From ketones to esters by a Cu-catalyzed highly selective C(CO)-C(alkyl) bond cleavage: Aerobic oxidation and oxygenation with air

Article abstract of DOI:10.1021/ja5073004

The Cu-catalyzed aerobic oxidative esterification of simple ketones via C-C bond cleavage has been developed. Varieties of common ketones, even inactive aryl long-chain alkyl ketones, are selectively converted into esters. The reaction tolerates a wide range of alcohols, including primary and secondary alcohols, chiral alcohols with retention of the configuration, electron-deficient phenols, as well as various natural alcohols. The usage of inexpensive copper catalyst, broad substrate scope, and neutral and open air conditions make this protocol very practical. ¹⁸O labeling experiments reveal that oxygenation occurs during this transformation. Preliminary mechanism studies indicate that two novel pathways are mainly involved in this process. (Chemical Equation Presented)

Full text of DOI:10.1021/ja5073004

Article
pubs.acs.org/JACS
From Ketones to Esters by a Cu-Catalyzed Highly Selective C(CO)−
C(alkyl) Bond Cleavage: Aerobic Oxidation and Oxygenation with Air
Xiaoqiang Huang,† Xinyao Li,† Miancheng Zou,† Song Song,† Conghui Tang,† Yizhi Yuan,†
and Ning Jiao*,†,‡
†State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking University, Xue Yuan Road 38,
Beijing 100191, China
‡Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200062, China
S
* Supporting Information
ABSTRACT: The Cu-catalyzed aerobic oxidative esterifica-
tion of simple ketones via C−C bond cleavage has been
developed. Varieties of common ketones, even inactive aryl
long-chain alkyl ketones, are selectively converted into esters.
The reaction tolerates a wide range of alcohols, including
primary and secondary alcohols, chiral alcohols with retention
of the configuration, electron-deficient phenols, as well as
various natural alcohols. The usage of inexpensive copper
catalyst, broad substrate scope, and neutral and open air conditions make this protocol very practical. 18O labeling experiments
reveal that oxygenation occurs during this transformation. Preliminary mechanism studies indicate that two novel pathways are
mainly involved in this process.
Recently, two elegant works on Cu-catalyzed aerobic
oxidative C(CO)−C bond cleavage of common ketones have
been developed.13,14 Bi and Liu et al. disclosed a chemo-
selective cleavage of methyl ketones to aldehydes (a1, Scheme
1).13 We reported the transformation of ketones to amides by
INTRODUCTION
■
Ketones are one of the most versatile and fundamental class of
compounds.1 The direct transformation of ketones has been
widely used in organic synthesis, in which various kinds of
chemical bonds could be constructed by the traditional
reduction, addition, α-functionalization, and cyclization process
through C−H and C−O bond cleavage.1,2 Nevertheless, the
direct transformation through the C(CO)−C bond cleavage of
ketones is still limited and attracts the continuous attention of
chemists. Encouraged by the traditional Baeyer−Villiger
reaction,3 Schmidt reaction,4 Haller−Bauer reaction,5 and
haloform reaction,6 some novel C(CO)−C bond functionaliza-
tions of ketones have been recently discovered through two
strategies: (1) chelation assisted strategy;7 (2) cleavage of
prefunctionalized ketones with the release of functional
fragments such as carbon dioxide, carboxylic acids, carbonyl
species, nitriles, or others.8 Despite the significance of these
elegant methods, these developed protocols suffer from the
limited substrate scopes, expensive noble transition-metal
catalysts, and stoichiometric oxidants such as peroxides and
toxic metal salts.
Scheme 1. Cu-Catalyzed Aerobic Oxidative C(CO)−C Bond
Cleavage of Ketones
Molecular oxygen is considered as an ideal oxidant in view of
green and sustainable chemistry, owing to its abundant, natural,
and environmental friendly character.9 Recently, the copper-
catalyzed aerobic oxidation10,11 and oxygenation10,12 reaction
with molecular oxygen has been a booming topic and has had
rapid advancement. Accordingly, the exploration of new and
environmental benign catalytic pathways for the direct
transformation of ketones through C(CO)−C bond cleavage
has been one of the most attractive but challenging tasks.
