Room temperature palladium-catalyzed decarboxylative acyl/aroylation using [Fe(III)(EDTA)(η2-O2)]3- as oxidant at biological pH

Article abstract of DOI:10.1002/adsc.201201085

The purple-coloured iron peroxo complex [Fe(III)EDTA(η²- O₂)]^3- as a novel reagent system for Pd-catalyzed decarboxylative ortho-acylation of acetanilides with α-oxocarboxylic acids at room temperature in aqueous media has been realized. This reaction provides an effective access to ortho-acylacetanilides under mild conditions. Copyright

Full text of DOI:10.1002/adsc.201201085

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DOI: 10.1002/adsc.201201085
Room Temperature Palladium-Catalyzed Decarboxylative Acyl/
Aroylation using [Fe
(III)
E
G
Biological pH
Sugandha Sharma,a Imran A. Khan,a and Anil K. Saxenaa,*
a
Medicinal and Process Chemistry Division, CSIR-Central Drug Research Institute, ChattarManzil Palace, Lucknow –
226-001, India
Fax : (+91)-522-2212411-18, extn. 4268; (+91)-522-351-638; e-mail: anilsak@gmail.com or ak_saxena@cdri.res.in
Received: December 11, 2012; Published online: February 22, 2013
The CDRI communication number allotted to this manuscript is 8400.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201085.
Abstract: The purple-coloured iron peroxo complex
[Fe(III)EDTA
(h2-O2)]3ꢀ as a novel reagent system
Enamides and their derivatives are intrinsically
useful intermediates[4] and have been successfully
N
ACHTUNGTRENNUNG
for Pd-catalyzed decarboxylative ortho-acylation of
acetanilides with a-oxocarboxylic acids at room
temperature in aqueous media has been realized.
This reaction provides an effective access to ortho-
acylacetanilides under mild conditions.
used as a coupling partner in the palladium catalyzed
C H bond activation reaction. However, methods
[5]
ꢀ
for direct olefin functionalization of enamides typical-
ly rely on the use of organometallic reagents,[4a,b]
acrylates,[4c] or arenes,[4e] and catalytic decarboxyla-
tive–dehydrogenative couplings of carboxylic acids
with enamides have thus become an innovative area
to been investigated. Herein, we report a versatile
palladium-catalyzed decarboxylative acylation of
cyclic enamides with a-oxocarboxylic acid to access
diaryl ketones and that it is equally efficient with aro-
Keywords: carboxylic acids; cross-coupling decar-
boxylation; electron-transfer mediators; enamides;
re-oxidation
ꢀ
matic acids to access diaryls via alkenyl C H bond ac-
tivation under mild conditions. The role of oxidants in
Over the past three decades, the tenacious efforts of the palladium-catalyzed cross-coupling is in the regen-
synthetic chemists to develop efficient methods for eration of the catalyst which is directly proportional
transition metal-catalyzed cross-coupling reactions to the turnover number.[6] The direct oxidation of the
have revolutionized the synthesis of unsymmetrical catalyst by molecular oxygen or hydrogen peroxide is
diaryl ketone and diaryl compounds.[1,2] Some of these often kinetically unfavoured.[7]
methods involving in particular the palladium-cata-
The use of coupled catalytic systems with electron-
lyzed cross-coupling reactions between aryl halides transfer mediators (ETMs, Scheme 1)[6d,e] usually facil-
and aryl organometallics are widely used in modern itates these procedures by transporting the electrons
academic and industrial laboratories, and have led to from the catalyst to the oxidant along a low-energy
core structures in medicinal chemistry and material pathway, thereby increasing the efficiency of the oxi-
sciences. Traditional cross-coupling methods require dation and thus complementing the direct oxidation
pre-activated coupling partners, such as expensive reactions.[8]As a result of the similarities with biologi-
and/or moisture-sensitive organometallic reagents, cal systems, there are reports regarding electron-
and frequently produce unwanted, often toxic, stoi- transfer mediators (ETMs) during the reoxidation of
chiometric side-products.[1] Therefore, attempts have palladium and evidence for metal-catalyzed oxidation
been made to develop more atom economical, milder in biological systems.[9] The extensively recognized,
and greener protocols.[2i,3] Although many well-de- purple-coloured iron peroxo complex [Fe(II)EDTA-
signed studies have demonstrated that high selectivity
(h2-O2)]3ꢀ has been shown to catalyze the oxidation of
ACHTUNGTRENNUNG
can be achieved in such direct arylation reactions by organic substrates[10] and the decomposition of
tuning directing groups, electronic effects, and steric H2O2.[11,12] Furthermore, it acts as a mimic for super-
factors, significant room for improvement still re- oxide dismutase.[13]
mains.[3]
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Sugandha Sharma et al.
