Visible light-catalytic dehydrogenation of benzylic alcohols to carbonyl compounds by using an eosin y and nickel-thiolate complex dual catalyst system

Article abstract of DOI:10.1039/c8gc03828g

We developed a simple and environmentally benign visible-light-driven dehydrogenation of benzylic alcohols to the corresponding aldehydes or ketones. By using the dual catalyst system consisting of eosin Y as a photocatalyst and a Ni(ii) complex as a proton reduction catalyst, we could dehydrogenate benzylic alcohols to aldehydes or ketones with excellent yields under mild conditions. The sole byproduct is hydrogen gas.

Full text of DOI:10.1039/c8gc03828g

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DOI: 10.1039/C8GC03828G
A simple and environmentally benign visible-light-driven dehydrogenation of benzylic
alcohols with H as the only byproduct is achieved by EY and nickel-thiolate complex dual
2
catalyst system.

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DOI: 10.1039/C8GC03828G
Journal Name
COMMUNICATION
Visible Light-catalytic Dehydrogenation of Benzylic Alcohols to
Carbonyl Compounds by Using Eosin Y and Nickel-Thiolate
Complex Dual Catalyst System
Received 00th January 20xx,
Accepted 00th January 20xx
Xiu-Jie Yang,*abc Yi-Wen Zheng, Li-Qiang Zheng, Li-Zhu Wu, Chen-Ho Tung and Bin Chen*ad
ac
c
ad
acd
DOI: 10.1039/x0xx00000x
www.rsc.org/
We developed a simple and environmentally benign visible-light- oxidant in the dehydrogenation step may destroy such
driven dehydrogenation of benzylic alcohols to corresponding functional group(s).
aldehydes or ketones. By using the dual catalyst system consisted
The straightforward transformation of alcohols into the
of eosin Y as photocatalyst and a Ni(II) complex as proton corresponding carbonyl compounds is the dehydrogenation
reduction catalyst, we could dehydrogenate benzylic alcohols to without using any sacrificial oxidant and yielding hydrogen as
aldehydes or ketones with excellent yields under mild conditions. the byproduct. Transition metal-catalyzed systems for this
The sole byproduct is hydrogen gas.
transformation have been developed (Scheme 1a). In most
cases such catalytic systems are based on precious metals such
Transformation of alcohols into corresponding aldehydes or
ketones is an important reaction in organic synthesis both in
4a
4b
as Ru, and Ir, and utilization of earth-abundant metals is
5
still challenging. Jones and coworkers used a nickel(II) catalyst,
1
laboratory and industrial level. Such transformation has been
R
R
TpNi(Q ) (Tp = tris(3,5-dimethylpyrazolyl)borate; Q = 4-
trifluoromethyl-2-hydroxyquinoline) to dehydrogenate
achieved by oxidation of substrate alcohol, and a variety of
oxidation methods have been developed. In most cases, the
5c
alcohols. However, in the above transformation making a C-O
linkage from a C-H and an O-H bond with loss of a hydrogen
2
oxidation processes use stoichiometric amount of oxidants,
2
a
2b
such as metal oxides, methyl sulfoxides, hypervalent
2
molecule (H ) is thermodynamically unfavorable, and has to be
iodine,2 and peroxides, that leads to low atom economy
and possible generation of toxic wastes. In this context,
utilization of oxygen or air as the terminal oxidant shows great
promising from both economic and green chemistry
viewpoints, since oxygen is cheap, and the oxidation produces
water as the by-product. However, molecular oxygen in
ground state is inactive in the reactions with organic
substrates. One approach to achieve the oxidation with oxygen
as oxidant is photosensitization that transforms molecular
oxygen into active species such as singlet oxygen or superoxide
c-d
2e
carried out under harsh conditions. We have developed an
environmentally benign strategy for dehydrogenation of a
6
variety of substrates. Our approach combines a photocatalyst
with a proton reduction catalyst to create a dual catalyst
system. The former catalyst uses light energy as the driving
Previous work:
a. Thermal dehydrogenation
OH
O
[
Ru], [Ir], [Co], [Ni]...
+
H2
3
anion. However, the selectivity in such oxidation is usually low.
organic solvent, reflux
R
R
Furthermore, in many cases the substrate alcohol may contain
functional group(s) that is sensitive to oxidation. Such group(s)
is often prepared via multistep processes. Utilization of
b. Photocatalyzed dehydrogenation in heterogeneous solution
OH
O
PC with co-Catalyst
Visible light, rt
R2
R2 + H2
R1
R1
PC: TiO2, CdS, CdSe, C3N4 ...
a. Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences,
Beijing 100190, China. E-mail: chenbin@mail.ipc.ac.cn
co-Catalyst: Pt, Ni, NiCl , NiP...
