Palladium-catalyzed synthesis of aromatic ketones and isoindolobenzimidazoles via selective aromatic C-H bond acylation

Article abstract of DOI:10.1002/adsc.201200743

A convenient and efficient palladium-catalyzed synthesis of aromatic ketones and isoindolobenzimidazoles has been developed via selective aromatic C-H bond acylation. The protocol uses palladium acetate as the catalyst, readily available carboxylic acids as the acylating reagents, trifluoroacetic anhydride as the activated agent of the acids, and the corresponding aromatic ketones and isoindolobenzimidazoles were obtained in good to excellent yields. This finding should provide a new and useful strategy for synthesis of aromatic ketones and isoindolobenzimidazoles.

Full text of DOI:10.1002/adsc.201200743

FULL PAPERS
DOI: 10.1002/adsc.201200743
Palladium-Catalyzed Synthesis of Aromatic Ketones and
À
Isoindolobenzimidazoles via Selective Aromatic C H Bond
Acylation
Juyou Lu,a Hao Zhang,a Xiaowu Chen,b Hongxia Liu,b Yuyang Jiang,b
and Hua Fua,b,*
a
Key Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry of Education), Department of
Chemistry, Tsinghua University, Beijing 100084, Peopleꢀs Republic of China
Fax : (+ 86)-10-6278-1695; e-mail: fuhua@mail.tsinghua.edu.cn
Key Laboratory of Chemical Biology (Guangdong Province), Graduate School of Shenzhen, Tsinghua University,
b
Shenzhen 518057, Peopleꢀs Republic of China
Received: August 18, 2012; Published online: January 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200743.
Abstract: A convenient and efficient palladium-cata- midazoles were obtained in good to excellent yields.
lyzed synthesis of aromatic ketones and isoindolo- This finding should provide a new and useful strat-
benzimidazoles has been developed via selective aro- egy for synthesis of aromatic ketones and isoindolo-
À
matic C H bond acylation. The protocol uses palla- benzimidazoles.
dium acetate as the catalyst, readily available car-
boxylic acids as the acylating reagents, trifluoroacetic Keywords: carboxylic acids; C H functionalization;
anhydride as the activated agent of the acids, and the Friedel–Crafts acylation; ketones; palladium cataly-
corresponding aromatic ketones and isoindolobenzi- sis
À
Introduction
Herein, we report a novel and efficient palladium-cat-
alyzed synthesis of aromatic ketones and isoindolo-
À
Aromatic ketones are important materials in manu- benzimidazoles via selective aromatic C H bond acy-
facturing of fine chemicals and pharmaceuticals, dyes, lation with readily available carboxylic acids.
fragrances and agrochemicals,[1] and Friedel–Crafts
acylation is one of the common methods for their syn-
thesis.[2] However, the acylation reactions have met Results and Discussion
great opposition from green chemistry[3] because the
traditional methods usually use large quantities of As shown in Table 1, acylation of 2-phenylpyridine
hazardous Lewis acids such as AlCl3, TiCl4, and FeCl3, (1a) with benzoic acid (2e) leading to phenyl(2-(pyri-
which are treated to give large volumes of waste in din-2-yl)phenyl)methanone (3a) was chosen as the
the course of work-up.[4] Therefore, the development model reaction to optimize the reaction conditions in-
of truly catalytic alternatives is highly desired. In the cluding catalysts, ratio of substrates, amount of tri-
past decades, some catalytic Friedel–Crafts acylations fluoroacetic anhydride (TFAA) and temperature.
have been developed under various catalytic condi- First, the ratio of 2e relative to 1a was investigated
tions,[5] but the product formation as isomeric mix- (entries 1–9), and 8 equiv. of 2e was suitable (entry 8).
