Metal-free UV-Vis-light-induced aerobic oxidative hydroxylation of arylboronic acids in the absence of a photosensitizer

Article abstract of DOI:10.1039/c3ra46516k

A simple and efficient method has been developed for the metal-free photosynthesis of phenols with UV-Vis (ultraviolet-visible) light in quartz tubes. The protocol uses readily available arylboronic acids as the starting materials, triethylamine as the sacrificial electron donor, and air as the oxidant. The method shows high efficiency, environmental friendliness and mild reaction conditions, without the addition of a photosensitizer. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ra46516k

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Metal-free UV-Vis-light-induced aerobic oxidative
hydroxylation of arylboronic acids in the absence of
a photosensitizer†
Cite this: RSC Adv., 2014, 4, 12977
ab
Min Jiang,a Yong Li,ab Haijun Yang,
Ruilong Zong,a Yunhe Jinb and Hua Fub
Received 8th November 2013
Accepted 20th January 2014
*
DOI: 10.1039/c3ra46516k
www.rsc.org/advances
A simple and efficient method has been developed for the metal-free
As is well known, about 8% of solar light is high energy
photosynthesis of phenols with UV-Vis (ultraviolet-visible) light in ultraviolet light that can activate organoboron compounds.9
quartz tubes. The protocol uses readily available arylboronic acids as Based on previous results, we applied photocatalysis to our study
the starting materials, triethylamine as the sacrificial electron donor, on the transformation of arylboronic acids to phenols. First, 4-
and air as the oxidant. The method shows high efficiency, environ- methoxylboronic acid (1i) was used as a model substrate to
mental friendliness and mild reaction conditions, without the addition investigate suitable reaction conditions, including using light
of a photosensitizer.
sources with different wavelengths, different solvents, different
amines as the sacricial electron donors, different reaction times
and different atmospheres. As shown in Table 1, the effect of the
solvent was investigated rst, using light from a high pressure
mercury lamp with all ranges of light wavelength as the light
source, triethylamine as the sacricial electron donor, and air as
the oxidant at room temperature for 8 h (entries 1–7); the mixed
solvent, CH3CN–H2O (1 : 10), afforded the highest yield (entry 7),
because of the good dissolving power of 1i in the mixed solvent.
Only trace amounts of the product were observed when the
reaction was performed under a nitrogen atmosphere (entry 8),
while an oxygen atmosphere provided a 97% yield (entry 9). The
effect of the light wavelength was determined using light lters
(10–13); the light with a full range of wavelengths afforded the best
result (entry 7), with the by-products, biphenyl and diphenyl ether,
determined by GC-MS (see the ESI).† When UV light (220–400 nm)
was used, a 76% yield was observed (entry 8); in fact, light from
the high pressure mercury lamp is of wide UV wavelength. Other
amines were also used (entries 15 and 16), and these gave slightly
lower yields. When the reaction time was shortened, the yields
were decreased (entries 17 and 18). The amount of triethylamine
was varied (entries 19–21), and the results showed that one equiv.
of triethylamine was suitable. Therefore, suitable conditions for
the photosynthesis of phenols from arylboronic acids are as
follows: a high pressure mercury lamp with all ranges of light
wavelength as the light source, triethylamine as the sacricial
electron donor, CH3CN–H2O (1 : 10) as the solvent, and air as the
oxidant, at room temperature for 8 h.
Chemists have been making great efforts to nd a green, effi-
cient method for the synthesis of organic compounds. Photo-
synthesis has shown good potential for the synthesis of organic
compounds as it is easily accessible and clean.1 A lot of light
sensitive materials have been reported to be good catalysts,
such as TiO2,2 C3N4,3 Ru complexes,4 Ir complexes5 and organic
compounds.6 In those studies, most attention was focused on
visible light catalysts, because there is about 50% visible light in
solar light. Recently, some progress has been made towards the
synthesis of phenols using arylboronic acids as the substrate.7
Very recently, Xiao and co-workers developed a visible-light
initiated aerobic oxidative hydroxylation method for arylbor-
onic acids using air as the oxidant, in the presence of
[Ru(bpy)3Cl2]$6H2O.8a Meanwhile, Scaiano's group reported the
photocatalytic hydroxylation of arylboronic acids using methy-
lene blue as the photosensitizer.8b Both of these focused on
visible light photocatalysis in glass tubes. Herein, we report a
simple and highly efficient metal-free UV-Vis-light-induced
hydroxylation method for arylboronic acids in quartz tubes,
without the addition of a photosensitizer.
aBeijing Key Laboratory for Analytical Methods and Instrumentation, Department of
Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail: cyhj@
tsinghua.edu.cn; Fax: +86-10-62788971
bKey Laboratory of Bioorganic Phosphorus Chemistry and Chemical Biology (Ministry
of Education), Department of Chemistry, Tsinghua University, Beijing 100084, P. R.