2
c. this work: Photocatalyzed dehydrogenation in homogeneous solution
OH
b.
State Key Laboratory of Heavy Oil Processing, School of Chemical Engineering,
O
China University of Petroleum, Qingdao, Shandong 266555, China.
E-mail: yangxiujie@upc.edu.cn
Key Laboratory of Colloid and Interface Chemistry of Ministry of Education,
Shandong University, Jinan, Shandong 250100, China.
School of Future Technology, University of Chinese Academy of Sciences, Beijing
EY, Ni-MPA
R2
R2
+ H2
aqueous solution
Green Light, rt
c.
R1
R1
d.
Scheme 1 Methods of the dehydrogenation of benzylic alcohol
1
00049, China.
Electronic Supplementary Information (ESI) available: [The experimental
procedure are provided]. See DOI: 10.1039/x0xx00000x
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Journal Name
[
a]
Table 1. Optimization of the photocatalytic reaction .
View Article Online
O
DOI: 10.1039/C8GC03828G
OH
EY, hv
H
H
+ H2
Ni-MPA, H O, rt
2
yield of
EY
mol%)
Ni-MPA
(mol%)
yield of
H
2
Entry
pH[b]
benzaldehyde
[
d]
(
[
c]
1
2
0
1
1
1
0.5
2
3
5
1
1
7.5
0
8.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
8.5
7.0
10.0
12.0
0
0
0
0
0
0
[
e]
3
4
5
6
7
8
9
7.5
7.5
7.5
7.5
7.5
7.5
2.5
5
89
52
86
83
38
36
54
26
54
41
98
56
96
95
37
35
59
30
51
43
Figure 1 Time profile of the photocatalytic dehydrogenation of benzyl alcohol.
(
2
Condition: 0.2 mmol benzyl alcohol; 7.5 mol% Ni-MPA; 1 mol% EY; 5 mL H O;
LED green light.)
dehydrogenation processes in detail by using transient
absorption technique.
1
1
1
1
0
1
2
3
The photocatalytic dehydrogenation of benzylic alcohols was
carried out in oxygen-free aqueous or organo-aqueous
solutions under argon atmosphere at room temperature. A 5
mL aqueous solution composed of the substrate (0.2 mmol, 40
mM), Ni-MPA catalyst (0.015 mmol nickel acetate and 0.15
mmol 3-mercaptopropionic acid, 3 mM) and the photocatalyst
(EY, 0.002 mmol, 0.4 mM) in a Pyrex tube was irradiated with a
1
1
1
7.5
7.5
7.5
[
2
a] Benzyl alcohol: 0.2 mmol; 5 mL H O; 1 atm Ar; LED (green light, 525 nm, 3W);
reaction time: 24 h. [b] The pH of the solution was adjusted by NaOH aqueous
solution. [c] Yields of benzaldehyde were determined by H NMR (CDCl
Yields of H were determined by GC analysis. [e] Without irradiation.
2
1
3
). [d]
3
W green LED lamp (λ= 525 nm). The progress of the reaction
force for the dehydrogenation, while the later catalyst may was monitored by the detection of the generated hydrogen
capture electrons from the substrate(s) and/or reaction with a gas chromatography (Figure S1). After irradiation, the
1
intermediates to reduce the protons eliminated from the product was isolated and identified by H NMR. Table 1 shows
substrate(s) into molecular hydrogen. Under mild conditions the results with benzyl alcohol as the model substrate. After
the dual catalyst system may afford dehydrogenation product 24 h of irradiation, 89% yield of the desired product
with excellent yields and equivalent amount of H as sole benzaldehyde was obtained (Table 1, entry 4). Beside
2
byproduct. Many reports have successfully utilized such benzaldehyde and hydrogen, no other product was detected.
strategy to carry out the dehydrogenation of alcohols. Several The photocatalyst, the catalyst and irradiation are all
7
dual catalysts of noble metal deposited on semiconductor
such as Pt/TiO
benzaldehyde production and hydrogen evolution. Xu and
necessary for efficient alcohol transformation and hydrogen
2
, Pt/CdS, have been adopted in simultaneous evolution (Table 1, entries 1-3).
