Palladium-catalyzed ortho-acylation of acetanilides with aldehydes through direct Ci-H bond activation

Article abstract of DOI:10.1002/chem.201101192

Easy access to o-acyl acetanilides: A new Pd-catalyzed ortho-acylation of acetanilides with both aromatic and aliphatic aldehydes has been developed based on a Ci-H activation process. In the presence of tert-butyl hydroperoxide (TBHP) as the ideal oxidant, this reaction provides an efficient access to ortho-acyl acetanilides in good yields (see scheme). Copyright

Full text of DOI:10.1002/chem.201101192

DOI: 10.1002/chem.201101192
Palladium-Catalyzed ortho-Acylation of Acetanilides with Aldehydes
À
through Direct C H Bond Activation
Chengliang Li,[a] Lei Wang,*[a, b] Pinhua Li,*[a] and Wei Zhou[a]
À
Transition-metal-catalyzed direct C H bond functionaliza-
tion, one of the hot topics in current organic chemistry, rep-
resents a burgeoning field because it enables the efficient
construction of carbon–carbon or carbon–heteroatom bonds
without the use of stoichiometric organometallic coupling
like to realize the palladium-catalyzed ortho-acylation of
acetanilides with commercially available, inexpensive alde-
À
hydes through direct C H bond activation. It is worth
noting that there are only a few examples of the transition-
metal-catalyzed direct access to ketones from aldehydes
[12]
À
À
reagents. Since the discovery of s-chelation directed C H
through C H cleavage of arenes.
To our pleasure, we
bond activation,[1] extensive efforts have been made in this
have achieved this assumed process. Herein, we will de-
scribe the palladium-catalyzed ortho-acylation of acetani-
lides with aldehydes in the presence of tert-butyl hydroper-
oxide (TBHP) as the ideal oxidant, affording the o-acyl ace-
tanilides in moderate to good yields (Scheme 1).
field.[2] The combination of transition metals and directing
À
groups is a useful strategy to facilitate C H bond cleavage,
which affords valuable transformations of C H bonds to
other covalent bonds. Among them, palladium-mediated C
À
À
H activation is one of the most attractive processes.[3] In this
area of research, various functional groups containing heter-
oatoms, such as acyl/acyloxy,[4] pyridyl,[5] oxazolyl,[6] N-me-
thoxy carbamoyl,[7] and acetoamino groups can provide an
anchor for either stoichiometric or catalytic ortho-metala-
tion of aromatic rings. With the acetamino group as a direct-
À
ing group, the ortho C H bond of acetanilide can be highly
regioselectively functionalized. Horino and co-workers have
reported the ortho-vinylation of acetanilides through cyclo-
palladation complexes in 1981,[8] and Tremont have de-
scribed the PdII-promoted direct functionalization of aceta-
nilides with alkyl iodides in 1984.[9] Recently, significant
progress in this area has been achieved with a variety of
coupling partners.[10]
o-Acylacetanilides are important structural units and syn-
thetic intermediates in biologically active natural products
and medicinal chemistry.[11] For their potential application,
the development of convenient synthetic pathway is highly
desirable. In 2010, Ge and co-workers have developed the
decarboxylative ortho-acylation of acetanilides with a-oxo-
Scheme 1.
Our initial investigation focused on the effect of a transi-
tion-metal catalyst on the ortho-acylation of acetanilides
with aldehydes. Several palladium, nickel, and iron sources
were screened in a model reaction of acetanilide (1a) with
para-chlorobenzaldehyde (2e), and the results are summar-
ized in Table 1. The ortho-acylation reaction of acetanilide
could be catalyzed by PdII salts, PdII, or Pd0 complexes, such
as Pd
(PCy3)2Cl2], [Pd
ence of tert-butyl hydroperoxide (TBHP) in toluene. It is
C
R
ACHTUGNTRNE(UNNG PPh3)2Cl2], [Pd-
À
carboxylic acids through palladium-catalyzed C H activa-
A
G
ACHTUNGTRENNUNG
tion to prepare o-acyl acetanilides.[10a] Inspired by the recent
studies of direct functionalization of acetanilides, we would
evident that PdACHTNGUTERNNU(G CF3CO2)2 was the most effective catalyst for
the reaction (Table 1, entries 1–7);[10a] probably the ortho-
acylation of acetanilide requires a highly cationic PdII spe-
cies to facilitate the formation of palladacycle intermediate
through the electrophilic substitution of arene,[13] and the
[a] C. Li, Prof. Dr. L. Wang, Dr. P. Li, W. Zhou
Department of Chemistry, Huaibei Normal University
Huaibei, Anhui 235000 (P.R. China)
Fax : (+86)561-3090518
dissociation of Pd
a cationic PdII species is easier than that of Pd
But, Pd/C, Ni(acac)2, Fe(acac)3, FeCl3, and FeBr2 failed to
ACHTUNGTRENNUNG
E-mail: leiwang@chnu.edu.cn
AHCTUNGTRENNUNG
pinhuali@yahoo.com.cn
A
ACHTUNGTRENNUNG
[b] Prof. Dr. L. Wang
facilitate the reaction (Table 1, entries 8–12). A variety of
oxidants were examined for their effect on the model reac-
tion and the results were also shown in Table 1. To our de-
light, the model reaction proceeded smoothly and generated
the desired product in 80% yield, representing one of the
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.201101192.
