Reactions catalyzed by a binuclear copper complex: Selective oxidation of alkenes to carbonyls with O2

Article abstract of DOI:10.1039/c7cy01757j

Terminal alkenes were selectively cleaved into ketones and aldehydes catalyzed by a binuclear copper catalyst bearing a simple salicylate ligand with O₂ as the oxidant. The reaction was carried out under an atmosphere of O₂ (balloon) with 0.5 mol% of catalyst and could be performed on a gram scale, providing a convenient and practical method for the cleavage of terminal alkenes into carbonyl compounds.

Full text of DOI:10.1039/c7cy01757j

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Received 00th January
20xx,
Reactions catalyzed by a binuclear copper complex: selective
oxidation of alkenes to carbonyls with O2
Yuxia Liu,a Dong Xue,a Chaoqun Li,a Jianliang Xiaoa,b and Chao Wang*a
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Previous work
Terminal alkenes were selectivitely cleavaged into ketones and
aldehydes catalyzed by a binuclear copper catalyst bearing simple
salicylate ligand with O2 as oxidant. The reaction was carried out
under an atmosphere of O2 balloon with 0.5 mol% of catalyst and
could be performed at a gram scale, providing a convenient and
practical method for the cleavage of terminal alkenes into
carbonyl compounds.
O
O
CuCl2, O2
Cl
+
H
electrocatalysis
no substrate scope
Cu containing
polyoxometalates, 5 mol%
R1HC CHR2
R1CHO
+
R2CHO
O2 (2 bar)
The selective cleavage of alkenes to carbonyl compounds is of
great importance for both industry and academia, as alkenes
are abundant raw chemicals from fossil fuel as well as biomass
and are common intermediates in organic synthesis.1 The
ozonolysis with oxone as oxidant, which has been developed
for decades, is still the most used method for this
transformation.2 Stoichiometric other oxidants, e.g. KMnO4,3
OsO4,4 PhIO/HBF4,5 m-CPBA,6 H2O2,7 and TBHP8 with or without
catalysts, have also been employed for selective cleavage of
alkenes. The generation of stoichiometric amount of waste
and the toxicity and safety issues associated with these
protocols call for the development of greener and safer
methods for alkene cleavage.
R2
R2
[Cu(µ-Cl)Cl(phen)]2, 5 mol%
HCHO
O
+
O2 (4 atm)
R1
R1
HO
This work
O
O
OH
C
O
O
II
Cu
II
Cu
H3C
C
N
O
N
CH3
O
O
O
OH
OH
Oxygen is an abundant, cheap and environmentally benign
oxidant.9 Efforts have been made to develop catalytic systems
for cleavage of alkenes with O2 as oxidant.1e Metal-free
catalysts, including photosensitizers10 and free radical
initiators,11 were reported to be able to cleave alkene with O2
as oxidant. Various metal catalysts, e.g. Pd,12 Ru,13 Au,14 Mn,15
Fe,16 Co17 and Ni,18 have shown activity for the scission of
alkene with O2. Despite the progress, inexpensive and readily
available catalysts with broad substrate scope, low catalyst
loading and low oxygen pressure are still highly desirable.
Cu catalyzed aerobic oxidation has found numerous
R2
R2
1, 0.5 mol%
HCHO
O
+
R1
R1
O2 (balloon)
Scheme 1. Copper-catalyzed aerobic alkene cleavage.