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Scheme 1. Schematic representation of the most plausible mechanism.
Our study commenced with the decarboxylative reoxidation of the catalyst. Under the optimized reac-
acylation of N-phenylacetamide1a with phenylglyox- tion conditions [Pd(OAc)2 1–2 mol%, Fe(III)-EDTA
ylic acid 2 to the N-(2-benzoylphenyl)acetamide 3a 1.0 equiv.], a variety of substituted phenylglyoxylic
using Fe(III)-EDTA-H2O2 {or [Fe(III)EDTA
(h2- acids and aryl/alkyl carboxylic acids were found to un-
O2)]3ꢀ} as the oxidant and Pd
(OAc)2 as the catalyst. dergo efficient decarboxylative cross-coupling with
A
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Optimization of the reaction conditions demonstrated cyclic enamides at room temperature to afford excel-
that the reaction was most productive using H2O as lent yields of the products (Figure 1). Specifically, a-
(h2-O2)]3ꢀ as phenylglyoxylic acids with ortho- or para-substituted
solvent and 1.1 equiv. of [FeIIIACTHUNTGRENNUG(EDTA)CAHTUNGTRENNUGN
oxidant, providing 3a in 98% yield at room tempera- electron-donating or electron-withdrawing groups are
ture within 0.5 h (entry 5, Table 1) and between a pH all successfully engaged in this reaction (3a–3i). It is
range of 7.2–7.8 (within the biological pH range). The noteworthy that electronic properties do not affect
exceptionally good yield anticipates the dual role of the ortho-substituted phenylglyoxylic acids since both
the iron peroxo complex as electron transfer-mediat- electron-donating and electron-withdrawing groups
ed oxidant and phase-transfer catalyst. Furthermore, gave good yields (3f–3h). Satisfactorily, the b-naph-
the study showed that Pd
N
ACHTUNGRTEN(NNUG OTf)2 and thyloxoacetic acid also reacted smoothly to give the
Pd
a
ACHTUNGTRENNUNG
PdACHTUNGTRENNUNG
three in the formation of 3a (entries 17–19, Table 1). ate yields under the above conditions, however on
It was also noted that both the catalyst (1.0– employing 2.0 equivalents of [Fe(III)EDTA
(h2-O2)]3ꢀ
2.0 mmol) and the oxidant (1.0–1.2 equiv.) were found with 4 mol% of Pd(TFA)2 promising yields were ob-
to be stoichiometrically precise when Fe(III) com- served (3j–3l).
plexes were used as the mediators (entries 4–6, 16–18 Further to expand the scope of this direct acylation
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
and 21–23, Table 1). The results obtained with reaction, we next investigated the decarboxylative
NH4S2O8, K2S2O8 and TBHP indicated moderate oxi- couplings of phenylglyoxylic acids (2a), a-oxocarbox-
dative properties in water due to the poor solubility ylic acids and aromatic carboxylic acids with cyclic en-
of the substrates (1a and 2a) in aqueous media.
