Development of a palladium-catalyzed decarboxylative cross-coupling of (2-azaaryl)carboxylates with aryl halides Dedicated to Professor Melanie Sanford on receipt of the Tetrahedron Young Investigator Award

Article abstract of DOI:10.1016/j.tet.2013.03.085

A catalytic method for the decarboxylative coupling of 2-(azaaryl) carboxylates with aryl halides is described. The decarboxylative cross-coupling presented is mediated by a system catalytic in both palladium and copper without requiring stoichiometric amounts of organometallic reagents or organoboronic acids. This method circumvents additional synthetic steps required to prepare 2-azaaryl organometallics and organoborates as nucleophilic coupling partners, which are prone to protodemetallation and protodeborylation and produce potentially toxic byproducts.

Full text of DOI:10.1016/j.tet.2013.03.085

Tetrahedron 69 (2013) 5732e5736
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Development of a palladium-catalyzed decarboxylative
cross-coupling of (2-azaaryl)carboxylates with aryl halides
*
Christopher K. Haley, Christopher D. Gilmore, Brian M. Stoltz
Warren and Katharine Schlinger Laboratory for Chemistry and Chemical Engineering, Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, CA 91125, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A catalytic method for the decarboxylative coupling of 2-(azaaryl)carboxylates with aryl halides is de-
scribed. The decarboxylative cross-coupling presented is mediated by a system catalytic in both palla-
dium and copper without requiring stoichiometric amounts of organometallic reagents or organoboronic
acids. This method circumvents additional synthetic steps required to prepare 2-azaaryl organometallics
and organoborates as nucleophilic coupling partners, which are prone to protodemetallation and pro-
todeborylation and produce potentially toxic byproducts.
Received 6 February 2013
Received in revised form 15 March 2013
Accepted 19 March 2013
Available online 27 March 2013
Dedicated to Professor Melanie Sanford
on receipt of the Tetrahedron Young
Investigator Award
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Decarboxylative cross-coupling
(2-Azaaryl)carboxylates
Palladium
Copper
1. Introduction
presence of catalytic palladium(II) triflate and silver carbonate. This
represented an important advance as it provided an alternative to
The 2-substituted pyridine motif (Fig. 1), is present in a number
of important small molecules,1 however 2-azaaryl nucleophiles
typically used in cross-coupling reactions, such as organozincs,
Grignards, organolithiums, organostannanes, and boronic acids,
are notoriously unstable and difficult to prepare.2 While some
modifications have been made to improve their synthetic viability,
especially within the realm of boronic acid derivatives, these ap-
proaches are often inconvenient or produce toxic waste.3 The de-
velopment of a decarboxylative method of generating these
organometallic species (e.g., 2) in situ from (2-azaaryl)carboxylates
(1) represents a desirable alternative to traditional aryl nucleo-
philes for the synthesis of 2-aryl pyridine structures (3). 2-(Azaaryl)
carboxylates are generally inexpensive, are stable to both air and
water, and represent a more ecologically friendly alternative to
their organometallic counterparts.
aryl halides and aryl pseudo-halides usually used in Heck reactions.
A critical development within the field of decarboxylative cross-
coupling was reported in 2006; Goossen and co-workers disclosed
a catalytic decarboxylative cross-coupling reaction, utilizing a dual-
catalyst system of copper and palladium. Mechanistically, this
dual-catalyst approach likely proceeds through a decarboxylative
cupration of the arylcarboxylate partner. The resulting aryl copper
species subsequently undergoes transmetallation onto the palla-
dium, which furnishes the coupled product through reductive
elimination (Fig. 2).5 Unlike typical cross-couplings, which require
stoichiometric aryl organometallic nucleophiles, decarboxylative
couplings proceed through an in situ generation of the nucleophilic
coupling partner. A variety of non-aryl carboxylates have proven to
act as efficient coupling partners in decarboxylative cross-coupling
reactions,6 including alkynes,7 -keto acids,8 and 2-(2-azaaryl)ace-
a
Myers andco-workers reported the first practical decarboxylative
cross-coupling in 2002,4 a palladium-catalyzed decarboxylative
Heck-type olefination (Fig. 2). This work demonstrated that olefi-
nated arenes are accessible from benzoic acids and olefins in the
tates.9 While these represent great advances within the field, the
robust coupling of a key class of molecules, namely 2-(azaaryl)car-
boxylates, has been elusive. Recently, during the course of our own
work on this topic, Wu and co-workers reported on the palladium-
catalyzed decarboxylative cross-coupling reactions of 2-picolinic
acid (Fig. 2).10 In light this work, we sought to supplement their
studies with our own findings.
* Corresponding author. E-mail address: stoltz@caltech.edu (B.M. Stoltz).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.03.085

C.K. Haley et al. / Tetrahedron 69 (2013) 5732e5736
5733
Table 1
Bn
O
O
Decarboxylative cross-coupling of picolinic acid with aryl halides
H
N
N
N
OH
MeO
N
H
N
H
N
O
O
HIV protease inhibitor
N
Entry Cu source
Pd source
Solvent
Heat source % Yielda
O
N
O
1
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
CuCl
CuBr
CuI
PdI2
PdI2
PdI2
PdCl2
NMP
NMP
NMP
NMP
NMP
NMP
NMP
NMP
Oil bath
6
36
0
9
12
6
8
14
6
13
14
7
16
31
20
7
Cy
NH
2
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
O
3b
4
HN
MMP-13 inhibitor
O
N
N
5
6
7
8
PdBr2
S
O
Pd(OAc)2
Pd(acac)2
Pd(TFA)2
O
O
Et
N
9
Pd(F6-acac)2 NMP
N
10
11
12
13
14
15
16c
17
18
19
20
Pd(PPh3)4
Pd2(dba)2
PdI2
PdI2
PdI2
NMP
NMP
NMP
NMP
NMP
NMP
NMP
DMSO
DMF
N
N
A3 adenosine receptor antagonist
tridente ligand
O
HO
CuOTf
Cu2O (1.0 equiv) PdI2
Cu2O
Cu2O
Cu2O
Cu2O
PdI2
N
O
PdI2
PdI2
PdI2
PdI2
12
4
18
2
O
OH
O
NMP:Quin(1:1)
Toluene
2-pyridyl-epothilone B
natrual product derivative
Conditions: 0.464 mmol 8, 1.39 mmol 9, Cu (10 mol %), Pd (5 mol %), PPh3 (15 mol %),
O
1,10 phen. (15 mol %), 0.5 mL solvent.
a
CuL2
R2
Yields determined by LCMS analysis with 4,40-di-tert-butyl biphenyl as internal
OH
Cu(I)
–CO2
Pd
R1
R1
R1
standard.
R2X
b
N
N
N
Performed at 120 ꢀC.
Utilized 1 equiv of Cu2O.
1
2
3
c
Fig. 1. The importance of 2-substituted pyridines.
2-phenyl pyridine (10) in 36% yield (Table 1, entry 2). Ultimately,
cuprous oxide (Cu2O), the copper source reported by Goossen and
co-workers, gave consistently higher yields than copper (I) halides
(Table 1, entries 12e16). Importantly, for both copper and palladium
sources, weakly coordinating counterions were favored. While
these results were encouraging, we sought to further improve the
system. Concurrent with these studies, we examined several other
solvents, however were limited to high boiling solvents as a conse-
quence of the temperatures required for the copper-catalyzed de-
carboxylation of picolinic acid (Table 1, entries 17e20).
With these copper and palladium sources, we next examined
several phosphine ligands. None showed improvement over the
triphenylphosphine used in our initial studies. During the course of
our studies we discovered that preheating the mixture at 50 ꢀC for
10 min prior to heating to 190 ꢀC was advantageous. Gratifyingly,
we discovered that changing from the bidentate ligand 1,10 phe-
nanthroline to the monodentate ligand pyridine caused a sub-
stantial increase in yield (Table 2, entry 6).
Myers's Decarboxylative Olefination
CO2H
R3
R3
R1
R1
Pd(O2CCF3)2, Ag2CO3
DMSO-DMF
R2
R2
4
80–120 ºC
5
R2 = OMe, NO2, F, Cl, Me
18 examples, 42–99% yield
Goossen's Decarboxylative Cross-Coupling
CO2H
R3
CuI (5 mol %), Pd(acac)2 (3 mol %)
R1
R3
+
R1
1,10 Phen. K2CO2, 3Å MS
R2 Br
R2
NMP, 160 ºC, 24 h
4
6
7
R2 = NO2, CHO, OMe
NHC(O)CH3, SO2CH3
28 examples, 28–99% yield
Wu's Decarboxylative Cross-Coupling
R2
CO2H
+
PdCl2, BINAP, Cu2O
DMA, 150 °C, 24 h, Ar
R1
R2
N
R1
Br
N
1
6
3
23 examples, trace–78% yield
Fig. 2. The decarboxylative olefination and cross-coupling of aryl carboxylates.
The implementation of a pre-formed potassium picolinate salt
lead to a 10% increase in yield over its in situ generated counterpart
(Table 2, entry 7). In examining the stereoelectronics of the N-ligand
we discovered sterically encumbered nitrogen-contain ligands
proved detrimental to the yield (Table 2, entries 8 and 13), while both
electron-rich and deficient ligands also did not improve yields. In-
terestingly, as observed with 1,10 phenanthroline, other bidentate
ligands, such as TMEDA lead to lower yields (Table 2, entry 15). We
hypothesize theformation of a stable 18 electron copper complex (12)
is detrimental to decarboxylation (Fig. 3). As seen, by LCMS analysis,
much of our reduced yield could be accounted for by the persistence
of picolinic acid, indicating the initial decarboxylation is challenging.
With a suitable N-ligand in hand we hoped to gain better insight
into the influence of the P-ligands on our catalytic system; like their
nitrogen counterparts, bidentate phosphorous ligands performed
poorly under the reaction conditions (Table 3, entries 8 and 13). We
also reexamined copper sources, this time including copper(II) salts
(Table 3, entries 16e20). All of these were inferior to copper(I)
2. Results and discussion
2.1. Optimization of a decarboxylative cross-coupling of
picolinic acid with bromobenzene
To develop a general cross-coupling methodology, we selected
simple substrates for optimization studies, namely picolinic acid (8)
and bromobenzene (9, Table 1). We began our investigations by
applying conditions similar to those reported by Goossen with
disappointing results (Table 1, entry 1). Using the same catalyst
system we examined microwave irradiation and observed an in-
crease in yield over heating in an oil bath (Table 1, entry 2). Similar
results were disclosed by Goossen and Crabtree who independently
reported enhanced yields using microwave irradiation.11
With these initial results in hand we set about screening palla-
dium and copper sources. Both palladium(II) and palladium(0)
sources were examined, with palladium(II) iodide providing

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C.K. Haley et al. / Tetrahedron 69 (2013) 5732e5736
Table 2
Table 3
Investigation of P- and N-ligands for the decarboxylative cross-coupling of picolinic
Investigation of P-ligands and copper sources for the decarboxylative cross-coupling
acid with aryl halides
of picolinic acid with aryl halides
Entry
R
Phosphine Nitrogen ligand
ligand
Temperature Heat Base % Yielda
Entry
R
Cu source
P-Ligand
% Yielda
(ꢀC)
source
1
2
3
4
5
6
7
8
K
K
K
K
K
K
K
K
K
K
K
K
K
Na
Cs
K
K
K
K
K
K
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
Cu2O
P(o-Tolyl)3
P(Cy)3
26
42
21
42
36
45
31
46
49
20
44
48
38
4
1
2
3
4
5
6
7
8
H
H
H
H
H
H
K
K
K
K
K
K
K
K
K
K
K
dppm
dppb
dppe
BINAP
PPh3/KF
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
1,10 Phen
1,10 Phen
1,10 Phen
1,10 Phen
1,10 Phen
Pyridine
Pyridine
2,6 Lutidene
Ethyl isonicotinate 50/190
DMAP
170
170
170
170
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
m
wave K2CO3
wave K2CO3 16
wave K2CO3 31
wave K2CO3
wave K2CO3
wave K2CO3 52
7
P(t-Bu)3
P(2-Furyl)3
P((CF3)2Ph)3
P(4-MeOePh)3
P(EtO)3
dppe
6
2
170
50/190
50/190
50/190
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
wave
d
d
d
d
d
d
d
d
d
d
d
62
46
49
56
43
43
38
60
27
51
62
9
Ph2PO
9
10
11
12
13
14
15
16
17
18
19
20
21
(PhO)2POH
JohnPhos
AsPh3
BINAP
PPh3
PPh3
PPh3
PPh3
PPh3
10
11
12
13
14
15
16
17
50/190
50/190
50/190
50/190
50/190
50/190
50/190
50/190
4-MeO pyridine
DABCO
Et3N
Hunig’s base
TMEDA
1
Cu(OAc)
Cu2(CO3)(OH)2
Cu(SO4)
CuO
Cu(OTf)2
Cu(OTf)2
34
29
47
27
31
62
(iPr)2NH
Quinuclidine
PPh3
PPh3
PPh3
Conditions: 0.464 mmol 11, 1.39 mmol 9, Cu (10 mol %), Pd (5 mol %), P-ligand
(15 mol %), N-ligand (30 mol %), 0.5 mL NMP.