applications in organic synthesis19 and many oxidative
enzymes contain Cu as active site.20 Cu-containing enzymes
have shown to catalyze alkene cleavage with O2 under mild
conditions.1c,
21
However, Cu-catalyzed oxidation of alkenes
with O2 has rarely been achieved (Scheme 1). In 1993, Sasaki
and co-workers reported the electrocatalytic oxidative
cleavage of styrene in the presence of CuCl2, albeit with low
product selectivity and yields.22 Recently, Neumann and co-
workers reported that Cu-containing polyoxometalates are
effective catalysts for cleavage of various alkenes under 2 bar
of O2 at 85 oC with a S/C (substrate/catalyst ratio) of 20.23
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Table 1. Optimization of reaction conditions for 1 catalyzed aerobic oxidation of 2aa
O
1 (0.5 mol%), TBAC (1 mol%)
R2
R1
R2
R1
NaBF4 (30 mol%), THF,
O2 (balloon), 60 oC,12 h
2
3
O
O
O
CH3
Et
Entry Catalyst
Additive
-
Solvent
THF
Yield (%)b
0
3c, 70% yielda
3a, 90% yield
3b, 85% yield
1
2
-
O
O
O
1
-
THF
THF
45
70
0
3
1
TBAC
OMe
F
Cl
4
-
TBAC
THF
3d, 71% yielda
3e, 88% yield
3f, 77% yield
5
1
TBAC+NaBF4
TBAC+TBA-BF4
TBAC+KPF6
TBAC+NaBF4
TBAC+NaBF4
TBAC+NaBF4
TBAC+NaBF4
THF
90
89
88
36
0
O
O
O
6c
7d
8e
9f
10
11
1
1
THF
F
F
THF
CF3
Br
1
THF
3g, 78% yield
3h, 82% yield
3i, 78% yield
1
THF
O
O
O
CuCl2
CuCl2
THF
0
CH3
CF3
THF/H2O (9:1)
0
Cl
CH3
CF3
3k, 80% yield
a Reaction conditions: 2a (0.5 mmol), 1 (0.5 mol%), solvent (0.5 mL), TBAC (1
b
mol% when added), NaBF4 (30 mol% when added), O2 balloon, 60 oC, 12 h.
3j, 78% yield
3l, 87% yield
c
d
Isolated yield. TBA-BF4 = Tetrabutylammonium tetrafluoroborate, 30 mol%.
O
O
O
with 30 mol% of KPF6. e The reaction was carried out under air. f The reaction was
S
carried out under argon.
F
F
F
Shyu and co-workers demonstrated that the [Cu(µ-
Cl)Cl(phen)]2 complex as well as CuCl2 could catalyze the
scission of aromatic gem-disubstituted alkenes under 4 atm of
O2 at 60 oC with a S/C of 20.24 Herein, we disclose a structurally
well-defined binuclear Cu catalyst with simple salicylate ligand
for cleavage of terminal alkenes. This new catalytic system
features easy catalyst preparation, relatively high productivity
(S/C = 200) and low oxygen pressure (O2 balloon), providing a
convenient method for the conversion of alkenes into ketones
and aldehydes.
3m, 89% yield
3n, 85% yield
3o, 95% yield
Scheme 2. Copper-catalyzed cleavage of gem-diarylalkenes into ketones with O2.
Reaction conditions: alkenes (0.5 mmol), 1 (0.0025 mmol, 0.5 mol%), TBAC (0.005
mmol, 1 mol%), NaBF4 (0.15 mmol, 30 mol%), THF (0.5 mL), O2 balloon, 60 oC, 12 h.
Isolated yields. a The reaction time was 24 h.
(Table 1, entry 5). TBA-BF4 (Tetrabutyl ammonium
tetrafluoroborate) and KPF6 also accelerated the reaction
(Table 1, entries 6 and 7). The reason for the acceleration
effect of these salts is unclear at the moment. The reaction
also took place under air, albeit with a lower yield of 36%
(Table 1, entry 8). As expected, no reaction took place under
an argon atmosphere (Table 1, entry 9). CuCl2 was an effective
catalyst for alkene cleavage under pressurised O2 as reported
by Shyu and co-workers;24 however, it is essentailly inactive
under our conditions (Table 1, entries 10 and 11). Examination
of solvent effect revealed that freshly distilled THF is the best
solvent for the reaction (See ESI, Table S1 for details).