amides, which were sucessfully accomplished to yield
In the initial trial, we were delighted that the de- 4a–4i. As shown in Figure 2, all of the 4-chromanone-
sired product 3a was obtained in 51% yield in the derived enamides bearing electron-donating or elec-
presence of the oxidant NH4S2O8 (entry 1, Table 1) in tron-withdrawing groups on the phenyl ring afforded
water. A screening of oxidants showed that the best the desired products in moderate to good yields (5a–
results wereobtained in the case of Fe
ACHTUNGTRENNUNG
{especially [Fe(III)EDTA
N
ACHTUNGTRENNUNG
K2S2O8, TBHP and H2O2 were only moderately effec- patible in this reaction, giving good to excellent yields
tive in the formation of product (entries 1–4 and 12– in the cross-coupling reaction, e.g., 2-[methoxy-
15). The effectiveness of the oxidant was also exam-
ined and it was found that increasing the stoichiome- yields of products 4c and 4e, for the tetralone-derived
try of [Fe(III)EDTA
(h2-O2)]3ꢀoffered a better yield of enamide only a modest amount of product (4i) was
the desired product with an optimal addition of 1.0– observed. As shown in Figure 2, a variety of substitut-
1.2 equiv. of catalyst [Fe(III)EDTA
(h2-O2)]3ꢀ which, ed phenylglyoxylic acids, including both those with
in turn, provides the high atom economy during the electron-donating and those with electron-withdraw-
ACHTUNGTREN(NGNU methyl)amino]-2-oxoacetic acid also gave excellent
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
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Room Temperature Palladium-Catalyzed Decarboxylative Acyl/Aroylation
Table 1. Optimization of reaction conditions.[a]
Entry
PdX2
Pd(OAc)2
Oxidant
Time [h]
Yield [%][b]
1
2
3
4
G
NH4S2O8
K2S2O8
TBHP
H2O2
16
12
16
4
51
34
21
10
5
6
7
8
Fe
Hgb-H2O2
Fe(III)-Cit-H2O2
urea-H2O2
BzOOBz
air
G
0.5
0.6
0.6
14
12
24
15
16
12
16
4
0.4
0.4
0.5
12
15
0.4
0.4
0.5
4.5
3.4
0.4
0.4
2.0
2.5
98
85
88
14
22
trace
36
62
45
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
oxone
Pd
G
NH4S2O8
K2S2O8
TBHP
H2O2
18
trace
<98
91
94
14
36
91
<88
84
19
Fe
Fe
U
Hgb-H2O2
urea-H2O2
oxone
Pd
Pd
Fe
N
Hgb-H2O2
Fe
Fe
ACHTUNGTRENNUNG
A
Hgb-H2O2
Fe(III)-EDTA-H2O2
Hgb-H2O2
Fe(III)EDTA-H2O2
Hgb-H2O2
14
74
55
26
PdCl2
T
Pd(OH)2/C
A
19
[a]
Reaction conditions: 1a (1.0 mmol), 2a (2.0 mmol), PdX2 (0.01 mmol), oxidant (1.0—1.2 equiv.), water (60 mmol) for 10–
30 min.
[b]
Isolated yields in percentage.
ing groups were compatible under the optimal reac- bonds of different electronic systems. This efficient
tionconditions. The coupling of 4-chromanone with 3- Pd-catalyzed ortho-acylation/aroylation process pro-
carboxyquinol-4-ones gave a good yield of 5i. The re- ceeds under mild reaction conditions. We believe this
sults on the compatibility of substituted acetanilide synthetic approach using [Fe
ACHTUNGTREN(GNUN III)ACHTUNGTREG(NNUN EDTA)ACUTHNGTRENNUNG
are presented in Figure 2. an oxidant generated in situ by reacting FeAHCTNUGTRENNNUG
The formation of 6aa, 6ab and 6b under the opti- with H2O2 at pH 7.2–7.8 will be useful for generating
mized reaction condition was also well done using versatile ortho-acetamidodiaryls and diaryl ketones
benzthiazole, benzoxazole, benzimidazole and N-ace- under mild and environmentally benign reaction con-
tylbenzimidazole, respectively. In the case of N-pivo- ditions
lylindole we obtained selectively the 3-substituted
product (6d) in 87% yield whereas with N-acetyl- and
N-benzoylindoles mixtures of 2- and 3-substituted
products (6e, 6f and 6g, 6h) were obtained.