10208
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Chem. Eur. J. 2011, 17, 10208 – 10212

COMMUNICATION
Table 1. Screening of catalysts, oxidants and solvents for the palladium-
optimal; when less than 2 equiv of TBHP was used, the re-
action did not go to completion; however, no significant im-
provement was observed with more 2 equiv of TBHP. The
effect of solvent on the reaction was also investigated;
among the solvents tested, toluene was the most suitable re-
action media for the ortho-acylation reaction of acetanilide.
Chlorobenzene, benzene, and 1,2-dichloroethane were infe-
rior and generated 3e in 56, 36 and 16% yields, respectively
(Table 1, entries 25–27). Unfortunately, no desired product
was isolated when the reactions were carried out in DMF,
DMSO, NMP, dioxane, THF, C2H5OH, diglyme, or CH3CN
(Table 1, entries 28–35). During the course of further optimi-
zation of the reaction conditions, the reaction was generally
completed within 24 h when it was performed at 1208C by
catalyzed ortho-acylation of acetanilide with para-chlorobenzaldehyde.[a]
Entry
Catalyst
Oxidant
Solvent
3e
[%][b]
1
2
3
4
5
6
7
8
Pd
Pd
[Pd
[Pd
[Pd
[Pd
N
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
(tert-C4H9O)2
DCP
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
C6H5Cl
benzene
DCE
80
66
33
17
34
38
35
N.R.
N.R.
N.R.
N.R.
N.R.
39
G
E
N
U
using 5 mol% of PdACTHNUTRGNEUN(G CF3CO2)2 in the presence of TBHP
PdCl2
Pd/C
(2 equiv) in toluene. Meanwhile, when the model reaction
was carried out at 1008C and 1508C, the desired product 3e
was isolated in 45 and 32% yields, respectively.
With the optimized conditions in hand, the scope of the
Pd-catalyzed ortho-acylation of acetanilides with aldehydes
9
Ni
ACHTUNGTRENNUNG
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Fe
ACHTUNGTRENNUNG
FeCl3
FeBr2
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
37
À
through direct C H bond activation was investigated with a
A
N.R
N.R
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
55
variety of aldehydes. The results are summarized in Table 2.
As can be seen from Table 2, the reactivity of both aliphatic
and aromatic aldehydes was observed, with the aromatic al-
dehydes shown to be much more reactive than aliphatic
ones (Table 2, 3a–3o vs. 3p–3r). The aromatic aldehydes
displayed high reactivity under the present reaction condi-
tions and good yields of desired ortho-acylation of acetani-
lide products were obtained. Aromatic aldehydes with both
electron-donating and electron-withdrawing functionalities,
such as methoxy, methyl, trifluoromethyl, fluoro, chloro,
bromo, and CO2CH3 groups, afforded the corresponding iso-
lated products in yields of 63–86% (Table 2, 3b–3o). As
one of the substrates, an aromatic aldehyde with a methyl
group at the meta position of the phenyl ring (compound
3h) gave a comparable product yield to that of benzalde-
hyde (3a). Meanwhile, an aromatic aldehyde with a methyl
group at the para position of the phenyl ring (3c) gave a su-
perior product yield to that of benzaldehyde (3a), and aro-
matic aldehyde with a methyl group at the ortho position of
the phenyl ring (compound 3k) gave inferior product yield
to that of benzaldehyde (3a). This ortho-position effect has
also been observed in the reaction of 2-fluorobenzaldehyde
or 2-chlorobenzaldehyde as one of the substrate (3l and
3m). It is known that ortho substitution on the phenyl rings
C6H5I
K2S2O8
CF3CO2Ag
Ag2O
ACHTUNGERTN(NUNG OAc)2
CuSO4
KBrO3
O2
I2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
36
16
DMF
DMSO
NMP
dioxane
THF
C2H5OH
diglyme
CH3CN
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
[a] Reaction conditions: acetanilide (1.0 equiv), para-chlorobenzaldehyde
(1.5 equiv), catalyst (0.05 equiv), oxidant (2.0 equiv), solvent
(1.5 mLmmolÀ1), sealed tube, 1208C, under air, 24 h. [b] acac=acetyl-
ACHTUNGTRENNUNGacetonate, DCP=dicumyl peroxide, DDQ=2,3-dichloro-5,6-dicyanoben-
zoquinone, TBHP=tert-butyl hydroperoxide, DCE=1,2-dichloroethane,
DMF=N,N-dimethylformamide, DMSO=dimethyl sulfoxide, NMP=N-
methyl-2-pyrrolidone. [d] Yields of isolated products after flash chroma-
tography.