tures from the acylations is often unavoidable. In The effect of temperature was studied (compare en-
order to solve the problems above, the palladium-cat- tries 8, 10–13), and 1008C was a good choice. Other
alyzed synthesis of aromatic ketones from aromatic catalysts were screened (entries 14 and 15), and
boronic acids and carboxylic acids or anhydrides has PdACHTNUTRGNEUNG(OAc)2 provided the highest yield (compare en-
been developed.[6] Recently, the direct functionaliza- tries 8, 14 and 15). The amount of Pd
tion of C H bonds has made great progress, and plored (compare entries 8, 16–18), and 10 mol%
(OAc)2 was ex-
[7]
À
À
some aromatic ketones were prepared via the C H Pd
(OAc)2 as the catalyst afforded an acceptable yield.
bond functionalization strategy using aldehydes,[8a–f] A slightly higher yield was observed when amount of
alcohols[8g] or a-oxocarboxylic acids[8h] as the partners. TFAA was increased (compare entries 8 and 19). The
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Table 1. Acylation of 2-phenylpyridine (1a) with benzoic acid (2e) leading to phenyl[2-(pyridin-2-yl)phenyl]methanone (3e):
optimization of conditions.[a]
Entry
Cat. (mol%)
Pd(OAc)2 (10)
2e [mmol]
TFAA [mmol]
Temperature [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
G
1.2
2
3
4
5
6
7
8
9
8
8
8
8
8
8
8
8
8
8
8
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
20
10
100
100
100
100
100
100
100
100
100
r.t.
trace
12
45
54
58
74
77
78
77
0
15
58
46
29
0
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
PdCl2 (10)
Cu(OAc)2 (10)
Pd(OAc)2 (5)
Pd(OAc)2 (1)
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
9
10
11
12
13
14
15
16
17
18
19
20
R
R
N
60
80
120
100
100
100
100
100
100
100
ACHTUNGTRENNUNG
T
ACHTUNGTRENNUNG
53
25
0
–
Pd
A
82
Pd
ACHTUNGTRENNUNG
73[c]
[a]
Reaction conditions: 2-phenylpyridine (1a) (1 mmol) in a sealed Schlenk tube, reaction time: 16 h.
Isolated yield.
Under nitrogen atmosphere.
[b]
[c]
reaction provided a lower yield when a nitrogen at-
According to the results above, a possible mecha-
mosphere was introduced (entry 20). Therefore, the nism for the palladium-catalyzed synthesis of aromat-
standard conditions for the palladium-catalyzed syn-
thesis of aromatic ketones are as follows: 10 mol%
PdACHTUNGTRENNUNG(OAc)2 as the catalyst, 20 equiv. of TFAA as the
activating agent, 8 equiv. of carboxylic acids as the
partners of the substituted 2-phenylpyridines, and the
reactions were performed at 1008C without exclusion
of air.
We then investigated the substrate scope for the
palladium-catalyzed synthesis of aromatic ketones. As
shown in Table 2, the examined substrates provided
good to excellent yields. For substituents R1, the sub-
strates (1c and 1d) with electron-withdrawing groups
showed slightly lower reactivity than those (1a and
1b) with electron neutral groups. For carboxylic acids,
the aliphatic carboxylic acids exhibited higher reactiv-
ity than the aromatic carboxylic acids. The reactions
could tolerate various functional groups including
À
À
ether (entry 7), C Br bonds (entries 8–10, 13), C Cl
bonds (entries 11, 12, 18–20), trifluoromethyl (en-
tries 13, 21 and 22), and nitro groups (entries 14 and
15).
Scheme 1. Possible mechanism on palladium-catalyzed syn-
thesis of aromatic ketones.
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Palladium-Catalyzed Synthesis of Aromatic Ketones and Isoindolobenzimidazoles
[a]
À
Table 2. Palladium-catalyzed synthesis of aromatic ketones via selective aromatic C H bond acylation.