Received: July 18, 2014
© XXXX American Chemical Society
A
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a
Table 1. Optimization of the Reaction Conditions
b
c
entry
catalyst
CuBr
ligand
Py
solvent
additive
atmosphere
yield of 3a(%)
d
1
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
xylene
DMSO
neat
O2
62
2
3
CuBr
none
Py
Ar
0
Py
O2
0
e
4
other metals
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
Py
O2
0
5
6
7
2,2′-Bpy
1,10-Phen
L-Proline
none
Py
O2
11
O2
0
O2
0
f
8
O2
0
9
O2
61
10
Py
O2
7
77
g
11
h
Py
O2
12
Py
PhCl
PhCl
PhCl
PhCl
PhCl
NHPl
O2
60
13
14
15
ij
16 ,
Py
HOAc
O2
53
Py
BF3·Et2O
BF3·Et2O
BF3·Et2O
O2
81
Py
open air
open air
83
Py
82 (82)
a
Reaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), catalyst (0.025 mmol), ligand (0.75 mmol), and additive (0.50 mmol) in solvent (1.0 mL)
b
c
was stirred at 130 °C for 16 h. The pressure of O2 is 1.0 atm. For the effects of reaction atmosphere, see Table S8 of SI. Determined by 1H NMR.
d
The number in parentheses was isolated yield. For the effects of CuBr loading, see Table S1 of SI; for the evaluation of Cu salts, see Table S6 of SI.
e
f
Other metals: Fe[Pc], CoBr2, Ni(OAc)2, Mn(OAc)3·2H2O, MnCl2, Ag2CO3, and Pd(OAc)2, respectively. For the effects of other amines, see Table
g
h
S4 of SI; for the effects of electronic and steric effects of pyridine, see Table S3 of SI. 1.00 mmol of 2a was employed. 0.0375 mmol of NHPI was
employed. For the effects of pyridine loading, see Table S2 of SI; for the effects of reaction temperature, see Table S5 of SI; for the effects of reaction
concentration, see Table S7 of SI. Py (0.50 mmol), BF3·Et2O (0.25 mmol), and PhCl (0.5 mL), 10 h. Py = pyridine; 2,2′-Bpy = 2,2′-bipyridine;
1,10-Phen = 1,10-phenanthrolin. NHPI = N-hydroxyphthalimide.
i
j
aerobic oxidative C(CO)−C bond cleavage (b1, Scheme 1).14
Despite the significance of these novel reactions, the aerobic
oxidative functionalization of simple ketones through C−C
bond cleavage with other kinds of nucleophiles such as alcohol
for the esterification is still a challenging task,15 because of the
following: (1) Both of the reported mechanisms (a1 and b1,
Scheme 1) could not guide the C(CO)−C bond esterification.
When a nucleophile was employed in the aldehyde formation
pathway, the nucleophile could not be retained in the ketone
partner (a2, Scheme 1). Alternatively, a nuleophile without a
leaving group could not undergo the amide formation process
(b2, Scheme 1). (2) The alcohol nucleophile could be oxidized
to aldehyde byproduct in the presence of transition-metal
catalyst under oxidative conditions such as pure dioxygen.16 (3)
The cleavage of inactive ketones with long-chain alkyl groups is
still challenging. (4) A new mechanism should be disclosed to
enable this kind of C−C bond esterification approach.
Herein, we report a Cu-catalyzed aerobic oxidative
esterification of simple ketones via C(CO)−C bond cleavage
with a broad range of ketones including inactive aryl long-chain
alkyl ketones (c, Scheme 1). Varieties of primary and secondary
alcohols and electron-deficient phenols, especially natural
alcohols, are tolerant under the present Cu−air system. A
novel mechanism and the detailed mechanistic studies are also
presented.
product was formed under argon atmosphere (entry 2). Copper
salt showed unique ability in this transformation, as the reaction
did not work without copper catalyst (entry 3, also see Tables
S1 and S6, SI). On the other hand, other metal catalysts such as
Fe[Pc], CoBr2, Ni(OAc)2, Mn(OAc)3·2H2O, MnCl2, Ag2CO3,
and Pd(OAc)2 could not accomplish the process (entry 4).
When 2,2′-bipyridine was employed as the ligand, the yield of
3a decreased to 11% with large amounts of 1a recovered (entry
5). 1,10-Phenanthrolin and L-proline resulted in no efficiency
(entries 6−7). Investigation on the effects of other amines
(Table S4, SI) and the electronic and steric effects of pyridine
(Table S3, SI) showed that pyridine was indispensable in this
reaction (entry 8). Xylene gave a similar yield with PhCl, while
the strong polar solvent DMSO led to only 7% yield (entries
9−10). It is noteworthy that the yield of 3a increased to 77%
under the neat condition with 2a (4.0 equiv) (entry 11). Then
we turned our attention to screen different additives (entries
12−14). It was found that the addition of BF3·Et2O delivered
3a in 81% yield (entry 14). Notably, 3a could be obtained in
83% yield by using air as the oxidant and oxygen source, which
makes it easy to operate. After further screening of pyridine
loading (Table S2, SI), temperature (Table S5, SI), and
reaction concentration (Table S7, SI), the aerobic C(CO)−C
bond cleavage of 1a gave 3a in 82% yield using the Conditions
A: CuBr (0.025 mmol), pyridine (0.50 mmol), and BF3·Et2O
(0.25 mmol) in PhCl (0.5 mL) with stirring at 130 °C open to
air for 10 h (entry 16, Table 1).
RESULTS AND DISCUSSION
■
Next, we explored the scope of the (het)aryl methyl ketones
(Table 2). It was observed that the electronic variation at the
para-substituents of the aryl ring of (het)aryl methyl ketones
did not affect the C−C bond cleavage efficiency (entries 1−11,
Table 2). Furthermore, the efficiency was affected by the
We commenced our study by Cu-catalyzed aerobic oxidative
C−C bond cleavage of 1a. To our delight, when 1a was treated
with 2a, 0.1 equiv of CuBr, and 3.0 equiv of pyridine in PhCl
under O2 (1.0 atm) at 130 °C, the desired product 3a was
obtained in 62% yield (entry 1, Table 1). As expected, no
B
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a
a
Table 2. Substrate Scope of (Het)Aryl Methyl Ketones
Table 3. Substrate Scope of Alkyl Ketones
a
a
Conditions B: ketone (0.25 mmol), nBuOH (1.00 mmol), CuBr
Conditions A: ketone (0.25 mmol), alcohol (0.50 mmol), CuBr (0.025
mmol), pyridine (0.50 mmol), and BF3·Et2O (0.25 mmol) in PhCl
(0.025 mmol), and pyridine (0.50 mmol) in PhCl (0.5 mL) with
stirring at 130 °C open to air for 24 h. Isolated yields. The number in
parentheses was GC yield. 28% of 4-methoxybenzaldehyde was
obtained as a byproduct. 54% of 4-ethoxybenzaldehyde was obtained
b
b
(0.5 mL) with stirring at 130 °C open to air for 10 h. Isolated yields.
c
c
Yield of the reaction in 50 mmol scale.
d
as a byproduct.
position of the substituents. For example, o-bromo-acetophe-
none 1o gave lower yield than p-/m-bromo-acetophenone 1g/
1n (cf. entries 7, 14, and 15). A wide range of functional groups
were tolerant in this C−C bond esterification. For instance,
ketones bearing halo-, NO2-, CN-, or CF3- groups afforded the
corresponding esters in moderate to good yields, respectively
(entries 5−10). Interestingly, the C(CO)−C bond of 1-(4-
(methylsulfonyl)phenyl)-ethanone 1k was selectively cleaved
with the retention of the methylsulfonyl group (entry 11). In
addition, 2-acetonaphthone 1q worked well, giving 3q in 72%
yield (entry 17). Obviously, the cleavage of various heteroaryl
methyl ketones, including benzofuran, benzothiophene, thio-
phene, and pyridine fragments, proceeded smoothly to generate
medicinally important heterocyclic esters in moderate to good
yields (entries 18−21). The reaction of 1b in 50 mmol scale
afforded 3b in 64% (8.21 g) yield. In some cases, the
corresponding aryl aldehydes as byproduct could be obtained.13
To highlight the broad substrate scope of this process, aryl
ketones with long-chain alkyl groups and aliphatic ketones/
aldehydes were investigated (Table 3). Remarkably, it is not
necessary to add BF3·Et2O as a Lewis acid when the methyl
group is replaced by a long-chain alkyl group. Employing 4.0
equiv of butan-1-ol as the nucleophile with longer reaction time
(Conditions B), phenyl long-chain alkyl ketones containing
various substituents provided the corresponding butyl benzoate
in good yields (entries 1−10, Table 3). Intriguingly, aliphatic
ketone/aldehyde 4k and 4l afforded 5b and 5l but in low yields
(30% and 31%, respectively, entries 11 and 12). These two
reactions might prefer the oxidative process to generate the
phenylglyoxal intermediate, as reported by Bi and Liu,13
followed by dehydrogenative coupling to generate the
corresponding α-ketoesters,11i which underwent fragmentation
under Conditions B (see eq S2, SI) to afford the corresponding
esters 5b and 5l. However, the methylene at the α-position of
the carbonyl group is necessary, as 4n failed to undergo the
transformation. These results proved the broad substrate scope
of this C−C bond esterification process. In some cases, such as
4i and 4j, the aryl long-chain alkyl ketones were not consumed
completely, leading to the low yields.
