Visible-light-induced selective aerobic oxidation of sp3 C-H bonds catalyzed by a heterogeneous AgI/BiVO4 catalyst

Article abstract of DOI:10.1039/c9gc03727f

An efficient oxidation of sp³ C-H bonds to esters and ketones has been developed using AgI/BiVO₄ as the photocatalyst and O₂ as the oxidant in water. Various substrates can be transformed into the desired esters and ketones in moderate to good yields. The synthetic utility of this approach has been demonstrated by gram-level experiments and consecutive oxidation experiments. A plausible mechanism has been proposed.

Full text of DOI:10.1039/c9gc03727f

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DOI: 10.1039/C9GC03727F
COMMUNICATION
3
Visible-Light-Induced Selective Aerobic Oxidation of sp C-H bonds
Catalyzed by Heterogeneous AgI/BiVO catalyst
4
Li-ya Jiang,a, c Jing-jing Ming, Lian-Yue Wang, Yuan-yuan Jiang, Lan-hui Ren, * Zi-cheng Wang,
Wen-chen Cheng
a, c
b
a
a,
a
Received 00th January 20xx,
Accepted 00th January 20xx
a
DOI: 10.1039/x0xx00000x
3
1. Previous studies
An efficient oxidation of sp C-H bonds to esters and ketones has
been developed using AgI/BiVO as photocatalyst and O as oxidant
(1) Lee, 2019
O
R
R' Eosin Y (5 mol%), K2CO3 (1.5 equiv), O2 (1 atm)
N
R
R
R'
4
2
N
NaN3 (2.0 equiv), DMSO (2 mL), 23 o
C
in water. Various substrates can be transformed into the desired
esters and ketones in moderate to good yields. The synthetic utility
of this approach has been demonstrated by gram-level experiments
and consecutive oxidation experiments. A plausible mechanism has
been proposed.
R
5
W Green LEDs,16 h
0
.5 mmol
(2)
Zhao and Muhler, 2019
O
O
R'
or
CdS (50 mg), NHPI (10 mg), MeCN (3 mL)
O2 (1 bar), 300 W visible light, 60 oC, 6 h
R'
R
R
or
0.2 mmol; R = -CH3; R' = -H, -CH3
(3) Das, 2018
O
R
N
Rose bengal (3-6 mol%), DBN (1.5-2.5 equiv), DMF (1 mL)
O2 balloon, 12 W blue LED, rt, 16-48 h
R
N
Oxygen-bearing groups (hydroxyl and carbonyl) are important
structures found in many bioactive molecules and
pharmaceutical agents, such as arpromidine, pheniramine,
chloropheniramine, triprolidine, doxylamine, isochromanones,
0
.3 mmol
(4) Cong, 2017
O
R'
R'
Eosin Y (2 mol%), TiO2-P25 (20 mg), acetone (4 mL)
Air balloon, 160 W blue LED, rt, 12 h
O
O
R
R
0
.1 mmol
1
3
etc. The direct oxidation of the sp C-H bonds is an attractive
way to construct oxygen-bearing groups. Traditionally,
stoichiometric quantities of hazardous oxidants, such as high-
valent metal salts, halides and peroxides, were used for these
chemical transformations resulting in large amounts of
(
5) Lei, 2015
O
R'
Acr+-MesClO4- (1 mol%), MeCN (2.0 mL), O2
R
R'
R
3
W Blue LEDs, 12 h
0
.5 mmol
(6) Oisaki and Kanai, 2014
O
R2
Ru(bpy)3Cl2 6H2O (1.5 mol%)
Cu(MeCN)4OTf (1.0 mol%)
O2 (1 atm), white LED (VIS)
MeCN (0.05 M), rt
R2
R1
R3
R1
R3
2
unwanted by-products. Therefore, development of cleaner
3
and cost-effective methodology for the oxidation of sp C-H
2
. This work
bonds is highly important and significant.
O
H
H
3
R'
R'
Heterogeneous reusable catalyst
Direct oxidation of the sp C-H bonds using environmentally
R
O
R
O
2
0wt% AgI/BiVO4 (15 mg), H2O (2 mL)
Visible light, O2 as oxidant, H2O as solvent
No additive agent
or
or
O
O2 balloon, 25 oC, 16 W white LED
benign oxidants (O
straightforward access to oxygen-bearing groups formation
which has advantages of easy accessible raw materials, H O as
2
or air) under mild conditions offers
H
H
Mild conditon
R1
R2
R1
R2
Good functional group tolerance
2
7 examples
2
up to 92% yield
by-product, high atom economy over the alternative
stoichiometric oxidation protocols. However, owing to the
triplet ground state structure, molecule oxygen is relatively
inert under mild conditions, especially towards inert C-H bonds.
3
Scheme 1. Visible-light-induced photocatalytic oxidation of the sp C-H bonds using O
or air as oxidants.
2
conditions (high temperature and pressure). Thus, the
development of simple and mild methods for selective
oxidation of the sp C-H bonds is highly desirable.