8
a
Three factors may have effect on the photocatalytic
Xiao used in situ photodeposition Ni/CdS nanoparticle to dehydrogenation efficiency. First, the photocatalytic
8
b
dehydrogenate alcohols. Wu used 3-mercaptopropionic acid dehydrogenation is very dependent on the amount of the
MPA) capped CdSe quantum dots as photocatalyst, and photocatalyst (Table 1, entries 4-8). Experiments showed that
nickel(II) chloride as proton reduction catalyst to perform the during irradiation the absorption spectrum of the sample in
(
8
c
dehydrogenation reaction of alcohols. Reisner et al. also visible light region did not change, suggesting the
achieved this process with a cyanamide surface functionalized photocatalyst is stable. The sample with photocatalyst in low
melon-type
bis(diphosphine) (NiP) catalyst system in heterogeneous solution. Therefore, longer irradiation time is needed to obtain
solution (Scheme 1b).
high conversion (Table 1, entry 5). On the other hand, for the
carbon
nitride
and
molecular
Ni(II) concentration cannot absorb all of the light reaching the
Here we report an example for photocatalytic sample with very concentrated photocatalyst, the
dehydrogenation of alcohols in homogeneous solutions. We photocatalyst on the surface of the reaction vessel would
employed a commercially available organic dye eosin Y (EY) as shield the light through the solution, which diminishes the
the photocatalyst to initiate the reaction, and a simple photocatalytic dehydrogenation (Table 1, entries 6-8).
complex (Ni-MPA)9 assembled from Ni(II) ion with 3- Secondly, concentration effect of Ni-MPA complex on the
mercaptopropionic acid as the proton reduction catalyst to photocatalytic dehydrogenation was also evaluated (Table 1,
capture the electrons and protons eliminated from the OH entries 4, 9-10), and 3 mM Ni-MPA complex (7.5 mol%) was
group and the α-C-H of the substrate alcohol. Irradiation of the found to be the best, indicating the proton reduction catalyst
sample solution in water with visible light at ambient condition has a greater impact on the reaction. Third, the pH value of the
led to the dehydrogenation of the alcohol into the desired solution is crucial for efficient photocatalytic dehydrogenation,
carbonyl compound, accompanying with hydrogen production and the optimal pH value is 8.5 (Table 1, entries 4, 11-13). This
in a quantitative yield (Scheme 1c). The homogeneous system is probably due to the fact that the proton reduction catalyst
allows us to study the dynamic of the photocatalytic
(Ni-MPA complex) can only be maintained stable at pH 8.5.
2
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H2
View Article Online
DOI: 10.1039/C8GC03828G
H+
e-
3EY*
Ni(II)
R
e-
Ni-MPA
Catalysis
hv
EY
R
Photo
Catalysis
Ph
O
Ni(III)-H-
5
EY
R
Ni(I)
H+
e-
H+
R
R
Ph
OH
Ph
OH
Ph
OH
Ph
OH
H
H
1
2
3
4
Scheme 2 Proposed mechanism for photocatalytic dehydrogenation of benzylic
alcohols.
3
7
-1 -
rate constant from benzyl alcohol to EY* to be 3.3 × 10 M ·s
.
1
The triplet EY was even more significantly quenched by Ni-
MPA. When adding Ni-MPA to the EY solution, the absorption
of EY* at about 560 nm was immediately observed and
gradually replaced by a new absorption at 460 nm (Figure 2c)
CN (3:2); (e) kinetic trace at 560 nm obtained on laser with a lifetime of 139 μs (Figure 2g), which is attributed to the
-
5
-5
3
Figure 2 Transient absorption spectra of (a) EY (1×10 M); (b) EY (1×10 M) and
-3
-5
-5
benzyl alcohol (BA, 1×10 M); (c) EY (1×10 M) and Ni-MPA (5×10 M); (d) EY
-3
-5
(
1×10-5 M), benzyl alcohol (1×10 M) and Ni-MPA (5×10 M) with 532 nm light in
de-aerated H O-CH
photolysis of EY (1×10 M); (f) kinetic trace at 560 nm obtained on laser
2
3
-5
+• 10
absorption of EY radical cation (EY ). Again, the decay of
photolysis of EY (1×10- M) and Ni-MPA (5×10 M); (g) kinetic trace at 460 nm
5
-5
obtained on laser photolysis of EY (1×10- M) and Ni-MPA (5×10 M); (h) kinetic
5
-5
3
EY* and the rise of EY+• occurred at the same time scale,
-5
trace at 460 nm obtained on laser photolysis of EY (1×10 M), benzyl alcohol
3
indicating electron transfer from EY* to Ni-MPA. At Ni-MPA
-5
(
1×10-3 M) and Ni-MPA (5×10 M)
-5
3
concentration of 5.0 × 10 M, the EY* lifetime was shortened
from 145 μs to 2.3 μs (Figure 2f). The rate constant of the
When employing 1 mol% of the photocatalyst, 7.5 mol% Ni-
MPA complex and performing the photocatalytic reaction in
solution pH of 8.5, complete conversion and excellent
selectivity can be achieved in a reasonable irradiation time.
