Merging Visible-Light Photocatalysis and Palladium Catalysis for C-H Acylation of Azo- and Azoxybenzenes with α-Keto Acids

Article abstract of DOI:10.1002/chem.201504530

An efficient C-H acylation of azo- and azoxybenzenes with α-keto acids has been developed by a combination of palladium catalysis and visible-light photoredox catalysis at room temperature under 1.5 W blue LED irradiation. This method tolerates a variety of disubstituted azo- and azoxybenzenes, as well as α-keto acids regardless of the nature of the substituents. A number of aryl ketones were obtained in good yields under mild reaction conditions.

Full text of DOI:10.1002/chem.201504530

DOI: 10.1002/chem.201504530
Communication
&
Photocatalysis
Merging Visible-Light Photocatalysis and Palladium Catalysis for
CÀH Acylation of Azo- and Azoxybenzenes with a-Keto Acids
Ning Xu,[a] Pinhua Li,[a] Zuguang Xie,[a] and Lei Wang*[a, b]
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system have focused on the use of Ru and Ir complexes as
photoredox catalysts, whereas combining organic dyes as pho-
toredox catalysts with transition-metal catalysts in synthetic
chemistry has not yet been explored.
Abstract: An efficient CÀH acylation of azo- and azoxy-
benzenes with a-keto acids has been developed by a com-
bination of palladium catalysis and visible-light photore-
dox catalysis at room temperature under 1.5 W blue LED
irradiation. This method tolerates a variety of disubstituted
azo- and azoxybenzenes, as well as a-keto acids regardless
of the nature of the substituents. A number of aryl ke-
tones were obtained in good yields under mild reaction
conditions.
In recent years, a-keto acids, as acylating reagents, have
shown high reactivity in the synthesis of ketones by a decar-
boxylative process to acyl-free radicals along with the extru-
sion of CO2.[3b,g,6] Meanwhile, a few examples of acridinium
salts, used as photoredox catalysts, have been reported in
a single catalyst system.[7] Herein, we have developed a combi-
nation of palladium and acridinium salt as a photoredox cata-
lyst under visible-light irradiation for the CÀH acylation of azo-
and azoxybenzenes with a-keto acids that provides a mild and
green methodology for the synthesis of azo-substituted aryl
ketones in good yields (Scheme 1).
Azo-substituted aryl ketones are very important in the chemi-
cal and pharmaceutical industries, and they have been widely
used in the fields of photochemical dyes, drug intermediates,
biosensors, and food additives. In addition, these compounds
can be easily converted into the corresponding amino or hy-
drazine products in organic synthesis.[1] As a result, a variety of
strategies has been established to realize them. Classic meth-
ods are the coupling of diazonium salts with arenes and the
oxidation of the corresponding azo-containing secondary alco-
hols.[2] However, these methodologies suffer from the harsh re-
action conditions and relatively limited substrate scope. In re-
cently years, much attention has been focused on the transi-
tion-metal-catalyzed oxidative sp2 CÀH acylation of azo- and
azoxybenzenes with aldehydes, aryl methanes, alcohols, and a-
oxocarboxylic acids.[3] Despite these important advances, most
of the current methods have some limitations. Drawbacks of
the CÀH acylation, in most cases, are stoichiometric amounts
of an external oxidant and higher reaction temperature.[3a–h]
Therefore, development of a mild, atom-efficient, and eco-
friendly method for the synthesis of azo-substituted aryl ke-
tones is highly desirable.
Scheme 1. Synthetic strategies for azo-substituted aryl ketones from azo-
and azoxybenzenes with a-oxocarboxylic acids.
