G
–
–
10
11
12
13
14
15
16
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Ag2O (1.0 equiv)
PivOH (1.0 equiv) 22
–
–
–
–
67[c]
73[d]
53[e]
68[f]
[a] Reaction conditions: Azobenzene (1a, 0.20 mmol), 2-oxo-2-phenyl-
acetic acid (2a, 0.30 mmol), Pd catalyst (5 mol%), K2S2O8 (0.30 mmol) in
DCE (1.0 mL) at room temperature under air for 36 h. [b] Isolated
yields. [c] 28 h. [d] 48 h. [e] 1a/2a=1:1. [f] 1a/2a=1:2.
1a with 2a at room temperature with K2S2O8 as the oxidant
and DCE as solvent. It should be noted that Pd
ACHTUNGRTEN(NUNG OAc)2 ex-
hibited high activity in the decarboxylative reaction, and
Subsequently, we attempted to further transform the acy-
lated azobenzenes into the corresponding indazoles, which
can exhibit unique biological activity in nature. Recently we
have established an efficient methodology, based on a Zn/
NH4Cl/MeOH system, for constructing the indazole back-
was found to be more active than [PdACTHNUTRGENUN(G TFA)2] (TFA = tri-
fluoroacetic acid) (Table 1, entry 7 vs entry 3).[4a] Several
other Pd sources were screened, including PdCl2, [Pd-
ACHTUNGTRENNUNG(CH3CN)2Cl2], and [PdCAHTUTGNERN(NUNG PPh3)2Cl2], but did not enhance the
yield of product 3a (Table 1, entries 8–10). The addition of
common additives, such as Ag2O or PivOH (pivalic acid), to
the reaction resulted in a reduced yield of 3a (Table 1, en-
tries 11 and 12). An optimization study of the reaction time
and the molar ratio of 1a/2a did not provide improved con-
ditions for the synthesis of 3a (Table 1, entries 13–16). Fur-
ther investigation of the solvent effect in the model reaction
showed that 1,2-dichloroethane was the most successful of
the solvents tested (See TS1 in Supporting Information for
details). Finally, the optimized reaction conditions were
bone.[12] Ellman et al. have reported indazole synthesis
III
À
through Rh -catalyzed C H functionalization from the re-
action of azobenzenes with aldehydes.[14] We have developed
a more efficient transformation of acylated azobenzenes
into the corresponding indazoles using a Cu2Cl2/NaBH4/
EtOH system at room temperature for 3 min. The results
are listed in Table 3 and show that the presence of electron-
donating and electron-withdrawing groups in acylated azo-
benzenes, such as MeO, tBu, Me, and Br, did not affect the
transformation and the corresponding products 4a–f were
obtained in excellent yields (97–99%).
found to be: PdACHTUNGTRENNUNG(OAc)2 (5 mol%), K2S2O8 (1.5 equiv) as the
oxidant and a molar ratio of (1:1.5) of azobenzene to 2-oxo-
2-phenylacetic acid in DCE at room temperature under air
for 36 h.
Following this, some control experiments were carried out
in order to probe the possible reaction mechanism. Firstly,
a process involving free radical species could be excluded by
a control experiment which showed that the addition of one
To evaluate the scope of this decarboxylative acylation re-
action, various a-oxocarboxylic acids were investigated
under the optimized reaction conditions, as shown in
Table 2. The results indicate that azobenzene 1a can react
with various 2-oxo-2-phenylacetic acids to generate the cor-
responding products in good yield. Notably, electron-donat-
ing and electron-withdrawing groups (Me, MeO, tBu, Br, Cl,
F and NO2) on the para-position of the phenyl ring of the 2-
oxo-2-phenylacetic acids underwent the decarboxylative
equivalent
of
2,2,6,6-tetramethylpiperidine
N-oxide
(TEMPO) into the system did not affect the yield of 3a.[15]
When the reaction of azobenzene 1a with 2-oxo-2-phenyl-
acetic acid (2a) was carried out in the presence of a stoichio-
metric amount of PdACHTUNGTRNENUG(OAc)2 in the absence of K2S2O8 in
DCE at room temperature for 36 h, none of the desired
product 3a was isolated. This demonstrated that the reaction
did not undergo the Pd0/PdII process. Recently, Sanford
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Pd-Catalyzed Decarboxylative Acylation of Azobenzenes
COMMUNICATION
Table 2. The scope of the acylation of azobenzene 1a with a-oxocarboxylic acids.[a]
[a] Reaction conditions: Azobenzene (1, 0.20 mmol), a-oxocarboxylic acid (2, 0.30 mmol), PdACHTUNGTRENGN(U OAc)2 (5 mol%), K2S2O8 (0.30 mmol) in DCE (1.0 mL) at
room temperature under air for 36 h. [b] Isolated yields. [c] At room temperature for 48 h.