Experimental Section
In summary, we have succeeded in showing that ar-
All solutions were prepared with deionized water. Unless
otherwise noted, reactions were magnetically stirred and
omatic and keto carboxylic acids that can act as the
ꢀ
acyl sources in the oxidative coupling with the C H monitored by thin layer chromatography. Solutions of
Adv. Synth. Catal. 2013, 355, 673 – 678
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675

Sugandha Sharma et al.
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Figure 1. Scope of aryl keto carboxylic acids in palladium-
catalyzed ortho-acylation of acetanilide.
[Fe
Na[Fe
molar quantity of Na2[H CTHUNGTRENNUNG
G
R
were
prepared
from
solid
U
ACHTUNGTRENNUNG
prevent the precipitation of iron hydroxide at higher pH
values.
General Procedure for the Cross-Coupling Reactions
To a solution of 33% H2O2 (10 mL), FeACTHNUTRGNEUNG(III)EDTA (0.1 mol)
was added then followed by addition of 0.01N NaOH
(3.0 mL) which as a result forms a purple-coloured complex
which was not isolated. The obtained purple-coloured solu-
tion was diluted to 100 mL to prepare the stock solution
which was stored under low light and cooled atmosphere
and was used as such.
Figure 2. Results of decarboxylative cross-coupling reactions
using electronically different enamides. Reaction conditions:
carboxylic acid (1.0 mmol), 2a (1.0 mmol), PdX2 (0.01 mmol,
1 mol%), oxidant (as indicated, 1.0–1.2 equiv), water
(60 mmol) for 10–30 min.
To the 5 mL stock solution of purple-coloured
Fe
2a (0.3 mmol) were added followed by the addition of
Pd(OAc)2 (0.7 mg, 1 mol%) and the mixture was vigourous-
ly stirred at room temperature for 30 min (as per TLC)
(NB: during the completion of the reaction the mixture gets
homogenized and the purple tinge disappears). After com-
pletion the reaction mixture was extracted with EtOAc (3ꢁ
5 mL), dried over anhydrous Na2SO4 and evaporated to
obtain the crude product which was purified through
column chromatography(silica gel 200–240 mesh) using
ACHTUNGTRENNUNG(III)EDTA solution 50 mg of 1a (0.3 mmol) and 45 mg of
1%!5% ethyl acetate:hexane as eluent to obtain 3a; yield:
70.2 mg (98%).
ACHTUNGTRENNUNG
N-[2-(4-Methylbenzoyl)phenyl]acetamide (3a): 1H NMR
(300 MHz, CDCl3): d=8.67 (d, J=8.1 Hz, 1H), 7.76–7.58
(m, 5H), 7.30 (t, J=7.7 Hz, 1H), 7.12 (d, J=8.2 Hz, 2H),
5.24 (Brs, 1H, NH + H2O), 2.36 (s, 3H), 2.17 (s, 3H);
13C NMR (40 MHz, CDCl3): d=201.61, 169.12, 143.65,
143.41, 135.77, 134.95, 132.42, 130.22, 128.86, 128.49, 123.18,
676
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Room Temperature Palladium-Catalyzed Decarboxylative Acyl/Aroylation
122.60, 78.26, 24.56, 21.06; ESI-MS: m/z= 254.4 (M+H+);
HR-MS: m/z=253.1109 [M+], calculated for C16H15NO2:
253.1103.
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are thankful to MOH and UGC, respectively, for funding
and also Mr. R. K. Purushottam, Mr. D. N. Viswakarma, Mr.
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