Our initial studies focused on a Pd-catalyzed model reaction
of CÀH acylation of azobenzene (1a) with 2-oxo-2-phenylacetic
acid (2a). Inspired by the reported literature,[4,5,8] Ru complex
was employed as a photocatalyst firstly, and blue LED was uti-
lized as the source of visible light. To our delight, the model re-
action underwent smoothly to generate the desired CÀH acyla-
tion product 3aa in 72% yield at room temperature in the
presence of Pd(TFA)2 (5.0 mol%) and [Ru(bpy)3]Cl2·6H2O
(2.0 mol%) in toluene under an oxygen atmosphere and 1.5 W
blue LED irradiation for 16 h at room temperature (Table 1,
entry 1). Subsequently, various Ru and Ir complexes, such
as [Ru(bpy)3][PF6]2·6H2O, [Ru(phen)3]Cl2·6H2O, [Ru(phen)3]
[PF6]2·6H2O, and fac-[Ir(ppy)3], were screened (entries 2–5), and
a slightly improved yield of 3aa was observed in the presence
of [Ru(bpy)3][PF6]2·6H2O (entry 2). In an attempt to improve the
yield of the desired product, a series of organic dyes, such as
Na2-eosinY, eosin Y, rose bengal, and 9-mesityl-10-methylacridi-
nium perchlorate (PC-A) were examined as photoredox cata-
lysts[4i,7,9] instead of Ru-complex.
Most recently, visible-light-induced photoredox catalysis has
emerged as an important platform for the development of
unique single electron-transfer pathway under remarkably mild
reaction conditions.[4] Particularly, dual catalysis realized by
merging photocatalysis with transition-metal catalysis can ac-
complish the novel organic transformations, which are unfeasi-
ble or not accessible by a single catalytic system.[5] In 2011,
Sanford and co-workers achieved a Pd-catalyzed CÀH arylation
by merging palladium catalysis with visible-light photoredox
catalysis.[5a] Subsequently, many efforts have been devoted to-
wards the construction of CÀC and CÀheteroatom bonds by
combining visible-light-induced photoredox and transition-
metal catalysis.[5b–j] However, recent studies in the dual catalytic
[a] N. Xu, Prof. P. Li, Z. Xie, Prof. Dr. L. Wang
Department of Chemistry, Huaibei Normal University
Huaibei, Anhui 235000 (P.R. China)
Fax: (+86)561-3090518
E-mail: leiwang@chnu.edu.cn
Gratifyingly, the use of PC-A enhanced the acylation of 1a
with 2a, leading to the formation of 3aa in 78% yield under
either oxygen or air atmosphere (Table 1, entries 6–10). It is evi-
dent that organic dyes, such as eosin Y and PC-A, are cheaper
and easier to be modified, and can be degraded compared
with transition-metal photoredox catalysts (Ru and Ir com-
[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.201504530.
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perchlorate (PC-A) was the best of choice among the ex-
amined acridinium salts, including 9-phenyl-10-methylacri-
dinium perchlorate (PC-B), 9-phenyl-10-methylacridinium
tetrafluoroborate (PC-C), 9-(tert-butylphenyl)-10-methylacri-
dinium perchlorate (PC-D), 9-(tert-butylphenyl)-10-methyla-
cridinium tetrafluoroborate (PC-E), 9-(2,4,6-trifluorophenyl)-
10-methylacridinium perchlorate (PC-F), and 9-(2,4,6-tri-
fluorophenyl)-10-methylacridinium tetrafluoroborate (PC-G;
entries 15–20). In addition, solvent screening indicated that
toluene was the best reaction medium for the model reac-
tion (entries 21–31). Further investigations on the light
source, palladium catalyst, molar ratio of substrates, and
catalyst loading are also presented in Tables S1, S2, and S3
in the Supporting Information.
