Ligand-free Pd-catalyzed carbonylative cross-coupling reactions under atmospheric pressure of carbon monoxide: Synthesis of aryl ketones and heteroaromatic ketones

Article abstract of DOI:10.1002/ejoc.201001685

The carbonylative Suzuki cross-coupling reactions of boronic acids with aryl iodides catalyzed by Pd₂(dba)₃ as a ligand-free catalyst under atmospheric pressure of carbon monoxide has been firstly developed. Under mild reaction conditions, a broad range of aryl/heteroaryl iodides and aryl/heteroaryl boronic acids were selectively coupled to afford the corresponding diaryl ketones in good to excellent yields at low catalyst loadings (0.05 to 2 mol-%). Moreover, the catalyst can also be recycled. The carbonylative Suzuki cross-coupling reactions of boronic acids with aryl iodides catalyzed by Pd₂(dba)₃ as a ligand-free catalyst under an atmosphere of carbon monoxide has been developed. A broad range of aryl/heteroaryl iodides and aryl/heteroaryl boronic acids were selectively coupled to afford the corresponding diaryl ketones in good to excellent yields. The catalyst can also be recycled.

Full text of DOI:10.1002/ejoc.201001685

FULL PAPER
DOI: 10.1002/ejoc.201001685
Ligand-Free Pd-Catalyzed Carbonylative Cross-Coupling Reactions under
Atmospheric Pressure of Carbon Monoxide: Synthesis of Aryl Ketones and
Heteroaromatic Ketones
Hongling Li,[a] Min Yang,*[a] Yanxing Qi,[a] and Jijun Xue*[b]
Keywords: Palladium / Carbonylation / Ketones / Ligand-free / Cross-coupling
The carbonylative Suzuki cross-coupling reactions of boronic
acids with aryl iodides catalyzed by Pd2(dba)3 as a ligand-
free catalyst under atmospheric pressure of carbon monoxide
has been firstly developed. Under mild reaction conditions,
a broad range of aryl/heteroaryl iodides and aryl/heteroaryl
boronic acids were selectively coupled to afford the corre-
sponding diaryl ketones in good to excellent yields at low
catalyst loadings (0.05 to 2 mol-%). Moreover, the catalyst
can also be recycled.
carbonylative cross-coupling of sterically hindered di-ortho-
substituted aryl iodides and ortho-substituted aryl boronic
acids under balloon pressure of CO.[5m]
Introduction
Diaryl ketones are important building blocks in the syn-
thesis of natural products and biologically active small mo-
lecules, and a number of approaches for their preparation
have been introduced.[1] One traditional approach is the
Friedel–Crafts acylation of substituted aromatic rings,
which requires excess amounts of Lewis acid.[2] Another al-
ternative strategy is the acylation of aryl metal species with
functional carboxylic acid derivatives.[3] An efficient and di-
rect route to diaryl ketones is the Pd-catalyzed three-com-
ponent coupling of aryl halides, boronic acids, and carbon
monoxide, which forms two carbon–carbon bonds in a sin-
gle operation, without the stepwise fashion of introducing
the ketone functional group.[4] Consequently, many efficient
palladium-based catalytic systems for the reactions have
been described.[5] Generally, to suppress unwanted competi-
tive direct Suzuki cross-coupling, a high pressure of CO is
employed to achieve good chemoselectivity of the reac-
tion,[5d–5g,5m,5n] and ligands are usually necessary to in-
crease reactivity and stability of the palladium catalysts. Up
to now, significant advances have been achieved by
palladium-based catalytic systems containing N/P li-
gands.[5b,5d–5h,5j–5n] For example, Martin and co-workers
have successfully developed a PEPPSI-IPr catalyst for the
However, ligand-free palladium catalyst would be more
desirable from an economical and ecological point of view.
Recently, Bhanage and co-workers firstly reported the li-
gand-free Pd complex catalyzed Suzuki carbonylative reac-
tions of aromatic substrates and a few heteroaryl substrates,
but an autoclave (100–200 psi of carbon monoxide) was
necessary to drive the reactions.[6] To the best of our knowl-
edge, there is no general study on the Suzuki carbonylative
reaction catalyzed by a ligand-free palladium catalyst under
atmospheric pressure of carbon monoxide. In continuation
of our program on phosphane-free Pd-catalyzed C–C bond-
formation reactions,[7] herein we demonstrate for the first
time that, without any additional ligand, the commercially
available simple Pd source Pd2(dba)3 (dba = dibenzylidene-
acetone) is an active catalyst for the Suzuki carbonylative
reactions of aryl/heteroaryl iodide under atmospheric pres-
sure of CO. This method allows the preparation of a variety
of diaryl ketones/aryl heteroaryl ketones in good to excel-
lent yields at low catalyst loadings under mild reaction con-
ditions.
Results and Discussion
[a] State Key Laboratory for Oxo Synthesis and Selective
Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences,
The Suzuki carbonylative reaction of 4-iodoanisole (1a)
with phenylboronic acid (2a) was chosen as the model reac-
tion and influences of different parameters were examined
to optimize the reaction conditions (Table 1). Several sol-
vents including THF, anisole, and toluene were evaluated
in the presence of Pd(OAc)2 (1 mol-%) and K2CO3
(3 equiv.) under balloon pressure of CO and the highest
yield and selectivity were observed in anisole (Table 1, En-
try 3 vs. Entries 1 and 2). Among the palladium catalysts
Lanzhou 730000, P. R. China
Fax: +86-931-4968190
E-mail: minyang1@gmail.com
[b] State Key Laboratory of Applied Organic Chemistry and
Department of Chemistry, Lanzhou University,
Lanzhou 730000, P. R. China
Fax: +86-931-8912582
E-mail: jjxuelzu@gmail.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001685.
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Ligand-Free Pd-Catalyzed Carbonylative Cross-Coupling Reactions
examined, Pd2(dba)3 gave the best result: 99% yield of 3a (Table 2, Entry 6). This selectivity is much better than that
(Table 1, Entry 5 vs. Entries 3 and 4). It is well known that reported for PdCl2(MeCN)2-catalyzed carbonylative cou-
π-acid olefins can not only affect the rate of oxidative ad- pling reaction of triphenylalane with 4-iodonitrobenzene,
dition through formation of an active palladium species, affording 41% yield of 4-nitrobenzophenone and 55% yield
but they might also play a role in stabilization of the cata- of 4-nitrobiphenyl.[5a,5d] The carbonylation of 4-bromo-
lyst.[8] Thus, we speculated that Pd2(dba)3 as an olefin-con- iodobenzene (1f) selectively occurred at the C–I bond,
taining catalyst would facilitate the catalytic reaction. The which gives an opportunity for further functionalization at
effect of temperature was not obvious and almost quantita- the intact bromide group in the product ketone (Table 2,
tive yield was obtained at either 80 or 100 °C (Table 1, En- Entry 8). To our delight, when the Pd2(dba)3 catalyst was
tries 5 and 6). Finally, two other bases were evaluated employed, the reaction also worked well with electron-
(Table 1, Entries 7 and 8). A slightly lower yield was ob- neutral iodobenzene (1b) and sterically hindered 1-iodo-
served using Cs2CO3 as the base, but a very poor yield of naphthalene (1g), providing corresponding ketones 3i and
3a was found when the organic base Et3N was used. There- 3j in 84 and 82% yield, respectively (Table 2, Entries 9 and
fore, the optimized conditions for Suzuki carbonylative re- 10).
action were Pd2(dba)3 (1 mol-%) as the catalyst, anisole as
the solvent, and K2CO3 as the base at 100 °C under balloon
pressure of CO.
Table 2. Suzuki carbonylative reaction of aryl iodides with aryl-
boronic acids.[a]
Table 1. Effects of reaction parameters on the Suzuki carbonylative
reaction of 4-iodoanisole (1a) with phenylboronic acid (2a).[a]
Entry
Catalyst
Conv.
