Room-Temperature Decarboxylative Couplings of α-Oxocarboxylates with Aryl Halides by Merging Photoredox with Palladium Catalysis

Article abstract of DOI:10.1002/chem.201502286

Enabled by merging iridium photoredox catalysis and palladium catalysis, α-oxocarboxylate salts can be decarboxylatively coupled with aryl halides to generate aromatic ketones and amides at room temperature. DFT calculations suggest that this reaction proceeds through a Pd⁰-Pd^II-Pd^III pathway, in which the Pd^III intermediate is responsible for reoxidizing Ir^II to complete the Ir^III-Ir^III-Ir^II photoredox cycle. Like a mergin': Enabled by merging iridium photoredox catalysis and palladium catalysis, palladium-catalyzed decarboxylative coupling of α-oxocarboxylates with aryl halides can proceed at room temperature. DFT calculations suggest that a Pd⁰-Pd^II-Pd^III catalytic cycle is merged with an Ir^III-Ir^III-Ir^II photoredox cycle, in which Pd^III is responsible for oxidizing Ir^II to complete the photoredox cycle.

Full text of DOI:10.1002/chem.201502286

DOI: 10.1002/chem.201502286
Communication
&
CÀC Coupling
Room-Temperature Decarboxylative Couplings of
a-Oxocarboxylates with Aryl Halides by Merging Photoredox
with Palladium Catalysis
Wan-Min Cheng, Rui Shang,* Hai-Zhu Yu, and Yao Fu*[a]
ered a photoredox/nickel dual catalytic system for the decar-
boxylative coupling of a-amino acids with halides.[9] Inspired
Abstract: Enabled by merging iridium photoredox cataly-
sis and palladium catalysis, a-oxocarboxylate salts can be
decarboxylatively coupled with aryl halides to generate ar-
omatic ketones and amides at room temperature. DFT cal-
culations suggest that this reaction proceeds through
a Pd0–PdII–PdIII pathway, in which the PdIII intermediate is
responsible for reoxidizing IrII to complete the IrIII–*IrIII–IrII
photoredox cycle.
by this work, we wondered whether photoredox catalysis
could be applied as a cocatalytic cycle to facilitate decarboxy-
lation in palladium-catalyzed decarboxylative coupling reac-
tions.[10] Herein, we report a room-temperature decarboxylative
coupling of a-oxocarboxylate and oxalate monoamide salt by
merging palladium catalysis and photoredox catalysis. The re-
action presented herein not only shows the feasibility of opti-
mizing the harsh condition of palladium-catalyzed decarboxy-
lative couplings but also indicates the potential of merging the
iridium-catalyzed photoredox process with palladium-catalyzed
aryl halide activation.[11]
Palladium-catalyzed selective formation of carbon–carbon
bonds through expulsion of carbon dioxide—namely decar-
boxylative coupling reactions[1]—benefits from the stability
and easy accessibility of the carboxylic acid coupling partner.
During the last decade, various types of catalytic systems have
been developed to extend the scope of palladium-catalyzed
decarboxylative couplings. Decarboxylative couplings catalyzed
solely by a palladium catalyst[2] or by bimetallic catalytic sys-
tems, such as Cu/Pd[3] and Ag/Pd,[4] in which a metal cocatalyst
is used to facilitate decarboxylation, have been developed.[5]
However, compared with traditional cross-coupling reactions,[6]
these previously developed methods have had drawbacks and
most of these catalytic systems need a high temperature of
around 120–1908C. Decarboxylative coupling of an a-oxocar-
boxylic acid with an aryl halide is a typical example. The decar-
boxylative coupling of ketone formation developed by Goos-
sen et al. using a Pd/Cu bimetallic system needed a high reac-
tion temperature of 1708C.[7] A related decarboxylative aryl
ester synthesis using an oxalate ester salt developed by our
group required a temperature of 1508C,[8] and the related
amide synthesis using an oxalate amide salt was unsuccessful