Table 1. Optimization of photoredox catalyst and solvent[a]
Entry
Photoredox Catalyst
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
[Ru(bpy)3]Cl2·6H2O
[Ru(bpy)3][PF6]2·6H2O
[Ru(phen)3]Cl2·6H2O
[Ru(phen)3][PF6]2·6H2O
fac-[Ir(ppy)3]
Na2-eosinY
eosin Y
rose bengal
PC-A
PC-A
–
PC-A
PC-A
PC-A
PC-B
PC-C
PC-D
PC-E
PC-F
PC-G
PC-A
PC-A
PC-A
PC-A
PC-A
PC-A
PC-A
PC-A
PC-A
PC-A
PC-A
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH3CN
MeOH
DCM
72
76
73
trace
trace
trace
trace
trace
78
With the optimized reaction conditions (1 (0.20 mmol), 2
(0.24 mmol), Pd(TFA)2 (5.0 mol%), and PC-A (2.0 mol%) in
toluene (2.0 mL) under air atmosphere and 1.5 W blue LED
irradiation at room temperature for 16 h) in hand, we next
explored the functional group compatibility of this transfor-
mation using a series of typical disubstituted azobenzenes.
As shown in Table 2, the reaction of various azobenzenes
with 2-oxo-2-phenylacetic acid (2a) proceeded well and
generated the desired acylated products in good yields.
Substituents on the aromatic moiety of the azo com-
pounds showed an electronic effect of the coupling reac-
tion. In general, aromatic azo compounds with electron-do-
nating groups were more reactive than that with electron-
withdrawing groups, providing higher yields of the desired
products. These results were also supported by the inter-
molecular competing experiments shown in Scheme 2. For
example, azobenzenes substituted with electron-donating
groups, such as Me, OMe, and iPr, at the para positions
generated the desired products (3ab–ad) in 68-70% yields.
Meanwhile, azobenzenes with electron-withdrawing
groups, including Cl and COOEt, at the para positions deliv-
ered the corresponding products 3ae and 3af in 67 and
58% yields, respectively. It should be noted that meta-sub-
stituted azobenzenes also worked well in the reaction to
afford the anticipated products 3ag–ai in 57–73% yields.
Notably, ortho-substituted azobenzenes gave relatively
lower product yields (3aj and 3ak) than para- or meta-sub-
stituted azobenzenes due to their steric hindrance. To our
delight, when tetra-substituted azobenzene 1l reacted
with 2a, the desired product 3al was obtained in 66%
yield. The reaction of unsymmetrical azobenzene 1m also
proceeded well and the acylation occurred on the electron-
rich aromatic ring selectively, providing the product 3am
in 63% yield.
9
10[c]
11
12[d]
13[e]
14[f]
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
78
N.R.
N.R.
N.R.
N.R.
73
76
76
75
48
63
trace
trace
58
60
65
37
71
67
57
DCE
DME
THF
acetone
CHCl3
PhCl
PhCF3
DMSO
trace
N. R.
[a] Reaction conditions: 1a (0.20 mmol), 2a (0.24 mmol), Pd(TFA)2 (5.0 mol%),
photoredox catalyst (2.0 mol%), 1.5 W blue LED, and solvent (2.0 mL) at room
temperature under oxygen for 16 h. [b] Isolated yield. [c] Under air. [d] In the
absence of Pd(TFA)2. [e] In the absence of light. [f] Under nitrogen. DME=1,2-
dimethoxyethane; DMSO=dimethyl sulfoxide; N.R.=no reaction.
Next, the representative a-oxocarboxylic acids were syn-
thesized and their performance was examined for the CÀH
acylation of azobenzenes under the optimized reaction
conditions. As can be seen in Table 2, 2-oxo-2-arylacetic
acids with electron-rich and -poor substituents, such as Me,
MeO, tBu, F, Cl, and Br on para positions of aromatic rings
gave the desired products (3ba–bf) in good yields. Generally,
2-oxo-2-arylacetic acids with electron-withdrawing groups on
the benzene rings were more suitable substrates and gave
plexes). Further experiments indicated that the photoredox
catalyst, palladium catalyst, visible-light irradiation, and molec-
ular oxygen were all essential for the reaction, and no reaction
occurred in their absence (entries 11–14). Further optimization
of acridinium salts showed that 9-mesityl-10-methylacridinium
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Table 2. The scope of acylation of azobenzenes with a-oxocarboxylic
acids.[a,b]
Scheme 2. Intermolecular competing experiments.
stituted a-oxocarboxylic acid 2n was evaluated, and a moder-
ate yield of the product 3bn was obtained under the opti-
mized reaction conditions. However, an aliphatic a-oxocarbox-
ylic acid, such as 2-oxopropanoic acid (2o), did not react with
1a, and the desired product 3bo was not obtained.
