WO3 nanospheres with improved catalytic activity for visible light induced cross dehydrogenative coupling reactions

Article abstract of DOI:10.1016/j.jphotochem.2018.05.018

Three tungsten oxides (WO₃-W1 to WO₃-W3) with different morphologies were prepared and characterized by XRD, SEM, TEM and HR-TEM measurements. The as-synthesized tungsten oxides were screened as photocatalysts for visible-light-drive

Full text of DOI:10.1016/j.jphotochem.2018.05.018

JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
WO3 nanospheres with improved catalytic activity for visible light induced
cross dehydrogenative coupling reactions
⁎
Shujing Lia,b, ,1, Deepak Prakash Shelarc,1, Chun-Chao Houc, Qian-Qian Chenc, Pan Dengb,
Yong Chenc,⁎⁎
a
Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University, Beijing 100048, PR China
Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing 100048, PR China
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & University of Chinese Academy of Sciences,
b
c
Chinese Academy of Sciences, Beijing 100190, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Three tungsten oxides (WO3-W1 to WO3-W3) with different morphologies were prepared and characterized by
XRD, SEM, TEM and HR-TEM measurements. The as-synthesized tungsten oxides were screened as photo-
catalysts for visible-light-driven cross dehydrogenative coupling (CDC) reactions along with commercially
available WO3-W4. Preliminary studies showed that WO3-W1 with hollow sphere morphology efficiently pho-
tocatalyzed oxidative CeH functionalization as compared to other tested samples, by using molecular oxygen as
a benign oxidant. The superior photocatalytic performance of WO3-W1 can be attributed to its larger surface area
and pore structure which was supported by nitrogen adsorption-desorption measurements. Further, this protocol
was used to synthesize α-functionalized tertiary amines in good to excellent yields by irradiating a mixture of
WO3-W1, tertiary amine, and nucleophiles (nitromethane or ketones) to visible light under aerobic conditions.
Moreover, WO3-W1 can be recycled and reused with no obvious change in catalytic activity, indicating that this
is an environmentally friendly and economical protocol and also underlines the robustness of the catalysts in
light mediated cross dehydrogenative coupling reactions. It is hoped that our results could offer useful in-
formation for designing of new heterogeneous semiconductors for photoredox catalytic organic reactions.
Tungsten oxide
Heterogeneous catalysis
Photoredox
CeC Coupling reaction
1. Introduction
and the difficulty in separating them from reaction mixtures. Further-
more, ruthenium and iridium compounds are highly toxic and sus-
Visible-light photoredox catalysis has attracted much attention in
organic synthetic community because it is environmentally benign and
has promising industrial applications [1–8]. In this context, asymmetric
alkylation of aldehydes [9–11], [2 + 2] cycloaddition of enones
[12–14], [3 + 2] cycloaddition of aryl cyclopropyl ketones [15–17],
reductive dehalogenation [18,19], radical addition to unsaturated
bonds [20–22], and coupling reactions [23–26] have been successfully
achieved. Among all these reactions, CeC bond forming reaction is the
most extensively studied reaction. Especially, CDC reaction has at-
tracted considerable interest among the CeC bond forming reactions
because it does not require substrate prefunctionalization and is thus
atom economic [27–30]. Recently, photo-induced CeC bond forming
reactions have been conducted by using molecular ruthenium or ir-
idium complexes as photocatalysts [29–31]. However, practical appli-
cations of these photocatalysts are hampered because of their high costs
pected carcinogens and these metals are scarce. For these reasons, the
identification of more convenient visible-light photocatalysts has be-
come an area of intense research. As a result of these efforts, several
reports have appeared where the commonly used Ru(II) and Ir(III)
polypyridyl complexes as catalysts are avoided. In this regard, organic
dyes [32–34] as well as heterogeneous semiconductors [35] have been
applied to the CeC bond forming reactions to avoid the use of ex-
pensive metals and the presence of traces of metals in final products.
Photocatalysts based on heterogeneous semiconductors [36–39]
have found some applications in the catalysis of organic reactions
[35,40–44]. Moreover, heterogeneous photocatalyst have ability to
recover and reuse and thus eliminate the contamination of the organic
product. For example, carbon nitride has been explored as an intriguing
catalyst for photoredox reactions due to its polymeric feature and facile
synthesis. Zhang et al., have summarized the application of carbon
⁎
Corresponding author at: Beijing Advanced Innovation Center for Food Nutrition and Human Health, Beijing Technology and Business University, Beijing 100048, PR China.
Corresponding author.
⁎⁎
E-mail addresses: lishujing@mail.ipc.ac.cn (S. Li), chenyong@mail.ipc.ac.cn (Y. Chen).
These authors contributed equally to this work.
1
https://doi.org/10.1016/j.jphotochem.2018.05.018
Received 5 March 2018; Received in revised form 13 May 2018; Accepted 14 May 2018
Availableonline19May2018
1010-6030/©2018ElsevierB.V.Allrightsreserved.

S. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
nitride in this field [45]. In addition, titanium dioxide is by far the
most-attractive photocatalytic materials owing to its low cost, excellent
chemical and photochemical stability, low toxicity, and high reactivity
[46,47]. However, because of its wide band gap (> 3 eV), TiO2 re-
sponds only to UV light, which seriously limits its use in visible-light-
driven photocatalysis. Although several CeC, CeH, or Ceheteroatom
bond forming reactions [48–50] and even asymmetric α-alkylation re-
actions of aldehydes that are promoted by visible light in the presence
of TiO2 have been reported, these reactions proceeded with low effi-
ciency [35]. In turn, low-band-gap semiconductors such as PbBiO2Br
[35] have been used as efficient visible-light photocatalysts for the CeC
coupling reactions. However, the toxicity of this material severely limits
its practical use. Inspired by these results, we have endeavored to
identify inexpensive, recyclable, environmentally benign, abundant
heterogeneous catalyst with narrow band gap.
Tungsten oxide (WO3) is one of the important members of transition
metal oxides and has attracted considerable attention because it pos-
sesses wide range of applications in electrochromic windows [51],
optical devices [51], electrochromic (EC) devices [52], photocatalysis
[53], and gas sensing [54]. With a suitable band gap (2.7 eV), WO3 is
also an important visible-light-responsive photocatalyst and has been
widely used in the photocatalytic degradation of organic pollutants
[55,56]. Recently, efforts have been made to synthesize nanostructured
WO3 for application in supercapacitors (SCs) and lithium-ion battery
(LIBs) [57,58]. Undertaking these reports and our continuous interest in
identification of less expensive photocatalyst [59], we herein report
that WO3 can be useful in visible light mediated cross dehydrogenative
coupling reactions as a robust and efficient photocatalyst. The as-syn-
thesized nanosphere WO3-W1 with a surface area of 14.1 m2/g and
main porous radius size distribution of 25.8 nm exhibited enhanced
activity in CDC reaction. Moreover, WO3 has been successfully reused
for at least five consecutive cycles without any loss of catalytic activity
and composition which revealed the robustness of the catalysts in light
mediated cross dehydrogenative coupling reactions.
solution was transferred to a Teflon-lined autoclave, which was then
sealed and maintained at 180 °C for 24 h. After cooling to room tem-
perature, the obtained black product was collected by centrifugation
and rinsed several time by water and ethanol. Then the as-prepared
PVA@Glau (1 g) and WCl6 (0.8 g) were soaked in anhydrous ethanol
(30 mL) and sealed, stirred in an ice-water bath for 12 h, followed by
filtration. After dried in vacuum, the obtained sample was heated from
room temperature to 450 °C over 7 h and annealed at 450 °C for a fur-
ther 1 h in air to obtain the final yellow product.
2.2.2. General procedure for the CDC reaction of nitroalkanes with
tetrahydroisoquinolines
A
mixture of 2-phenyl-1,2,3,4-tetrahydroisoquinoline (42 mg,
0.2 mmol), and WO3-W1(10 mg) was dissolved in nitromethane (10 mL)
in a 15 mL Pyrex tube equipped with a rubber septum and magnetic stir
bar. The mixture was bubbled with a stream of oxygen for 30 min. The
tube was then sealed and irradiated by a 0.5 W LED lamp at ambient
temperature. The progress of the reaction was monitored by thin-layer
chromatography at regular intervals. After 24 h of irradiation, solvent
was removed under vacuum, and the residue was purified by column
chromatography on silica gel to afford the corresponding products.
2.2.3. General procedure for the Mannich reaction of acetone with
tetrahydroisoquinolines
A
mixture of 2-phenyl-1,2,3,4-tetrahydroisoquinoline (42 mg,
0.2 mmol), WO3-W1 (10 mg) and L-proline (4.6 mg, 0.04 mmol) was
dissolved by a mixture of acetone (6 mL) and CH3OH (4 mL) in a 15 mL
Pyrex tube equipped with a rubber septum and magnetic stir bar. The
mixture was bubbled with a stream of oxygen for 30 min. The tube was
then sealed and irradiated by a 0.5 W LED lamp at ambient tempera-
ture. After 24 h of irradiation, solvent was removed under vacuum, and
the residue was purified by column chromatography on silica gel to
afford the corresponding products.
2.3. Characterization
2. Materials and methods
Electrospray Ionization (ESI) mass spectra were performed on a
Finnigan LCQ quadrupole ion trap mass spectrometer (samples were
dissolved in HPLC grade methanol). 1H, 13C and 19F NMR spectra were
recorded using a Bruker Avance DP X 400 MHz instrument. LED light:
λ > 420 nm, 0.5 W x 30. The emission spectrum of the LED light is as
shown in Fig. S1.
2.1. Materials
Commercially available reagents and solvents were used without
further purification unless indicated otherwise. The solvents used for
photophysical measurements were of HPLC grade. All the N-arylte-
trahydroisoquinolines 1 needed for CDC reactions were prepared by
using the reported procedure [60] and purified through column chro-
matography. The commercial WO3 (bulk material) was purchased from
Aladdin Industrial Corporation.
2.3.1. TEM, HR-TEM, SEM and SEM-EDX measurements
The powder samples were dispersed in ethanol solutions which were
sonicated for at least 30 min. Subsequently the suspended sample was
dropped and placed on an ultra-thin carbon film copper mesh and al-
lowed to dry in the air at room temperature. The TEM, HRTEM mea-
surements were performed on a TEM (JEM 2100F) which was operated
at an accelerating voltage of 200 kV. The scanning electron microscopes
(SEM) and Energy-dispersion X-ray spectroscopy (EDX) measurements
were performed on the HITACHI S-4800.
2.2. Synthesis of materials
2.2.1. Synthesis of tungsten oxides (WO3) with different morphologies
The nanosphere WO3 was synthesized using a pyrolysis of W18O49
precursor method [61]. In a typical procedure, a W6+ precursor (WCl6,
2 g) was dissolved in 100 mL absolute ethanol, and the transparent
solution was then transferred to the Teflon-lined stainless steel auto-
clave and heated at 180 °C for 24 h. The obtained blue precipitate was
collected by centrifugation and washed several times by ethanol, fol-
lowed by drying under vacuum for 12 h. Then, this dry precipitate was
further annealed in air at 550 °C in a box furnace for 2 h, after which the
nanosphere WO3 was obtained. The same procedure was used to pre-
pare sea-urchin like nano WO3 except the concentration of WCl6 pre-
cursor is 0.5 g/100 mL.
The hollow-shell vesicle WO3 was synthesized using the PVA@Glau
hybrid polymer as template. The PVA@Glau was synthesized as fol-
lowed: PVA (1 g, polyvinyl alcohol, DP = 1750
(3 g) were dissolved in 100 mL distilled water under a heated tem-
perature until a clear solution was obtained. Then the transparent
2.3.2. X-ray diffraction
Powder X-ray diffraction (PXRD) analysis was carried out on a dif-
fractometer (Bruker D8 Focus) using CuKa radiation (λ = 1.5406 Å)
between 5° and 90° (2θ).
2.3.3. N2-physisorption
The WO3 samples were dried at 150 °C on a vacuum line for more
than 24 h before the measurement. BET surface areas were measured by
N2 adsorption and desorption at 77 K on a Quadrasorb SI-MP Analyzer
from Quantachrome Instrument Company and analyzed by means of a
computer using quadrasorb win software. The pore size distribution
was calculated by the BJH method and all the total pore volumes of
WO3 were calculated at 0.9 relative pressures.
50) and glucose
45

S. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
Fig. 1. Characterizations of nanosphere WO3-W1. (a) Powder XRD pattern. (b) SEM image, Inset: the corresponding TEM image. (c) Magnified TEM image of the
surface of nanosphere. (d) TEM for the nanoparticles of WO3. (e) SAED pattern and (f) HRTEM image.
2.3.4. ESR measurement
adsorption edge at 470 nm, demonstrating that the obtained WO3 na-
noparticles have sufficient adsorption under visible light irradiation
(Fig. S3). Scanning electron microscopy (SEM) and transmission elec-
tron microscope (TEM) analyses were performed to examine the mor-
phology of the as-prepared WO3 samples and commercial WO3-W4.
SEM and TEM images clearly showed that the WO3-W1 has a sphere-
like structure with a diameter of 500 nm (Fig. 1b). A close inspection
revealed that these sphere-like structures are composed of nanoparticles
with a size of 20–30 nm (Figs. 1c, d, S3). From the HR-TEM, it is clear
that the WO3-W1 has lattice spacing of about 0.365 nm, which corre-
sponds to the monoclinic WO3 phase (002) face (Fig. 1f). Selected area
electron diffraction (SAED) pattern of WO3 nanoparticles showed sharp
and ordered spots, which can be indexed to (002), (-311), (024) and
(141) planes of monoclinic WO3 (Fig. 1e). Moreover, the energy dis-
perse X-ray analysis (EDX) in random areas of WO3-W1 demonstrated
that W and O elements were abundantly present in the nanosphere
sample (Fig. S5). Other WO3 with different morphologies were also
ESR spectra were recorded at room temperature using a Bruker ESP-
300E spectrometer at 9.8 GHz, X-band with 100 Hz field modulation.
Samples were quantitatively injected into specially made quartz capil-
laries for ESR analysis before being purged with argon or oxygen for
30 min in the dark and illuminated directly in the cavity of the ESR
spectrometer with a 100 W mercury lamp.
3. Results and discussion
Three nanoscale tungsten oxide materials (WO3-W1 to WO3-W3)
were prepared according to literature methods (see experimental sec-
tion). The as-synthesized samples were first confirmed by the X-ray
diffraction (XRD) analysis (Fig. 1a and S2). The entire diffraction peaks
agree well with the reported JCPDS (43-1035) card, which can be in-
dexed to the pure monoclinic phase of WO3. No other impurities were
detected in the prepared products. The UV–vis spectrum shows the
46

S. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
Table 1
did not proceed in the dark (Table 1, entry 6) and gave trace amount of
conversion and product in the absence of oxygen (Table 1, entry 7).
Finally we examined the viability of the reaction in the absence of a
photocatalyst. When the reaction mixture was irradiated for 24 h in the
absence of a semiconductor, only 15% conversion was observed
(Table 1, entry 8). These last results (Table 1, entries 6–8) clearly show
that nanostructured WO3 employed in this study are real photocatalyst
and especially WO3 with nanosphere morphology facilitates the CeC
bond formation under irradiation with visible light under aerobic
conditions. It should be noted that other visible light responsive semi-
conductors including CdS, BiVO4, BiOI were also tested for this reac-
tion, and it was found that these semiconductors are less efficient re-
lative to WO3 under the same experimental conditions.
To understand the difference of photocatalytic performance of these
tungsten oxides with various morphologies, we have tested their BET
surface areas and pore structures through nitrogen adsorption-deso-
rption measurements. In general, large specific surface areas and wide
pore size distributions of photocatalysts can provide more surface ac-
tive sites and make charge carrier transport easier, thus leading to an
enhancement of the photocatalytic performance. Fig. 2 shows the ni-
trogen adsorption–desorption isotherms and the corresponding BJH
pore size distribution for all samples. All these isotherms belonged to
type IV hysteresis loops at relative pressure of P/P0 above 0.8, in-
dicating the formation of mesoporous structure. Based on the nitrogen
adsorption–desorption isotherms in Fig. 2a, the surface area of WO3-
W1, WO3-W2, WO3-W3, and WO3-W4 is determined to be ca. 14.1, 7.5,
25.4, and 3.5 m2 g−1, respectively. Furthermore, Fig. 2b shows that the
pore size distribution is narrow and determined to be 29.0 and 34.0 nm
for WO3-W1 and WO3-W2, respectively, while WO3-W3 and WO3-W4
do not show the pore size distribution with certain order. Therefore, it
seems that the observed photoactivity order, i.e., WO3-W1 > WO3-
W2 > WO3-W3 > WO3-W4, could be correlated closely with the two
aspects: the surface area and the pore size distribution. The highest
photoactivity WO3-W1 shows a rather large surface area and wide pore
size distribution.
Optimization of CDC reaction of tetrahydroisoquinoline 1a with nitromethane.
Entry
Conditionsa
Conv.[%]b
Yield [%]c
1
2
3
4
5
6
7e
8
WO3-W1
WO3-W2
WO3-W3
WO3-W4
WO3-W1 in air
No light, WO3-W1
WO3-W1
100
100
100
59
100
0
96
88
82
44
76
0
trace
15
trace
12
No catalyst
dIsolated yield based on 1a.
a
Reaction conditions: 1a (0.2 mmol), CH3NO2 (10 mL) and 10 mg of pho-
tocatalyst. The reaction was saturated with O2 before irradiation unless in-
dicated otherwise; reaction time of 24 h.
b
After reaction, the solvent was evaporated, and the residue was subjected
to 1H NMR spectroscopic analysis. Conversions were determined by crude 1H
NMR spectroscopic analysis using 4,4′-dimethyl-2,2′-bipyridine as an internal
standard and are based on 1a.
c
Yields on the basis of the starting amount of 1a and were determined by 1
H
NMR analysis.
e
The reaction was deoxygenated.
characterized and confirmed (Figs. S5, S7 and S8). On the basis of all
characterization including XRD, SEM, TEM and HRTEM, we assigned
the synthesized WO3 along with commercially available WO3 as WO3-
W1: nanospheres, WO3-W2: sea-urchin like, WO3-W3: hollow-shell ve-
sicle and commercially available WO3-W4: bulk materials.
Although the application of tungsten oxides in photocatalytic de-
gradation of organic pollutants is well-known [54,55], their use as a
catalyst for organic synthesis has been neglected. Therefore, the as-
synthesized WO3 along with commercially available WO3 were
screened for their use in light-mediated organic reactions as photo-
catalyst. Initially, the aza-Henry reaction was selected as a test reaction
to apply WO3 as inorganic heterogeneous photocatalyst. The time
course of photocatalytic CDC reaction over WO3-W1 was given in Fig.
S9. Quite gratifyingly, full conversion was achieved in 24 h for all tested
WO3 but WO3-W1 with hollow sphere morphology showed slightly
better catalytic performance than others (Table 1, entries 1–3). We next
explored the use of commercial WO3-W4 powder in the reaction. Under
previously tested reaction conditions only 59% conversion was re-
corded after 24 h and the desired aza-Henry product 3a was obtained
with a low yield of 44% (Table 1, entry 4). When oxygen was replaced
by air, the system also worked well but slightly lower yield was ob-
tained (Table 1, entry 5). Control experiments showed that the reaction
With the optimized reaction conditions established, we then in-
vestigated the scope of this protocol (Table 2). In general, different N-
aryltetrahydroisoquinolines 1a–h were examined in the light-mediated
WO3-W1 catalyzed oxidative aza-Henry reaction. It is found that elec-
tron-donating as well as electron-withdrawing groups on the aromatic
rings are compatible under the reaction conditions. They can uniformly
furnish the desired products 3a–h in good to excellent yields (Table 2).
This new protocol allows an economic and cheap oxidative cross de-
hydrogenative coupling reaction with respect to metal-catalyzed pho-
tochemical reaction.
According to our control experimental results (Table 1, entries 6–8)
and literature reports [62], the reaction mechanism of WO3 photo-
catalyzed aza-Henry reaction is proposed (Scheme 1). Briefly, upon
visible light irradiation, photogenerated electrons in the conduction
Fig. 2. (a) Nitrogen adsorption–desorption isotherm. (b) The corresponding BJH pore size distribution curve of WO3.