a
Yields determined by LCMS analysis with 4,40-di-tert-butyl biphenyl as internal
standard.
Conditions: 0.464 mmol 11, 1.39 mmol 9, Cu (10 mol %), PdI2 (5 mol %), P-ligand
(15 mol %), pyridine (30 mol %), 0.5 mL NMP.
a
Yields determined by LCMS analysis with 4,40-di-tert-butyl biphenyl as internal
standard.
Table 4
Examination of the equivalents of bromobenzene in the decarboxylative cross-
coupling of picolinic acid with aryl halides
Entry
Bromobenzene (equiv)
% Yielda
1
2
3
4
3.0
2.0
1.0
0.75
62
65
55
57
Conditions: 0.464 mmol 13, Cu2O (10 mol %), PdI2 (5 mol %), PPh3 (15 mol %),
pyridine (30 mol %), 0.5 mL NMP.
a
Yields determined by LCMS analysis with 4,40-di-tert-butyl biphenyl as internal
standard.
Fig. 3. Proposed mechanism of the decarboxylative cross-coupling of picolinic acid
with aryl halides.
Table 5
Examination of reaction time in the decarboxylative cross-coupling of picolinic acid
with aryl halides
oxide. We investigated other counterions of the picolinate salts,
such as cesium and sodium (Table 3, entries 14 and 15). None of
these fared as well as potassium; confirming a trend was also ob-
served in Goossen’s studies.
One of the unproductive side-reactions of our decarboxylative
cross-coupling is the formation of biphenyl through an Ullmann-
type dimerization of bromobenzene. By modulating the stoichi-
ometry of bromobenzene (9) we attempted to suppress the un-
productive dimerization and increase cross-coupling (Table 4).
Ultimately, reducing the equivalents of bromobenzene from 3.0 to
2.0 was slightly beneficial to our yield (Table 4, entry 5).
Entry
Time (h)
% Yielda
1
2
3
4
5
6
0.5
1.0
2.5
4.0
5.0
8.0
15
38
64
62
69
72
Concurrent to these studies, we examined the impact of reaction
times on the yield of our cross-coupling. Though a reaction time of
8 h provided the highest yield of 2-phenyl pyridine at 72%, it rep-
resented only a marginal improvement over shorter times (Table 5).
Conditions: 0.464 mmol 13, 0.928 mmol 9, CuO2 (5 mol %), PdI2 (5 mol %), PPh3
(15 mol %), pyridine (30 mol %), 0.5 mL NMP.
a
Yields determined by LCMS analysis with 4,40-di-tert-butyl biphenyl as internal
standard.

C.K. Haley et al. / Tetrahedron 69 (2013) 5732e5736
5735
In order to better understand the reaction mechanism, we
performed a series of control experiments in which we omitted
each of the reagents. We discovered pyridine was not able to fa-
cilitate the necessary deprotonation of picolinic acid in situ (Table 6,
entry 1). Omission of triphenylphosphine substantially decreased
the yield of 2-phenyl pyridine (10) (Table 6, entry 2). Both copper
and palladium proved crucial for our reaction; removal of either
metal lead to little or no reactivity (Table 6, entries 3 and 4). Ulti-
mately, pyridine was not necessary to facilitate the reaction (Table
6, entry 5). However this is most likely due to an unproductive
protodecarboxylative side reaction that produces pyridine from
picolinic acid.12 2-Phenyl pyridine is a common substrate for
functional group directed CeH activation and can serve as a ligand
for copper or palladium.13 Consequently, we explored the possi-
bility of product inhibition. However, yields were not affected by
addition of 2-phenyl pyridine to the reaction (Table 6, entry 6),
ruling out this inhibitory interaction.
of the major byproducts of reactions using electron deficient aryl
bromides was an Ullmann-type coupled product. Heteroaromatic
halides produced coupled products in disappointingly low yields
(22 and 23). Other (2-azaaryl)carboxylates performed well under
the reaction conditions (24 and 25), representing an expansion of
the substrate scope detailed in Wu’s report.