The scope of the reaction with respect to alcohols was
investigated too (Table 4). We do not observe the oxidation of
alcohols including benzylic alcohols in these reactions. The
reactions of various substituted 2-phenylethanols and benzyl
alcohols worked well, regardless of the electronic nature and
position of the substituent groups (entries 1−14, Table 4).
Significantly, an ester group and pyridine ring could be present
C
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design.18 Therefore, it is of great interest to modify natural
ketones and alcohols via the present protocol.
a
Table 4. Substrate Scope of Alcohols
We were delighted to figure out that the current esterification
reaction was capable of tolerating a wide range of natural
alcohols in good yields (Table 5). For instance, nerol, bearing
a
Table 5. Modification of Natural Ketones and Alcohols
a
b
All reactions were performed under Conditions A. Isolated yields.
0.75 mmol of pyridine, and 0.50 mmol of BF3·Et2O were employed,
c
a
b
All reactions were performed under Conditions A. Isolated yields.
and the reaction was run for 24 h.
c
Double bond isomerizations were not observed in these reactions.
in the substrates (entries 11 and 14). Primary alkyl alcohols
bearing vinyl and azido groups were well tolerated (entries 16
and 18). Interestingly, highly fluorinated alcohol was also
proved to be reactive (entry 17). Delightfully, secondary
alcohols, which are vulnerable to oxidative systems,16,17
survived well under the oxidative conditions, giving the
corresponding esters in moderate yields (entries 19−21).
Phenols with strong electron-withdrawing groups performed
well in the present C−C bond cleavage transformation (entries
22−23), while electron-rich phenols tended to decompose.
Unfortunately, propargylic alcohols are not tolerated under the
current oxidative conditions. Within the above scope of
alcohols, the present method might provide an alternative
pathway for esters synthesis from aryl ketones.
an allyl alcohol, was proved to be competent (entry 2, Table 5).
Notably, secondary natural alcohols, such as menthol,
(−)-borneol, and cholesterol participated in the present
transformation, highlighting the broad substrate scope and
the potential utility of this protocol (entries 3−5). In addition,
α-ionone and β-ionone, important spices, underwent C−C
cleavage to provide 7f and 7g in acceptable yields, respectively,
without the observation of double bond isomerization (entry
6−7).
Notably, the configuration of the product was totally
retained, while the Mitsunobu reaction19 featured complete
inversion of configuration (eq 1).
Interestingly, when substrate 9 was employed using reaction
Conditions A, the intramolecular esterification product 10 was
obtained in 37% yield (eq 2).
To explore the mechanism of this transformation, some
potential intermediates were subjected to the reaction system
(Table 6). In contrast to the acetophenone (11a), the reactions
Ester moieties are widely found as key motifs in naturally
existent and biologically active substances. In medicinal
chemistry, the ester group is a good hydrogen bond acceptor
and the esterification strategy is widely used in pro-drug
D
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phenylglyoxal intermediates13 and α-ketoester intermediates11i
(see eq S2, SI). The reactions of 1-phenylvinylboronic acid
pinacol ester 11g under Conditions A and Conditions B were also
investigated, respectively (entry 7, Table 6). These results
indicate that 11g is not involved in the transformation.
Although silyl enol ether 11h could be converted to 5a in
medium yield (entry 8 Table 6), the weakly basic conditions, as
well as the absence of silicon and peroxyacid reagents in our
system, exclude the possibility of a Rubottom-type oxidation.20
On the basis of the above control experiments, it might be
concluded that the initial interaction of alcohols and ketones
might occur before the oxidative process and the C−C bond
cleavage process.
Table 6. Control Experiments
As shown in entries 1−2 of Table 1, O2 is crucial to this
reaction. Therefore, isotope experiments were investigated
under 18O2 atmosphere. To our surprise, 33% of carbonyl
oxygen atom was labeled (3a-18O16O: 3a-16O 16O = 33:67, eq
3). Further control experiments proved that ester product 3a
could not undergo oxygen exchange with water (eq 4).
Moreover, when the reaction was conducted in the presence of
2.0 equiv of H218O open to air, only 10% of 3a was labeled (eq
5), which could be reasonably explained by the oxygen
exchange between acetophenone 1a and water (eq 6). These
two reactions suggest that the oxygen atoms of the ester do not
originated from water. Therefore, the above results demon-
strate that the incorporation of O2 to the ester products is one
potential pathway involved in this novel transformation with a
new mechanism.
Additionally, when employing TEMPO (2,2,6,6-tetramethyl-
piperidinooxy), BHT (2,6-di-tert-butyl-4-methylphenol), or 1,1-
diphenylethene as radical trappers, the reactions were almost
inhibited (see eq S8−S11, SI). To get more information about
this transformation, EPR (electroparamagnetic resonance)
experiments were conducted with the addition of free radical
spin trapping agent DMPO (5,5-dimethyl-1-pyrroline N-oxide).
To our delight, some signals of organic radicals were observed,
and these signals would disappear with the addition of
superoxide dismutase (SOD) (Figure S6−S8, SI). These results
imply that some unknown radicals derived from superoxide
compounds might exist during the reaction.12c,g
of the corresponding C−C bond cleavage intermediate
benzaldehyde (11b) and benzoic acid (11c), as well as the
oxidized intermediates 11d and 11e with nBuOH afforded ester
5a in much lower yields under Conditions A or Conditions B.
These results might rule out the possibility of 11b−11e as
intermediates of the reaction. One of the interesting results is
that when phenylglyoxal monohydrate 11f was employed as the
substrate, the reaction afforded 5a in 3% yield under Conditions
A, but 54% yield under Conditions B (in the absence of BF3·
Et2O). These results indicate that 11f is not involved in the
transformation under Conditions A. But, 11f might be an
alternative minor pathway under Conditions B (because the
yield is still lower than 65%, cf. entries 1 and 6, Table 6).
Furthermore, this result can reasonably account for the reaction
of 4k and 4l (entries 11−12, Table 3) through the relay of
To explore the effects of the pyridine and BF3·Et2O in this
transformation, some EPR experiments were carried out
(Figure 1). The signals of CuII species did not appear when
CuBr was stirred open to air at 130 °C (a, Figure 1). Similarly,
Figure 1. EPR spectra (X band, 9.7 GHz, RT) of conditions: (a) CuBr
(0.025 mmol) in PhCl (0.5 mL) with stirring at 130 °C open to air for
1 h; (b) CuBr (0.025 mmol) and BF3·Et2O (0.25 mmol) in PhCl (0.5
mL) with stirring at 130 °C open to air for 1 h; (c) CuBr (0.025
mmol) and Py (0.50 mmol) in PhCl (0.5 mL) with stirring at 130 °C
open to air for 1 h. The mixture was then analyzed by EPR,
respectively.
E
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no CuII signals were detected in the sample of CuBr and BF3·
Et2O in PhCl under air at 130 °C (b, Figure 1). Notably, when
the pyridine was added to the above system, the signals
corresponding to the CuII were observed clearly (c, Figure 1).21
The above EPR results, in agreement with our previous
reports,15b indicated that pyridine could enable the reaction of
CuI with O2 to generate CuII and superoxide radicals.22 Based
on the results in Tables 2 and 3, as well as the above
experiments, we suggested that BF3·Et2O played as a Lewis acid
that could coordinate to the carbonyl oxygen of the methyl
ketones and activate the carbonyl group.23
[PyH].10h Then, the carbon-centered radical intermediate is
intercepted by molecular oxygen, generating superoxide
intermediate B.26 Then, the SET reduction and subsequent
protonation of intermediate B by CuI and [PyH] generates
hydroperoxide intermediate C.27 Further rearrangement of
hydroperoxide intermediate C leads to C−C bond cleavage,
affording ester along with the formation of aldehyde F.28 In this
way, the carbonyl group remains intact without the O atom
incorporation.
Alternatively, in pathway b, dehydration of hemiketal A
generates vinyl ether D reversibly. Subsequently, CuII and
superoxide radical, which are formed through the reaction of
CuI and O2 in the presence of pyridine,22 react with vinyl ether
D to produce dioxetane intermediate E.29 Finally, the C−C
bond and O−O bond cleavage of dioxetane intermediate E
affords ester and aldehyde byproduct F.29 In pathway b, the
oxygen atom of the carbonyl group is derived from molecular
oxygen, which is consistent with the labeling experiment (eq 3).