China
With the optimized conditions for the photosynthesis of
phenols from arylboronic acids in hand, we investigated the
scope for the transformation of arylboronic acids to phenols. As
shown in Table 2, the examined substrates provided good to
† Electronic supplementary information (ESI) available: General procedure for
synthesis, characterization data, and 1H and 13C NMR spectra of compounds
2a–w. See DOI: 10.1039/c3ra46516k
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Table 1 Photosynthesis of 4-methoxyphenol (2i) from 4-methox- Table 2 Photosynthesis of phenols (2) from arylboronic acids (1)a
yphenylboronic acid (1i): optimization of conditionsa
2 (yieldb)
Entry
Light source
Solvent
Amine
Yield (%)b
1
2
3
4
5
6
7
8
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
>400 nm
>350 nm
>280 nm
254 nm
220–400 nm
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
UV-Vis
H2O
CH3OH
CH3CN
C2H5OH
Toluene
DMSO
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
N(Et)3
iPrN(Et)2
NH(Et)2
N(Et)3
N(Et)3
N(Et)3
N(Et)3
—
80
62
67
54
24
84
97
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
A/W (1 : 10)
Tracec
97d
Trace
10
14
54
76
96
9
10
11
12
13
14
15
16
17
18
19
20
21
89
68e
82f
97g
90h
Tracei
a
Reaction conditions: 4-methoxylboronic acid (1i) (1 mmol), solvent
(2.0 mL), reaction time (8 h), N(Et)3 (1 mmol), under air for entries 1–
7 and 10–21, under N2 for entry 8 and under O2 for entry 9, UV-Vis:
full wavelength light from a high pressure mercury lamp without a
b
c
lter. Isolated yield. A/W ¼ acetonitrile/water. Under nitrogen
d
e
atmosphere. Under oxygen atmosphere. Reaction time (4 h).
Reaction time (6 h). N(Et)3 (2 mmol). N(Et)3 (0.5 mmol). In the
absence of N(Et)3.
f
g
h
i
a
Reaction conditions: arylboronic acid (1) (1.0 mmol), N(Et)3
excellent yields. For arylboronic acids, their reactivities were (1.0 mmol), CH3CN–H2O (1 : 10) (2.0 mL), reaction temperature (rt,
ꢀ25 ꢁC), reaction time (8 h), under high pressure mercury lamp.
affected by the steric and electronic effects of the substrates; the
substrates with electron-donating groups provided higher yields
b
Isolated yield.
than those with electron-withdrawing groups, and the substrates
with a bigger steric hindrance gave lower yields. Obviously, the
substrates with uoro (2l) or triuoromethyl (2r) provided lower methylene blue, rst obtained an electron from amine, then
yields. The reactions had high tolerances for functional groups, transferred the electron to oxygen to form a superoxide anion,
including ether (2i–k), carbon-halo bonds (2l–n), nitro (2o, 2p), and the reaction of the superoxide anion with arylboronic acid
cyano (2q), carboxyl (2s and 2t), and ester (2u). Therefore, the then afforded phenol. However, the reports did not include the
present method showed a good way to synthesise phenols from intermediates produced from arylboronic acids, and the
arylboronic acids under light irradiation.
source of the by-products, such as biphenyls and diphenyl
Compared with Xiao and Scaiano's studies, the present ether, were not shown. In order to explore the present mech-
reactions were performed in quartz tubes instead of glass tubes anism, electron paramagnetic resonance (EPR) was used to
because quartz tubes are of a better transmittance for ultraviolet monitor the reaction process, and a radical trapper was
light than glass tubes. As shown in Scheme 1, two control applied to trap the free radicals generated during the reac-
experiments were carried out. The reaction of methox- tion.9 As shown in Scheme 2, dimethyl pyridine N-oxide
yphenylboronic acid (1i) to 4-methoxyphenol (2i) in a quartz (DMPO) was chosen as the radical trapper to determine the
tube performed well, but a lower yield (12%) was obtained when oxide radicals. 0.1 equiv. of phenylboronic acid was quickly
the experiment was carried out in a glass tube. Therefore, mixed with DMPO (50 mM) in 2 mL of CH3CN–H2O (1 : 10),
ultraviolet light could be a key factor for the reactions in Table 2. and the mixture was moved into an EPR at cell. In situ EPR
In previous work,8 the reaction was proposed to be a free spectra were elicited under irradiation from a high pressure
radical process: the light sensor, such as [Ru(bpy)3Cl2]$6H2O or mercury lamp for 4 min (Fig. 1).
