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T. Xu et al. / Journal of Catalysis 378 (2019) 63–67
then illuminated by a SUN-Q-Light photoreactor (Q-Lab Corpora-
tion, Xe-1-BC, USA) under shaker oscillation. The filtered liquid
was then quantitatively analyzed through ultra-performance liq-
uid chromatography with a PDA detector (UPLC, Waters) and a
HSS T3 column (1.8
water as the mobile phase by external standard method.
l
m, 2.1 ꢁ 100 mm) using acetonitrile and
3. Results and discussion
Initially, the hydroxylation of 4-methoxyphenylboronic acid
(0.1 mmol) was chosen as a model reaction for the optimization
of reaction conditions. First, the optimal reaction time of 4-
methoxyphenylboronic acid to 4-methoxyphenol was evaluated,
and the yield of phenol increases with the prolonged irradiation
time (Table 1, entries 1–4). To our surprise, g-C3N4 showed satisfy-
ing catalytic activity after 12 h with visible light irradiation at
room temperature, giving the product 2a in 97% yield. Then several
reactions were carried out as controls and to explore the effects of
solvents and amines. First, we used different solvents including
MeOH, MeCN, DMSO, DMF (Table 1, entries 5–8), to see the best
conversion. While 2-PrOH has given best results, reaction in other
solvents has not afforded good results. Moreover, the reaction did
not take place when the photocatalyst was not present (Table 1,
entry 9). When Et3N was added as the amine, the reaction gave
the product in lower yields of 82% (Table 1, entry 10), and we found
that iPr2NEt (5.0 equiv) was the ideal amine. Interestingly, the
reaction gave no yield in the absence of iPr2NEt (Table 1, entry
11), which might act as a sacrificial electron donor. Then the reac-
tion was carried out under O2 and N2 atmosphere, respectively. It
was found that the reaction proceeded smoothly under O2 condi-
tions but with a yield of 92% (Table 1, entry 12). However, the pro-
duct was formed under N2 atmosphere but with very poor yield of
16% (Table 1, entry 13). Additionally, the superoxide radical cap-
ture agent 4-benzoquinone (BQ) was added as the scavengers of
super oxide radical (ÅO2ꢀ), the reaction could proceed in the pres-
ence of BQ but gave the 4-methoxyphenol in low yields (21%). It
suggested that the key step in the oxidative of arylboronic acids
was the generation of the superoxide radical (ÅO2ꢀ) and it was fur-
ther proved by EPR. As can be seen in Fig. S8A, the DMPO-ÅO2ꢀ signal
could be detected when the g-C3N4 were irradiated for 2 min,
while no signals appeared without light irradiation. It also indi-
Scheme 1. Schematic diagrams of possible reaction mechanism over g-C3N4/LMPET
photocatalysts under solar light irradiation.
Graphitic carbon nitride (g-C3N4) as the organic semiconductor
material is a metal-free and visible-light-responsive heteroge-
neous photocatalyst with low cost, chemical stability and appeal-
ing electronic band structure [17]. It can be easily synthesized by
pyrolysis of cheap precursors like melamine, urea and dicyandi-
amide. g-C3N4 has also been exploited for the synthesis of value
added commodities and complex organic molecules [18]. However,
powdery g-C3N4 is inclined to aggregate and deposit in solutions,
restricting its application. It is necessary to find an appropriate car-
rier to immobilize g-C3N4 to avoid the time consuming trouble of
filtration and recovery. Fiber materials possess the characteristics
of the large surface area and easy weaving [19]. Taking advantage
of these superiorities, g-C3N4 was loaded onto the low melting
point sheath-core composite polyester fibers (LMPET) to prepare
g-C3N4-based artificial photosynthetic catalytic fabric (g-C3N4/
LMPET) with large light receiving area, which is mimicking the role
of leaves in natural plants for the two-dimensional planar reaction
system.
Herein, g-C3N4 and g-C3N4/LMPET fabric dominated by superox-
ide radicals (ÅOꢀ2 ) was firstly reported for the conversion of aryl-
boronic acids to phenols with isopropanol (2-PrOH) as solvent in
the presence of N,N-diisopropylethylamine (iPr2NEt) under the vis-
ible light or solar irradiation (Scheme 1). g-C3N4/LMPET with large
receiving light area simulates the structural characteristics of
green leaves and solves an intrinsic problem of powdered catalyst
separation from the reaction mixture. The characterization data of
g-C3N4 and g-C3N4/LMPET, such as SEM images, DSC curves, FTIR
spectra, DRS spectra, XRD and EPR spectra, can be found in the Sup-
porting Information, Figs. S1–S8. g-C3N4 and g-C3N4/LMPET fabric
as the metal-free heterogeneous photocatalysts both show high
selectivity in the reaction of arylboronic acid to phenolic com-
pounds. The reaction takes place under mild conditions, air, ambi-
ent temperature and pressure.
Å
cates that Oꢀ2 plays a major role for the hydroxylation of boronic
acid in this study, similar to previous reports. When using other
photocatalysts, TiO2 gave only the desired phenol in 29% yield
(Table 1, entry 15) and the reaction could hardly proceed in the
presence of WO3. Thus, we determined the optimal reaction in
the presence of g-C3N4 (10 mg) and i-Pr2NEt (5.0 equiv) in iso-
propanol (5 mL) under air atmosphere by visible light irradiation
at room temperature.
2. Materials and methods
To explore the tolerance of the present method toward more
reactive functional groups, a series oxidative hydroxylation of aryl-
boronic acids with the optimal reaction conditions (Table 1, entry
3) were carried out, and the results were summarized in Table 2.
With g-C3N4 as photocatalyst, a wide range of arylboronic acids,
bearing either electron-donating group such as methoxy, biphenyl
and naphthalenyl or electron withdrawing groups such as cyano,
fluoride, bromide, and nitro proceeded smoothly to the
corresponding aryl alcohols in satisfactory yields. The conversion
efficiency was affected by electronic effects of the substrates. For
example, 4-fluorophenylboronic acid afforded the product 2c in a
100% yield after 12 h, while 4-methylphenylboronic acid gave the
corresponding product 2g in a 35% yield after 12 h. It was clear that
arylboronic acids bearing electron-withdrawing groups (Table 2,
2b-2f) showed higher reactivity than those bearing electron-
donating groups (2g). What’s more, this photocatalytic method
In a typical reaction, 10 mg of g-C3N4 or about 250 mg g-C3N4/
LMPET (containing 3.18% g-C3N4), 0.5 mmol of N,N-
diisopropylethylamine (iPr2NEt), and 0.1 mmol of boronic acid
were added to 5 mL of isopropanol (2-PrOH) in the reactor. When
g-C3N4 was used as the photocatalyst, the reaction mixture was
illuminated by a CEL-HXF300 Xenon lamp with a UV filter (filtered
light at k > 400 nm; CEL-HXF300, Beijing Education Au-light Co.,
Ltd, China) under magnetic stirring. For post-reaction analysis,
the g-C3N4 was separated from the reaction mixture through filtra-
tion. The filtered liquid was then quantitatively analyzed through a
GC (Agilent 6890N) equipped with a mass spectrometric detector
(MS, Agilent 5973) and Agilent Technology HP-5MS capillary col-
umn (30 m ꢁ 0.25 mm ꢁ 0.25 mm) using high-purity He as the
carrier gas and dodecane as the internal standard. When g-C3N4/
LMPET was used as the photocatalyst, the reaction mixture was