3. Conclusion
Decarboxylative coupling is an attractive alternative to tradi-
tional cross-coupling reactions. The starting materials are stable
and inexpensive while the byproducts of these reactions are less
toxic and easier to dispose of than traditional organometallic re-
agents and boronic acids. 2-Metallated heteroarenes are particu-
larly unstable, difficult to prepare and expensive. Herein we have
reported an approach to circumvent the need to use these un-
desirable reagents. We have shown that 2-(azaaryl)carboxylates
Table 6
Control experiments for the decarboxylative cross-coupling of picolinic acid with aryl halides
Entry
R
Cu source
Pd source
Solvent
Phosphine ligand
Nitrogen ligand
Additive
% Yielda
1
2
3
4
H
K
K
K
K
K
Cu2O
Cu2O
d
PdI2
PdI2
PdI2
d
NMP
NMP
NMP
NMP
NMP
NMP
PPh3
d
Pyridine
Pyridine
Pyridine
Pyridine
d
Pyridine
d
5
22
9
0
62
65
PPh3
PPh3
PPh3
PPh3
d
Cu2O
Cu2O
Cu2O
d
5
PdI2
PdI2
d
6b
Pyridine
2-Phenyl pyridine
Conditions: 0.464 mmol 11, 0.928 mmol 9, Cu2O (5 mol %), PdI2 (5 mol %), PPh3 (15 mol %), pyridine (30 mol %), 0.5 mL NMP, 50/190 ꢀC,
mwave.
a
Yields determined by LCMS analysis with 4,40-di-tert-butyl biphenyl as internal standard.
b
2-Phenyl pyridine (0.464 mmol) was added to the reaction mixture.
2.2. The cross-coupling of (2-azaaryl)carboxylates with aryl
halides
can be effectively used in decarboxylative cross-coupling reactions
with aryl halides to furnish 2-(azaaryl)arenes, a motif present in
a wide variety of important small molecules.
With our best conditions to date identified, we turned our at-
tention to examination of the substrate scope of the decarboxylative
cross-coupling (Fig. 4). Electronically neutral aryl halides (10 and 16)
performed well under the reaction conditions. Both electron rich
and electron deficient aryl halides fared modestly (17 and 18). One
4. Selected experimental
4.1. General
Spectral data for compounds 10, 16, 17, 18, 19, 20, 21, 22, 23, 24,
25 were consistent with previously reported data.10,14,15
A representative procedure for the decarboxylative cross-
coupling of (2-azaaryl)carboxylates with aryl halides.
4.2. 2-Phenyl pyridine (10)
74.9 mg (0.464 mmol) 13, 18.3 mg (0.0697 mmol) PPh3, 8.36 mg
(0.0464 mmol) PdI2, and 6.6 mg (0.046 mmol) Cu2O were added to
a flame-dried 2.0 mL microwave vial equipped with a spin vane.
The vial was sealed, evacuated and back-filled with argon (three
times). 0.5 mL NMP (degassed with Ar, >10 min) and 97.6
ml
(0.929 mmol) bromobenzene were added sequentially via syringe.
The mixture was stirred at room temperature for 10 min. The
mixture was then irradiated in the microwave, with a 90 s pres-
tirring period followed by 10 min at 50 ꢀC and increase in tem-
perature to 190 ꢀC. The reaction was heated at this temperature for
6 h. The crude mixture was purified on a silica column.
Acknowledgements
The authors wish to thank NIH-NIGMS (R01GM080269) for
Fig. 4. The decarboxylative cross-coupling of picolinic acid with aryl halides.
financial support.

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Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 3683e3686; (m) Dick, G. R.; Woerly, E. M.;
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