[%]
Yield of 3a Selectivity of 3a
[%][b]
[%]
1[c]
2[d]
3[e]
4[e]
5[e]
6
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
42
76
Ͻ1
60
82
89
99
100
96
32
Ͻ1
79
100
100
100
100
99
Ͼ99
100
100
100
97
Pd(CH3CN)2Cl2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
7[f]
8[g]
82
85
[a] Reaction conditions: 1a (0.2 mmol), 2a (0.2 mmol), catalyst
(1.0 mol-%), K2CO3 (3 equiv.), CO (balloon pressure), and anisole
(2 mL) at 100 °C for 20 h. [b] Determined by GC using biphenyl
as an internal standard. [c] THF (2 mL) as the solvent at 60 °C.
[d] Toluene (2 mL) as the solvent at 80 °C. [e] At 80 °C. [f] Cs2CO3
(3 equiv.) as base. [g] Et3N (3 equiv.) as base.
Further investigation of the substrate scope was carried
out under the optimized reaction conditions. In Table 2,
various arylboronic acids as well as aryl iodides were exam-
ined and good functional group tolerance of the Pd2(dba)3
catalytic system was observed. For example, both 4-methyl-
benzeneboronic acid (2b) and 4-chlorophenylboronic acid
(2c) reacted smoothly with aryl iodides to give unsymmetri-
cal biaryl ketones in high yields (Table 2, Entries 1–4). In
addition, aryl iodides containing electron-withdrawing
groups including CH3OCO, Br, and Cl produced desired
ketones 3f, 3c, and 3h in good to excellent yields (Table 2,
Entries 5, 7, 8). It is noteworthy that up to 85% yield of 3g
was obtained for 1d with the strong electron-withdrawing
substituent NO2, known to promote the direct coupling re-
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol), Pd2(dba)3
(1 mol-%), K2CO3 (3 equiv.), CO (balloon pressure), anisole (2 mL)
action, to provide a mixture of biaryl and aryl ketone at 100 °C for 20 h. [b] Isolated yield.
Eur. J. Org. Chem. 2011, 2662–2667
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H. Li, M. Yang, Y. Qi, J. Xue
FULL PAPER
Heteroaryl-substituted pyridine derivatives that can be CO insertion into the pyridyl–palladium intermediate, the
formed through Suzuki carbonylative reactions are a very use of a higher pressure of carbon monoxide has been a
important family of compounds in diverse areas of chemis- general method for suppressing such a side reaction.[5g,12]
try such as metal-coordination complexes,[9] pharmaceuti- To demonstrate the generality of our catalyst system, iodo-
cal agents,[10] and molecular electronic device materials.[11] pyridines were treated with various aryl boronic acids under
However, based on the detrimental electron-withdrawing ef- atmospheric pressure of CO to provide pyridyl aryl ketones
fect of the nitrogen atom of the pyridine ring that disfavors in good to excellent yields (Table 3, Entries 1–6). Catalyzed
Table 3. Suzuki carbonylative reactions of heteroaryl iodides or heteroaryl boronic acids.[a]
[a] Reaction conditions: 1 (0.2 mmol), 2 (0.2 mmol), Pd2(dba)3 (1 mol-%), K2CO3 (3 equiv.), CO (balloon pressure), anisole (2 mL) at
100 °C for 20 h. [b] Isolated yield.
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Ligand-Free Pd-Catalyzed Carbonylative Cross-Coupling Reactions
by ligand-free Pd2(dba)3 (1 mol-%), 2-iodopyridine (1i) was ylative reaction of electrically neutral iodobenzene (1b) with
found to be less reactive than 3-iodopyridine, providing 4-methoxyphenylboronic acid (2d) led to a slight decrease
69% yield of corresponding ketone 3p (Table 3, Entry 6 vs. in the yield of the corresponding ketone (i.e., 3a), whereas
1), which is comparable to that reported by Miyaura and the TON grew from 184 (0.5 mol-%) to 910 (Table 4, En-
co-workers using PdCl2(PPh3)2 (3 mol-%, 66% yield).[5d] On tries 4 and 5).
the other hand, there are few reports on the Pd-catalyzed
carbonylative Suzuki cross-coupling reactions of S-het-
Table 4. Catalytic efficiency of the palladium-catalyzed Suzuki
carbonylative reaction.[a]
eroaryl/O-heteroaryl substrates due to the formation of
stable, catalytically inert compounds.[13] To expand the gen-
erality of our catalyst system, carbonylative Suzuki cross-
coupling reactions of O- and S-heteroaryl substrates were
investigated (Table 3, Entries 7–18). The reactions of 2-
Entry
1
2
Pd loading
[mol-%]
Yield
[%][b]
TON[c]
iodothiophene (1j) proceeded smoothly to give good to ex-
cellent yields of the corresponding ketones (Table 3, En-
tries 7–10). Notably, despite the possibility of competitive
Heck, Suzuki, and carbonylative Heck reactions, the Suzuki
carbonylative reaction of 4-vinylphenylboronic acid (2f)
with 2-iodothiophene (1j) proceeded selectively to afford a
moderate yield (66%) of 4-vinylphenyl 2-thienyl ketone (3u;
Table 3, Entry 11), an important intermediate that could be
easily hydroxycarbonylated to Suprofen, a commercial non-
steroidal anti-inflammatory drug.[14] To the best of our
knowledge, this is the first time that ketone 3u was synthe-
sized by ligand-free catalysis under atmospheric pressure of
carbon monoxide, which demonstrates the high selectivity
and catalytic activity of our catalyst system.[5n] Further-
more, thiophen-3-ylboronic acid (2g) and furan-2-ylboronic
acid (2h) worked well with a variety of aryl iodides, provid-
ing good yield of the expected heteroaryl ketones (63–87%;
Table 3, Entries 12–17). Even sterically hindered 1-iodon-
aphthalene (1g) was found to proceed smoothly with furan-
2-ylboronic acid (2h), affording 67% yield of 3B (Table 3,
Entry 18). From a synthetic viewpoint, it was significant to
note that various combinations of heteroaryl iodides with
heteroaryl boronic acids could be coupled efficiently by our
catalyst system. The carbonylative Suzuki reaction of 3-
iodopyridine (1h) with furan-2-ylboronic acid (2h) and thio-
phen-3-ylboronic acid (2g) resulted in 82 and 87% yield of
the corresponding heteroaryl ketones, respectively (Table 3,
Entries 19 and 20). Moreover, 2-iodothiophene (1j) reacted
smoothly with furan-2-ylboronic acid (2h) and thiophen-3-
ylboronic acid (2g), providing 83 and 68% yield of the de-
sired heteroaryl ketones, respectively (Table 3, Entries 21
and 22).
1
2
3
4
5
1a
1a
1a
1b
1b
2a
2a
2a
2d
2d
0.5
0.1
0.05
0.5
100
99
85
92
91
200
990
1700
184
0.1
910
[a] Reaction conditions: 1 (0.5 mmol), 2 (0.5 mmol), Pd2(dba)3,
K2CO3 (3 equiv.), CO (balloon pressure), anisole (2 mL) at 100 °C
for 20 h. [b] GC yield with the use of biphenyl as an internal stan-
dard. [c] TON: turnover number, defined as mmol (product)/mmol
(Pd).
Encouraged by the high efficiency of our catalyst, we
then examined the reusability of Pd2(dba)3 (Table 5). The
catalyst was recovered simply by filtration, washing with
ethyl acetate and drying under vacuum, and then reused. It
was found that the activity of the catalyst remained after
four consecutive runs. This result might indicate that the
active species was only slightly leached during these experi-
ments, which is significant from a practical point of view.
Table 5. Recycling study of the catalyst.[a]
Run
1
2
3
4
Yield [%][b] 100
90
81
71
[a] Reaction conditions: 1a (1 mmol), 2a (1 mmol), Pd2(dba)3
(1 mol-%), K2CO3 (3 equiv.), CO (balloon pressure), anisole
(10 mL) at 100 °C for 20 h. [b] GC yield with the use of biphenyl
as an internal standard.