Next, the optimization of the reaction conditions for the CÀ
H acylation of azoxybenzene 4a with 2-oxo-2-phenylacetic
acid (2a) showed that the reaction did indeed occur with 76%
yield of the desired product (5a) in the presence of Pd(TFA)2
(5.0 mol%), PC-A (2.0 mol%), 1.5 W blue LED, and DCE (2.0 mL)
at room temperature in air for 20 h (for details of the optimiza-
tion, see Table S4–7 in the Supporting Information). To explore
the applicability of this protocol, some typical disubstituted
azoxybenzenes were examined and the results are listed in
Table 3. The reaction of azoxybenzenes with 2-oxo-2-phenyl-
acetic acid (2a) provided the corresponding products 5a–e in
66–77% yields, and tolerated a variety of functional groups, in-
cluding Me, MeO, and iPr groups, in the substrates of azoxy-
benzenes. However, ortho-substituted azoxybenzene 4 f gener-
ated 5 f in lower yield due to the steric hindrance.
[a] Reaction conditions:
(5.0 mol%), PC-A (2.0 mol%), 1.5 W blue LED, and toluene (2.0 mL) at
room temperature in air for 16 h. [b] Isolated yield.
1
(0.20 mmol),
2
(0.24 mmol), Pd(TFA)2
To gain insight into the mechanism of this transformation,
some control experiments and an intermolecular competing ki-
netic isotope effect (KIE) were carried out. The addition of
1.5 equivalent of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy)
to the mixture of azobenzene 1a and 2-oxo-2-phenylacetic
acid (2a) suppressed the reaction completely (Scheme 3a).
Moreover, TEMPO trapped the benzoyl radical to generate 6
(Scheme 3b). The intermolecular kinetic isotopic effect was ob-
served with kH/kD =3.7 (Scheme 3c), indicating the sp2 CÀH
bond cleavage involved in the rate-determining step of the re-
action.
higher yields than 2-oxo-2-arylacetic acids with electron-donat-
ing groups on the benzene rings (3ba–bc versus 3bd–bf). 2-
Oxo-2-(meta-methylphenyl)acetic acid (2g) worked well in the
reaction, and the desired product 3bg was obtained in 73%
yield. Evidently, the steric hindrance effect was observed in the
reaction of 2-oxo-2-arylacetic acids with azobenzenes, generat-
ing the products (3bh–bj) in 57–69% yields under the present
reaction conditions. Furthermore, 2-oxo-2-(2,4-dimethylphenyl)-
acetic acid (2k) showed good reactivity in the reaction and
provided product 3bk in 72% yield. It should be noted that re-
actions of 2-(naphthalen-1-yl)-2-oxoacetic acid (2l) and 2-
(naphthalen-2-yl)-2-oxoacetic acid (2m) with 1a afforded the
corresponding products 3bl and 3bm in 77 and 84% yields,
respectively. Encouraged by these results, the heterocyclic-sub-
On the basis of our preliminary mechanistic study and previ-
ous related literature,[4,5,7,10] a possible mechanism of this trans-
formation is proposed in Scheme 4. The reaction begins with
the photoexcitation of mesityl acridinium catalyst (PC-A) by
visible light to generate its excited state (PC-A*), which is sub-
sequently oxidized by the molecular oxygen (single-electron
process) to afford PC-A·, along with the generation of superox-
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Table 3. The scope of acylation of azoxybenzenes with 2-oxo-2-phenyl-
acetic acid.[a,b]
Scheme 3. Preliminary mechanistic study.