Entry
1
1
3
Yield [%][b]
68
1a
1a
1a
1a
3a
3b
3c
3d
2
3
4
81
80
88
5
6
7
1a
1a
1a
3e
3f
3g
86
65
69
8
1a
1a
1a
1a
3h
3i
54
60
73
9
10
3j
11
3k
82
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Juyou Lu et al.
Yield [%][b]
98
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Table 2. (Continued)
Entry
1
3
12
13
1a
1a
3l
3m
47
14
15
1a
1a
3n
3o
45
50
16
17
18
19
20
21
1b
1b
1c
1c
1c
1d
3p
3q
3r
3s
3t
90
79
75
87
75
69
3u
22
1d
3v
61
[a]
Reaction conditions: PdACHTNUTRGNENG(U OAc)2 (0.05 mmol for entries 1–15; 0.025 mmol for others), 1a (0.5 mmol), 1b–d (0.25 mmol), car-
boxylic acid (2 mmol for entries 1–4; 4 mmol for entries 5–15; 1 mmol for entries 16–22), TFAA (10 mmol for entries 1–
15; 5 mmol for entries 16–22), temperature (1008C), reaction time (16 h) in a sealed Schlenk tube.
Isolated yield.
[b]
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Palladium-Catalyzed Synthesis of Aromatic Ketones and Isoindolobenzimidazoles
In order to confirm the formation of mixed anhy-
dride intermediate (II) in Scheme 1, we attempted
a palladium-catalyzed acylation of 2-benzylpyridine
(1e) in the presence of 2,2,2-trifluoroacetic acid and
acetic anhydride, and the target product, 1-(2-((pyri-
din-2-yl)methyl)phenyl)ethanone (3v), was obtained
in 85% yield (Scheme 2). However, the reaction was
not possible in the absence of trifluoroacetic acid.
Inspired by the excellent results above, we investi-
gated palladium-catalyzed reactions of substituted 2-
phenyl-1H-benzimidazoles (4) with carboxylic acids
Scheme 2. Acylation of 2-benzylpyridine (1e) under the
standard conditions.
ic ketones is proposed in Scheme 1. Treatment of car- (2) (see Table 3). To our delight, the reactions provid-
boxylic acid (1) with Pd(OAc)2 provides catalyst I ed isoindolobenzimidazoles (5). The cascade reactions
[because of the appearance of 40 equiv. of carboxylic underwent sequential selective aromatic C H bond
AHCTUNGTRENNUNG
À
acid (1) relative to PdACHTNUTRGNENG(U OAc)2 in the system], and reac- acylation and intramolecular cyclization (see Table 3
tion of TFAA with 2 gives mixed anhydride (II). Co- and Scheme 3). It is known that isoindoles[9] and ben-
ordination of I with substituted 2-phenylpyridine (1) zoimidazoles[10] display a variety of biologically and
affords III,[8a] treatment of III with II (oxidative addi- medicinal activities, but the previous methods for syn-
tion) yields complex IV,[6a,b] and reductive elimination thesis of N-fused heterocycles, isoindolobenzimida-
of IV provides the target product (3) regenerating the zoles, from isoindoles and benzoimidazoles are very
Pd(II) catalyst.
rare thus far.[11] Therefore, the present method should
provide a useful strategy for the synthesis of this kind
of compounds.
In order to explore the mechanism of the palladi-
um-catalyzed synthesis of isoindolobenzimidazoles,
two control experiments were performed as shown in
Scheme 3. Reaction of 2-phenyl-1H-benzimidazoles
(4a) with acetic anhydride leading to 6 in 75% yield
(Scheme 3, A), and only small amount (21% yield) of
isoindolobenzimidazole (5a) was obtained when 6 was
treated under the standard conditions (Scheme 3, B).