À
et al. have carried out detailed research on Pd-catalyzed C
H activation and a reaction mechanism involving a PdII/PdIV
process was proposed and confirmed.[16] Inspired by San-
fordꢁs excellent work, a complex of PdIV with azobenzene
was prepared in situ[16,17] and was then reacted with 2-oxo-2-
phenylacetic acid (2a) in DCE at room temperature for
36 h, after which time 3a was isolated with a yield of 47%
(see the control experiments in Supporting Information for
details). Based on the above results, a reaction mechanism
involving a PdII/PdIV process for this system was proposed
and is shown in Scheme 2. Although we failed to isolate the
possible intermediate from the reaction of PdACTHNUGTRENUNG(OAc)2 with
À
azobenzene 1a through C H insertion, the complex I is still
considered as the key active species for the next functionali-
zation, because similar five-membered intermediates have
previously been reported.[18] Subsequently, cyclopalladated
complex I is oxidized in the presence of K2S2O8 to provide
the PdIV intermediate II. The anion exchange between com-
plex II and a-keto acid 2a can then lead to the formation of
complex III and the release of HOAc. Decarboxylation of
complex III will generate complex IV along with extrusion
of CO2.[19] Finally, reductive elimination of IV can afford the
product 3a, and PdII intermediate I for the next cycle.
Scheme 2. Proposed reaction mechanism.
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L. Wang et al.
Table 3. Transformation of the acylated azobenzenes into the corre-
sponding indazoles.[a]
solvent was evaporated under reduced pressure and the residue was puri-
fied by flash chromatography (silica gel, ethyl acetate/petroleum ether
1:3–1:5, v/v), affording the desired product 4a as a white solid (53.0 mg,
98%).
Acknowledgements
We are grateful to the National Natural Science Foundation of China
(21372095 and 21172092) for financial support.
Keywords: azobenzenes · decarboxylation · ortho-acylation ·
palladium catalysis · oxocarboxylic acids
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(0.10 mmol), NaBH4 (0.20 mmol), EtOH (0.50 mL), room temperature,
3 min. [b] Isolated yields.
In summary, we have developed a novel Pd-catalyzed
ortho-acylation of azobenzenes with a-oxocarboxylic acids
in the presence of K2S2O8 at ambient temperature without
the presence of an additive. The reactions of azo compounds
with a variety of a-oxo-2-arylacetic acids proceeded smooth-
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The reaction is highly efficient and has a broad substrate
scope. In addition, several indazole derivatives were effi-
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a Cu2Cl2/NaBH4/EtOH system with excellent yields. A more
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Experimental Section
Typical procedure for the palladium-catalyzed ortho-acylation of azoben-
zenes with a-keto acid 2a: Under an atmosphere of air, a 5.0 mL sealed
tube was charged with azobenzene (1a, 36.4 mg, 0.20 mmol), 2-oxo-2-
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(1.0 mL) at room temperature for 36 h. H2O (20.0 mL) was added to the
resulting solution and extracted with dichloromethane (2ꢂ5.0 mL). The
organic layers were combined, dried over Na2SO4, and concentrated to
yield the crude product, which was further purified by flash chromatogra-
phy (silica gel, ethyl acetate/petroleum ether 1:20–1:50, v/v), affording
the desired product 3a as a red solid (43.0 mg, 76%).
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a
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Pd-Catalyzed Decarboxylative Acylation of Azobenzenes
COMMUNICATION
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Received: May 12, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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L. Wang et al.
Palladium Catalysis
H. Li, P. Li, H. Tan,
L. Wang* ...................... &&&& — &&&&
A Highly Efficient Palladium-Cata-
lyzed Decarboxylative ortho-Acylation
of Azobenzenes with a-Oxocarboxylic
Acids: Direct Access to Acylated Azo
Compounds
Avoiding additives: A highly efficient
and mild Pd-catalyzed decarboxylative
ortho-acylation of azobenzenes with a-
oxocarboxylic acids was developed
that provides an alternative route to
acylated azo compounds. This decar-
boxylative acylation process was com-
pleted in the absence of any additives
at ambient temperature, to afford the
acylated azobenzenes in moderate to
good yields (see scheme).
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www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!