Pd(ii)-catalyzed decarboxylative cross-coupling of oxamic acids with potassium phenyltrifluoroborates under mild conditions

Article abstract of DOI:10.1039/c1cc11635e

A novel Pd-catalyzed decarboxylative cross-coupling of oxamic acids with potassium phenyltrifluoroborates has been realized under mild reaction conditions. This method provides an efficient access to N-mono- or N,N-disubstituted benzamides and benzoates.

Full text of DOI:10.1039/c1cc11635e

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COMMUNICATION
Pd(II)-catalyzed decarboxylative cross-coupling of oxamic acids with
potassium phenyltrifluoroborates under mild conditionswz
Mingzong Li, Cong Wang, Ping Fang and Haibo Ge*
Received 21st March 2011, Accepted 11th April 2011
DOI: 10.1039/c1cc11635e
A novel Pd-catalyzed decarboxylative cross-coupling of oxamic
acids with potassium phenyltrifluoroborates has been realized under
mild reaction conditions. This method provides an efficient access to
N-mono- or N,N-disubstituted benzamides and benzoates.
demonstrated by Minisci and coworkers.16 Thus, we carried
out our investigation on decarboxylative cross-coupling of
N,N-diethyloxamic acid (2a) with potassium phenyltrifluoroborate
(1a) in the presence of 10 mol% Pd(OAc)2 and 2 equiv. of K2S2O8
at room temperature (Table 1). After an extensive solvent
screening, the mixture of DMSO/CH3CN/H2O (4 : 4 : 2, v/v/v)
was found to be optimal, and the desired product 3a was isolated
in 84% yield (entry 4). Both palladium and persulfate were required
for this transformation, and no desired product was detected in the
absence of either of these two reagents (entries 6 and 7). Further
optimization of the reaction conditions showed that good to
high yields could also be obtained with Pd(TFA)2, Pd(acac)2
or [Pd(MeCN)4](BF4)2 as catalyst (entries 8, 9 and 10). Not
surprisingly, K2S2O8 could be replaced with (NH4)2S2O8, albeit
with a lower yield (entry 11) while other oxidants such as oxone,
cerium ammonium nitrate (CAN), CuX2, AgX, and benzo-
quinone gave no desired product. It was also noted that good
yields could be obtained with a reduced amount of catalyst and
increased persulfate (entries 12 and 13).
Palladium-catalyzed decarboxylative cross-coupling has attracted
a considerable interest in recent years.1 Along with the extensively
studied decarboxylative arylation,2 Pd(0)-catalyzed decarboxyl-
ative couplings of organo halides with alkenyl3 and alkynyl acids,4
a-oxocarboxylates,2f,5 and monooxalates6 have also been reported.
Furthermore, Pd(II)-catalyzed decarboxylative Heck-type olefina-
tion has been established by Myers et al.7 while arylation
and acylation of unactivated arenes through C–H activation/
C–C coupling have also been demonstrated by others8 and our
group.9 However, Pd-catalyzed decarboxylative amidation with
oxamic acids has not been explored.
Arylamides are important structural units in organic and
medicinal chemistry, and considerable effort has been devoted to
their synthesis through transition-metal catalysis.10 In many cases,
these methods suffer from the use of toxic carbon monoxide10a–e
and/or limited functional group tolerance.10f–h Addition of
arylborates to isocyanates provides an appealing alternative.
However, excess arylborates were required for the Rh-catalyzed
process,11 while low to modest yields were generally observed for
the Pd-catalyzed approach.12 On the basis of the successful
decarboxylative cross-coupling of arylborates and a-oxocarboxylic
acids by Yamamoto et al.13 and us,14 herein we report the first,
Pd(II)-catalyzed decarboxylative cross-coupling of oxamic acids
with potassium phenyltrifluoroborates15 under mild conditions for
the formation of arylamides.
Table 1 Optimization of reaction conditionsa
Oxidant
(equiv.)
DMSO/CH3CN/ Yieldb
Entry PdX2 (mol%)
H2O (v/v/v)
(%)
1
2
3
4
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
0/4/1
4/0/1
3/5/2
4/4/2
o10
68
75
88
(84)c
76
0
In a recent report from our laboratory, it was shown that
Pd-catalyzed decarboxylative cross-coupling of arylborates and
a-oxocarboxylic acids could be achieved at room temperature in
the presence of stochiometric persulfate,14 presumably through an
acyl radical mediated process.16 In addition, room-temperature
persulfate-mediated decarboxylation of oxamic acids was also
5
6
7
8
9
10
Pd(OAc)2 (10)
Pd(OAc)2 (100)
—
Pd(TFA)2 (10)
Pd(acac)2 (10)
K2S2O8 (2)
—
K2S2O8 (2)
K2S2O8 (2)
K2S2O8 (2)
5/3/2
4/4/2
4/4/2
4/4/2
4/4/2
4/4/2
0
81
70
84
[Pd(MeCN)4](BF4)2 K2S2O8 (2)
(10)
Department of Chemistry and Chemical Biology,
11
12
13
Pd(OAc)2 (10)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
(NH4)2S2O8 (2) 4/4/2
K2S2O8 (2)
K2S2O8 (3)
70
75
83
Indiana University Purdue University Indianapolis (IUPUI),
402 N. Blackford St. Indianapolis, IN 46202, USA.
E-mail: geh@iupui.edu; Fax: +1 317-274-4701
w This article is part of a ChemComm web-based themed issue on new
advances in catalytic C–C bond formation via late transition metals.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc11635e
4/4/2
4/4/2
a
Conditions: 1a (0.3 mmol), 2a (0.6 mmol), PdX2, oxidant, 6 mL
b
solvent, rt, overnight. Yields determined by crude 1H NMR using an
c
internal standard based on 1a. Isolated yield based on 1a.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6587–6589 6587