47

S. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
Table 2
Table 3
Substrate scope of the oxidative aza-Henry reaction.a
Mannich reaction of N-aryltetrahydroisoquinolines with ketone.a
Entry
Substrate
R1
R2
Conv.b
Yieldc
Entry
Substrate
R1
R2
Conv.b
Yieldc
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
H
H
H
H
H
H
OCH3
OCH3
H
100
100
100
100
100
100
100
100
96 (91)d
94 (88)d
90 (87)d
94 (88)d
88 (82)d
90 (84)d
84 (77)d
78 (70)d
CH3
OCH3
Cl
Br
F
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
H
CH3
Cl
Br
F
H
CH3
CH3
CH3
CH3
100
100
100
100
100
100
100
100
82 (88)d
82 (87)d
80 (86)d
76
CH3
79 (85)d
76
1g
1h
H
Br
CH2CH3
CH2CH3
CH2CH3
1g
1h
CH3
Cl
70
72
a
Reaction conditions: 1 (0.2 mmol), WO3-W1 (10 mg), nitroalkane (10 mL),
a
irradiated at ambient temperature in the presence of oxygen for 24 h.
Reaction conditions:
1
(0.2 mmol), WO3-W1 (10 mg), L-proline
b
After reaction, the solvent was evaporated, and the residue was subjected
(0.04 mmol), ketone (6 mL), CH3OH (4 mL), irradiated at ambient temperature
in the presence of oxygen for 24 h.
to 1H NMR spectroscopic analysis. Conversions were determined by crude 1H
NMR spectroscopic analysis using 4,4′-dimethyl-2,2′-bipyridine as an internal
standard and are based on 1.
b
After reaction, the solvent was evaporated, and the residue was subjected
to 1H NMR spectroscopic analysis. Conversions were determined by crude 1H
NMR spectroscopic analysis using 4,4′-dimethyl-2,2′-bipyridine as an internal
standard and are based on 1.
c
Yields on the basis of the starting amount of 1 and were determined by 1H
NMR analysis.
d
c
Isolated yields based on substrate 1.
Isolated yields based on substrate 1.
d
Yields on the basis of the starting amount of 1 and were determined by 1
H
NMR analysis.
catalyst for light mediated oxidative cross dehydrogenative coupling
reactions, next we decided to examine reusability of WO3-W1 by re-
peating the same reaction procedure using the recovered catalyst. The
solid catalyst was recycled by centrifuging the residue mixture, washing
with ethanol repeatedly and drying in vacuum at 50 °C for 2 h. As
shown in Fig. 3b and Table S1, WO3-W1 can be efficiently reused for at
least 5 consecutive catalytic cycles without a distinct loss of its catalytic
activity which proves catalytic efficiency of WO3-W1 catalyst. To obtain
information about the catalyst structure we compared the fresh and
used WO3-W1 in the photocatalytic reactions by SEM and XRD tech-
niques. As displayed in Fig. 3a, XRD patterns of WO3-W1 before and
after photocatalytic reaction are rather identical. SEM analyses also
showed that there is no significant change in the shape and composition
of the catalyst (Fig S11). Therefore, the as-prepared WO3-W1 semi-
conductor is a stable visible-light-driven photocatalyst for cross dehy-
drogenative coupling reactions.
Scheme 1. Proposed reaction mechanism for aerobic photocatalytic aza-Henry
reaction catalyzed by WO3.
band (CB) of WO3 enable O2 to undergo a two-electron transfer process
(E0(O2/H2O2= +0.695 vs NHE)), which is thermodynamically per-
missible. An ESR spin-trapping technique was employed to confirm our
mechanism. After the addition of substrate 1a, the radical signal was
obviously observed (Fig. S10). Meantime, photogenerated holes in the
valance band (VB) of WO3 are reductively quenched by tertiary amine
1a via a single electron transfer process to form an aminyl cation ra-
dical 4. Hydrogen abstraction of aminyl cation radical 4 by radical gives
the iminium ion 5. Finally, iminium ion 5 can undergoes nucleophilic
addition to provide desired product 3a.
To further evaluate the catalytic performance of WO3-W1 and to
underline the efficiency of this heterogeneous photocatalyst, we ex-
tended this protocol to the oxidative Mannich reaction. This reaction is
fundamentally an important carbon-carbon bond-forming reaction in
organic synthesis and offers a practical approach to valuable β-amino-
ketones. When WO3-W1 was used to catalyze the photochemical oxi-
dative reaction of amine (1a-e) with acetone in the presence of a co-
catalyst, L-proline [23], the corresponding Mannich-type products were
obtained in good to excellent yields (Table 3, entries 1–5). When
acetone was replaced by methyl-ethyl ketone, similar products could be
obtained in moderate to good yields (Table 3, entries 6–8)
4. Conclusion
In summary, we synthesized WO3 with different morphology and
evaluated their photocatalytic activity for cross dehydrogenative cou-
pling reactions. Among all tested WO3, WO3-W1 exhibited the highest
efficiency in catalyzing the CDC reaction, which can be attributed to its
larger surface area and wide pore size distribution. The heterogeneous
catalysts can catalyze aza-Henry and Mannich reactions in high yields,
which are comparable to those of homogeneous reactions using tran-
sition metal complexes or organic dyes as catalysts. Furthermore, WO3-
W1 was successfully recycled and reused for five consecutive cycles
without any loss of catalytic activity, which underlines the robustness
of the catalysts in light mediated cross dehydrogenative coupling re-
actions.
Conflict of interest
The authors declare that they have no conflict of interest.
After identifying inexpensive, readily available, heterogeneous
48

S. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
Fig. 3. (a) XRD analysis for recycled WO3. (b) Recycling experiments for the WO3 catalyzed aza-Henry reaction between N-aryl-tetrahydroisoquinoline and ni-
tromethane.
Acknowledgements
[16] L. Wang, H. Zou, X. Zhang, G. Huang, Mechanism and origins of selectivity in
rhodium-catalyzed intermolecular [3+2] cycloadditions of vinylaziridines with
allenes, Organ. Chem. Front. 4 (2017) 587–596.
[17] H. Huo, K. Harms, E. Meggers, Catalytic, enantioselective addition of alkyl radicals
to alkenes via visible-light-activated photoredox catalysis with a chiral rhodium
complex, J. Am. Chem. Soc. 138 (2016) 6936–6939.
[18] L. Furst, J.M.R. Narayanam, C.R.J. Stephenson, Total synthesis of (+)-gliocladin C
enabled by visible-light photoredox catalysis, Angew. Chem. Int. Ed. 50 (2011)
9655–9659.
This work was supported by the Strategic Priority Research Program
of the Chinese Academy of Sciences (XDB17000000) and the Natural
Science Foundation of China (21773275, 21371175). We thank CAS-
Croucher Funding Scheme for Joint Laboratories.
[19] J.D. Nguyen, J.W. Tucker, M.D. Konieczynska, C.R. Stephenson, Intermolecular
atom transfer radical addition to olefins mediated by oxidative quenching of pho-
toredox catalysts, J. Am. Chem. Soc. 133 (2011) 4160–4163.
[20] J.W. Tucker, J.M. Narayanam, S.W. Krabbe, C.R.J. Stephenson, Electron transfer
photoredox catalysis: intramolecular radical addition to indoles and pyrroles, Org.
Lett. 12 (2010) 368–371.
[21] A.U. Meyer, A.L. Berger, B. Konig, Metal-free C-H sulfonamidation of pyrroles by
visible light photoredox catalysis, Chem. Commun. 52 (2016) 10918–10921.
[22] I. Ghosh, L. Marzo, A. Das, R. Shaikh, B. Konig, Visible light mediated photoredox
catalytic arylation reactions, Acc. Chem. Res. 49 (2016) 1566–1577.
[23] M. Rueping, C. Vila, R.M. Koenigs, K. Poscharny, D.C. Fabry, Dual catalysis: com-
bining photoredox and Lewis base catalysis for direct Mannich reactions, Chem.
Commun. 47 (2011) 2360–2362.
[24] P.V. Pham, D.A. Nagib, D.W. MacMillan, Photoredox catalysis: a mild, operationally
simple approach to the synthesis of alpha-trifluoromethyl carbonyl compounds,
Angew. Chem. 50 (2011) 6119–6122.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi: https://doi.org/10.1016/j.jphotochem.2018.05.
018.
References
[1] T.P. Yoon, M.A. Ischay, J. Du, Visible light photocatalysis as a greener approach to
photochemical synthesis, Nat. Chem. 2 (2010) 527–532.
[2] J.M. Narayanam, C.R. Stephenson, Visible light photoredox catalysis: applications
in organic synthesis, Chem. Soc. Rev. 40 (2011) 102–113.
[3] D. Kalyani, K.B. McMurtrey, S.R. Neufeldt, M.S. Sanford, Room-temperature C-H
arylation: merger of Pd-catalyzed C-H functionalization and visible-light photo-
catalysis, J. Am. Chem. Soc. 133 (2011) 18566–18569.
[4] D.P. Hari, P. Schroll, B. Konig, Metal-free, visible-light-mediated direct C-H aryla-
tion of heteroarenes with aryl diazonium salts, J. Am. Chem. Soc. 134 (2012)
2958–2961.
[5] A. McNally, C.K. Prier, D.W. MacMillan, Discovery of an alpha-amino C-H arylation
reaction using the strategy of accelerated serendipity, Science 334 (2011)
1114–1117.
[6] J.D. Nguyen, E.M. D’Amato, J.M. Narayanam, C.R. Stephenson, Engaging un-
activated alkyl, alkenyl and aryl iodides in visible-light-mediated free radical re-
actions, Nat. Chem. 4 (2012) 854–859.
[7] X. Zhu, Z. He, Q.-Y. Li, X.-J. Wang, Direct C(sp2)–H amination of aryl aldehyde-
derived hydrazones via visible light promoted photoredox catalysis, Rsc. Adv. 7
(2017) 25171–25174.
[8] P. Qian, Y. Deng, H. Mei, J. Han, J. Zhou, Y. Pan, Visible-light photoredox catalyzed
oxidative/reductive cyclization reaction of N-cyanamide alkenes for the synthesis of
sulfonated quinazolinones, Org. Lett. 19 (2017) 4798–4801.
[9] D.A. Nicewicz, D.W. MacMillan, Merging photoredox catalysis with organocata-
lysis: the direct asymmetric alkylation of aldehydes, Science 322 (2008) 77–80.
[10] M. Neumann, S. Fuldner, B. Konig, K. Zeitler, Metal-free, cooperative asymmetric
organophotoredox catalysis with visible light, Angew. Chem. Int. Ed. 50 (2011)
951–954.
[11] P. Xu, G. Wang, Y. Zhu, W. Li, Y. Cheng, S. Li, C. Zhu, Visible-light photoredox-
catalyzed C-H difluoroalkylation of hydrazones through an aminyl radical/polar
mechanism, Angew. Chem. Int. Ed. 55 (2016) 2939–2943.
[12] J. Du, T.P. Yoon, Crossed intermolecular [2+2] cycloadditions of acyclic enones via
visible light photocatalysis, J. Am. Chem. Soc. 131 (2009) 14604–14605.
[13] M.A. Ischay, Z. Lu, T.P. Yoon, [2+2] cycloadditions by oxidative visible light
photocatalysis, J. Am. Chem. Soc. 132 (2010) 8572–8574.
[14] C. Brenninger, A. Pothig, T. Bach, Bronsted acid catalysis in visible-light-induced
[2+2] photocycloaddition reactions of enone dithianes, Angew. Chem. Int. Ed. 56
(2017) 4337–4341.
[25] C. Dai, J.M. Narayanam, C.R. Stephenson, Visible-light-mediated conversion of al-
cohols to halides, Nat. Chem. 3 (2011) 140–145.
[26] R.S. Andrews, J.J. Becker, M.R. Gagné, Investigating the rate of photoreductive
glucosyl radical generation, Org. Lett. 13 (2011) 2406–2409.
[27] M. Fagnoni, D. Dondi, D. Ravelli, A. Albini, Photocatalysis for the formation of the
C−C bond, Chem. Rev. 107 (2007) 2725–2756.
[28] N. Hoffmann, Photochemical reactions as key steps in organic synthesis, Chem. Rev.
39 (2008) 1052–1103.
[29] C.K. Prier, D.A. Rankic, D.W.C. MacMillan, Visible light photoredox catalysis with
transition metal complexes: applications in organic synthesis, Chem. Rev. 113
(2013) 5322–5363.
[30] D.M. Schultz, T.P. Yoon, Solar synthesis: prospects in visible light photocatalysis,
Science 343 (2014) 985–1005.
[31] J.A. Terrett, M.D. Clift, D.W. MacMillan, Direct beta-alkylation of aldehydes via
photoredox organocatalysis, J. Am. Chem. Soc. 136 (2014) 6858–6861.
[32] Y. Pan, S. Wang, C.W. Kee, E. Dubuisson, Y. Yang, K.P. Loh, C.H. Tan, Graphene
oxide and Rose Bengal: oxidative C–H functionalisation of tertiary amines using
visible light, Green Chem. 13 (2011) 3341–3344.
[33] Q. Liu, Y.N. Li, H.H. Zhang, B. Chen, C.H. Tung, L.Z. Wu, Reactivity and mechanistic
insight into visible-light-induced aerobic cross-dehydrogenative coupling reaction
by organophotocatalysts, Chem. Eur. J. 18 (2012) 620–627.
[34] D.P. Hari, B. Konig, Synthetic applications of eosin Y in photoredox catalysis, Chem.
Commun. 50 (2014) 6688–6699.
[35] M. Cherevatskaya, M. Neumann, S. Fuldner, C. Harlander, S. Kummel,
S. Dankesreiter, A. Pfitzner, K. Zeitler, B. Konig, Visible-light-promoted stereo-
selective alkylation by combining heterogeneous photocatalysis with organocata-
lysis, Angew. Chem. Int. Ed. 51 (2012) 4062–4066.
[36] M.R. Hoffmann, W. Choi, D.W. Bahnemann, Environmental applications of semi-
conductor photocatalysis, Chem. Rev. 95 (1995) 69–96.
[37] K. Rajeshwar, N.R.D. Tacconi, C.R. Chenthamarakshan, Semiconductor-based
composite materials: preparation, properties, and performance, Chem. Mater. 32
(2001) 2765–2782.
[38] N. Serpone, A.V. Emeline, Semiconductor photocatalysis - past, present, and future
outlook, J. Phys. Chem. Lett. 3 (2012) 673–677.
[15] Z. Lu, M. Shen, T.P. Yoon, [3+2] cycloadditions of aryl cyclopropyl ketones by
visible light photocatalysis, J. Am. Chem. Soc. 133 (2011) 1162–1164.
49

S. Li et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry363(2018)44–50
[39] H. Kisch, Semiconductor photocatalysis-mechanistic and synthetic aspects, Angew.
Chem. Int. Ed. 52 (2013) 812–847.
fabrication of a WO3·2H2O thin film with improved electrochromic properties, J.
Mater. Chem. 22 (2012) 19904.
[40] N. Zeug, J. Buecheler, H. Kisch, Catalytic formation of hydrogen and C-C bonds on
illuminated zinc sulfide generated from zinc dithiolenes, J. Am. Chem. Soc. 107
(1985) 1459–1465.
[41] B. Ohtani, S. Kusakab, S.I. Nishimoto, M. Matsumura, Y. Nakato, Improvement of
photocatalytic activity and product selectivity by cadmium metal deposited in situ
on suspended cadmium(II) sulfide particles, Chem. Lett. 24 (1995) 803–804.
[42] S. Marinković, N. Hoffmann, Diastereoselective radical tandem addition-cyclization
reactions of aromatic tertiary amines by semiconductor-sensitized photochemical
electron transfer, Eur. J. Org. Chem. 2004 (2004) 3102–3107.
[43] T. Mitkina, C. Stanglmair, W. Setzer, M. Gruber, H. Kisch, B. Konig, Visible light
mediated homo- and heterocoupling of benzyl alcohols and benzyl amines on
polycrystalline cadmium sulfide, Org. Biomol. Chem. 10 (2012) 3556–3561.
[44] X. Lang, X. Chen, J. Zhao, Heterogeneous visible light photocatalysis for selective
organic transformations, Chem. Soc. Rev. 43 (2014) 473–486.
[53] M. Sadakane, K. Sasaki, H. Kunioku, B. Ohtani, R. Abe, W. Ueda, Preparation of 3-D
ordered macroporous tungsten oxides and nano-crystalline particulate tungsten
oxides using a colloidal crystal template method, and their structural character-
ization and application as photocatalysts under visible light irradiation, J. Mater.
Chem. 20 (2010) 2092–2100.
[54] L.F. Zhu, J.C. She, J.Y. Luo, S.Z. Deng, J. Chen, N.S. Xu, Study of physical and
chemical processes of H2 sensing of Pt-coated WO3 nanowire films, J. Phys. Chem.
C 114 (2010) 15504–15509.
[55] D. Chen, L. Gao, A. Yasumori, K. Kuroda, Y. Sugahara, Size- and shape-controlled
conversion of tungstate-based inorganic-organic hybrid belts to WO3 nanoplates
with high specific surface areas, Small 4 (2008) 1813–1822.
[56] G. Xi, B. Yue, J. Cao, J. Ye, Fe3O4/WO3 hierarchical core-shell structure: high-
performance and recyclable visible-light photocatalysis, Chem. Eur. J. 17 (2011)
5145–5154.
[45] Z. Zhou, Y. Zhang, Y. Shen, S. Liu, Y. Zhang, Molecular engineering of polymeric
carbon nitride: advancing applications from photocatalysis to biosensing and more,
Chem. Soc. Rev. 47 (2018) 2298–2321.
[46] X.B. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, mod-
ifications, and applications, Chem. Rev. 38 (2007) 2891–2959.
[47] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles,
mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758.
[48] X.-H. Ho, M.-J. Kang, S.-J. Kim, E.D. Park, H.-Y. Jang, Green organophotocatalysis.
TiO2-induced enantioselective α-oxyamination of aldehydes, Catal. Sci. Technol. 1
(2011) 923–926.
[49] M. Rueping, J. Zoller, D.C. Fabry, K. Poscharny, R.M. Koenigs, T.E. Weirich,
J. Mayer, Light-mediated heterogeneous cross dehydrogenative coupling reactions:
metal oxides as efficient, recyclable, photoredox catalysts in C-C bond-forming re-
actions, Chem. Eur. J. 18 (2012) 3478–3481.
[57] S. Yoon, E. Kang, J.K. Kim, C.W. Lee, J. Lee, Development of high-performance
supercapacitor electrodes using novel ordered mesoporous tungsten oxide materials
with high electrical conductivity, Chem. Commun. 47 (2011) 1021–1023.
[58] S. Yoon, C. Jo, S.Y. Noh, C.W. Lee, J.H. Song, J. Lee, Development of a high-per-
formance anode for lithium ion batteries using novel ordered mesoporous tungsten
oxide materials with high electrical conductivity, Phys. Chem. Chem. Phys. 13
(2011) 11060–11066.
[59] B. Wang, D.P. Shelar, X.Z. Han, T.T. Li, X. Guan, W. Lu, K. Liu, W.F. Fu, C.M. Che,
Long-lived excited states of zwitterionic copper(I) complexes for photoinduced
cross-dehydrogenative coupling reactions, Chem. Eur. J. 21 (2015) 1184–1190.
[60] F.Y. Kwong, A. Klapars, S.L. Buchwald, Copper-catalyzed coupling of alkylamines
and aryl iodides: an efficient system even in an air atmosphere, Org. Lett. 4 (2002)
581–584.
[61] G. Xi, S. Ouyang, P. Li, J. Ye, Q. Ma, N. Su, H. Bai, C. Wang, Ultrathin W18O49
nanowires with diameters below 1 nm: synthesis, near-infrared absorption, pho-
toluminescence, and photochemical reduction of carbon dioxide, Angew. Chem. 51
(2012) 2395–2399.
[50] C. Vila, M. Rueping, Visible-light mediated heterogeneous C-H functionalization:
oxidative multi-component reactions using a recyclable titanium dioxide (TiO2)
catalyst, Green Chem. 15 (2013) 2056–2059.
[51] J. Zhou, Y. Ding, S.Z. Deng, L. Gong, N.S. Xu, Z.L. Wang, Three-dimensional
tungsten oxide nanowire metworks, Adv. Mater. 17 (2010) 2107–2110.
[52] Z. Xie, L. Gao, B. Liang, X. Wang, G. Chen, Z. Liu, J. Chao, D. Chen, G. Shen, Fast
[62] B. Yuan, R. Chong, B. Zhang, J. Li, Y. Liu, C. Li, Photocatalytic aerobic oxidation of
amines to imines on BiVO4 under visible light irradiation, Chem. Commun. 50
(2014) 15593–15596.
50