Oxidative C(OH)[sbnd]C bond cleavage of secondary alcohols to acids over a copper catalyst with molecular oxygen as the oxidant

Article abstract of DOI:10.1016/j.jcat.2017.02.017

Selective oxidative cleavage of C[sbnd]C bond is pivotal for producing functionalized molecules, useful for organic synthesis and biomass utilization. We herein report the oxidative C(OH)[sbnd]C bond cleavage of secondary alcohols to acids over a copper/1, 10-phenanthroline complex with molecular oxygen as the oxidant. A wide range of secondary alcohols are converted into acids with up to 98% yields. More interestingly, it is effective for breaking up lignin model systems into acids, which is rarely achieved in previous studies. Density functional theory (DFT) calculations indicate a copper-oxo-bridged oxygen dimer is the active species for the C[sbnd]H bond cleavage which is the rate-determining step for C[sbnd]C bond.

Full text of DOI:10.1016/j.jcat.2017.02.017

Journal of Catalysis 348 (2017) 160–167
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Oxidative C(OH)AC bond cleavage of secondary alcohols to acids
over a copper catalyst with molecular oxygen as the oxidant
Min Wang a, Jianmin Lu a, Lihua Li a, Hongji Li a,b, Huifang Liu a,b, Feng Wang a,
⇑
a State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
b University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective oxidative cleavage of CAC bond is pivotal for producing functionalized molecules, useful for
organic synthesis and biomass utilization. We herein report the oxidative C(OH)AC bond cleavage of sec-
ondary alcohols to acids over a copper/1, 10-phenanthroline complex with molecular oxygen as the oxi-
dant. A wide range of secondary alcohols are converted into acids with up to 98% yields. More
interestingly, it is effective for breaking up lignin model systems into acids, which is rarely achieved in
previous studies. Density functional theory (DFT) calculations indicate a copper-oxo-bridged oxygen
dimer is the active species for the CAH bond cleavage which is the rate-determining step for CAC bond.
Ó 2017 Elsevier Inc. All rights reserved.
Received 15 December 2016
Revised 17 February 2017
Accepted 17 February 2017
Keywords:
Carboxylic acids
Copper
Catalysis
Oxidative cleavage
Molecular oxygen
1. Introduction
bonds in organic molecule, is present in biomass, such as lignin
fragments, and thus its cleavage is not only useful in the organic
The CAC bond dominantly constitutes the backbone of the
organic materials. Its cleavage and functionalization provide an
useful molecule-tailoring method for the synthesis of value-
added chemicals and degradation of polymer pollute, and more
importantly, show promising applications in the depolymerization
of biomass into chemicals and fuels for the sustainability [1–11].
But the nonpolar, thermodynamically stable and kinetically inert
characters of CAC bonds render a challenge to be broken up under
mild conditions. Traditional success is limited to the ring-strained
activated CAC bond, and/or relies on strong oxidants, such as per-
oxides and metal salts, or vapor phase cracking at high tempera-
ture [12–21]. Recent researches were devoted to new and
environmental benign catalytic pathways for the selective break-
ing up CAC bond in functionalized molecules [22–24].
The substituents at C(X)AC position greatly influence the reac-
tivity of CAC bond. Pioneer works by Jiao [25–27], Jiang [28], Bi
[29], Berreau [30] and Schoenebeck [31], have realized the
unstrained C(@O)AC bond cleavage of aromatic ketone to alde-
hyde, acid, ester and amide [32–39]. Very recently, we found that
copper catalyst enables the selective cleavage of strong C(N)AC
bond of tertiary amine to amide, and C(@O)AC bond cleavage of
ketone to acid and imide [40–42]. C(OH)AC bond, one of the basic
synthesis, but also shows potential in lignin valorization. Com-
pared to the ketones, the inert hydroxyl groups and hydrogen-
bonding characters render the C(OH)AC bond more difficult to be
cleaved. Csp2ACsp3 (position 1, Scheme 1) of secondary benzyl alco-
hols was broken up via N-containing groups directed reductive
cleavage over Rh complex [43], or insert of oxygen atom via
Baeyer-Villiger oxidation with performic acid as oxidant [44].
Herein, we report the selective breaking up Csp3ACsp3 bond at
position 2 of secondary benzyl alcohols (Scheme 1). We discovered
that a copper/1,10-phenanthroline complex is effective for the
oxidative cleavage of broad secondary benzyl alcohols to acids with
up to 98% yields with molecular oxygen as oxidant. Kinetic isotopic
experiment revealed that CAH bond oxygenation is the rate-
determining step for CAC bond cleavage. DFT calculations revealed
that a copper-oxo-bridged oxygen dimer is the active center for the
CbAH bond oxidation with Ea = 69 kJ molÀ1. More importantly, this
method is effective for the oxidative cleavage of lignin linkages,
which shows promising for the production of acid from lignin.
2. Experimental
2.1. General considerations
⇑
All of the chemicals were of analytical grade, mostly purchased
from J&K Chemicals and Aladdin Chemicals, and were used without
Corresponding author.
E-mail address: wangfeng@dicp.ac.cn (F. Wang).
http://dx.doi.org/10.1016/j.jcat.2017.02.017
0021-9517/Ó 2017 Elsevier Inc. All rights reserved.

