Palladium-catalyzed oxidative C-H bond acylation of acetanilides with benzylic alcohols

Article abstract of DOI:10.1002/adsc.201100417

An efficient and clean method to construct C-C bonds has been developed via the acylation reaction of acetanilides with benzylic alcohols using tert-butyl hydroperoxide (TBHP) as oxidant catalyzed by palladium acetate in the presence of triflic acid (TfOH). The acylation reactions exhibit excellent reactivities, and up to 95% yield of the corresponding aryl ketone could be obtained under the optimal conditions. Copyright

Full text of DOI:10.1002/adsc.201100417

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DOI: 10.1002/adsc.201100417
À
Palladium-Catalyzed Oxidative C H Bond Acylation of
Acetanilides with Benzylic Alcohols
Yu Yuan,a,* Duanteng Chen,a and Xiaowei Wanga
a
College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, Peopleꢀs Republic of China
E-mail: yyuan@yzu.edu.cn
Received: May 22, 2011; Revised: July 19, 2011; Published online: December 6, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100417.
À
Abstract: An efficient and clean method to con-
struct C C bonds has been developed via the acyla-
dant.[9] Acetanilides have been utilized in C H func-
tionalization reactions widely[10] and the correspond-
ing amino products could be readily transferred to
other functional compounds which are useful in or-
ganic synthesis[11]. Herein, we report the acylation re-
action of acetanilides and benzylic alcohols using
TBHP as the oxidant in the presence of catalytic pal-
ladium acetate via oxidative cross-coupling.
À
tion reaction of acetanilides with benzylic alcohols
using tert-butyl hydroperoxide (TBHP) as oxidant
catalyzed by palladium acetate in the presence of
triflic acid (TfOH). The acylation reactions exhibit
excellent reactivities, and up to 95% yield of the
corresponding aryl ketone could be obtained under
the optimal conditions.
Initially, acetanilide (1a) and phenylmethanol (2a)
were treated with TBHP and 10 mol% PdACTHNURGTNEUNG(OAc)2 in
Keywords: acylation; benzylic alcohols; oxidative
C H activation; palladium catalysis; triflic acid
acetic acid at 908C for 5 h in the presence of
50 mol% p-toluenesulfonic acid (p-TsOH), conditions
À
À
that were applied in the cleavage of ortho-C H bonds
by Pd(II) in some cases[10a,12]. A moderate yield
(55%) of the corresponding ketone (3aa) was ob-
tained, while a slightly better yield (60%) of 3aa re-
Aryl ketones are very important compounds used in sulted when triflic acid (TfOH) was used instead of p-
the pharmaceutical, fragrance, dye, and agrochemical TsOH (Table 1, entries 1 and 2). It was found that the
industries.[1] The traditional methods for synthesis of reaction could be carried out well in an AcOH/N,N-
aryl ketones mainly rely on the Friedel–Crafts acyla- dimethylacetylamine (DMA) mixture (4:1 v/v), but
tion of aromatic compounds in the presence of Lewis no reactions were detected without the additive or in
acids or Brønsted acid[2] and oxidation of the corre- toluene and trifluoroacetic acid (entries 3–6). Differ-
sponding secondary alcohols by various oxidants[3]. ent palladium sources were screened as the catalyst in
À
Recently, the direct C H bond acylation of aromatic this acylation reaction of acetanilide (1a) and phenyl-
compounds has been reported by some groups.[4] The methanol (2a) (entries 11–13). Only the catalytic ac-
combination of transition metals and directing groups tivity of Pd
is a useful strategy to facilitate C H bond cleavage, The oxidants oxygen and K2S2O8 when employed in
(PPh3)4 was similar to that of Pd
E
À
À
À
which affords valuable transformations of C H to C
the reaction were shown to afford poor reactivities
C bonds.[5,6,7] This kind of acylation is a more atom (entries 14 and 15).
economic and environmentally friendly alternative for
aryl ketone synthesis in comparison to the Friedel–
Next, a variety of substituted anilides and aryl-
methanols were reacted with TBHP catalyzed by
(OAc)2 under the optimized conditions (Table 2).
AHCTUNGTRENNUNG
Crafts acylation. Cheng and Li reported the Pd-cata- Pd
G
2
À
lyzed oxidative sp C H acylation of 2-arylpyridine The acylation reaction proceeded efficiently with
with aldehydes in good yields, respectively.[8] The acy- electron-donating moieties in acetanilide, which is
lation reaction of aromatic oximes and aldehydes was consistent with an electrophilic palladation mecha-
catalyzed by palladium via oxidative coupling to nism,[13] whereas the poor yields were obtained using
obtain the corresponding ketone compounds in high acetanilide bearing electron-withdrawing groups at
yields.[8c] Moreover, alcohols could be employed as the aromatic ring. Furthermore, m, p-disubstituted
substrates in the acylation reaction of 2-arylpyridine acetanilide also provided a good yield (entry 6). Un-
using tert-butyl hydroperoxide (TBHP) as the oxi- fortunately o-substituted acetanilide failed to undergo
Adv. Synth. Catal. 2011, 353, 3373 – 3379
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Table 1. Optimization of the reaction conditions.[a]
Entry
Pd (mol%)
Additives (equiv.)
