Pd/Cu-cocatalyzed aerobic oxidative carbonylative homocoupling of arylboronic acids and CO: A highly selective approach to diaryl ketones

Article abstract of DOI:10.1002/asia.201402326

A highly selective Pd/Cu-cocatalyzed aerobic oxidative carbonylative homocoupling of arylboronic acids has been developed. This method employs a simple catalytic system, readily available boronic acids as the substrates, molecular oxygen as the oxidant, and 1 atm of CO/O₂, which makes this method practical for further applications.

Full text of DOI:10.1002/asia.201402326

COMMUNICATION
DOI: 10.1002/asia.201402326
Pd/Cu-Cocatalyzed Aerobic Oxidative Carbonylative Homocoupling of
Arylboronic Acids and CO: A Highly Selective Approach to Diaryl Ketones
Long Ren[a] and Ning Jiao*[a, b]
Abstract: A highly selective Pd/Cu-cocatalyzed aerobic oxi-
dative carbonylative homocoupling of arylboronic acids has
been developed. This method employs a simple catalytic
system, readily available boronic acids as the substrates, mo-
lecular oxygen as the oxidant, and 1 atm of CO/O2, which
makes this method practical for further applications.
Diaryl ketones are important synthetic intermediates and
[1]
À
common building blocks for C C bond formation. As
ubiquitous scaffolds, symmetric diaryl ketones also exist
widely in functional molecules,[2] advanced materials,[3] natu-
ral extracts,[4] bioactive molecules, and pharmaceuticals;[5]
examples are the electrical material precursor I,[2a] the leish-
Figure 1. Important molecules and derivatives with a symmetric diaryl
ketone scaffold.
manicidal lead II,[5b] the antibacterial agent III,[5a] and the
drug difenidol IV (Figure 1).[5c] Due to the growing impor-
tance of diaryl ketones, much attention has been paid to de-
velop new methods for their synthesis.
Generally, the procedures require hazardous/energy-inten-
sive acylating reagents with excess Lewis acids (Friedel–
Crafts acylation) (a, Scheme 1)[6] or highly reactive organo-
metal reagents (b, Scheme 1).[7] The catalytic carbonylation
with CO[8] provides a more straightforward route for the
synthesis of diaryl ketones,[9] and thus the carbonylative cou-
pling of aryl halides and arylmetal reagents has become
a versatile procedure for the synthesis of diaryl ketones (c,
Scheme 1). Nonetheless, these procedures usually need high
CO pressure, high temperature, aryliodides as electro-
philes,[10] and also complicated ligands to keep the carbony-
lation intermediate stable enough to reduce direct Suzuki
coupling products.[11]
The carbonylative homocoupling of aryl halides provides
an approach that requires equivalent or excess external re-
Scheme 1. Strategies for the synthesis of symmetric diaryl ketones.
ductive reagents (d, Scheme 1).[12,13] >Furthermore, difficul-
[a] L. Ren, Prof. N. Jiao
ties in substrate preparation, poor chemoselectivity, or the
State Key Laboratory of Natural and Biomimetic Drugs
use of an organic halide oxidant makes the carbonylative
School of Pharmaceutical Sciences
homocoupling strategy of arylmetal reagents very limited
Peking University
Xue Yuan Road 38, Beijing 100191 (China)
for further applications.[12,14] By contrast, nontoxic arylbor-
onic acids are readily available and air/moisture-stable re-
agents, which makes them an excellent source for the asym-
metric synthesis of diaryl ketones with aryl halides.[15] To the
best of our knowledge, work on the carbonylative homocou-
pling of arylboronic acids has rarely been reported.[16] The
symmetric diaryl ketones from arylboronic acids were usual-
ly documented as side products and obtained with poor se-
[b] Prof. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences
Shanghai 200032 (China)
Fax : (+86)10-8280-5297
E-mail: jiaoning@bjmu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402326.
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Ning Jiao et al.
lectivities.[16] Due to our continuing interest in aerobic oxi-
dative catalysis,[17] we aimed at developing a highly selective
synthesis of symmetric diaryl ketones from arylboronic acids
through a catalytic carbonylation process with a CO/O2 mix-
ture (e, Scheme 1). Unfortunately, compared with the meth-
ods with two partners (a-c, Scheme 1) which could give un-
symmetrical diaryl ketones, the current method does not
have this option.
firms that O2 works as an oxidant in this system (entry 5).
After solvent screening, a yield of up to 83% of 2a was ob-
tained when the reaction was carried out in DMF in the ab-
sence of an additional base (entry 6). The yield remained
the same when we increased the volume ratio of CO/O2 to
2:1 and decreased the loading of CuCl to 5.0 mol%