Figure 1 shows the time profile of the photocatalytic
dehydrogenation of benzyl alcohol. Evidently, benzaldehyde
and hydrogen were produced simultaneously during
irradiation of the sample.
3
electron transfer from EY* to Ni-MPA was calculated to be 8.6
9
-1 -1
×
10 M ·s . This quenching constant is ca. 260 times of that
by benzyl alcohol. In our photocatalytic experiments the
concentration of benzyl alcohol used was only 13 times greater
3
than that of Ni-MPA. We infer that EY* is mainly quenched by
+•
Ni-MPA via electron transfer, and the generated EY can
undergo electron transfer with benzyl alcohol producing the
alcohol radical cation and ground EY. As evidenced in Figure
The photocatalytic dehydrogenation of benzylic alcohols has
been proposed to originate from photoinduced electron
transfer. We first examined the quenching of EY excited states
by the substrate benzyl alcohol and the catalyst Ni-MPA. The
fluorescence of EY in water/acetonitrile solution (3:2, v/v) can
only be slightly quenched by benzyl alcohol and Ni-MPA at
high quencher concentration. It is therefore reasonable to
consider that the photoinduced electron transfer come from
2
d, when adding benzyl alcohol to the mixture solution of EY
+•
and Ni-MPA, the EY absorption around 460 nm is significantly
quenched by benzyl alcohol. In the presence of 1.0 × 10 M
benzyl alcohol, the lifetime of EY is shortened from ca. 139 μs
to 1.9 μs (Figure 2h). The rate constant of the electron transfer
from benzyl alcohol to EY was calculated to be ca. 5.2 × 10
-3
+•
+•
8
-1 -1
M ·s .
Based on the above results, a proposed mechanism for the
photocatalytic dehydrogenation of benzylic alcohols is
3
the EY triplet state ( EY*), due to the fast decay of the EY
singlet excited state via intersystem crossing. Then, we carried
out the flash-photolysis study to shed more light on the
3
summarized in Scheme 2. The electron transfer from EY* to
+•
Ni-MPA (Ni(II)) results in the formation of EY and a reactive
3
process of the reaction. The EY* absorption with λmax at 560
9
a
9
-1 -1
reduced Ni(I) species with rate constant of 8.6 × 10 M ·s .
nm were observed when the EY aqueous solution was excited
with laser at 532 nm (Figure 2a). Upon addition of benzyl
+•
Subsequently, EY accepts an electron from benzyl alcohol (1)
8
-1 -1
with rate constant of 5.2 × 10 M ·s to regenerate EY and
produce benzyl alcohol radical cation (2, Scheme 2). Further, 2
eliminates a proton leading to the formation of benzyl alcohol
radical (3). Protonation of the Ni(I) species provides Ni(III)-H .
Then, 3 gives an electron to Ni(III)-H species and eliminates
3
alcohol into the aqueous solution of EY, the EY* absorption at
5
4
60 nm was gradually replaced by a new absorption centred at
06 nm (Figure 2b), which is assigned to the absorption of EY
-
11
-•
10
radical anion (EY ) according to the literatures. The decay of
-
3
-•
EY* and the rise of the EY occur simultaneously, suggesting
the second proton, yielding product benzaldehyde. Upon
3
the electron transfer from benzyl alcohol to EY*. The lifetime
of EY* was shortened from 145 μs in the absence of benzyl
-
accepting electron, Ni(III)-H can react with a proton eliminated
3
2
from 4 to release H eventually and regenerate the Ni(II)
alcohol (Figure 2e) to 25 μs at benzyl alcohol concentration of
1
1
catalyst. The elimination of a proton from 2 to generate 3
and that from 4 to produce 5 in the above proposed
-3
1
.0 × 10 M (Figure S2). This in turn gives the electron transfer
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Journal Name
[
a]
OH OH
O
OH
Table 2 Photocatalytic Reaction of Benzyl Alcohol Derivatives .
View Article Online
O
EY, Green light
DOI: 10.1039/C8GC03828G
OH
+
H
2
EY, Green light.
Ni-MPA, H O, r.t.
2
R2
R2 + H2
Ni-MPA, Solvent, rt
24 h
Conversion: 70.4%
Selectivity: 100%
R1
R1
Yields[b]
100
> 99
100
72[d]
100
100
100
100
100
100
0
H
2
[c]
Entry
R
1
R
2
Solvent
t/h Conv.