Due to its ecocompatibility, easy availability, safe handling
and everlasting abundance as an energy source, visible-light has
received central attentions from various research groups.
Visible-light-induced photocatalysis have been developed into
powerful tools for the functionalization of organic molecules
3
Although the oxidation of the sp C-H bonds forming oxygen-
bearing groups using molecular oxygen has been studied, to this
day, many promising results were obtained using transition-
3
metal catalysts3 or non-metal catalysts under relatively harsh
-8
9
a.
College of Pharmacy, Weifang Medical University, Weifang 261053, P. R. China;
E-mail: Ren_lanhui@163.com.
b.
Dalian Institute of Chemical Physics, the Chinese Academy of Sciences and
1
0
under mild conditions. The molecular oxygen had been used
as oxidant in many visible-light-induced catalytic oxidation of
the sp C-H bonds. Many exciting results were obtained
Dalian National Laboratory for Clean Energy, DNL, 457 Zhongshan Road, Dalian,
1
16023, P. R. China.
Li-ya Jiang and Jing-jing Ming contributed equally to this work.
c.
3
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
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COMMUNICATION
Journal Name
involving semiconductors or organic dyes or organic small hexagonal β-AgI (JCPDS File No. 09-0374). The XRD spectra of
View Article Online
DOI: 10.1039/C9GC03727F
4
molecules photocatalytic oxidation systems, such as eosin 20wt% AgI/BiVO presented a good coexistence of AgI and
1
1a
11b
11c,
11h
Y/K
2
CO
3
/NaN
3
,
CdS/NHPI,
DDQ/TBN/AcOH,
rose bengal/DBN,
BiVO
4
. The result showed that AgI was successfully introduced
1
1d
11e
14
hydroquinone/CuCl
2
2
• 6H O,
4
eosin into the BiVO systems.
1
1f
,11g riboflavin tetraacetate (RFT)/Fe
Y/TiO
2
-P25
,
CBr
4
A
B
1
1i
+
-
complex, 9-mesityl-10-methylacridinium ion (Acr -MesClO
4
1
1j
11k
),
2-chloroanthraquinone (2-Cl-AQN), Ru(bpy)
3
Cl
2 2
• 6H O/
Some photocatalytic
1
1l
11m
Cu(MeCN)
4
OTf
3 4
and g-C N /NHPI.
oxidation systems are listed in Scheme 1-1.
For years, we have been studying the clean oxidation of the
3
sp C-H bonds, and we have reported eco-friendly protocols for
3
the oxidation of the sp C-H bonds adjacent to pyridine
moiety.9
h, 12
In line with our ongoing interest in green chemistry
C
D
and uninterruptible studies on the selective aerobic oxidation
3
of sp C-H bonds, herein, we present an efficient oxidation of
3
sp C-H bonds to esters and ketones (Scheme 1-2). This
transformation features a visible-light-induced heterogeneous
reusable AgI/BiVO
4 2
catalyzed reaction with O as green oxidant
and H O as solvent. The white LED lamp bought from market is
2
used as light source. Notably, this transformation do not require
any additive. This method is efficient and applicable to a broad
range of substrates with good functional group compatibility.
As we know, many visible-light-induced catalytic oxidation
4
Figure 1. SEM (A and B) and TEM (C and D) images of the 20wt% AgI/BiVO .
3
of the sp C-H bonds that the oxidant is molecular oxygen
20wt% AgI/BiVO
.
.
.
4
4
.
..
.. .
. .
•
-
11
.
.
.


involve superoxide radical anion (O
literature,13 AgI/BiVO
was excited by visible light to generate
the photoelectrons which could reduce the dissolved molecular
2
). According to the
.
.
4
•-
oxygen on the surface of the AgI/BiVO
4
material forming O
could damage cell walls and resulted in antimicrobial
effects.
2
.
.
BiVO
•-
O
2
3
We envisioned that the oxidation of the sp C-H bonds might
be accomplished via a visible-light-induced process using

AgI
AgI/BiVO
effort to test this hypothesis, AgI/BiVO
prepared by an in situ deposition-precipitation procedure (see
Supporting Information) reported in the literature. Scanning
electron microscope (SEM), transmission electron microscope
4
as catalyst and molecular oxygen as oxidant. In an
materials were
10
20
30
40
Theta (degree)
4
Figure 2. The XRD patterns of pure AgI, pure BiVO and 20wt% AgI/BiVO .
50
60
70
4
2
4
1
4
Survey
O 1s
V 2p
Bi 4f
A
Ag 3d
Ag 3d3/2
73.7 eV
Ag 3d5/2
B
367.9 eV
I 3d
3
(
TEM), X-ray diffraction (XRD) and X-ray photoelectron
spectroscopy (XPS) were used for characterizing the AgI/BiVO
materials (Figure 1-3). The morphological evolution of 20wt%
AgI/BiVO material was examined by SEM (Figure 1A and 1B)
and TEM (Figure 1C and 1D) analysis. Both two types of
morphologies (AgI and BiVO ) were found in 20wt% AgI/BiVO
nanocomposite. A large number of irregular AgI nanoparticles
distributed on the surface of BiVO , indicating that AgI and
BiVO were well combined with each other. The sizes of AgI
particles were about 13.08-95.94 nm. The length of BiVO
particles were about 1.03-2.06 um, and the width were about
Ag 3d C 1s
4
8
00
600
400
200
0
380 378 376 374 372 370 368 366 364 362
Binding Energy (eV)
Binding Energy (eV)
4
Bi 4f5/2
I 3d
I 3d3/2
C
Bi 4f
D
I 3d5/2
Bi 4f7/2
159.0 eV
630.9 eV
164.3 eV
6
19.6 eV
4
4
6
40
635
630
625
620
615
170 168 166 164 162 160 158 156 154
Binding Energy (eV)
Binding Energy (eV)
4
V 2p
E
O 1s
O 1s
529.4 eV
F
4
V 2P3/2
V 2P1/2
24.2 eV
515.9 eV
4
5
5
26 524 522 520 518 516 514 512 510 508 538
Binding Energy (eV)
536
534
532
530
528
526
0
.15-0.77 um.
Binding Energy (eV)
Figure 3. The XPS spectras of 20wt% AgI/BiVO
d, (D) Bi 4f, (E) V 2p and (F) O 1s.
4
sample: (A) survey scan, (B) Ag 3d, (C) I
4
The phase compositions of pure AgI, pure BiVO and 20wt%
3
AgI/BiVO
4
were characterized by X-ray diffraction (XRD). As
o
showed in Figure 2, the peaks of BiVO
4
at 2θ values of 18.61 ,
o
o
o
o
o
o
o
o
4
The survey XPS spectrum of 20wt% AgI/BiVO (Figure 3A)
2
5
8.90 , 30.56 , 35.24 , 39.80 , 42.50 , 45.37 , 46.73 , 49.89 ,
o
o
o
showed that the predominant elements of the samples were C,
Bi, V, O, Ag and I. The carbon element was ascribed to the
adventitious hydrocarbons from the XPS instrument itself. In
the XPS spectrum of Ag 3d, two peaks were observed at 367.9
3.36 , 55.27 and 59.27 could be attributed to (011), (121),
(
(
040), (002), (211), (051), (231), (240), (202), (161), (321) and
123) crystal planes of BiVO (JCPDS File No. 14-0688),
4
respectively. The characteristic peaks of AgI were ascribed to
2
| J. Name., 2012, 00, 1-3
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Journal Name
and 373.3 eV, which originated respectively from Ag 3d5/2 and 20wt% AgI/BiVO
COMMUNICATION
O as solvent, and O
4
as catalyst, 2 mL H
2
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2
o
DOI: 10.1039/C9GC03727F
Ag 3d3/2 (Figure 3B). The two peaks could not been divided into balloon as oxidant under a 16 W white LED at 25 C.
+
four peaks, indicating only the existence of Ag among
Table 1. Optimization of Reaction Conditions.
4
AgI/BiVO composites. Figure 3C presented that the high-
O
resolution XPS spectrum of I 3d, the peaks appeared at binding
energy (BE) of 619.6 and 630.9 eV were ascribed to I 3d5/2 and I
H
H
AgI/BiVO , solvent (2 mL), 12 h
O
4
O
3
d3/2, respectively. The peaks located at 159.0 and 164.3 eV
o
O balloon, 25 C, 16 W white LED
2
were belonged to Bi 4f7/2 and Bi 4f5/2 (Figure 3D), respectively.
Two obvious peaks situated at banding energies of 515.9 and
1
a, 1 mmol
2a
a
Entry
1
Cat. (mg)
20wt% AgI/BiVO
20wt% AgI/BiVO
-
Solvent
MeCN
MeCN
MeCN
MeCN
H O
2
EtOH
MeOH
Yield (%)
28
5
24.2 eV were clearly perceived in Figure 3E, which should
4
4
(10)
(10)
belong to V 2p3/2 and V 2p1/2, respectively. The peaks at 529.4
eV in Figure 3F were attributed to O 1s. The XPS results were
b
2
n. r.
n. r.
n. r.
78
35
10
16
18
5
16
33
n. r.
10
41
66
79
67
63
83
3
4
further to confirm the successful combination of BiVO and
c
4
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
4
4
4
4
4
(10)
(10)
(10)
(10)
(10)
1
3, 14
AgI.