because of the reluctant decarboxylation, even at 1808C. The
high temperatures required limit the scope and cause prob-
lems with the reaction operation. To achieve Pd-catalyzed de-
carboxylative coupling under mild conditions is a long-stand-
ing problem.[1a,b] Recently, MacMillan and co-workers discov-
A typical example, optimized after considerable experimen-
tation, is described in Table 1, Eq. (1). A transparent Schlenk
tube charged with 4-bromotoluene (0.2 mmol), 2-oxo-2-
phenylacetic acid (0.4 mmol), sodium acetate (0.4 mmol),
[Ir{dF(CF3)ppy}2(dtbbpy)]PF6
(1.0 mol%),
[Pd(PhCN)2Cl2]
(2.0 mol%), and Nixantphos (2.0 mol%) in N,N-dimethylform-
amide (DMF) was exposed to irradiation by a 36 W blue LEDs
at 258C. After irradiation, generation of bubbles from the reac-
tion solution could be observed. After 20 h irradiation, aque-
ous work-up followed by column chromatography gave the
desired isolated product, phenyl(p-tolyl)methanone (3a), in
84% yield. Some parameters affecting the decarboxylative cou-
pling reaction were also tested (Table 1). For coupling with 4-
bromotoluene, this reaction needs a suitable diphosphine
ligand (Nixantphos) to support the palladium because under
ligand-free conditions or using other phosphine ligands, such
as 1,3-bis(diphenylphosphino)propane (dppp) or tri-o-tolyl-
phosphine (P(o-Tol)3), only trace amounts of 3a were formed
(Table 1, entries 2 and 3). Increasing the amount of photocata-
lyst to 2.0 mol% significantly improved the yield of 3a to 97%.
The conditions outlined in Eq. (1) can also be applied for the
decarboxylative coupling of 4-iodotoluene (Table 1, entry 5).
Further optimization showed that for 4-iodotoluene, after
changing the solvent, base, and palladium source to DMA,
K2HPO4, and [Pd2(dba)3], respectively, the product (3a) could
be obtained in 95% yield under phosphine ligand-free condi-
tions (Table 1, entry 6). The difference in ligand effect between
using 4-iodotoluene and 4-bromotoluene can be explained by
the easier oxidative addition of 4-iodotoluene to a Pd0 catalyst.
4-Chlorotoluene was a totally ineffective substrate (Table 1,
entry 7). We also tested the Ir/Ni system that had been suc-
cessfully merged for decarboxylative arylation[9]/alkenylation[10a]
of C(sp3)ÀCOOH by MacMillan and co-workers for our reaction,
[a] W.-M. Cheng, Dr. R. Shang, Dr. H.-Z. Yu, Prof. Dr. Y. Fu
iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province
Key Laboratory of Biomass Clean Energy, Department of Chemistry
University of Science and Technology of China, Hefei 230026 (P.R. China)
E-mail: faint123@mail.ustc.edu.cn
fuyao@ustc.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502286.
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Table 1. Study of parameters for decarboxylative arylation of benzoylfor-
mic acid with aryl halides.[a]
Table 2. Scope evaluation of aryl halides.[a]
Entry
X
Variations from conditions outlined in Eq. (1)
Yield
[%][b]
1
2
3
4
Br
Br
Br
Br
I
I
Cl
Br
none
dppp instead of Nixantphos
P(o-Tol)3 instead of Nixantphos
2mol% photocatalyst
none
84[g]
trace
trace
97
94
95
5
6[c,d,e]
without Nixantphos
none
10 mol% NiCl2, 15 mol% dtbbpy instead of
[Pd(PhCN)2Cl2] and Nixantphos
without light
7
8[f]
0
0
9
10
11
Br
Br
Br
0
0
0
without photocatalyst
without Pd source
[a] Reaction conditions: For aryl bromide (1.0 equiv, 0.2 mmol), benzoyl-
formic acid (2.0 equiv), Ir cat. (2.0 mol%), [Pd(PhCN)2Cl2] (2.0 mol%), Nix-
antphos (2.0 mol%), NaOAc (2.0 equiv), DMF (2.5 mL), irradiation with
36 W blue LEDs, 258C, 20 h; for aryl iodide (1.0 equiv, 0.2 mmol), benzoyl-
formic acid (2.0 equiv), Ir cat. (1.0 mol%), [Pd2(dba)3] (1.0 mol%), K2HPO4
(2.0 equiv), DMA (2.5 mL), irradiation with 36 W blue LEDs, 258C, 20 h.
[a] Reaction conditions: 1a (2.0 equiv) and 2 (1.0 equiv, 0.2 mmol), Ir cat.
(1.0 mol%), palladium source (2.0 mol% of Pd), Nixantphos (2.0 mol%),
base (2.0 equiv), DMF (2.5 mL), irradiation with 36 W blue LEDs, 258C,
20 h; [b] yields determined by GC analysis; [c] using K2HPO4 instead of
NaOAc; [d] using [Pd2(dba)3] instead of [Pd(PhCN)2Cl2]; [e] using DMA in-
stead of DMF; [f] 3a was formed in 11 % yield when 4-iodotoluene was
used; [g] yield of isolated product. Nixantphos=4,6-bis(diphenylphos-
phino)phenoxazine;
dimethylformamide.
DMA=N,N-dimethylacetamide;
DMF=N,N-
tionality was also tolerated (3n). This shows that the decarbox-
ylative reaction has orthogonal reactivity with aryl–aryl Suzuki
coupling, which may be ascribed to the mild reaction condi-
tions and mild base in the optimized reaction system. More-
over, besides aryl halides, heteroaryl halides are also suitable
substrates. Thiophenyl halide, furanyl halide, pyridinyl halide,
and indolyl halide were all suitable coupling partners, as
shown by 3o, 3p, 3q, and 3r, respectively.
The generality of this reaction with regard to the a-oxocar-
boxylic acids is summarized in Table 3. Electronic effects on the
carboxylate affect the reaction outcome, as shown by 3s and
3t. Whereas 2-oxo-2-phenylacetic acid, with an electron-donat-
ing methoxy substituent on the phenyl ring, gave an excellent
yield, an electron-withdrawing group such as trifluoromethyl
sharply decreased the yield. This observation contrasts with
previous observations reported in metal-catalyzed (Cu, Pd) de-
carboxylative couplings,[2,3] whereby an electron-withdrawing
substituent on carboxylate always facilitated the reaction, and
suggests that a free-radical decarboxylation is more likely.[12]
Both 2-(4-chlorophenyl)-2-oxoacetic acid (3u) and 2-mesityl-2-
oxoacetic acid (3v) were good substrates. 2-Heteroaryl-2-oxo-
acetic acids, such as 2-(furan-2-yl)-2-oxoacetic acid and 2-(ben-
zo[b]thiophen-2-yl)-2-oxoacetic acid, were also amenable sub-
strates (3w, 3x).
but no product was obtained (Table 1, entry 8). Further testing
showed that this reaction cannot proceed at all in the absence
of irradiation (Table 1, entry 9) or either photocatalyst or palla-
dium catalyst (entries 10 and 11), indicating the necessity of
merging the two kinds of catalysis for this reaction. It should
be noted that, under the conditions outlined in Table 1,
entry 1, about 5% of benzil was detected in the reaction mix-
ture.
Evaluation of the scope of aryl halides is summarized in
Table 2. The ortho-, meta-, and para-substituted aryl halides
were all amenable substrates, as demonstrated by 3a, 3g, and
3h. The electronic property of the aryl halide has no significant
influence on the reaction outcome; as demonstrated by 3b
and 3c, aryl bromides and iodides containing both strong elec-
tron-donating (OMe) and electron-withdrawing (CF3) groups
gave excellent yields. Functional groups such as fluoro (3q),
chloro (3d), bromo (3j), unprotected amine (3i), cyano (3e),
ester (3 f), acetal (3k), and aldehyde (3l) were also all well tol-
erated. A tert-butoxycarbonyl (Boc-)-protected phenylalanine
ester was functionalized on the phenyl ring without any race-
mization (3m). Interestingly, an aryl boronic pinacol ester func-
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Table 3. Scope evaluation of a-oxocarboxylic acids.[a]
Table 4. Decarboxylative coupling of potassium oxalate monoamides
with aryl halides.[a]
[a] Reaction conditions: 4-Iodotoluene (1.0 equiv, 0.2 mmol), carboxylic
acid (2.0 equiv), Ir cat. (1.0 mol%), [Pd2(dba)3] (1.0 mol%), K2HPO4
(2.0 equiv), DMA (2.5 mL), irradiation with 36 W blue LEDs, 258C, 20 h.
[a] Reaction conditions: For aryl bromide (1.0 equiv, 0.2 mmol),
4
(2.0 equiv), Ir cat. (1.0 mol%), [Pd(PhCN)2Cl2] (4.0 mol%), Nixantphos
(4.0 mol%), DMF (2.5 mL), irradiation with 36 W blue LEDs, 258C, 20 h; for
In addition to 2-aryl-2-oxoacetic acids, alkyl-2-oxoacetic acids
also worked well, as demonstrated by 3y, 3z, and 3za. Enoliz-
able a-hydrogen of ketone was tolerated, and no ketone a-
arylation[13] product was formed. The mild reaction conditions
allowed utilization of this decarboxylative coupling to function-
alize more complex molecules, as demonstrated by the modifi-
cation of an estradiol derivative (3zb).
aryl iodide (1.0 equiv, 0.2 mmol),
4 (2.0 equiv), Ir cat. (1.0 mol%),
[Pd2(dba)3] (1.0 mol%), Xantphos (2.0 mol%), DMA (2.5 mL), irradiation
with 36 W blue LEDs, 258C, 20 h.
Mechanistic studies were carried out to further elucidate this
reaction mechanism. First, a radical trapping experiment by
As a special kind of a-oxocarboxylate, potassium oxalate
monoamide can generate an amide synthon through decar-
boxylation. Although the corresponding potassium oxalate
monoester can undergo decarboxylative coupling with aryl
halide at 1508C in the presence of a palladium catalyst,[8] be-
cause of the electron-donating ability of amine, the amide
salts could not be successfully decarboxylatively coupled with
aryl halides, even at 1808C in the presence of Cu/Pd or Ag/Pd
bimetallic catalysts. Given that an electron-donating substitu-
ent on a-oxocarboxylate facilitates this reaction (see Table 3),
we hypothesized that this dual system may solve the problem.
We were delighted to find this kind of carboxylate coupled
smoothly under the merged catalytic system. Table 4 demon-
strates the scope of the decarboxylative amide synthesis with
regard to both coupling partners. Various kinds of N,N-dialkyl-
benzamides were generated in good yields (5a–d). A secon-
dary amide with a free NÀH was also generated in excellent
yield (5e). Potassium 2-(diphenylamino)-2-oxoacetate gave the
product in low yield (5 f), which may be ascribed to the poor
solubility of this salt in DMA solvent. A diverse range of hetero-
aryl halides were suitable substrates for generating heteroaro-
matic amides containing furan (5i), benzothiophene (5j), pyr-
azole (5k), and pyrimidine (5l) rings. A phenylalanine deriva-
tive can also be modified to introduce an amide onto the
phenyl ring (5m).
adding
a radical scavenger, (2,2,6,6-tetramethylpiperidin-1-
yl)oxyl (TEMPO), was carried out (Scheme 1). The yield of 3a
Scheme 1. Radical trapping experiment.
dropped sharply to 25% when 2.0 equivalents of TEMPO were
added. Addition of 4.0 equivalents of TEMPO totally shut down
the reaction, and 2,2,6,6-tetramethylpiperidin-1-yl benzoate (6)
was isolated in 199% yield (based on 4-iodotoluene). In this
experiment, excess TEMPO works as an oxidant to regenerate
the photocatalyst. This result strongly suggests a radical mech-
anism for the decarboxylation, in which the excited iridium
complex (*IrIII) oxidizes the carboxylate anion to generate the
carboxyl radical, which rapidly loses carbon dioxide to deliver
an acyl radical.[9,10] Another important mechanistic issue con-
cerns how the palladium catalytic cycle merged with the IrIII–
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Figure 1. Proposed mechanism of Ir/Pd-catalyzed decarboxylative coupling.
*IrIII–IrII redox cycle. Three possible pathways for merging these
two catalytic cycles are proposed (Figure 1a). All three path-
ways (path a/b/c) are initiated by the oxidative addition of aryl
iodide on Pd0 to generate a PdII intermediate (IN1). In path a,
IN1 captures the acyl radical delivered by the iridium photo-
redox cycle to generate a PdIII intermediate (IN2). IN2 oxidizes
IrII to IrIII to complete the photoredox cycle and to form a PdII
intermediate (IN3). IN3 undergoes reductive elimination to de-
liver the product and finishes the palladium catalytic cycle. In
path b, IN2 undergoes reductive elimination to deliver the
product and to generate the PdI intermediate. The PdI inter-
mediate oxidizes IrII to IrIII to finish the photoredox cycle and
to regenerate Pd0 to finish the palladium catalytic cycle. In
path c, the oxidative addition intermediate IN1 oxidizes IrII to
IrIII and generates PdI intermediate (IN4), the PdI intermediate
(IN4) captures the acyl radical to generate IN3, and this is fol-
lowed by reductive elimination to deliver the product and to
complete the palladium catalytic cycle.