The compatibility of substituted oxalic acids is summarized
in Table 2. N,N-Disubstituted oxamic acids were well tolerated
under the optimal or slightly modified reaction conditions, and
good yields were obtained for both acyclic and cyclic benzamides
(entries 1–5). N-Monosubstituted oxamic acids were also com-
patible under the optimized conditions. In this case, preheating
(70 1C, 10 min) of the reaction mixture along with an addition
of a catalytic amount of Ag2CO3 (20 mol%) is required for
producing good yields of products (entries 6–8). Decarboxyl-
ative cross-coupling of oxamic acid (H2NCOCO2H) failed
under the current catalytic system, presumably due to the
unsuccessful decarboxylation process of this substrate.16 However,
decarboxylative cross-coupling of potassium oxalate monoester
could be achieved under the optimized conditions with good yields
(entries 9 and 10).
Table
3 Coupling of potassium phenyltrifluoroborates with
N,N-diethyloxamic acida
Product
Yieldb (%)
Product
Yieldb (%)
66c
65
85c
92c
73c
86c
65c
73c
Table 2 Scope of substituted oxalic acidsa
Entry
1
Starting material 2
Product 3
Yieldb (%)
81
83
65
2
3
4
5
76c
82
46c
47c
a
Conditions: 1 (0.3 mmol), 2 (0.6 mmol, 2 equiv.), Pd(OAc)2 (0.015 mmol,
5 mol%), K2S2O8 (0.6 mmol, 2 equiv.), DMSO/MeCN/H2O (4:4:2,
83
b
v/v/v, 6 mL), rt, overnight. Isolated yields based on 1. c With 10 mol%
of Pd(OAc)2 and 3 equiv. of K2S2O8.
63c
As shown in Table 3, a variety of functional groups including
methoxy, methyl, halogen, acetyl, and trifluoromethyl at
meta and para-positions of the potassium phenyltrifluoroborate
are tolerated for decarboxylative cross-coupling with N,N-diethyl-
oxamic acid. However, ortho-substituted potassium phenyl-
trifluoroborates failed for this reaction and it is believed that
steric effects play a role here due to competing homo-coupling of
potassium phenyltrifluoroborates.17
6
7
8
9
75c,d
62c,d
71c,d
Results from the decarboxylative cross-coupling of N-cyclo-
hexyloxamic acid with potassium phenyltrifluoroborates are
surveyed in Table 4. It is noteworthy that in this case, a catalytic
amount of Ag2CO3 was added to facilitate the decarboxylation
process.16 This reaction shares similar functional group compati-
bility as N,N-diethyloxamic acid. As expected, ortho-substituted
potassium phenyltrifluoroborates provided moderate yields of
products presumably due to the reduced steric effects and thus
favorable cross-coupling compared with homo-coupling of
potassium phenyltrifluoroborates.
73d
79d
10
a
Conditions: 1 (0.3 mmol), 2 (0.6 mmol, 2 equiv.), Pd(OAc)2 (0.015 mmol,
5 mol%), K2S2O8 (0.9 mmol, 3 equiv.), DMSO/MeCN/H2O (4:4:2,
v/v/v, 6 mL), rt, overnight. b Isolated yields based on 1a. c 20 mol%
In summary, a novel palladium-catalyzed decarboxylative
cross-coupling of oxamic acids with potassium phenyltrifluoro-
borates under mild conditions has been developed. This transfor-
mation provides an efficient approach for the preparation
d
of Ag2CO3 was added. Preheated at 70 1C for 10 min, and then rt
overnight.
c
6588 Chem. Commun., 2011, 47, 6587–6589
This journal is The Royal Society of Chemistry 2011

Table 4 Coupling of potassium phenyltrifluoroborates with N-cyclo-
hexyloxamic acida
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63
81
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81
80
77
76
66
51
60
60
46
47
a
Conditions: 1 (0.3 mmol), 2 (0.6 mmol, 2 equiv.), Pd(OAc)2 (0.03 mmol,
10 mol%), K2S2O8 (0.9 mmol, 3 equiv.), Ag2CO3 (0.06 mmol, 20 mol%),
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We gratefully acknowledge Indiana University Purdue
University Indianapolis (IUPUI) for financial support. The Bruker
500 MHz NMR was purchased using funds from an NSF-MRI
award (CHE-0619254).
Notes and references
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c
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Chem. Commun., 2011, 47, 6587–6589 6589