Therefore, a possible mechanism for the synthesis of
isoindolobenzimidazoles is suggested as shown in
Scheme 4. First, reaction of substituted 2-phenyl-1H-
benzimidazole (4) with TFAA gives V, and palladi-
um-catalyzed acylation of V with carboxylic acid (2)
provides VI, freeing water. Hydrolysis of VI leads to
VII, nucleophilic attack of NH to carbonyl in VII af-
fords VIII, and dehydration of VIII gives the target
product (5).
Scheme 3. (A) Reaction of 2-phenyl-1H-benzimidazole (4a)
with acetic anhydride leading to 6. (B) Treatment of 6 under
the standard conditions.
Scheme 4. Possible mechanism for the palladium-catalyzed synthesis of isoindolobenzimidazoles.
Adv. Synth. Catal. 2013, 355, 529 – 536
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[a]
À
Table 3. Palladium-catalyzed synthesis of isoindolobenzimidazoles through selective aromatic C H bond acylation.
Entry
1
4
Temperature [8C]/Time [h]
5
Yield [%][b]
51
4a
4a
4a
4a
4b
4b
4b
4b
4c
4c
4c
120/48
5a
5b
5c
5d
5e
5f
5g
5h
5i
2
140/24
140/24
140/24
120/48
140/24
140/24
140/24
140/24
140/24
140/24
73
71
77
60
75
82
70
69
64
65
3
4
5
6
7
8
9
10
5j
11
5k
[a]
Reaction conditions: PdACHTNUTRGNENG(U OAc)2 (0.025 mmol), substituted 2-phenyl-1H-benzo[d]imidazoles (4) (0.25 mmol), carboxylic
acid (1 mmol), TFAA (5 mmol), PhF (2 mL), temperature (120 or 1408C), reaction time (24 or 48 h) in a sealed Schlenk
tube.
Isolated yield.
[b]
Conclusions
agents, TFAA as the activating agent of the acids, and
the corresponding aromatic ketones and isoindolo-
We have developed a convenient and efficient palladi- benzimidazoles were obtained in good to excellent
um-catalyzed synthesis of aromatic ketones and isoin- yields. The novel and practical method should find
À
dolobenzimidazoles via selective aromatic C H bond wide application and attract much attention in aca-
acylation. The protocol uses Pd
A
readily available carboxylic acids as the acylating re-
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Palladium-Catalyzed Synthesis of Aromatic Ketones and Isoindolobenzimidazoles
7.50–7.45 (m, 2H), 7.32–7.26 (m, 2H), 5.58 (d, 2H, J=
Experimental Section
8.6 Hz); 13C NMR (CDCl3, 75 MHz): d=156.7, 149.0, 139.1,
138.3, 130.9, 130.2, 130.0, 128.1, 124.0, 123.3, 121.9, 121.3,
121.1, 110.8, 93.3; HR-MS: m/z=219.0924, calcd. for
C15H11N2 [M+H]+: 219.0922.
General Procedure for Palladium-Catalyzed Synthesis
of Aromatic Ketones (3)
A 25-mL Schlenk tube was charged with substituted 2-phe-
nylpyridine (1) (0.5 mmol for 1a; 0.25 mmol for 1b–d), car-
boxylic acid (2 mmol for entries 1–4, 4 mmol for entries 5–
15, 1 mmol for entries 16–22), trifluoroacetic anhydride
(10 mmol for entries 1–15; 5 mmol for entries 16–22), and
Compound 5k: petroleum ether/ethyl acetate (20:1);
yield: 89 mg (65%); yellow solid; mp 158–1608C. H NMR
1
(CDCl3, 300 MHz,): d=7.79–7.76 (m, 2H), 7.70 (d, 1H, J=
7.9 Hz), 7.62–7.59 (m, 1H), 7.29–7.25 (m, 3H), 6.28 (t, 1H,
J=7.6 Hz), 2.75–2.67 (m, 2H), 2.46 (s, 3H), 1.75–1.68 (m,
2H), 1.09 (t, 3H, J=7.2 Hz); 13C NMR (CDCl3, 150 MHz):
d=155.8, 148.8, 139.6, 135.8, 133.1, 130.8, 130.7, 128.8, 124.5,
123.4, 122.7, 122.4, 120.8, 116.2, 110.7, 29.3, 23.1, 21.6, 13.9;
HR-MS: m/z=275.1545, calcd. for C19H19N2 [M+H]+:
275.1548.
