Aerobic photooxidative hydroxylation of boronic acids catalyzed by anthraquinone-containing polymeric photosensitizer

Article abstract of DOI:10.1039/d0ra00176g

We report herein the synthesis of a polymeric photosensitizer and its application in aerobic photooxidative hydroxylation of boronic acids. The polymeric photosensitizer was synthesized by the condensation of anthraquinone-2-carbonyl chloride (AQ-2-COCl) with poly (2-hydroxyethyl methacrylate) (PHEMA). The photo-oxidative hydroxylation of boronic acids using anthraquinone-containing-poly (2-hydroxyethyl methacrylate) (AQ-PHEMA) was then explored and shown to exhibit high efficiency and broad scope. Moreover, AQ-PHEMA could be easily recovered and reused for more than 20 times without significant loss of the catalytic activity.

Full text of DOI:10.1039/d0ra00176g

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Aerobic photooxidative hydroxylation of boronic
acids catalyzed by anthraquinone-containing
polymeric photosensitizer†
Cite this: RSC Adv., 2020, 10, 7927
a
a
b
*
*
Yang Chen, Jianhua Hu and Aishun Ding
We report herein the synthesis of a polymeric photosensitizer and its application in aerobic photooxidative
hydroxylation of boronic acids. The polymeric photosensitizer was synthesized by the condensation of
anthraquinone-2-carbonyl chloride (AQ-2-COCl) with poly (2-hydroxyethyl methacrylate) (PHEMA). The
photo-oxidative hydroxylation of boronic acids using anthraquinone-containing-poly (2-hydroxyethyl
methacrylate) (AQ-PHEMA) was then explored and shown to exhibit high efficiency and broad scope.
Moreover, AQ-PHEMA could be easily recovered and reused for more than 20 times without significant
loss of the catalytic activity.
Received 8th January 2020
Accepted 17th February 2020
DOI: 10.1039/d0ra00176g
rsc.li/rsc-advances
hydroxylation of boronic acids is another hot topic, which has
received much attention in the past decade.29–32 In this trans-
Introduction
formation, the superoxide radical anion generated by single
Oxidative hydroxylation of boronic acids is one of the most
important methods for the synthesis of phenols or alcohols.1–7
Strong oxidants such as ozone,8 high-valence halogen
compounds,9–11 peroxides and N-oxides12 are efficient reagents
for such transformations (Scheme 1a). However, these methods
generate large amounts of wastes detrimental to the environ-
ment. Oxygen in air is a green oxidant with environment-
friendly feature. The methods using oxygen have been well
studied.13,14 Some Ru,15 Pd,16 Fe,17 Cu,18–20 complexes have been
reported to be effective catalysts in this reaction (Scheme 1b).
However, the catalysts are expensive, hard to synthesis, and not
environmentally friendly. The metal-free catalyst processes
provide a promising alternative (Scheme 1c)21 wherein good
catalytic activity could be envisioned.
Scheme
1 Oxidative hydroxylation of boronic acids with small
molecular catalysts.
Visible light is an inexhaustible green energy.22–24
Photosensitizer-catalyzed transformations have shown very
important applications in photochemistry.25–27 Numerous
achievements have been reported in this eld. For example, Guo
2
group reported selective cleavage and formation of Csp –I bond,
using carefully-designed thioxanthone derivative.28 This was the
2
rst example of Csp –I bond cleavage by means of photosensitiz-
ing, which highlights the potential of small molecular photosen-
sitizer. Visible light photosensitizer-catalyzed oxidative
aState Key Laboratory of Molecular Engineering of Polymers, Department of
Macromolecular Science, Fudan University, 2005 Songhu Road, Shanghai 200438,
PR China. E-mail: hujh@fudan.edu.cn; Fax: +86-21-31242888; Tel: +86-21-55665280
bDepartment of Chemistry, Fudan University, 2005 Songhu Road, Shanghai 200438,
PR China. E-mail: shunzi0522@126.com; Fax: +86-21-31249190; