Ligand-Free Palladium-Catalyzed Oxidative Carbonylative Homocoupling of Arylboron Reagents at Ambient Pressure

Article abstract of DOI:10.1002/ejoc.201600689

Arylboronic acids or potassium aryltrifluoroborates were readily oxidatively carbonylated to their corresponding diaryl ketones in high yields with high selectivities by ligand-free palladium-catalyzed homocoupling at atmospheric pressure. This novel method employs molecular oxygen or iodine as the oxidant and offers an attractive alternative to transition-metal-based oxidant systems.

Full text of DOI:10.1002/ejoc.201600689

A Journal of
Accepted Article
Title: Ligand-Free Palladium Catalyzed Oxidative Carbonylative Homocoupling of
Arylboron Reagents at Ambient Pressure
Authors: Wei Han; Hongyuan Zhao
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201600689
Link to VoR: http://dx.doi.org/10.1002/ejoc.201600689
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European Journal of Organic Chemistry
10.1002/ejoc.201600689
COMMUNICATION
Ligand-Free Palladium Catalyzed Oxidative Carbonylative
Homocoupling of Arylboron Reagents at Ambient Pressure
Hongyuan Zhao[a] and Wei Han*[a]
Abstract: Arylboronic acids or potassium aryltrifluoroborates were
readily oxidatively carbonylated to their corresponding diaryl ketones
in high yields with high selectivities by ligand-free palladium-
catalyzed homocoupling at atmospheric pressure. This novel method
employs molecular oxygen or iodine as the oxidant and offers an
attractive alternative to transition-metal based oxidant systems.
seminal example of Pd(PPh3)2Cl2-catalyzed synthesis of
symmetric diaryl ketones from arylboronic acids was reported by
Jiao group. Notably, this method employed environmental-
friendly oxygen as oxidant, albeit with CuCl as co-oxidant.[9]
Organotrifluoroborate is a valuable alternative to boronic acid
and boronate ester. The organotrifluoroborate doesn’t undergo
undesirable side reactions with commonly used bases, organic
acids, and nucleophiles due to their tetracoordinate nature.[10]
Thus, organotrifluoroborates, in contrast to most other boron
reagents, can be employed to build molecular complexity, while
leaving the carbon-boron bond intact.[11] To the best of our
knowledge, although organotrifluoroborates present all of the
advantages aforementioned, the carbonylative homocoupling of
these species into the corresponding diaryl ketones has never
been reported.[12]
Diaryl ketones have attracted continuous attention in many
areas serving as important synthetic intermediates and common
building blocks for C-C bond formation.[1] As core scaffolds,
diaryl ketones also exist in a tremendous range of natural
products,
pharmaceuticals,
advanced
materials,
and
photosensitizers.[2] Due to their siginificant importance, much
interest has been paid to the synthesis of diaryl ketones.
Arylboronic acid derivatives have high stability toward air,
water, and heat, ready availability, and low toxicity. For these
advantageous properties, they have found widely applications in
synthetic chemistry.[3] The transformation of these compounds
into diaryl ketones is typically accomplished through palladium-
catalyzed carbonylatve Suzuki coupling or carbonylative
homocoupling using gaseous CO as carbonyl source.[4] CO is a
strong -acidic ligand, decreases the electron density of Pd(0)
species, and thus increases the difficulty of the oxidative
addition of organic halides towards Pd(0) species.[5]
Consequently, the carbonylative Suzuki coupling usually need
high CO pressure and high temperature. Moreover, costly
ligands are almost always required to achieve efficient catalysis.
Recently, oxidative carbonylative homocoupling of arylboronic
acids with CO has emerged as an attractive new alternative to
the classic carbonylation strategy. This is because the oxidative
carbonylative process is on the basis of high-valent Pd(II)
species that circumvents the reluctant oxidative addition
Therefore, we focused on developing a more practical and
inexpensive method for catalytic carbonylative homocoupling of
aryltrifluoroborates and arylboronic acids in the absence of a
transition-metal oxidant and a phosphine ligand. Herein, we
report
the
oxidative
carbonylative
homocoupling
of
aryltrifluoroborates and arylboronic acids using oxygen or iodine
as the oxidant at atmopsheric pressure.
On the basis of our previous research where catalytic
carbonylations proceeded successfully in PEG-400 solvent,[13]
investigations were initiated on the oxidative carbonylation of
phenylboronic acid in PEG-400 using Na3PO4 as base under
ambient pressue (CO/O2 = 5:1) (Table 1). We found that an
iodide additive played a decisive role in the transformation
(Table 1, entries 1-4). To our delight, NaI as the iodide additive
gave 50 % yield of the desired product 2a, albeit with the
concomitant generation of side product 2a’ in 20% yield (entry 2).
Encouraged by this result, various bases were screened (entries
6-15), with the finding that KHCO3 turned out to be the best base
and enhanced the carbonylative coupling to give 2a in 85% yield
step.[5a,6]
However,
the
Pd-catalyzed
carbonylative
homocoupling of arylboronic reagents has underdeveloped.
Frequently, the homocouplings from arylboronic reagents were
reported as side reactions with poor selectivity.[7] Recently, Xiao
group demonstrated a highly selective Pd(PPh3)4-catalyzed
carbonylative homocoupling of arylboronic acids to give diaryl
ketones in the presence of AgNO3 as oxidant.[8] Subsequently, a
o
with high selectivity even at 80 C (entry 15).[14] Replacing PEG-
400 with NHD (polyethylene glycol dimethylether with an
average molecular weight of 250 Da) or dioxane as the solvent
resulted in failure of the reaction (entries 18-19).
When 4-methoxyphenylboronic acid (1b) as substrate was
subjected to the reaction conditions, a poor yield of the
desired product (2b) was obtained. To get better result, the
amount of NaI and the ratio of CO to O2 were optimized:
almost no reaction happened without oxygen, whereas with
the increase of the proportion of O2 to CO, homocoupling
side product 2b’ readily formed (Table 2, entries 1-4). When
the ratio of CO to O2 was 4:1, an ideal result can be obtained
in the presence of NaI (30 mol%) (Table 2, entry 5).
Having identified the optimal conditions, we turned to
examine the scope of arylboronic acids (Scheme 1). The model
substrate phenylboronic acid (1a) gave the carbonylative
product 2a in 85% yield. Arylboronic acids bearing either
electron-rich or electron-deficient substituents, regardless of
[a]
Jiangsu Collaborative Innovation Center of Biomedical Functional
Materials, Jiangsu Key Laboratory of Biofunctional Materials, Key
Laboratory of Applied Photochemistry, School of Chemistry and
Materials Science, Nanjing Normal University,
Wenyuan Road No.1, 210023 Nanjing (China).
E-mail: whhanwei@gmail.com
Homepage:http://hky.njnu.edu.cn/szqk/20125/140121_961265.html
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/ejoc.xxxxxxxxx.

European Journal of Organic Chemistry
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COMMUNICATION
Table 2. Further Optimization.[a]
their positions (o-, m-, p-), gave rise to the desired products in
good yields. The reaction times were found to be only loosely
correlated with the electronic nature of the substituents on the
phenyl ring. A variety of functionalities, such as methoxy (2b),
methyl (2c-f), fluoro (2g and 2i), chloro (2h and 2j), trifluoro (2k),
and trifluoromethoxy (2l) were well tolerated under normal
conditions. When naphthalene-1-boronic acid was emloyed, a
moderate yield of 2m can be abtained.
NaI
(mol %)
Entry
CO/O2
2:1
Yield of 2b (%)
Yield of 2b’ (%)
1
2
3
4
5
6
7
15
29
45
44
20
5
Table 1. Optimization of Reaction Conditions.[a]
2:1
30
30
4:1
55
5:1
30
48
10
5
4:1
4:1
40
74
100
40
67
10
trace
Yield
of 2a
(%)
1:0
trace
Yield of
2a’ (%)
Entry
[I]
Base
Tem.
Sol.
[a] Reaction conditions (unless otherwise noted): Reaction conditions
(unless otherwise noted): 1b (0.5 mmol), Pd(OAc)2 (1 mol%), NaI, KHCO3
(2 equiv), CO/O2, 80 oC, and 24 h.
1[b]
2[b]
TBAI
NaI
Na3PO4
Na3PO4
100 oC
100 oC
PEG-400
PEG-400
35
50
30
20
3[b]
4[b]
5
KI
Na3PO4
Na3PO4
Na3PO4
NaF
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
100 oC
80 oC
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
NHD
10
Trace
65
50
Trace
10
-
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
NaI
I2
6
10
65
7
CsF
10
70
8
Na2CO3
NaHCO3
K2CO3
K3PO4
KHCO3
Et3N
75
5
9
Trace
15
5
10
11
12
13
14
15
16
17
18
19
20
25
20
10
85
3
5
Trace
Trace
4
DBU
30
Scheme 1. Pd-catalyzed carbonylative homocoupling of arylboronic acids.
Reaction conditions (unless otherwise stated): 1 (0.5 mmol), KHCO3 (1.0
mmol), NaI (40 mol%), Pd(OAc)2 (1 mol %), PEG-400 (2 mL), CO/O2 (4:1, 1
atm), and 80 oC.
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
KHCO3
85
50 oC
80
Trace
Trace
Trace
Trace
64
25 oC
Trace
Trace
Trace
5
Considering the advantages aforementioned of organot_
rifluoroborates,[10,11] we further extended the protocol to
80 oC
aryltrifluoroborates. Unfortunately, only
a
trace of
80 oC
dioxane
carbonylative product was obtained when potassium
phenyltrifluoroborate was used under the above standard
conditions. After a concise optimization, we found out that
using iodine (1.0 equiv) as the oxidant instead of oxygen the
carbonylative homocoupling was greatly enhanced to give the
desired product 2a in 80% yield. Additionally, replacing
KHCO3 with Na2CO3 as the base resulted in a higher yield
80 oC
PEG-400
[a] Reaction conditions (unless otherwise noted): phenylboronic acid (0.5
mmol), Pd(OAc)2 (1 mmol%), iodide (15 mmol%), base (2 equiv), CO/O2
(5:1, 1 atm), PEG-400 (2 ml), 100 oC, and 9 h. [b] Base (4 equiv).

European Journal of Organic Chemistry
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COMMUNICATION
Pd(II) species by NaI/O2 or iodine.[5-6,18]
Scheme 3. Proposed mechanism for the present transformation.
In summary, we have developed the first phosphine-free
palladium-catalyzed
homocoupling of
arylboronic acids. This protocol allows for highly selective
transformation of the organoborons into the corresponding
symmetric diaryl ketones using O2 or iodine as the ultimate
oxidant without the the assistance of a transition-metal oxidant
aerobic
potassium
oxidative
aryltrifluoroborates
carbonylative
and
Scheme 2. Pd-catalyzed carbonylative homocoupling of potassium
aryltrifluoroborates. Reaction conditions (unless otherwise stated): 3 (0.5
mmol), Na2CO3 (1.0 mmol), I2 (0.5 mmol), Pd(OAc)2 (2 mol %),
deoxygenized PEG-400 (2 mL), CO (1 atm), and 80 oC. [a] KHCO3 (1.0
mmol) replaced Na2CO3. [b] 100 oC.
under
1
atm atmosphere. Further elucidation of the
mechanism and the extension to other substrates are in
progress.
(90%) of 2a (Scheme 2). When catalytic iodine (50 mol%) was
used, the yield of 2a was decreased to 71%. Under the newly
established conditions, we next examined the carbonylative
homocoupling
reactions
of
various
potassium
Experimental Section
aryltrifluoroborates (Scheme 2). To our delight, a broad scope
of potassium aryltrifluoroborates could be carbonylated in
good to excellent yields with high selectivities, including ortho-,
para-, and meta-substituted potassium aryltrifluoroborates.
General Procedure for aerobic oxidation of arylboronic acid to
biarylketone with ambient oxygen. A flask was charged with aryl
boronic acid 1 (0.5 mmol), Pd(OAc)2 (0.005 mmol, 1.1 mg), NaI (30.4 mg.
0.2 mmol), KHCO3 (101.2 mg, 1.0 mmol), and PEG-400 (2 ml) before
standard cycles evacuation and backfilling with mixture of CO and O2
(4:1, 1 atm). Then, the mixture was stirred at 80 °C for the indicated time.
At the end of the reaction, the reaction mixture was extracted with diethyl
ether (3 × 15 mL). The organic phases were combined, and the volatile
components were evaporated under reduced pressure. The crude
product was purified by column chromatography on silica gel (petroleum
ether / diethyl ether).
Both
electron-rich
and
electron-poor
potassium
aryltrifluoroborates achieved comparable results. Fluoro-
substituted potassium aryltrifluoroborates underwent facile
carbonylative homocoupling to provide fluorinated biaryl
ketones (2i and 4c) that are potentially important
intermediates in the synthesis of drugs and organic materials.
Indeed, 4c is a key intermediate in the preparation of
denagliptin employed for the treatment of type II diabetes.[15]
Chloro substitute, a reactive group in Pd-catalyzed cross-
coupling reactions, could be tolerated by this protocol (2j).
Notably, considerably reactive hydroxyl group proved to be
Acknowledgements
compatible
functional
group
(4f).
Potassium
The work was sponsored by the Natural Science Foundation of
China (21302099), the SRF for ROCS, SEM, the Qing Lan
project, and the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
aryltrifluoroborates bearing cyclic amide moiety were also
effective substrates with this method (4g-h).
On the basis of the above results and previous reports,[5-6,16-
a possible mechanism for the present transformation is
18]
outlined in Scheme 3 (anionic ligands omitted for clarity). The
intermediate A is first generated by the transmetalation of the
arylboronic acid with Pd(II) catalyst.[16] Subsequently, the
coordination of CO with the intermediate A and insertion into
the Pd–C bond affords a palladium acyl intermediate B, which
further reacts with another arylboronic acid to form a palladium
intermediate C.[17] Finally, reductive elimination of C gives the
ketone product 2 and Pd(0) which can be reoxidized to the
Keywords: Biaryketones • Carbonylation • Oxidation• Palladium
• Synthetic methods
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European Journal of Organic Chemistry
10.1002/ejoc.201600689
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Palladium-catalyzed
oxidation
of
Hongyuan Zhao, and Wei Han*
arylboronic acids or potassium
aryltrifluoroborates to biarylketones
with oxygen or iodine as the sole
oxidant at ambient pressure is
Page No. – Page No.
Ligand-Free Palladium Catalyzed
Oxidative
Homocoupling of Arylboron Reagents
at Ambient Pressure
Carbonylative
reported. This protocol features
a
good substrate scope of arylborons
bearing a wide range of functional
groups and gives desired products in
high yields with high selectivities.