In addition, when aryl methyl ketones (R′ = H, Scheme 2)
were subjected to Conditions B (in the absence of BF3·Et2O as
Lewis acid), the oxidation of this kind of substrates to
phenylglyoxals might be accompanied by the above pathways
a and b. Then the subsequent α-ketoesters formation11i and
fragmentation (see eq S2, SI), to afford the corresponding
esters, could not be excluded as a minor pathway (entry 6,
Table 6).
In order to identify the fragment of the ketones, substrate 12
was tested under Conditions B (eq 7). Apart from the
generation of 5a, 13% yield of 5b and 34% yield of 14 were
isolated. Moreover, only trace amount (<1%) of 13 was
detected by GC-MS. There were still some other unknown
products observed by GC-MS, which could not be isolated and
identified yet. On the basis of these results, we could conclude
that the corresponding 2-(4-methoxyphenyl)acetaldehyde was
produced as the early stage byproduct, which then underwent
the C−C bond cleavage process to get aldehyde 1413 or the
esterification process to form ester 5b, which is in accord with
the reaction of 4l (entry 12, Table 3).
Furthermore, we invested the proton/deuterium kinetic
isotope effect of the reaction. The value of kH/kD is 1.17, which
indicates that the C−H bond cleavage might not be involved in
the rate-determining step (eq 8).24
CONCLUSION
■
In summary, we have developed a new type of copper-catalyzed
C(CO)−C(alkyl) bond cleavage of common ketones for the
synthesis of esters via an aerobic oxidation and oxygenation
process with air. The usage of inexpensive copper catalyst under
open air conditions makes this protocol very green and
practical. A wide range of inactive ketones including more
challenging aryl long-chain alkyl ketones selectively undergo the
C(O)−C bond esterification. The reaction tolerates a variety of
primary and secondary alcohols and electron-deficient phenols,
especially natural alcohols. 18O labeling experiments demon-
strate that part of the carbonyl oxygen atom of ester is
originated from molecular oxygen. Preliminary mechanism
studies indicate that two pathways are mainly involved in this
transformation. Further investigations of the synthetic
applications are ongoing in our group.
On the basis of the above results and previous reports, two
possible pathways were proposed in Scheme 2. Initially,
hemiketal A is formed by the nucleophilic addition of ketone
with alcohol in a reversible way before the aerobic oxidation
process.25 In pathway a, the single electron transfer (SET) of A
and CuII forms a carbon-centered radical intermediate, CuI, and
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, analytical data for products, NMR
spectra of products. This material is available free of charge via
the Internet at http://pubs.acs.org.
Scheme 2. Proposed Mechanistic Pathways
AUTHOR INFORMATION
■
Corresponding Author
jiaoning@bjmu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Nos. 21325206, 21172006), the National
Young Top-notch Talent Support Program, and the Ph.D.
Programs Foundation of the Ministry of Education of China
F
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(h) Allen, S. E.; Walvoord, R. R.; Padilla-Salinas, R.; Kozlowski, M. C.
Chem. Rev. 2013, 113, 6234. (i) Wang, J.-R.; Deng, W.; Wang, Y.-F.;
Liu, L.; Guo, Q.-X. Youji Huaxue 2006, 26, 397. (j) Wendlandt, A. E.;
Suess, A. M.; Stahl, S. S. Angew. Chem., Int. Ed. 2011, 50, 11062.
(11) For some selected examples about copper-catalyzed reactions
using molecular oxygen as oxidant in the recent years, see:
(a) Esguerra, K. V. N.; Fall, Y.; Petitjean, L.; Lumb, J.-P. J. Am.
Chem. Soc. 2014, 136, 7662. (b) Yang, Y.; Dong, W.; Guo, Y.; Rioux,
R. M. Green Chem. 2013, 15, 3170. (c) Liu, C.-Y.; Li, Y.; Ding, J.-Y.;
Dong, D.-W.; Han, F.-S. Chem.Eur. J. 2014, 20, 2373. (d) Yu, J.; Jin,
Y.; Zhang, H.; Yang, X.; Fu, H. Chem.Eur. J. 2013, 19, 16804.
(e) Feng, Q.; Song, Q. J. Org. Chem. 2014, 79, 1867. (f) Jover, J.;
Maseras, F. Chem. Commun. 2013, 49, 10486. (g) Boess, E.;
(No. 20120001110013) is greatly appreciated. We thank Wujie
Zou in this group for reproducing the reactions of 1h and 4i.
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