Oxidants (equiv.)
Solvents
Yield [%][b]
1
2
3
4
5
6
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
N
p-TsOH (0.5)
TfOH (0.5)
TfOH (0.5)
TfOH (0.5)
TfOH (0.5)
TfOH (0.5)
–
TfOH (0.5)
TfOH (0.5)
TFA (0.5)
TfOH (0.5)
TfOH (0.5)
TfOH (0.5)
TfOH (0.5)
TfOH (0.5)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
TBHP (4)
O2 (1 atm.)
K2S2O8 (4)
AcOH
55
60
0
AcOH
TFA[c]
toluene
0
AcOH/DMA[d]
AcOH/DMA
AcOH/DMA
AcOH/DMA
AcOH/DCE
AcOH/DMA
AcOH/DMA
AcOH/DMA
AcOH/DMA
AcOH/DMA
AcOH/DMA
85
45
0
34
72
56
20
79
0
7
8[e]
9
10
11
12
13
14
15
PdCl
Pd
CHTUNGTRENNUNG
CHTUNGTRENNUNG
Pd
Pd
0
trace
[a]
Reaction conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst, oxidant, additive (0.1 mmol), solvent (1.0 mL), 908C, 5 h.
Isolated yield.
TFA: trifluoroacetic acid, DMA: N,N-dimethylacetylamine.
[b]
[c]
[d]
[e]
4:1 ratio
ACHTUNGTRENNUNG(v/v).
At 508C.
À
the acylation reaction under the current catalytic con-
The mechanism of the C H acylation reaction was
ditions. This is similar with the results mentioned in studied based on some related studies. It was found
the decarboylative acylation using o-substituted aceta- that this reaction could be divided into two steps con-
nilides as substrates.[10i] When an amino group was nected by an aldehyde. The benzylic alcohols could
connected with a pivaloyl or benzoyl instead of acetyl be converted to aldehydes readily in TBHP irrespec-
group in the anilides, the yields of corresponding aryl tive of whether palladium acetate was present in the
À
ketones were somewhat lower in this acylation reac- system or not. Meanwhile, the C H acylation reaction
tion (entries 10–13). It was found that 59% 3-(2-acet- could be carried out well using the corresponding al-
amido)-naphthyl phenyl ketone[14] was isolated when dehyde as substrate under the same conditions. How-
b-acetylnaphthylamine was reacted with phenylme- ever, in the second step no product was detected in
thanol as substrates (entry 9).
the absence of TBHP even when palladium acetate
As shown in Table 3, various substituted arylmetha- was used in a stoichiometric amount. Thus a plausible
À
nols, including those with methyl, methoxy, halogen, mechanism of the catalytic C H acylation reaction
and trifluoromethyl groups, were compatible under was described in Scheme 1. The acylation reaction
the optimal reaction conditions. The results of the re- may be initiated by electrophilic attack of a Pd(II)
action of acetanilide with the arylmethanols bearing center to the aromatic ring with the assistance of the
electron-deficient groups were better than with those acetamino group. The anion connected with Pd was a
bearing electron-rich groups. Moreover, para-substi- OTf anion to enhance the reactivity since TfOH was
tuted arylmethanols could furnish the best results in present in the system.[11d] At the same time, the alco-
the acylation reaction, in comparison with substitu- hol was oxidized to aldehyde by TBHP and then
ents at other positions on the aromatic ring (en- transferred to an acyl radical according to the litera-
tries 2–4). On combination of the optimal substrates ture.[8c,4h] The Pd(II) intermediate A would react with
screened above, a 95% yield of aryl ketone (3fj) the acyl radicals to afford the Pd(IV) intermediate
could be achieved when 3,4-dimethylacetanilide (1f) B.[8b,9,16] In our opinion, the Pd
and 4-fluorobenzyl alcohol (2j) were catalyzed by ly to exist in the system due to the high temperature
Pd
(OAc)2 in the presence of 50 mol% TfOH and oxidative conditions.[15,16] The Pd(IV) intermedi-
ACHTUNGERTN(NUNG III) complex is unlike-
ACHTUNGTRENNUNG
(entry 11).
ate B would undergo reductive elimination to pro-
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Palladium-Catalyzed Oxidative C H Bond Acylation of Acetanilides with Benzylic Alcohols
Table 2. Acylation reaction of anilides with phenylmethanol.[a]
Entry
1
Anilide 1
Product 3
Yield [%][b]
85
1a
1b
1c
3aa
3ba
3ca
2
3
92
93
4
1d
3da
57
5
6
1e
1f
3ea
3fa
73
94
7
8
1g
1h
3ga
3ha
21
23
9
1i
3ia
59
10
11
1j
3ja
65
77
1k
3ka
Adv. Synth. Catal. 2011, 353, 3373 – 3379
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Yu Yuan et al.