À
hampers the Pd-catalyzed C H insertion at ortho position,
displaying the obvious “ortho-substituent” effect.[10g, n, 15] It is
important to note that aromatic aldehydes with an electron-
withdrawing group, such as trifluoromethyl, fluoro, chloro
and bromo group at the para position of the phenyl ring, af-
forded a better yield than that of benzaldehyde (3d–3g vs.
3a). Moreover, substrate with the strong electron-donating
methoxy group at the para position of the phenyl ring (com-
pound 3b) delivered a lower yield than its counterparts sub-
stituted by the methyl group (3b vs. 3c). When both the
ortho- and para positions of the phenyl ring were occupied
by a chloro group, the product yield was comparable to that
best results when 2 equiv of TBHP was used as oxidant
(Table 1, entry 1). It was found that (t-C4H9O)2 and dicumyl
peroxide (DCP) were much more inferior oxidants and gen-
erated the desired product in 39 and 37% yields, respective-
ly (Table 1, entries 13 and 14). Unfortunately, other oxi-
dants, such as (C6H5COO)2, DDQ, C6H5IACHTNUGTRNEUNG(OAc)2, K2S2O8,
CF3CO2Ag, Ag2O, CuSO4, KBrO3, O2, and I2 were no
longer effective oxidants in this reaction and no desired
product was isolated (Table 1, entries 15–24). With respect
to the oxidant loading, 2 equiv of TBHP was found to be
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L. Wang, P. Li et al.
Table 2. The scope of aldehydes in palladium-catalyzed ortho-acylation of ace-
yields of the desired products 3p–3r were obtained, respec-
tively (Table 2).
tanilides with aldehydes.[a]
To expand the scope of acetanilide, a diverse array of sub-
stituted acetanilides were surveyed. The substituted acetani-
lides, with either electron-donating or electron-withdrawing
groups on the benzene rings of acetanilides, could be ap-
plied to afford the desired products 3s–3y in good yields
(Table 3). The similar phenomenon has also been observed
in the ortho-acylation of substituted acetanilides with p-
chlorobenzaldehyde. A substituted acetanilide with a methyl
group at the meta- or para position of the benzene ring gave
comparable product yields to that of acetanilide (Table 3, 3s
and 3v vs. 3e). At the same time, substituted acetanilides
with a weak electron-withdrawing group, such as chloro or
bromo group at the para- or meta position of the benzene
ring, also afforded a comparable product yield to that of
acetanilide (Table 3, 3t, 3w or 3x vs. 3e). On the other
hand, substituted acetanilides with a strong electron-donat-
Product
3
Yield Product
[%][b]
3
Yield
[%][b]
3a 73
3b 74
3j 71
3k 68
3c 78
3d 82
3e 80
3 f 74
3l 65
3m 63
3n 71
3o 67
Table 3. The scope of acetanilides in palladium-catalyzed ortho-acylation
of substituted acetanilides with para-chlorobenzaldehyde.[a]
Product
3
Yield Product
[%][b]
3
Yield
[%][b]
3s 80
3t 71
3u 67
3v 80
3w 71
3x 75
3y 68
3g 86
3h 72
3i 75
3p 64
3q 57
3r 60
3z trace
3aa trace
[a] Reaction conditions: acetanilide (1.0 mmol), aldehyde (1.5 mmol), Pd-
ACHTUNGTRENNUNG(CF3CO2)2 (0.05 mmol), TBHP (2.0 mmol), toluene (1.5 mL), sealed tube,
1208C, under air, 24 h. [b] Yields of isolated products after flash chromatogra-
phy.
of benzaldehyde (3a vs. 3n) due to the combination of elec-
tronic effect and steric hindrance. Fortunately, the aliphatic
aldehydes, such as n-butyraldehyde, iso-butyraldehyde and
n-heptaldehyde, also reacted with acetanilide smoothly
under present reaction conditions and 57–64% isolated
[a] Reaction conditions: substituted acetanilide (1.0 mmol), para-chloro-
benzaldehyde (1.5 mmol), Pd(CF3CO2)2 (0.05 mmol), TBHP (2.0 mmol),
toluene (1.5 mL), sealed tube, 1208C, under air, 24 h. [b] Yields of isolated
AHCTUNGTRENNUNG
products after flash chromatography.