PdACHTUNGTRENNUNG(OAc)2 (0.05 mmol, 11.2 mg for entries 1–15; 0.025 mmol,
5.6 mg for entries 16–22). The tube was sealed and the mix-
ture was allowed to stir at 1008C for 16 h (see Table 2). The
resulting solution was cooled to room temperature, and
water (2 mL) and NaOH (25 mmol, 1 g) were added to the
resulting solution. The resulting aqueous solution was ex-
tracted with ethyl acetate (3ꢂ10 mL), the combined organic
phase was concentrated with the aid of a rotary evaporator,
and the residue was purified by column chromatography on
silica gel to provide the desired product (3). Two representa-
tive examples are shown as follows:
1-(2-(Pyridin-2-yl)phenyl)butan-1-one (3c): petroleum
ether/ethyl acetate (10:1); yield: 90 mg (80%); pale yellow
oil. 1H NMR (CDCl3, 300 MHz): d=8.60 (s, 1H), 7.75 (t,
1H, J=7.6 Hz), 7.61 (q, 2H, J=7.2 Hz), 7.49–7.45 (m, 3H),
7.25–7.21 (m, 1H), 2.46 (t, 2H, J=7.2 Hz), 1.63 (tꢂq, 2H,
J=7.6 Hz, J=7.2 Hz), 0.85 (t, 3H, J=7.6 Hz); 13C NMR
(CDCl3, 75 MHz): d=207.3, 157.5, 149.3, 142.0, 138.5, 136.9,
130.0, 129.0, 128.7, 127.5, 122.4, 122.3, 45.1, 18.0, 13.9; HR-
MS: m/z=226.1235, calcd. for C15H16NO [M+H]+:
226.1232.
Acknowledgements
The authors wish to thank the National Natural Science
Foundation of China (Grant Nos. 20972083 and 21172128),
and the Ministry of Science and Technology of China (Grant
No. 2012CB722605) for financial support.
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1
yellow oil. H NMR (CDCl3, 300 MHz): d=8.59 (d, 1H, J=
4.5 Hz), 7.73 (tꢂd, 1H, J=7.6 Hz, J=2.0 Hz), 7.56–7.52 (m,
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General Procedure for Palladium-Catalyzed Synthesis
of Isoindolobenzimidazoles (5)
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A 25-mL Schlenk tube was charged with substituted 2-
phenyl-1H-benzimidazole (4) (0.25 mmol), carboxylic acid
(2) (1 mmol), PdACHTUNGTRENNUNG(OAc)2 (0.025 mmol, 5.6 mg) and fluoro-
benzene (2 mL). The tube was sealed and the mixture was
allowed to stir at 120 or 1408C for 24 or 48 h (see Table 3).
After the reaction had finished, the resulting solution was
cooled to room temperature, and water (2 mL) and NaOH
(12.5 mmol, 0.5 g) were added. The resulting aqueous solu-
tion was extracted with ethyl acetate (3ꢂ10 mL), the com-
bined organic phase was concentrated with the aid of
a rotary evaporator, and the residue was purified by column
chromatography on silica gel to provide the desired product
(5). Two representative examples are shown as follows:
Compound 5a: petroleum ether/ethyl acetate (20:1);
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yield: 56 mg (51%); yellow solid; mp 123–1258C. H NMR
(CDCl3, 300 MHz): d=7.94 (d, 1H, J=7.6 Hz), 7.79 (d, 1H,
J=8.9 Hz), 7.69 (d, 1H, J=7.2 Hz), 7.59 (d, 1H, J=8.3 Hz),
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