In conclusion, enabled by merging photoredox catalysis and
palladium catalysis, we have achieved decarboxylative cou-
pling of a-oxocarboxylates with aryl halides at room tempera-
ture. A diverse range of aromatic and heteroaromatic ketones
and amides can be generated in good yields with excellent
functional-group compatibility. Mechanistic studies revealed
that a photoredox cycle of iridium-catalyzed radical decarboxy-
lation merged with a Pd0–PdII–PdIII catalytic cycle, in which the
PdIII intermediate oxidizes IrII to connect the two cycles.
Experimental Section
General procedure for decarboxylative arylation of a-oxocarboxylic
acid with aryl iodide: Aryl iodide (1.0 equiv, 0.2 mmol), a-oxocar-
boxylic acid (2.0 equiv, 0.4 mmol), K2HPO4 (2.0 equiv, 0.4 mmol),
[Ir{dF(CF3)ppy}2(dtbbpy)]PF6 (1.0 mol%, 2.2 mg), and [Pd2(dba)3]
(1.0 mol%, 1.8 mg) were placed in a transparent Schlenk tube
equipped with a stirring bar. The tube was evacuated and filled
with argon (three times). To these solids, anhydrous N,N-dimethyl-
acetamide (DMA, 2.5 mL) was added via a gastight syringe. The re-
action mixture was then stirred under the irradiation of a 36 W
blue LED (distance approximately 3.0 cm from the bulb) at 258C
for 20 h. After 20 h, the mixture was quenched with water and ex-
tracted with ethyl acetate (3”10 mL). The organic layers were com-
bined and the solvent removed under reduced pressure. The crude
residue was purified by flash column chromatography on silica gel
(ethyl acetate/petroleum ether=1:50).
The key to comparing these three pathways is to under-
stand which palladium species (PdI, PdII, or PdIII) oxidizes IrII.
Since redox potentials of [ArPdI] and [ArPdIII] species lack litera-
ture data and are difficult to measure accurately, standard den-
sity functional theory (DFT) methods[14] were used to compare
these three mechanisms.[15] DFT studies showed that the oxida-
tion of IrII to IrIII by all three palladium intermediates (IN1, IN2,
and IN5) was thermodynamically favored (Figure 1b). Accord-
ing to the calculation results, the reduction of PdII by IrII in
path c is less feasible than the acyl ligation step in paths a and
b because of the slightly higher free energy of IN4 relative to
that of IN2 (À4.0 vs. À5.5 kcalmolÀ1). From IN2, the reduction
by IrII (in path a) is thermodynamically highly favored and re-
sults in the highly stable intermediate IN3, while the direct re-
ductive elimination has to overcome the activation barrier of
6.3 kcalmolÀ1 (TS1). The formed intermediate IN3 in path a can
easily undergo the final reductive elimination through a low-
energy barrier (TS2) to finish the catalytic cycle. The energies
of all intermediates and transition states in path a are below
the lowest energy level among the three pathways. Therefore,
path a is thermodynamically most favored; that is, the relay
between the two catalytic cycles takes place on the PdIII inter-
mediate, and the PdI intermediate is unlikely to form during
the whole catalytic cycle.[16]
Acknowledgements
This work was supported by the 973 Program (2012CB215306),
NSFC (21325208, 21172209, 21361140372), IPDFHCPST
(2014FXCX006), CAS (KJCX2-EW-J02), FRFCU, and PCSIRT.
Keywords: CÀC coupling · decarboxylation · homogeneous
catalysis · palladium · photocatalysis
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·
[16] The possibility of generating ÀCOPh from COPh and IrII was also taken
into account. However, this step is significantly endergonic by 13.2 kcal
molÀ1; thus, the pathway for forming acyl anion reacting with IN1 can
be excluded accordingly.
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Received: June 11, 2015
Published online on July 31, 2015
Chem. Eur. J. 2015, 21, 13191 – 13195
www.chemeurj.org
13195
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