Recently, we have discovered that a binuclear copper
salicylate complex, [Cu(Sal)2(NCMe)]2 (Sal = salicylate) 1, is able
to catalyze the aerobic cross dehydrogenative coupling of N-
aryltetrahydroisoquinolines25 as well as aerobic oxidation of
amines to amides with the aid of a VB1 analogue.26 The copper
complex 1 could be readily prepared from simple CuCl and
salicylic acid as fine green crystals.25 In continuing our interest
in aerobic oxidation reactions,16c, 25-26 we found that
also serve as catalyst for alkene cleavage.
1 could
With the optimized conditions in hand, the scope of the
binuclear copper catalyzed aerobic cleavage of alkenes into
ketones was examined. Various gem-diarylalkenes could be
converted to diarylketones in good to excellent yields (Scheme
2). Both electron-donating (OMe, Me, Et) and electron-
withdrawing (Br, Cl, F, CF3) groups on the aromatic rings of 1,1-
diarylethylenes were tolerated to afford the expected
As shown in Table 1, 1,1-diphenylethylene 2a was cleaved
into benzophenone 3a in 45% yield with 0.5 mol% of in the
1
o
presence of O2 (balloon) in THF at 60 C in 12 h (Table 1, entry
2). Interestingly, addition of TBAC (tetrabutylammonium
chloride, 1 mol%) was found to improve the activity of 1 (Table
1, entry 3). The role of TBAC might be stabilisation of active
catalytic intermediate via chloride anion coordination.25
Background experiments showed that no reaction took place
products (Scheme 2, 3a
with substituents on para- or meta-positions of phenyl rings all
reacted well (Scheme 2, 3i 3n). Excellent yield was obtained
-3n). Di- and tri-substituted substrates
in the absence of
1 (Table 1, entries 1 and 4). Much to our
-
surprise, when a catalytic amount (30 mol%) of NaBF4 was
for the substrate with a thiophene group (Scheme 2, 3o). It is
introduced, the yield of 3a was boosted remarkably to 90%
worth noting that the diary ketone products are important
2 | J. Name., 2012, 00, 1-3
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Scheme 4. Copper-catalyzed cleavage of styrene and its derivatives into aldehydes with
O2. Reaction conditions: alkenes (0.5 mmol), 1 (0.0025 mmol, 0.5 mol%), TBAC (0.005
mmol, 1 mol%), NaBF4 (0.15 mmol, 30 mol%), THF (0.5 mL), O2 balloon, 60 oC, 12 h.
Isolated yields. a The reaction time was 24 h.
Scheme 3. Copper-catalyzed cleavage of gem-arylalkyl disubstituted alkenes into
ketones with O2. Reaction conditions: alkenes (0.5 mmol), 1 (0.0025 mmol, 0.5 mol%),
TBAC (0.005 mmol, 1 mol%), NaBF4 (0.15 mmol, 30 mol%), THF (0.5 mL), O2 balloon, 60
oC, 12 h. Isolated yields. a The reaction time was 24 h.
structure motif for pharmaceuticals, natural products and
functional materials and could serve as building blocks for the
synthesis of important molecules.27
Scheme 5. Gram scale preparation of benzophenone.
The substrate scope could be extended to gem-arylalkyl
substituted alkenes (Scheme 3). An array of 1-aryl-1-
methylethenes could be converted to acetophenone and its
Scheme 5, benzophenone 3a was isolated in 90% yield at a 10
mmol scale.
The mechanism of the reaction was then considered. There
are several facts, which might have mechanistic implications. 1)
The reaction only works well in ether solvents, such as THF,
dioxane and diglyme, indicating that the solvent molecule
might be involved in the oxidation process. 2) The cyclopropyl
ring structure is preserved in product 4k (Scheme 3),
suggesting that carbon-based radicals from the olefin
substrate may not be generated during the aerobic
cleavage.16c 3) The reaction was fully inhibited in the presence
of catalytic amount of radical scavengers, such as BHT, TEMPO,
p-benzoquinone, and diphenylamine, indicating that radicals
are involved in the product formation (See ESI, section 4.1). 4)
Formaldehyde could be trapped after cleavage of 2a (See ESI,
section 4.2). 5) THF could be oxidized to gamma-butyrolactone
under standard conditions in the absence of alkene substrates.
In aerobic oxidation reaction catalyzed by Cu complexes38
as well as in biological systems28, it is generally believed that
CuI complexes, rather than CuII species, could react with
dioxygen to form reactive intermediates for oxidation.
derivatives (Scheme 3, 4a
electron-donating and electron-withdrawing substituents are
viable (4a 4h) and the thiophene ring was tolerated (4i).
Replacing the methyl group with more bulky substituents
resulted in lower activity (Scheme 3, 4j 4l). Interestingly, the
-4i). Again, aryl rings with both
-
-
cyclopropyl ring was well preserved after the oxidation, which
is not only synthetic useful but also shed light on the reaction
mechanism (vide infra).
Styrene and its derivatives were further examined as
substrates and the results are shown in Scheme 4. These
substrates gave poor yields in the Cu catalytic system reported
by Shyu and co-workers24 and might have selectivity issues.1e
Delightfully, aldehyde products were formed selectively in high
yields with our catalytic system. Electronic effect of
substituents appears to have little impact on the activity of
substrates (Scheme 4, 5a-5h). Good yields were obtained even
for the very electron-deficient ones, e.g. with -CN (5g) and -
NO2 (5h) groups. 2-Naphthaldehyde was obtained in moderate
yield from 2-vinylnaphthalene (5i).
Complex
which is likely to be preserved during catalysis as shown by our
1
is a CuII-CuII complex, the binuclear structure of
The practical usefulness of this method is demonstrated by
a gram scale reaction of aerobic cleavage of 2a. As shown in
previous studies.25-26 Complex
1
could be reduced by N-
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aryltetrahydroisoquinolines.25-26 The reduction of CuII to CuI by
amines was also reported by Klussmann and co-workers.29
Point 1) and 5) in the above paragraph suggest that THF is
essential for the reaction and could interact with the catalyst.
In a related study, Lippard and co-workers suggest that THF
could reduce an FeIII-FeIII complex to an FeIII-FeII complex,
generating an THF radical, which could then react with
dioxygen to give a peroxy radical intermediate.30 Based on the
above analysis, we thus tentatively propose that the CuII-CuII
complex might somehow promote the formation of a THF
radical, which could then react with dioxygen to form a 2-
peroxytetrahydrofuran radical, and it is this peroxy species
that is responsible for the oxidative cleavage of alkenes to
carbonyl groups, as suggested by Shyu and co-workers24.
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In conclusion, the structurally well-defined binuclear Cu
complex
1
reported by us before25-26 has now been found to
be an efficient, selective catalyst for the cleavage of aromatic
alkenes into ketones and aldehydes. The catalyst is prepared
from simple, cheap metal salt and ligand and the reaction can
be performed with an oxygen balloon in gram scale, providing
a convenient and practically useful alternative to the existing
alkene cleavage chemistry.
16 (a) S. Ma, J. Liu, S. Li, B. Chen, J. Cheng, J. Kuang, Y. Liu, B.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This research was supported by the National Natural Science
Foundation of China (21473109, 21773145), Science and
Technology Program of Shaanxi Province (2016KJXX-26),
Projects for the Academic Leaders and Academic Backbones,
Shaanxi Normal University (16QNGG008), the Program for
Changjiang Scholars and Innovative Research Team in
University (IRT_14R33), and the 111 project (B14041).
18 G. Urgoitia, R. SanMartin, M. T. Herrero and E. Domínguez,
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Table of Contents
A binuclear copper complex bearing simple salicylate ligand catalyses the efficient cleavage
of styrenes into ketones and aldehydes with O2 as oxidant. The reaction works under a
balloon atmosphere of O2 with 0.5 mol% of catalyst and could be performed at gram scale,
providing an alternative to ozonolysis.