(entry 7). Notably, only trace amounts of 2a were obtained
in the absence of CuCl catalyst (entry 8), which indicates
that CuCl plays a significant role not only for the chemose-
lectivity but also for the promotion of the efficiency of this
transformation. Even when the loading of catalyst was re-
duced to 2.5 mol%, the yield was unaffected (entry 9). A
further increase in the temperature and further decrease in
the CuCl loading did no improve the yield of the reaction
(entry 10, Table 1).
With the optimized reaction conditions in hand, the sub-
strate scope was then investigated (Scheme 2). The reaction
of phenylboronic acid (1b) gave the product 2b in 76%
yield. Substrates with electron-donating groups at the para-
position of the benzene ring underwent the reaction smooth-
ly to afford the desired products in good yields (2a, 2e, 2 f,
2h–2j). When the electron-donating groups were present at
the meta-position, the products were also obtained in good
yield (2c, 2d, 2k). By contrast, ortho-substitution resulted in
lower yields of products 2g (47%) and 2l (30%), thus re-
Our study commenced with the reaction of (4-methoxy-
phenyl)boronic acid (1a) catalyzed by [PdACHTUNGTRNEG(UN PPh3)2Cl2], pro-
ducing the desired product bis(4-methoxyphenyl)methanone
(2a) in 15% yield and the ester 3a as the by-product in
17% yield (entry 1, Table 1). In order to suppress the forma-
tion of the by-product, we changed the solvent from acetoni-
trile to the less polar toluene and searched for an efficient
co-catalyst. We found that the presence of copper salts re-
sulted in an improved performance. When CuSO4
(10 mol%) was loaded in this reaction, the yield of 2a in-
creased to 36% (entry 2), while the formation of 3a was
completely suppressed when CuCl was employed (entry 3).
However, no product was detected when the Pd catalyst was
removed, which signifies that [PdACTHUNGTRNEG(UN PPh3)2Cl2] but not CuCl is
the essential catalyst in this transformation (entry 4). When
the reaction was carried out under pure CO without O2,
only a trace amount of product 2a was obtained, which con-
Table 1. Pd/Cu cocatalyzed carbonylative homocoupling of aryl boronic
acid 1a.[a]
Entry [Pd
(PPh3)2Cl2] [Cu]
Solvent
Yield of
Yield of
2a [%][b]
3a [%][b]
1
5.0 mol%
5.0 mol%
5.0 mol%
0
–
MeCN
(0.12 m)
toluene
15
36
40
0
17
16
0
2
CuSO4
(10.0 mol%) (0.12 m)
CuCl toluene
(10.0 mol%) (0.12 m)
CuCl toluene
(10.0 mol%) (0.12 m)
CuCl toluene
(10.0 mol%) (0.12 m)
CuCl DMF
(10.0 mol%) (0.12 m)
CuCl DMF
(5.0 mol%) (0.20 m)
3
4
0
5[c]
6[d]
7[d,e]
8[d,e]
9[d,e]
5.0 mol%
5.0 mol%
5.0 mol%
5.0 mol%
2.5mol%
<5
83
83
<5
83
76
0
0
0
–
DMF
(0.20 m)
DMF
0
CuCl
0
(5.0mol%) (0.20 m)
CuCl DMF
(2.5 mol%) (0.20 m)
10[d,e,f] 2.5 mol%
0
[a] Reaction conditions (unless otherwise noted): 1a (0.4 mmol), CO/O2
(1:1, balloon), triethylamine (TEA, 0.50 mmol), 808C. [b] Isolated yields.
[c] Pure CO without O2. [d] Without TEA. [e] CO/O2 (2:1, balloon).
[f] At 1008C.
Scheme 2. Pd/Cu cocatalyzed carbonylative homocoupling of aryl boronic
acids 1. Standard conditions: 1 (0.4 mmol), [PdACHTNUGTRNEG(UN PPh3)2Cl2] (0.01 mmol),
CuCl (0.02 mmol), CO/O2 (2:1, balloon), DMF (2.0 mL), 808C, 24 h. Iso-
lated yields.
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Ning Jiao et al.
vealing a strong steric effect. However, no obvious differ-
ence in the efficiency was observed when a phenoxy group
was present at the ortho- or para-position, as the corre-
sponding products 2m and 2n were obtained in 63% and
61% yield, respectively. This might be attributable to the
steric flexibility of the phenoxy group.
When substrates with electron-withdrawing substituents at
the benzene ring were used, moderate yields were obtained
(2o, 2p, 2r, 2s). Interestingly, the chloro- and bromo-substi-
tuted aryl boronic acids are well tolerated in this transfor-
mation (2q, 2s, 2v). Naphthalen-1-ylboronic acid (1u) also
underwent the reaction smoothly under the standard condi-
tions, thus affording 2u in 71% yield. To our delight, sub-
strates with functional groups such as the nitro, cyano, hy-
droxy, and vinyl group were well compatible with this
system, and a high yield of 91% (2z) was obtained. These
products offered opportunities for further transformations
(2v–2a). Some heteroaromatic substrates, such as furanyl
and thiophenyl boronic acids, also perfomed well, although
the yields were somewhat low (2b–2g, 45-47%). It should
be noted that a small amount of biphenyl product was gen-
erally generated in some reactions.
Scheme 3. Possible mechanism of the carbonylative homocoupling of 1.
with the generation of the Pd0 species which can be reoxi-
dized to the PdII species by the Cu/O2 oxidative system to
complete the catalytic circle (path a). However, we cannot
rule out the possibility of transmetalation of 1 with the
copper species before the palladium intermediate A or C is
formed (path b, c).[19] The selectivity enhancement by CuCl
in this system still requires further understanding.
In conclusion, a highly selective Pd/Cu cocatalyzed aero-
bic oxidative carbonylative homocoupling of arylboronic
acids has been developed. This method uses the nontoxic ar-
ylboronic acids as the aryl source and the readily available
O2 as the ultimate oxidant. The use of simple catalysts
under 1 atm atmosphere and the tolerance of various func-
tional groups make this transformation applicable for the
synthesis of symmetric diaryl ketones. Further extension of
its application and elucidation of the mechanism is ongoing
in our laboratory.
As mentioned above, O2 served as the ultimate oxidant in
this catalytic system (entry 5, Table 1). In order to elucidate
the function of CuCl, we conducted experiments in the pres-
ence of 2 equivalents CuCl and in the presence of 5 mol%
CuCl plus an additional 2 equivalents of CuACTHNURGTNEUNG(OAc)2 under
1 atm pure CO without O2 (Table 2). These experiments af-
forded 2a in yields of 35% and 36%, respectively, thus sug-
gesting that CuCl is not only the cocatalyst that enhances
Experimental Section
Table 2. Pd/Cu cocatalyzed carbonylative homocoupling of 1a.[a]
Typical synthesis procedure: Compound 1a (60.8 mg, 0.4 mmol), [Pd-
AHCTUNGTREN(GNUN PPh3)2Cl2] (7.0 mg, 0.01 mmol), and CuCl (2.0 mg, 0.02 mmol) were
added to a 25 mL Schlenk tube equipped with a magnetic stir bar. After
three evacuate/refill cycles with a mixture of CO/O2 (2:1 ratio in a bal-
loon), DMF (2.0 mL) was added via a syringe. The reaction mixture was
then stirred at 808C for 24 h. Next, the solution was cooled to room tem-
perature, followed by dilution with EtOAc (10 mL). The organic phase
was then washed twice with brine (10 mL), dried over Na2SO4, and fil-
tered. After evaporation of the filtrate under vacuum, the residue was
purified by flash column chromatography on silica gel (petroleum ether/
EtOAc=20:1) to afford 2a as a pale yellow solid.
Entry
CuCl
Cu
0
(OAc)2
Yield [%][b]
1
2 equiv
5 mol%
35%
36%
2[c]
2 equiv
[a] Reaction conditions: 1a (0.4 mmol), [PdACTHUNTGRNEG(UN PPh3)2Cl2] (0.01 mmol), CO,
DMF (2.0 mL), 808C, 24 h. [b] Isolated yields. [c] With 38% yield of 4,4’-
dimethoxy-1,1’-biphenyl.
the selectivity of the product but also serves as an oxidant
(cf. entry 5, Table 1 with entries 1–2, Table 2). However, the
Cu salt is not an efficient oxidant in this transformation.
These results also indicate that molecular oxygen is a very
active partner to render an efficient Cu/O2 oxidative system.
On the basis of the above results, a possible mechanism is
outlined in Scheme 3 (ligands neglected). Under the stan-
dard conditions, arylpalladium species A is initially generat-
ed through the transmetalation of 1 with the PdII catalyst.[18]
After CO insertion, the carbonylated intermediate B is
formed. Then, another transmetalation of 1 with B occurs
and affords the palladium intermediate C.[11d,e] Finally, the
product 2 is produced via the reductive elimination of C
Acknowledgements
Financial support from the National Science Foundation of China (Nos.
21325206, 21172006), National Young Top-notch Talent Support Pro-
gram, and the Ph.D. Programs Foundation of the Ministry of Education
of China (No. 20120001110013) are greatly appreciated. We thank Mian-
cheng Zou in this group for reproducing the results of 2d and 2i.
Keywords: aerobic oxidation
·
arylboronic acids
·
carbonylation · ketones · palladium
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Ning Jiao et al.
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Received: April 3, 2014
Revised: April 25, 2014
Published online: && &&, 0000
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Chem. Asian J. 2014, 00, 0 – 0
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

COMMUNICATION
As simple as that: A highly selective
Pd/Cu cocatalyzed aerobic oxidative
carbonylative homocoupling of aryl-
boronic acids has been developed. This
method employs a simple catalytic
system, readily available boronic acids
as the substrates, molecular oxygen as
the oxidant, and a simple CO/O2
Carbonylation
Long Ren, Ning Jiao*
&&&& — &&&&
Pd/Cu-Cocatalyzed Aerobic Oxidative
Carbonylative Homocoupling of Aryl-
boronic Acids and CO: A Highly
Selective Approach to Diaryl Ketones
atmosphere, which makes this method
practical for further applications.
5
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Chem. Asian J. 2014, 00, 0 – 0
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ÞÞ