M. Wang et al. / Journal of Catalysis 348 (2017) 160–167
161
Scheme 1. The methods for the cleavage of the secondary alcohols.
further purification. 1H and 13C NMR (Nuclear Magnetic Reso-
nance) spectra were obtained with a Bruker AVANCE III 400 MHz
spectrometer with tetramethylsilane used as the internal
reference.
sidered to be located when the residual forces were less than
0.02 eV/Å and the SCF convergence criterion was set to 10À7 eV.
3. Results and discussion
2.2. Catalytic tests
3.1. Reaction condition optimization
The catalytic reactions were performed in a 10-mL autoclave
reactor with an internal Teflon insert. Typically, 0.5 mmol of sec-
ondary alcohols, 0.04 mmol of Cu(OAc)2 and 0.04 mmol of ligand,
1 mmol of base, and 2 mL of solvent were added to the reactor.
Then, the reactor was charged with 0.4 MPa O2 and heated to the
desired temperature under magnetic stirring. When the reaction
reached completion, the reaction mixture was diluted with 4 mL
of methanol, and the catalyst was separated via centrifugation.
Copper catalyst was widely used in the oxidation reactions [51].
We commenced our study with phenylethanol (1a) oxidation over
copper catalyst. Simple copper salts, such as Cu(OAc)2 did not work
for the alcohol oxidation (Table 1, entry 1). Then, various organic
ligands and base additives were screened (Table 1 and Fig. 1).
The use of 1, 10-phenanthroline (L1) ligand slightly increases the
activity with 13% conversion of 1a and 54% selectivity for benzoic
acid (2a) (Table 1, entry 2). The bases showed obvious effect on the
reaction. The activity increased with the base strength. The weak
organic bases, such as pyridine and DBU, showed little effect on
the reaction (Table 1, entries 3–4). KOH, NaOH and Na2CO3 were
favored for oxidative cleavage reaction (Table 1, entries 5–7), and
among these bases, the strongest base KOH showed the best per-
formance with 87% conversion of 1a and 84% selectivity for 2a
(Table 1, entry 7). Replacement of DMSO with other solvents
resulted in both lower conversion and CAC bond cleavage selectiv-
ity (Table 1, entries 8–11).
Various ligands were further investigated. Among the ligands,
8-hydroxyquinoline (L2) showed the highest activity with
89% conversion of 1a, but poor ability for CAC cleavage with
acetophenone (3a) as the major product. Salen (L3) and 2,
6-pyridinedicarboxylic acid (L4) showed relative higher selectivity
for 2a, but lower conversion (<50%). L1 ligand was the most effec-
tive ligand for breaking up CAC bond with 79% conversion and 83%
selectivity for benzoic acid in the presence of KOH at 120 °C for 2 h.
Preliminary results showed that Cu(OAc)2/L1/KOH is an effective
catalytic system for the oxidative cleavage of secondary alcohols
to acids.
The acid product was esterified with addition of 40
l
l of BF3ÁOEt2
at 100 °C for 6 h in Ar atmosphere. In reaction condition optimiza-
tion experiment, the products were identified and quantified using
gas chromatography-mass spectrometry (GC–MS) and an Agilent
7890A/5975C instrument equipped with an HP-5 MS column
(30 m in length, 0.25 mm in diameter). p-Xylene was used as the
internal standard. In the substrate scope experiment, the product
was isolated and identified by NMR. The procedure for the isolation
of the product is as follows: after the reaction completed, the reac-
tor was cooled to room temperature in water and vented the gas.
The reaction mixture was acidified with HCl 1.0 M (pH ꢀ 1–2,
15 mL) and then extracted with Et2O (3 Â 20 mL). Next, the com-
bined organic layers were washed with HCl 1.0 M (pH ꢀ 1–2,
3 Â 10 mL), dried over anhydrous Na2SO4, and filtered and the
Et2O was rotary evaporated. Solid products obtained were vacuum
dried for 10 h at 60 °C.
2.3. DFT calculations
The DFT calculations were performed using the Vienna ab initio
simulation package (VASP) [45] with project augmented wave
(PAW) potential and the Perdew-Burke-Ernzerhof (PBE) functional
[46]. A plane wave cutoff of 400 eV was used. The copper/1,10-
phenanthroline complex, copper-oxo-bridged dimer and copper
superoxide monomer were fully relaxed in a cubic box with a side
length of 30 Angstroms until the residual forces were less than
3.2. Oxidative cleavage of various secondary alcohols
The substrate scope was then tested with Cu(OAc)2/L1/KOH cat-
alytic system (Table 2). Under the optimized conditions, a wide
range of secondary alcohols were oxidized to the corresponding
acid with 33–98% yields. Electron-donating or electron-
withdrawing substitutes showed little effect on the reaction. Ary-
lethanols with Cl-, Me-, or MeO- groups afforded 90–93% acid
yields (Table 2, 2b-2d). 1-(2-Naphthyl)ethanol (1e) showed moder-
ate yield of 2e (70%). The change of methyl group to ethyl group
makes it more difficult to be cleaved and higher temperature
was needed for 1-phenyl-2-propanol (1f) with 69% yield of 2a. 1-
Phenylpropan-2-ol (1g) was relatively inert but still achieved 33%
0.02 eV/Å. The
C point of Brillouin zone was used in the k-point
mesh sampling. Transition states for elementary reaction steps
were determined by a combination of the nudged elastic band
(NEB) method [47] and the dimer method [48–50]. In the NEB
method, the path between the reactant and product is discretized
into a series of structural images. The image that is closest to a
likely transition state structure was then employed as an initial
guess structure for the dimer method. The transition state is con-

162
M. Wang et al. / Journal of Catalysis 348 (2017) 160–167
Table 1
a
Screening the reaction conditions.
Entry
Catalyst
Base/solvent
Conv. /%
Product distribution /%
2a
3a
4a
1
Cu(OAc)2
—/DMSO
0
0
0
0
2
3
4
5
6
7
8
9
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
Cu(OAc)2/L1
—/DMSO
13
16
16
66
72
87
36
34
9
54
51
63
66
93
94
57
0
28
26
47
34
4
4
3
99
27
96
18
23
0
0
3
2
0
0
34
4
Py./DMSO
DBU/DMSO
K2CO3/DMSO
NaOH/DMSO
KOH/DMSO
KOH/MeCN
KOH/THF
10
11
KOH/MeOH
KOH/MePh
39
0
69
a
Reaction conditions: 0.5 mmol phenylethanol, 0.04 mmol Cu(OAc)2, 0.04 mmol L1 ligand, 1 mmol KOH, 2 mL DMSO, 0.4 MPa O2, 120 °C, 6 h. L1: 1,10-phenanthroline;
DMSO: dimethyl sulfoxide. Py.: pyridine. THF: tetrahydrofuran. DBU: 1,8-Diazabicyclo[5.4.0]undec-7-ene. The results were determined by GC–MS.
Fig. 1. Screening the organic ligand. Reaction conditions: 0.5 mmol phenylethanol, 0.04 mmol Cu(OAc)2, 0.04 mmol ligand, 1 mmol KOH, 2 mL DMSO, 0.4 MPa O2, 120 °C, 2 h.
yields of 2a, possible via oxidative cleavage of the C ACb bond.
3.3. Oxidative cleavage of ketones
a
Heteroaromatic secondary alcohol, such as 2-(1-hydroxyethyl)
pyridine (1h), which usually poisons noble metal, was successfully
converted to 2h with 79% yield. 31% yield of 2a was obtained from
1, 2-diphenylethanol (1i). Differently, cyclic secondary alcohols
selectively transformed to the anhydride (2j, 96% yield).
Previous work showed that alcohols could be oxidized to ketone
over copper catalyst. The alcohol cleavage probably occurred via
ketone intermediate. We then monitored the product distribution
as the reaction time (Fig. 2). In the initial stage, 1a oxidation to

M. Wang et al. / Journal of Catalysis 348 (2017) 160–167
163
Table 2
Substrate scope of secondary alcohols.a
a
Reaction conditions: 0.5 mmol alcohols, 0.04 mmol Cu(OAc)2, 0.04 mmol L1, 1 mmol KOH, 2 mL DMSO, 0.4 MPa O2, 130 °C,
12 h.
for a wide range of ketones (Table 3). Under the optimized condi-
tions, either electron-donating (Me-, or MeO-) or electron-
withdrawing (Cl-, NO2-) substituted ketones afforded 74–97%
yields of the corresponding acids (Table 3). 1-Indanone trans-
formed to o-phthalic anhydride (2j) with 90% yield. Compared with
1i, higher yield (74%) of 2a was obtained from diphenylethanone
(3i).
3.4. Intermediate analysis and isotopic experiment
(1)
Fig. 2. The reaction time profile. Reaction conditions: 0.5 mmol phenylethanol,
0.04 mmol Cu(OAc)2, 0.04 mmol L1, 1 mmol KOH, 2 mL DMSO, 0.4 MPa O2, 40 °C.
(2)
acetophenone (3a) was very fast even at 40 °C. As the reaction pro-
ceeded, the selectivity of 3a decreased accompanied by the
increase of 2a selectivity. The results indicated 1a was first oxi-
dized to 3a which further converted to 2a via CAC bond oxidative
cleavage. Cu(OAc)2/Phen/KOH catalytic system was very effective
We then studied the reaction mechanism. The gas products
made the lime water cloudy (Fig. S1), indicating that the methyl
group was converted into CO2. 2-Hydroxy-1-phenylethanone

164
M. Wang et al. / Journal of Catalysis 348 (2017) 160–167
Table 3
Substrate scope of ketones.a
a
0.5 mmol alcohols, 0.04 mmol Cu(OAc)2, 0.04 mmol L1, 1 mmol KOH, 2 mL DMSO, 0.4 MPa O2, 130 °C, 12 h.
quickly undergone CAC cleavage to 2a within 0.5 h even at 40 °C
and it is a possible active intermediate (eq.1). Benzoylformic acid
was detected as minor product, but only 8% were convert to 2a,
indicating it is not an intermediate (Eq. (2)).
Then, the kinetic isotopic effect (KIE) for CAC cleavage was
investigated (Fig. 3). A secondary KIE was observed when AOH,
ACHA and ACH3 groups of 1a were deuterated. A primary KIE
was observed when the ACH3 group of 3a was deuterated. These
results demonstrated that 1a oxidation to 3a is fast and the CbAH
oxidation of 3a is the key elementary step for the CAC bond
cleavage.
oxidation of 3a to 2-hydroxy-1-phenylethanone, and (c) the oxida-
tive cleavage of 2-hydroxy-1-phenylethanone to 2a with release of
a CO2. The CbAH oxidation of 3a to 2-hydroxy-1-phenylethanone is
the rate-determining step. The kinetic study showed the CbAH
bond activation energy is 69 kJ molÀ1 (Fig. S2).
3.5. DFT calculation of the CAH bond abstraction
As previously reported, in Cu(OAc)2/L1 complex, copper and L1
formed a four coordinated structure and the coordination of ligand
makes the electron density of copper increased, resulting in the
efficient activation of oxygen [40]. In enzyme or mimicked copper
complex, oxygen is activated to copper-oxo-bridged oxygen dimer
or copper superoxide monomer depends on the ligand structure,
which is active in H-abstraction [52].
Based on the above results, it becomes clear that the reaction
contains three typical steps: (a) the oxidation of 1a to 3a, (b) the
Considering the CbAH oxidation of acetophenone (3a) to 2-
hydroxy-1-phenylethanone is the rate-determining step for the
oxidative cleavage of 1a. We then explored the reactivity of the
two possible active species in the H-abstraction of acetophenone
in the rate-determining step by DFT calculation (Fig. 4). After opti-
mization, copper superoxide monomer and acetophenone are in
the same plane and come close into each other, resulting in the
H transfer from carbon to the terminal oxygen. Acetophenone per-
pendicularly comes close to the copper-oxo-bridged oxygen
dimers, and then the H is abstracted by one of the bridging oxygen.
The H abstraction step is thermodynamically favored as evidenced
by negative reaction enthalpy for both the monomer and the
dimer. The Ea value of the H abstraction by the dimer (0.21 eV)
is much lower than that of the monomer (1.37 eV). These results
Fig. 3. Kinetic isotopic experiment. Reaction conditions: 0.5 mmol substrate,
0.04 mmol Cu(OAc)2, 0.04 mmol L1, 1 mmol KOH, 0.4 MPa O2, 2 mL DMSO, 40 °C,
0.5 h.