To investigate the efficiency of this simple catalyst, we
further decreased the catalyst loading to 0.05 mol-% under
the optimized reaction conditions (Table 4). It is note-
worthy that this ligand-free catalytic system exhibited high
activity for the Suzuki carbonylative reaction of 4-iodo-
Conclusions
In summary, we have described the first efficient and
anisole (1a) with phenylboronic acid (2a) without any fur- convenient alternative for the preparation of a variety of
ther prolonged reaction time (0.1 mol-% of Pd, 99% yield, biaryl ketones by using ligand-free Pd2(dba)3 as a catalyst
TON = 990; Table 4, Entry 2), whereas the same reaction under atmospheric pressure of carbon monoxide. Com-
affording 3a in 90% yield needed to be carried out under pared with previous reports, several features of the present
2 mol-% of Pd(tmhd)2 (tmhd = 2,2,6,6-tetramethyl-3,5-hept- results were established: (1) The catalytic system is efficient
anedionate).[6] By further decreasing the Pd loading to and general not only for a broad range of aryl iodides and
0.05 mol-% a desirable yield could still be obtained (85% aryl boronic acids but also for various combinations of het-
yield, TON = 1700; Table 4, Entry 3). Meanwhile, lowering eroaryl iodides with heteroaryl boronic acids. (2) A variety
the palladium amount to 0.1 mol-% in the Suzuki carbon- of functional groups, such as electron-donating groups, in-
Eur. J. Org. Chem. 2011, 2662–2667
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H. Li, M. Yang, Y. Qi, J. Xue
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cluding CH3 and CH3O, electron-withdrawing groups, in-
cluding CH3OCO, Br, and Cl, and even the strongly elec-
tron-withdrawing substituent NO2, are tolerated. (3) The
reactions demonstrated high selectivity (up to 100% for the
carbonylative Suzuki reaction of 4-iodoanisole with phen-
ylboronic acid) for the catalytic system, and the important
pharmaceutical intermediate 4-vinylphenyl 2-thienyl ketone
could be prepared selectively in good yield. (4) This ligand-
free Pd2(dba)3-catalyzed carbonylative Suzuki coupling re-
action provided high TONs up to 1700. (5) The catalyst can
also be recycled. Further efforts to extend the application
of this system in other coupling transformations are un-
derway in our laboratory.
[2]
[3]
Experimental Section
Typical Experimental Procedure for the Carbonylative Suzuki Cou-
pling Reaction: Aryl iodide 1a (47.8 mg, 0.2 mmol), aryl boronic
acid 2a (24.9 mg, 0.2 mmol), Pd2(dba)3 (1.9 mg, 1 mol-%), and
K2CO3 (83.8 mg, 3 equiv.) were mixed in anhydrous anisole (2 mL)
under balloon pressure of CO, and the reaction mixture was stirred
at 100 °C for 20 h. The reaction mixture was then filtered through
Celite, which was washed three times with ethyl acetate and ana-
lyzed by gas chromatography (GC) with biphenyl as the internal
standard. The crude product was purified by column chromatog-
raphy on silica gel (hexanes/EtOAc) to give desired product 3a. 1H
[4]
[5]
3
NMR (400 MHz, CDCl3):[15] δ = 7.82–7.80 (d, JH,H = 7.2 Hz, 2
H, Hphenyl), 7.75–7.73 (d, 3JH,H = 4.2 Hz, 2 H, 4-MeOC6H4), 7.56–
3
7.52 (t, JH,H = 7.2 Hz, 1 H, Hphenyl), 7.46–7.43 (t, 3JH,H = 7.4 Hz,
2 H, Hphenyl), 6.95–6.93 (d, 3JH,H = 7.6 Hz, 2 H, 4-MeOC6H4), 3.85
(s, 3 H, OCH3) ppm.
Typical Experimental Procedure for the Recycling Study of the Cata-
lyst: Aryl iodide 1a (238.8 mg, 1 mmol), aryl boronic acid 2a
(124.4 mg, 1 mmol), Pd2(dba)3 (9.5 mg, 1 mol-%), and K2CO3
(418.8 mg, 3 equiv.) were mixed in anhydrous anisole (10 mL) un-
der balloon pressure of CO, and the reaction mixture was stirred
at 100 °C for 20 h. After each run, the catalyst was recovered by
filtration through Celite. The catalyst was then washed with ethyl
acetate (3ϫ10 mL) and then dried under vacuum. The obtained
catalyst was used for the next run.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and characterization for all products.
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Received: December 15, 2010
Published Online: March 28, 2011
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