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Scheme 3 Control experiment using 18O-labeled water as the solvent.
Scheme 1 Photosynthesis of 4-methoxyphenol (2i) from 4-methox-
yphenylboronic acid (1i) in a quartz tube or a glass tube.
As we have determined the involvement of a carbon central
free radical and a hydroxyl free radical as mentioned above, a
possible mechanism for the photosynthesis of phenols from
arylboronic acids is proposed in Scheme 4. In the presence of
triethylamine, arylboronic acid (1) is activated by UV-Vis light to
form I, and the treatment of I with oxygen generates a super-
oxide free radical (II) and a nitrogen cation (III). The reaction of
II with a proton from water provides an HO2c free radical (IV),11
and the treatment of IV with III affords a hydroxyl free radical
(V), leaving a cation, VI.8a The reaction of V with arylboronic
acids gives a carbon central free radical (VII), leaving boronic
acid. The combination of VII with V affords the target product
(2) (Route 1). In addition, the reaction of phenol with V gives
a free radical (VIII), the conjugation of VIII with VII leads to a
byproduct (3) (Route 2), and the dimerization of VII also yields a
byproduct (4) (Route 3).
In conclusion, we have developed a simple and efficient
method for the metal-free photosynthesis of phenols with UV-
Vis (ultraviolet-visible) light in quartz tubes. The protocol uses
readily available arylboronic acids as the starting materials,
triethylamine as the sacricial electron donor, CH3CN–H2O
(1 : 10) as the solvent, and air as the oxidant, and the method
shows high efficiency, environmental friendliness and mild
reaction conditions, without the addition of a photosensitizer.
The EPR experiments show the existence of a carbon free radical
and a hydroxyl free radical in the reactions, which is helpful for
Scheme 2 Photosynthesis of phenol (2a) from phenylboronic acid (1a)
trapped by DMPO.
It was shown that both hydroxyl free radicals (star) and
carbon central free radicals (dot) existed in the reaction
(Fig. 1a). According to the simulated spectra of the hydroxyl free
radical (Fig. 1b) and the carbon central free radical (Fig. 1c),
there was a good agreement with the measured spectrum
(Fig. 1a) and simulated spectrum for the hydroxyl free radical
and carbon central free radical (Fig. 1d). The A-values coincided
with the literature values as reported by Mason and Kalya-
naraman (for the hydroxyl free radical, AH ¼ 1.5 mT, AN
¼
1.5 mT),10a,b and Jones (for the phenyl radical which is a carbon
central radical, AN ¼ 1.60 mT, AH ¼ 2.46 mT).10c We therefore
supposed that the carbon central free radical was produced
from arylboronic acids.
A control experiment was performed using 18O-labeled water
as the solvent. As shown in Scheme 3, no 18O-labeled product
was found by mass spectrometry, and the result indicated that
the oxygen in 2i was from the O2 in air (not from water). A
similar result was reported by Xiao's group.8a Water provided a
proton and a hydroxyl anion in the reaction. However, the
hydroxyl anion didn't take part in the formation of the product.
Fig. 1 In situ EPR spectra for the reaction shown in Scheme 2. (a)
Measurement spectrum; (b) simulated spectrum for a hydroxyl free
radical (g ¼ 2.0023, AH ¼ 1.50 mT, AN ¼ 1.50 mT); (c) simulated
spectrum for a carbon central free radical (g ¼ 2.0023, AH ¼ 2.40 mT, Scheme 4 Proposed mechanism for the photosynthesis of phenols
AN ¼ 1.58 mT); (d) simulated spectrum for a hydroxyl free radical and from arylboronic acids under irradiation from
a high pressure
carbon central free radical.
mercury lamp.
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the elucidation of the reaction mechanism. Further investiga-
tion on this mechanism is in progress.
The authors wish to thank the National Natural Science
Foundation of China (Grant no. 21105054) for nancial
support.
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