Scheme 3 Selective oxidation of 1-methyl-3-phenyl-propylene glycol.
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
O
24
CN 24
CN 24
24
89
94
> 99
68
89
90
100
100
99
76
0
98
of 72% reveals the application value of our method in organic
synthesis (details are shown in Supporting Information).
CH
3
O/CH
O/CH
O
3
99
OCH
3
3
> 99
90
CH(CH
3 2
)
Conclusions
Cl
O/CH
O/CH
O
3
CN 24
CN 24
24
94
In summary, we have successfully developed a noble-metal
free dual catalyst system for photocatalytic dehydrogenation
of benzylic alcohols in homogeneous solutions. Employing EY
as the photocatalyst and Ni-MPA as the proton reduction
catalyst, benzylic alcohols could be dehydrogenated to
aldehydes or ketones with high selectivity, and an equivalent
Br
3
92
OH
99
OCH
H
3
CH
CH
3
2
O/CH
3
CN 24
> 99
98
CH
3
O
24
amount of
H
2
is generated as the only byproduct.
Furthermore, the photocatalytic dehydrogenation shows high
selectivity to benzylic alcohols. The homogeneous catalyst
system makes it possible to study the detail dynamic of the
reaction processes by using flash- photolysis technique and the
results revealed that the photocatalytic dehydrogenation of
benzylic alcohols was initiated by single electron transfer from
1
1
1
0
1
2
H
Phenyl
O
30
81
3-phenyl-1-propanol H
2
O/CH
3
CN
24
24
trace
0
4-phenyl-2-butanol
H
2
O/CH
3
CN
0
0
[
a] Light source: LED (green light, 525 nm, 3W); room temperature; substrate
amount: 0.2 mmol; EY: 1 mol%; Ni-MPA: 7.5 mol%; pH=8.5; 1 atm Ar; 5 mL
solution. [b] Yields of aldehydes were based on the consumed starting material
3
+•
EY* to Ni-MPA generating EY , that can oxidize benzylic
alcohol to produce alcohol cation radical and regenerate EY.
and determined by 1H NMR (CDCl
). [c] Yields of H were determined by GC The dehydrogenation involved multiple electron transfer and
3
2
analysis. [d] The only byproduct is the corresponding carboxylic acid.
proton transfer processes, and Ni-MPA played a decisive role
in hydrogen production.
mechanism were further supported by the photocatalytic
dehydrogenation of benzyl alcohol in deuterated water. We
performed the photocatalytic dehydrogenation of benzyl
alcohol in D O solution. D instead of H was yielded as the
2 2 2
only byproduct (Figure S3) without alternation of the reaction
efficiency. This observation suggests that the released protons
Conflicts of interest
There are no conflicts to declare.
from the substrate are quickly exchanged with D
2
O, and the
Acknowledgements
yielding deuterons are reduced to D
2
.
This work was supported by the Ministry of Science and Technology
of China (2017YFA0206903), the National Natural Science
Foundation of China (21390404 and 21502227), the Strategic
With the understanding of the reaction mechanism, the
photocatalytic dehydrogenation was further extended to the
benzyl alcohol derivatives. Table 2 summarizes the results for a
variety of substrates. For the substrates with low solubility in Priority Research Program of the Chinese Academy of Science
water, we used organic-water mixture solvents. Evidently, (XDB17000000), the Key Research Program of Frontier Sciences,
almost for all benzyl alcohol derivatives (Table 2, entries 1-10) CAS (QYZDY-SSW-JSC029), State Key Laboratory of Heavy Oil
the photocatalytic dehydrogenation is very efficient, and the Processing (SLKZZ-2017009).
yields based on the consumed starting materials are excellent.
In contrast, aliphatic alcohols can not undergo such
photocatalytic dehydrogenation, and no oxidation products Notes and references
were detected (Table 2, entries 11-12). In order to further
1
2
(a) T. Mallat and A. Baiker, Chem. Rev. 2004, 104, 3037; (b)
S. Caron, R. W. Dugger, S. G. Ruggeri, J. A. Ragan and D. H. B.
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investigate the dehydrogenation selectivity of benzyl alcohols,
the photocatalytic dehydrogenation of polyhydroxy
a
compound, 1-methyl-3-phenyl-propylene glycol was carried
out and gave 100% selectivity of benzyl alcohol
dehydrogenation product (Scheme 3).
A
gram-scale
photocatalytic dehydrogenation of benzylic alcohol with yield
4
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3
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