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Palladium-Catalyzed ortho-Acylation
COMMUNICATION
ing methoxy group at the para position of the benzene ring
afforded an inferior yield to that of the unsubstituted one
(Table 3, 3u vs. 3e). It should be noted that the substrates
without symmetry, such as meta-methylacetanilide and meta-
chloroacetanilide, underwent the regiospecific ortho-acyla-
tion to generate 3s and 3t, respectively, because of the
steric effect. Other anilides, except for acetanilide, were also
examined. Regrettably, tert-butyl phenylcarbamate and N-
phenylpivalamide reacted with para-chlorobenzaldehyde
under the recommended reaction conditions in much lower
reactivity and only trace amounts of the desired products
(Table 3, compounds 3z and 3aa) were detected by TLC,
owing to the steric hindrance of tert-butoxycarbonyl and piv-
alyl groups.
Scheme 3.
À
be noted that the catalytic C H acylation was suppressed by
radical scavenger, such as ascorbic acid in a dose-dependent
manner.[19]
We also tried the competitive ortho-acylation of acetani-
lide between the aromatic aldehyde and aliphatic aldehyde.
The results indicated that 78% yield of 3e was isolated and
only a trace amount of 3q was detected when acetanilide
underwent ortho-acylation with 2-chlorobenzaldehyde and
iso-butyraldehyde under the present reaction conditions
(Scheme 4). It also supports the possible mechanism in
Scheme 2 because the stability of benzoyl radical is more
than that of butyryl radical.
A plausible mechanism of the palladium-catalyzed ortho-
À
acylation of acetanilide with aldehyde through direct C H
bond activation is depicted in Scheme 2. The reaction occurs
probably involving 1) the formation of a cyclopalladated in-
À
termediate (I) by chelate-directed C H activation of the
In summary, an efficient approach for the direct ortho-
acylation of acetanilides has been developed based on a pal-
À
ladium catalyzed C H activation process. This novel
method provides easy access to o-acyl acetanilides by direct
ortho-acylation of acetanilides using the commercially avail-
able, inexpensive aromatic and aliphatic aldehydes as the
À
acylation reagents for C H bond functionalization in the
presence of tert-butyl hydroperoxide (TBHP) as the ideal
oxidant, affording the o-acyl acetanilides in good yields. Fur-
ther investigation on the application of this kind of catalyst
in asymmetric catalysis is underway in our laboratory.
Experimental Section
General procedure: Under an air atmosphere, a sealable reaction tube
equipped with
(1.0 mmol), aldehyde (1.5 mmol), PdAHCUTNGTRENNUNG
a
magnetic stir bar was charged with acetanilide
(CF3CO2)2 (0.05 mmol), TBHP
Scheme 2. Possible mechanism of the palladium-catalyzed ortho-acylation
of acetanilide with aldehyde.
(2.0 mmol) and toluene (1.5 mL). The rubber septum was then replaced
by a Teflon-coated screw cap, and the reaction vessel placed in an oil
bath at 1208C. After stirring the mixture at this temperature for 24 h, it
was cooled to room temperature and diluted with dichloromethane. The
resulting solution was directly filtered through a pad of silica gel using a
sintered glass funnel, and concentrated under reduced pressure. The resi-
due was purified by flash chromatography on silica gel (eluant: hexane/
ethyl acetate=3:1, v/v) to give the desired ortho-acylation product. The
identity and purity of known products was confirmed by 1H and
13C NMR spectroscopic analysis and the new products were fully charac-
terized. See the Supporting Information for full details.
benzene ring of acetanilide with PdACTHNUTRGNEUNG
(CF3CO2)2;[8,9] 2) the re-
action of obtained I with the acyl radical, which was gener-
ated in situ by hydrogen-atom abstraction of the aldehyde,
forming either reactive PdIV[16] or the dimeric PdIII[17] inter-
mediate; and finally, 3) carbon–carbon bond formation
through reductive elimination affording the ortho-acylation
of acetanilide product and regenerating the PdII species for
the next run. A very similar
prepared palladacycle I’ dimer
could stoichiometrically trans-
form to the ortho-acylation
product 3e under the present
reaction
conditions
(Scheme 3), which offered
a
further proof for this proposed
Scheme 4. The competitive ortho-acylation of acetanilide between 2-chlorobenzaldehyde and iso-butyralde-
hyde.
mechanism.[8,10c,i,k,18] It should
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L. Wang, P. Li et al.
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5760.
Acknowledgements
We are grateful to the National Natural Science Foundation of China
(No. 20972057, 21002039) for financial support.
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À
Keywords: acetanilides
palladium · synthetic methods
·
aldehydes
·
C H activation
·
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Received: April 18, 2011
Published online: August 2, 2011
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