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Table 2. (Continued)
Entry
Anilide 1
Product 3
Yield [%][b]
12
1l
3la
42
13
1m
3ma
75
[a]
Reaction conditions: 1 (0.2 mmol), 2a (0.6 mmol), PdACHTUNTRGNE(UNG OAc)2 (10 mol%), TBHP (4 equiv.), TfOH (0.1 mmol), solvent
(1.0 mL), 908C, 5 h.
Isolated yield.
[b]
Table 3. Acylation reaction of acetanilides with arylmethanol.[a]
Entry
1
1
Arylmethanol 2
Product 3
Yield [%][b]
1a
1a
2b
2c
3ab
3ac
75
2
81
3
4
1a
1a
2d
2e
3ad
3ae
55
61
5
6
1a
1a
2f
2g
3af
76
73
3ag
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Palladium-Catalyzed Oxidative C H Bond Acylation of Acetanilides with Benzylic Alcohols
Table 3. (Continued)
Entry
1
Arylmethanol 2
Product 3
Yield [%][b]
7
1a
1a
1b
2h
2i
3ah
3ai
3bj
85
8
9
66
90
2j
10
11
12
13
1b
1f
1f
1f
1f
2c
2j
3bc
3fj
3fc
3ff
3fh
84
95
84
83
85
2c
2f
2h
14
[a]
Reaction conditions: 1 (0.2 mmol), 2 (0.6 mmol), PdACHTUNTRGNE(NUG OAc)2 (10 mol%), TBHP (4 equiv.), TfOH (0.1 mmol), solvent
(1.0 mL), 908C, 5 h.
Isolated yield.
[b]
duce the desired aryl ketone and regenerate the Experimental Section
Pd(II) species.
In conclusion, we have developed an efficient and Typical Procedure
À
clean method to construct C C bonds from acetani-
Pd
0.2 mmol), glacial acetic acid (0.8 mL), N,N-dimethylacet-
amide (0.2 mL), phenylmethanol 2a (65.0 mg, 0.6 mmol),
and TfOH (15 mg, 0.1 mmol) were placed inside a 15-mL
glass vial. The vial equipped with a Teflon cap was sealed
and immersed in an oil bath at 908C. TBHP (146 mL,
0.8 mmol, ~5.5M in decane) was added dropwise with a sy-
ringe and the reaction was kept under stirring for 5 h. After
ACHTUNGRTEN(NUNG OAc)2 (4.5 mg, 0.02 mmol), acetanilide 1a (27.0 mg,
lides and benzylic alcohols catalyzed by palladium(II)
acetate in the presence of TfOH. The corresponding
aryl ketones could be obtained in high yields. This re-
action protocol allows us to generate an array of
ortho-amino-substituted ketones in a simple and ef-
fective preparation while removing the acetyl group
from acetanilide.
AHCTUNGTRENNUNG
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Yu Yuan et al.
COMMUNICATIONS
Scheme 1. Plausible mechanism for the oxidative acylation.
cooling to room temperature, the reaction mixture was fil-
tered through Celite, and the filtrate was concentrated
under vacuum to afford an oily substance. The crude prod-
uct was purified by column chromatography on silica gel,
and the fraction was collected and concentrated to provide
Crafts Acylation Reactions, CRC Press, Boca Raton,
FL, 2010.
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3aa as a pale yellow solid; yield: 40.6 mg (85%). H NMR
(600 MHz, CDCl3): d=10.81 (s, 1H), 8.63 (d, J=7.9 Hz,
1H), 7.70 (d, J=7.0 Hz, 2H), 7.59 (d, J=8.1 Hz, 1H), 7.58–
7.52 (m, 2H), 7.49 (t, J=6.7 Hz, 2H), 7.08 (t, J=6.9 Hz,
1H), 2.22 (s, 3H); 13C NMR (150 MHz, CDCl3): d=198.7,
168.2, 139.5, 137.7, 133.3, 132.5, 131.5, 128.9, 127.3, 122.2,
121.0, 120.5, 24.3; IR (KBr): n=3307, 3065, 2962, 2926,
1697, 1636, 1581, 1520, 1444, 1260, 914, 745 cmÀ1; MS (EI):
m/z=239 (M+) 196, 167, 134, 120, 105, 92.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20702043) and by Program for New
Century Excellent Talents in Yangzhou University. A project
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions is acknowledged.
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