We employed commercially isochroman (1a) as model
substrate in the presence of 10 mg 20wt% AgI/BiVO using
atmosphere under a 16 W white LED at
5
6
7
8
9
4
MeCN as solvent in O
2
CH
2
Cl
2
o
2
5 C for 12 h, giving the desired product (1-isochromanone, 2a)
20wt% AgI/BiVO (10)
Toluene
DMF
1,4-Dioxane
DCE
4
in 28% yield (Table 1, entry 1). No desired product was observed
with neither AgI/BiVO nor light which indicated that the
AgI/BiVO and light were essential to this process (Table 1,
entries 2 and 3). The reaction did not happen in the absence of
10
11
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
4
4
4
4
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(10)
(15)
4
1
1
1
1
1
1
1
1
2
2
3
4
5
6
7
8
9
0
4
EA
1wt% AgI/BiVO
5wt% AgI/BiVO
4
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
O
2
(Table 1, entry 4). Various reaction parameters were tested
o
4
in oxygen atmosphere under a 16 W white LED at 25 C for the
optimization of reaction parameters including solvents,
catalysts, and reaction time to identify the optimal reactions
conditions. First, as shown in Table 1, different solvents, such as
10wt% AgI/BiVO
40wt% AgI/BiVO
60wt% AgI/BiVO
80wt% AgI/BiVO
20wt% AgI/BiVO
4
4
4
4
4
H
(
2
O, EtOH, MeOH, CH
DMF), 1,4-dioxane and 1,2-dichloroethane (DCE) were
screened, yielding 1-isochromanone (2a) in 78%, 35%, 10%,
2 2
Cl , toluene, N, N-dimethylformamide
21
22
20wt% AgI/BiVO (20)
4
H O
2
84
81
61
75
78
82
72
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
20wt% AgI/BiVO
4
4
4
4
4
4
(25)
(15)
(15)
(15)
(15)
(15)
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
d
1
1
6%, 18%, 5%, 16% and 33% isolated yields, respectively (Table
, entries 5-12). This oxidation reaction did not occur in ethyl
23
2
2
2
2
4e
5f
2
acetate (EA) (Table 1, entry 13). H O was found to be more
effective for this transformation which might be because the
water-insoluble characteristic of isochroman led to its
g
6
7
h
a
b
c
d
concentration on the surface of heterogeneous AgI/BiVO
catalyst. Therefore, H O was the best choice for the next
optimization study. Next, a survey of different AgI/BiVO
catalysts (different AgI contens) was carried out, revealing that
0wt% and 40wt% AgI/BiVO catalyzed this process more
efficient than other AgI/BiVO catalysts (Table 1, entries 14-19;
Supporting Information, Table S1, entries 1-7). Considering the
control of AgNO (raw material for AgI/BiVO catalyst) on
account of its easy detonation, we chose the 20wt% AgI/BiVO
4
Isolated yield, n. r. = no reaction. In the dark. Under N
time was 6 h. Reaction time was 8 h. Reaction time was 10 h. Reaction time was
6 h. The oxidant is air (air balloon).
2
atmosphere. Reaction
e
f
g
2
h
1
4
With the optimal conditions established, various ether
2
4
compounds were investigated and the results were summarized
in Scheme 2. Both electron-rich and electron-deficient ether
compounds could be employed efficiently in this photocatalytic
oxidation process, and good to excellent yields of the
corresponding esters were obtained. We studied the direct
oxidation of 7-chloroisochromane under this photocatalytic
oxidation process affording the 7-chloroisochroman-1-one (2b)
in 92% yield (Scheme 2, 2b). Isochroman-1-one compounds
were important structure found in a number of natural products
and bioactive molecules. A good range of benzyl methyl ethers
with ortho, meta, para or multi functional groups were well
tolerated in this photocatalytic oxidation process (Scheme 2, 2c-
4
3
4
4
as target catalyst to perform the oxidative reaction.
Subsequently, brief screening of the loadings of 20wt%
AgI/BiVO
4
provided a satisfactory oxidation product yield using
(Table 1, entry 20). Increasing of the
1
5 mg 20wt% AgI/BiVO
4
catalyst-loading to 20 or 25 mg did not further improve the
product formation (Table 1, entries 21 and 22). Different
reaction times were examined. Increase of reaction time to 16
h did not improve the product formation, and decrease of
reaction time led to the decrease of the product yield (Table 1,
entries 23-26). It was worth noting that acceptable product
yield was obtained when the reaction was performed under air
2
m). Under the optimal conditions, benzyl methyl ether was
converted into methyl benzoate (2c) in 87% yield (Scheme 2,
c). Chlorine substituent had slight influence on the oxidation
reaction, and the yields of products decreased (Scheme 2, 2c-
2
(Table 1, entry 27; Supporting Information, Table S1, entry 8).
2
f). By comparison, substrates bearing bromine substituent
Finally, the optimized catalytic system was obtained, viz., 15 mg
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Journal Name
3
provided higher yields of methyl benzoates (Scheme 2, 2g and mentioning that the sp C−H oxidation on ibuprofen methyl
2
View Article Online
h). It might be due to the stronger electronegativity of ester (drug molecule) occurred on the lDeOssI: 1h0in.1d03e9r/eCd9GbCe0n3z7y2l7iFc
chlorine. Substrates bearing methyl substituent and methoxy position with the formation of the ketone in 32% yield (Scheme
substituent followed similar trend, and afforded ester 3, 4f).
compounds in 89% and 91% yields, respectively (Scheme 2, 2i
and 2j). The substrates which contained multiple chemically corresponding aromatic aldehydes using O
In organic chemistry, the oxidation of benzyl alcohols to the
as oxidant is one of
2
different C-H bonds oxidative positions were tested, and the the most important transformations and of great significance
sole products were benzoates (Scheme 2, 2a, 2b and 2i). The both in theory and practice. When the dosage of benzyl alcohols
electron-withdrawing group such as -CF could afford the were reduced to 0.1 mmol, the oxidation of benzyl alcohols was
3
desired ester product in moderate yields (Scheme 2, 2k). Ether performed smoothly in our system with moderate yields of the
bearing two substituents afforded the corresponding benzoates corresponding aromatic aldehydes (Scheme 4).
in good yields (Scheme 2, 2l and 2m). Benzyl ethyl ethers and
O
benzyl ether were oxidized smoothly by this photocatalytic
20wt% AgI/BiVO4 (15 mg), H2O (2 mL)
oxidation protocol, for example, benzyl ethyl ether, 1-chloro-4-
ethoxymethyl)benzene and benzyl ether underwent this
protocol to afford ester products in 80%, 83% and 78% yields
respectively (Scheme 2, 2n, 2o and 2p). Interestingly, ethers
containing carbon-carbon double bond which was sensitive to
oxidation were transformed to the corresponding esters in 71%,
R
OH
6 W White LED, O2 balloon, 25 oC, 12 h
R
H
1
(
5a-5i, 0.1 mmol
Cl
6a-6i, yield
O
O
O
Cl
O
O
H
H
H
H
H
F
Cl
H3CO
6
a, 48%
6b, 45%
6c, 44%
6d, 42%
6e, 53%
4
2
9%, 69% and 59% yields respectively (Scheme 2, 2q, 2r, 2s and
t). The carbon-carbon double bond was left untouched.
O
H
O
O
O
H3CO
H3CO
H3CO
H
H
H
H
H
O
2
0wt% AgI/BiVO4 (15 mg), H2O (2 mL)
16 W White LED, O2 balloon, 25 oC, 12 h
a-1t, 1 mmol
F3C
R'
R'
R
O
R
O
6f, 51%
6g, 57%
6h, 37%
6i, 40%
1
2a-2t, yield
Scheme 4. Selective Oxidation of Benzyl Alcohols (isolated yield).
O
O
Cl
Cl
Cl
O
O
O
O
O
O
OCH3
OCH3
2d, 71%
OCH3
To highlight the synthetic practicality of the present
protocol, this sp C-H oxidation methodology was evaluated on
2a, 83%
2b, 92%
Br
2c, 87%
2e, 74%
3
O
O
O
O
Cl
Br
OCH3
2g, 80%
H3C
OCH3
2h, 88%
H3CO
OCH3
2i, 89%
gram-scale (10 mmol). The desired products were obtained in
good isolated yields (Scheme 5), indicating that this protocol
was a practical process for the preparation of esters.
OCH3
OCH3
2
f, 72%
2j, 91%
Cl
Cl
O
O
O
O
O
F3C
Cl
OCH3
Cl
OCH2CH3
OCH3
OCH3
OCH2CH3
2o, 83%
H
H
O
Cl
2
0wt% AgI/BiVO4 (150 mg), H2O (20 mL)
2
k, 67%
2l, 74%
2m, 71%
2n, 80%
R'
16 W White LED, O2 balloon, 25 oC, 72 h
R'
O
R
O
R
O
O
O2N
O
O
O
10 mmol
OCH2Ph
p, 78%
OCH3
OCH3
OEt
OCH2Ph
O
2
2q, 71%
2r, 49%
2s, 69%
2t, 59%
O
O
O
Scheme 2. Selective Oxidation of Ether Compounds (isolated yield).
O
OCH3
OCH2Ph
OCH3
H
H
O
20wt% AgI/BiVO4 (15 mg), H2O (2 mL)
2a, 67%
2d, 69%
2q, 63%
2r, 54%
R' 16 W White LED, O2 balloon, 25 oC, 12 h
R
R
R'
Scheme 5. Gram Scale Reactions (isolated yield).
3
a-3g, 0.1 mmol
4a-4g, yield
O
O
O
O
O
Table 2. Leaching experiment and mercury poisoning experiments.
N
CH3
O
H
H
4
a, 31%
4b, 44%
4c, 40%
4d, 42%
4e, 46%
O
20wt% AgI/BiVO4 (15 mg), H2O (2 mL)
16 W white light LED, O2 balloon, 25 oC, time
O
O
CH3
O
H3C
a
Entry
Reaction time
3 h
3 h + centrifugation + 9 h
3 h + mercury + 9 h
Yield (%)
21
CH3
COOMe
CH3
1
2
3
4
20
28
n. r.
4
f, 32%
4g, 41%
Scheme 3. Selective Oxidation of Alkylbenzenes (isolated yield).
Catalyst + mercury, stirring for 3 h; then adding
isochroman, 12 h
Inspired by the reactivity of ether compounds, we
aIsolated yield, n. r. = no reaction.
subsequently studied the oxidation of alkylbenzenes with
AgI/BiVO
When the dosage of alkylbenzenes were reduced to 0.1 mmol,
alkylbenzenes could be oxidized to the corresponding ketone
compounds in moderate yields (Scheme 3). It is worth
o
4
and O
2
under a 16 W white LED at 25 C for 12 h.
The stability of the 20wt% AgI/BiVO
4
catalyst was
certificated by the metal leaching test. The solid catalyst was
removed by centrifugation at 3 h, and the reaction continued
4
| J. Name., 2012, 00, 1-3
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Journal Name
COMMUNICATION
for another 9 h. The yield of 1-isochromanone (2a) did not
increased without 20wt% AgI/BiVO
and 2). The results showed that this oxidation transformation radical anion (O
In the photocatalytic oxidation process, many reactive
View Article Online
+
DOI: 10.1039/C9GC03727F
4
catalyst (Table 2, entries 1 oxygen species, such as photogenerated holes h , superoxide
•−
•
2
), hydroperoxyl radical (HOO ) or hydroxyl
•
14, 15
was a heterogeneous catalytic system. Mercury can react with radicals (HO ), were supposed to be involved.
We
other metals forming alloys (amalgamation) or adsorb on other conducted many quenching tests to recognize reactive
metal surface resulting in the deactivation of metal catalyst. oxidative species (Table 4). After adding 2,2,6,6-tetramethyl-1-
This method is often used to detect whether the reaction occurs piperidinyloxyl (TEMPO), 2,6-di-tert-butyl-4-methylphenol
on the surface of metal particles. When mercury is added into (BHT) or 1,1-diphenylethylene (DPE) to the reaction mixture, no
the reaction system, the reaction is inhibited, indicating that the product was obtained which proved this photocatalytic
target reaction occurs on the surface of metal particles. The oxidation process was radical pathway (Table 4, entries 1-6).
mercury poisoning experiments demonstrated this oxidation Ammonium oxalate (AO), benzoquinone (BQ) and FeSO
4
acted
and HOO were introduced into
the photocatalytic oxidation process, respectively. The results
+
−•
•
reaction occurred on the surface of the 20wt% AgI/BiVO
catalyst (Table 2, entries 1, 3 and 4).
4
as the scavengers for h , O
2
+
−•
•
showed the presence of h , O
2
and HOO (Table 4, entries 7-
to the oxidation mixture
70
60
50
40
30
20
10
0
10). Furthermore, addition of CuCl
2
resulted in no reaction occur which clearly revealed the
involvement of single electron transfer in the photocatalytic
oxidation process (Table 4, entry 11).
Table 3. Control experiments.
O
H
H
O Standard conditions
O
Isochroman
0
1
2
3
4
5
6
a
Entry
Controlled parameter
Standard conditions
UV filter
In the dark
2
Under N atmosphere
Yield (%)
83
Recycle time
1
2
3
4
5
6
7
8
Figure 4. Recycle of 20wt% AgI/BiVO
4
for selective oxidation of isochroman. Reaction
(150 mg), H O (20 mL), O balloon,
6 W white LED, 25 C, 72 h. Isolated yield after column chromatography.
84
condition: isochroman (10 mmol), 20wt% AgI/BiVO
4
2
2
o
n. r.
n. r.
Trace
28
12
24
1
Catalyst is BiVO
Catalyst is AgI
4
As we known, recyclability for catalyst was an important and
essential feature in large-scale catalytic reactions. The 20wt%
AgI/BiVO catalyst could been easily separated from the
reaction mixture by centrifugation or filter, and then washed
with ethanol three times and further reused after drying. In
Catalyst is recycled AgI
Catalyst is AgI (3 mg) + BiVO (12 mg)
4
4
a
Isolated yield, n. r. = no reaction. Standard conditions: isochroman (1 mmol),
o
2
0wt% AgI/BiVO
4
(15 mg), H
2
O (2 mL), O
2
balloon, 16 W white LED, 25 C, 12 h.
4
order to verify the reusability of 20wt% AgI/BiVO catalyst,
consecutive oxidation experiments were performed using
isochroman as substrate on gram-scale under the optimized
catalytic condition. Notably, the 20wt% AgI/BiVO catalyst was
4
successfully recycled up to six times without any significant loss
of activity (Figure 4).
The yield of isochromanone (2a) was almost no change in
the presence of UV cut-off filter which showed the oxidation
reaction was irradiated by visible light (Table 3, entry 2). No
The electron paramagnetic resonance (EPR) measurements
were performed to confirm the reactive oxygen species evolved
in the photocatalytic oxidation process with 5, 5-dimethyl-1-
pyrroline N-oxide (DMPO) as trapping reagent. As presented in
4
Figure 5A, four obvious signals with 20wt% AgI/BiVO in
methanol were obtained which indicated the existence of
−•
DMPO-O
2
under visible light irradiation. This EPR spectra
illustrated that the dissolved molecular oxygen on the surface
product was produced in the absence of light or O
entries 3 and 4) which clearly showed the significance of light
and O in this reaction. Trace amount of 1-isochromanone (2a)
was obtained when 15 mg BiVO was used as catalyst (Table 3,
2
(Table 3,
of the AgI/BiVO
produce superoxide radical anion (O
quadruple peaks of DMPO- OH were not displayed (Figure 5B).
The hydroxyl radicals (HO ) may not be involved in the
4
was reduced by photogenerated electrons to
−•
2
). The characteristic
2
•
4
•
entry 5). While low yield (28%) of 1-isochromanone (2a) was
obtained in the cases of AgI (Table 3, entry 6), and a lower yield
photocatalytic oxidation process which was good agreement
14
with previous report. As shown in Figure 5C and 5D, DMPO-
(12%) of 1-isochromanone (2a) was obtained when recycled AgI
−•
O
2
signals were also observed in the presence of pure AgI
was used as catalyst (Table 3, entry 7) which might be because
AgI was unstable under illumination. Furthermore, 1-isochrom
anone (2a) was obtained in 24% isolated yields in the presence
photocatalysts, whereas no corresponding signals were
observed in the presence of pure BiVO photocatalysts. These
demonstrated that photogenerated electrons located on the
4
of 3 mg AgI and 12 mg BiVO
to the AgI-catalyzed result. These results illustrated AgI/BiVO
was not simple mixture of AgI and BiVO
4
(Table 3, entry 8) which was similar
conduction band (CB) of AgI could reduce O
2
to superoxide
4
•−
radical anion (O
2
) while those on the conduction band (CB) of
4
.
could not.16 According to the reports,14 the
BiVO
4
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Journal Name
4
could not reduce electron-hole pairs. The photogenerated electrons from the CB
photogenerated electrons on the CB of BiVO
View Article Online
DOI: 10.1039/C9GC03727F
•−
O
2
to produced superoxide radical anion (O
more positive edge potential of BiVO (+0.46 eV) than that of photogenerated electrons on the VB of AgI recombined with the
(-0.33 eV vs NHE). The photogenerated electrons in the holes there, which was faster than the recombination between
CB of AgI were transferred to O absorbed on the surface of the CB- and hVB+ of AgI itself. A single electron transferred from
AgI/BiVO and promoted superoxide radical anion (O ) yields ether to the VB of BiVO , then generated cation radical A. The
was
) (-0.33 reduced by the photogenerated electrons located on the CB of
2 4
), owing to the of BiVO flowed into the VB of AgI. The transferred
4
•−
2 2
O /O
2
e
•−
4
2
4
because the CB edge potential of AgI (-0.42 eV) was more dissolved molecular oxygen on the surface of AgI/BiVO
negative than the standard redox potential E (O
4
0
•−
2
/O
2
−•
eV vs NHE).
2
AgI to produce superoxide radical anion (O ), which could then
react with cation radical A to afford radical B and hydroperoxyl
Table 4. Quenching experiments.
•
radical (HOO ). Radical B reacted with hydroperoxyl radical
H
H
O
•
11a
(
HOO ) to generate the hydroperoxidate intermediate C. The
2
0wt% AgI/BiVO4 (15 mg), H O (2 mL)
transformation from intermediate C to ester was supported by
the Kornblum-type decomposition pathway.
O
2
O
1
6 W White LED, quenchers, O2 balloon, 25 oC, 12 h
a
Entry
Quenchers
TEMPO
TEMPO
BHT
Equivalents
0.5
Yield (%)
n. r.
Conclusions
Radical
Radical
Radical
Radical
Radical
Radical
The used 20wt% AgI/BiVO4
1
2
3
4
5
6
7
8
9
1.0
0.5
1.0
0.5
1.0
0.5
1.0
1.0
n. r.
n. r.
n. r.
n. r.
BHT
DPE
DPE
AO
AO
BQ
n. r.
JCPDF No. 41-1402
+
Trace
Trace
Trace
14
h
h
O
2
+
−
•
•
1
1
0
1
FeSO
CuCl
4
2.0
1.0
HOO
SET
2
n. r.
aIsolated yield, n. r. = no reaction.
JCPDF No. 04-0782
A
2
B
0wt% AgI/BiVO4, DMPO-O2-

20wt% AgI/BiVO4, DMPO- OH

30
35 40 45 50 55 60 65 70
Theta (degree)
2
3
450
3500
3550
3450
3500
3550
4
Figure 6. The XRD pattern of the used 20wt% AgI/BiVO .
Magnetic field (Gauss)
Magnetic field (Gauss)
C
D
AgI, DMPO-O2-

BiVO4, DMPO-O2-

O2
O2
-
- - -
e e e e
CB
3
450
3500
3550
3450
3500
3550
Magnetic field (Gauss)
Magnetic field (Gauss)
- - - -
e e e e
CB
e-
Figure 5. The EPR spectras of radical adducts trapped by DMPO spin-trapping under
visible light irradiation for 10 min: (A) in methanol dispersion for DMPO-O
−•
2
catalyzed
•
by 20wt% AgI/BiVO
AgI/BiVO ; (C) in methanol dispersion for DMPO-O
dispersion for DMPO-O
4
; (B) in aqueous dispersion for DMPO- OH catalyzed by 20wt%
e-
−•
4
2
catalyzed by AgI; (D) in methanol
+
+
+
+
VB
−•
h h h h
2
4
catalyzed by BiVO .
VB
+ + + +
h h h h
The used 20wt% AgI/BiVO
XRD (Figure 6). The XRD pattern of the used 20wt% AgI/BiVO
catalyst was compared with standard card of Ag (JCPDS No. 04-
782 and 41-1402) carefully, and no characteristic diffraction
peaks of metallic Ag were observed, which illustrated that no
AgI decomposed to Ag in the photocatalytic oxidation process.
On the basis of the above results and pertinent literatures, a
possible Z-scheme mechanism for this photocatalytic oxidation
process was proposed in Scheme 6. Under visible light, AgI and
4
catalyst was characterized by
4
H
H
SET
O2
R'
R'
O
R'
OOH
R'
R
O
R
R
O
0
A
B
O
OOH
R'
R
O
R
O
H O
C
2
Scheme 6. Possible Reaction Pathway.
4
BiVO were simultaneously excited, and electrons in valence
band (VB) transferred to the conduction band (CB) forming
6
| J. Name., 2012, 00, 1-3
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Punta, F. Recupero, F. Fontana and G. Franco PVeiedwuAllriti,clJe.OOnrlinge.
Conclusions
Chem. 2002, 67, 2671.
DOI: 10.1039/C9GC03727F
In summary, we had developed an efficient method for
oxidation of sp C-H bonds to esters and ketones via a visible-
light-induced process using AgI/BiVO as photocatalyst and O
as oxidant in water. There were no hazardous oxidants, organic
solvents, additives involved, thus, the oxidation process was
environmentally friendly. The photocatalytic oxidation protocol
exhibited a broad substrate scope and excellent regioselectivity.
7
8
Pd catalytic system: G. Urgoitia, R. Sanmartin, M. T. Herrero
and E. Domínguez, Green Chem. 2011, 13, 2161.
3
Bimetallic catalytic system: (a) J. Luo, K. Xuan, Y. Wang, F. Li,
F. Wang, Y. Pu, L. Li, N. Zhao and F. Xiao, New J. Chem. 2019,
4
2
4
3, 8428. (b) H. Wang, L. Wang, J. Zhang, C. Wang, Z. Liu, X.
Gao, X. Meng, S. J. Yoo, J. G. Kim, W. Zhang and F. S. Xiao,
ChemSusChem 2018, 11, 3965. (c) S. Fan, W. Dong, X. Huang,
H. Gao, J. Wang, Z. Jin, J. Tang and G. Wang, ACS Catal. 2017,
7
, 243.
4
These AgI/BiVO catalysts showed good reusability and were
9
Non-metal catalytic system: (a) L. Wang, Y. Zhang, R. Du, H.
successfully recycled six times without any significant loss of
activity. The synthetic utility of the photocatalytic oxidation
protocol was demonstrated through gram-scale experiments.
This photocatalytic oxidation protocol could also be used for the
oxidation of benzyl alcohols to aromatic aldehydes. A plausible
mechanism had been proposed. Further studies will be aimed
at extending the scope of substrates, mass transfer of this
catalytic system and further investigations into the reaction
mechanism.
Yuan, Y. Wang, J. Yao and H. Li, ChemCatChem 2019, 11, 2260.
(b) K. J. Liu, Z. H. Duan, X. L. Zeng, M. Sun, Z. L. Tang, S. Jiang,
Z. Cao and W. M. He, ACS Sustain. Chem. Eng. 2019, 7, 10293.
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(
Fan and J. Li, Molecules 2018, 23, 1922. (d) H. Zhang, Y. Wang,
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1
1
0 (a) X. Zhang, K. P. Rakesh, L. Ravindar and H. L. Qin, Green
Chem. 2018, 20, 4790. (b) X. Y. Wang, Z. P. Shang, G. F. Zha, X.
Q. Chen, S. N. A. Bukhari and H. L. Qin, Tetrahedron Lett. 2016,
There are no conflicts to declare.
5
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1 (a) K. C. C. Aganda, B. Hong and A. Lee, Adv. Synth. Catal. 2019,
61, 1124. (b) G. Zhao, B. Hul, G. W. Busser, B. Peng and M.
Acknowledgements
3
We gratefully acknowledge the Research Foundation of
Weifang Medical University, the Innovative Drug Research and
Development Center of Weifang Medical University and the
National Natural Science Foundation of China (No. 21773227)
for support of this research.
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