M. Wang et al. / Journal of Catalysis 348 (2017) 160–167
165
Fig. 4. DFT calculation of the H abstraction from CAH bond of acetophenone by copper-oxo-bridged dimer and copper-superoxide monomer, respectively. Red O, orange Cu,
gray C, white H.
indicate that the copper-oxo-bridged oxygen dimer is the major
active species for the oxidative cleavage of the secondary alcohols.
3.7. Oxidative cleavage of lignin models
Lignin is the mostly abundant aromatic biomass resource in the
nature. The depolymerization of lignin is a promising way to pro-
vide aromatics for human society and aroused a great research
interest [55–63]. The key for the lignin depolymerization lies in
breaking up the major linkages. b-O-4 is the dominant linkage in
lignin with about 50% percentages (Fig. S3) [64]. Previously, we
reported a two-step method for the oxidative cleavage of b-O-4
models, that is first oxidation of benzyl CAOH of b-O-4 to b-O-4
ketone over VOSO4/TEMPO (2,2,6,6-Tetramethylpiperidine 1-
oxyl) catalyst in MeCN solvent, and then oxidative cleavage of b-
O-4 ketone over copper catalyst in methanol solvent [61,65].
Herein, using Cu(OAc)2/L1/KOH catalytic system, the b-O-4 lignin
linkage models were successfully oxidized to aromatic acid with
49–79% yields in one step (Table 4). Compared with arylethanol,
a milder condition (lower temperature and short reaction time:
100 °C, 2 h) was employed for the complete breaking down the
linkages. The methoxyl substituents showed little effect on the cat-
3.6. Proposed mechanism
Based on the above-mentioned results, we may come to a tenta-
tive reaction mechanism (Scheme 2). In the initial step, 1-
phenylethanol is fast oxidized to acetophenone. As well studied in
the literature, the base favors for the oxidation of alcohol to car-
bonyl compounds via deprotanation of OAH bond [53,54]. The
addition of KOH facilitates the fast oxidation of 1-phenylethanol
to acetophenone. C AH bond of acetophenone is oxidized by the
a
copper-oxo-bridged oxygen dimer to give 2-hydroxy-1-
phenylethanone which is further oxidized to phenylglyoxal [29].
Phenylglyoxal can be oxidized to benzoylformic acid by-product,
which is hardly converted to benzoic acid under the reaction condi-
tions. Acetalation of phenylglyoxal intermediate with water forms
a, a-bis(hydroxy)acetophenone. After a 1,2-hydride shift, CAC
bond cleavage takes place, leading to the formation of benzalde-
hyde and CO2 [29]. Benzaldehyde is further oxidized by O2 to ben-
zoic acid. The oxygen of the acid product is derived from O2 [28,42].
alytic performance. The complex linkages with C -OH (1s) were
c
more inert and higher temperature (140 °C) was applied.
Scheme 2. Proposed reaction mechanism.

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M. Wang et al. / Journal of Catalysis 348 (2017) 160–167
Table 4
Oxidative cleavage of lignin models.a
a
Reaction conditions: 0.2 mmol alcohols, 0.04 mmol Cu(OAc)2, 0.04 mmol L1, 0.4 mmol KOH, 2 mL DMSO, 0.4 MPa O2, 100 °C, 2 h.
4. Conclusion
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