[a] Reaction conditions:
(5.0 mol%), PC-A (2.0 mol%), 1.5 W blue LED, and DCE (2.0 mL) at room
temperature in air for 20 h. [b] Isolated yield.
4
(0.20 mmol), 2a (0.24 mmol), Pd(TFA)2
initiated by a CÀH activation of the azobenzene to form the
palladacyclic intermediate E. Palladacycle E then reacts with
acyl radical B, generated from the decomposition of 2a in situ,
to afford PdIV or PdIII species F.[3a,11] The reductive elimination of
F leads to the desired acylated product 3a and generates the
PdI intermediate G, which is reoxidized by superoxide anion C
to regenerate PdII catalyst to complete the catalytic cycle
ide anion C. Meanwhile, a single-electron oxidation of 2a by
the formed PC-A· regenerates ground-state photocatalyst PC-A
for the next run, and generates the corresponding carboxyl
radical species, which undergoes the decomposition to from
the critical benzoyl radical species B, along with extrusion of
CO2 (for FTIR analysis of the resulting CO2 gas, see the Support-
ing Information). On the other hand, Pd catalytic cycle can be
2À
along with the formation of O2 (D) and H2O2.
To support the generation of superoxide radical anion C
À
(O2 ·) in proposed mechanism, 5,5-dimethyl-1-pyrroline-N-
oxide (DMPO) was used as a probe to capture the active spe-
cies.[12a,c] As shown in Figure 1, when a toluene solution of
Scheme 4. Plausible reaction mechanism.
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Keywords: acylation · azo compounds · blue LED · palladium ·
visible-light photocatalysis
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b) under blue LED irradiation for 60 s.
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À
radical O2 · was clearly observed (Figure 1b). It revealed that
À
the superoxide radical anion (O2 ·) is generated from the mo-
lecular oxygen by single-electron transfer (SET)[5d,e,h,12] under
the present reaction conditions.
In summary, this communication describes a mild and gener-
al approach combined with palladium catalysis and visible-
light photocatalysis for the CÀH acylation of azo- and azoxy-
benzenes with a-keto acids. The use of a catalytic amount of
the photoredox catalyst under the irridation of 1.5 W blue LED
avoids a typical high loading of external oxidant. This transfor-
mation, performed at room temperature, made the acylation
of azo- and azoxybenzenes with a wide spectrum of substrates
and reagents. Further investigations on the combination of or-
ganic dyes as photocatalyst with transition-metal catalysts in
organic transformations are currently underway in our labora-
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Experimental Section
Typical procedure for the acylation of azoxybenzenes
A mixture of azobenzene (1a, 36.4 mg, 0.20 mmol), 2-oxo-2-phe-
nylacetic acid (2a, 36 mg, 0.24 mmol), Pd(TFA)2 (3.3 mg,
0.01 mmol), and 9-mesityl-10-methylacridinium perchlorate (PC-A;
1.6 mg, 0.004 mmol) was dissolved in toluene (2.0 mL) in a 10 mL
oven-dried reaction vessel equipped with a magnetic stirring bar.
The reaction vessel was irradiated using 1.5 W blue LED for 16 h at
room temperature under air. After the reaction was completed, the
resulting mixture was extracted with EtOAc (2”5.0 mL). The organ-
ic layers were combined, dried over Na2SO4, and concentrated to
yield the crude product, which was further purified by flash chro-
matography (silica gel, ethyl acetate/petroleum ether 1:50, v/v) af-
fording the desired product 3aa as a yellow liquid (44.6 mg, 78%
yield).
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Acknowledgements
We gratefully acknowledge the National Natural Science Foun-
dation of China (Nos. 21572078, 21372095) for financial sup-
port of this work.
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Received: November 11, 2015
Published online on January 7, 2016
Chem. Eur. J. 2016, 22, 2236 – 2242
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim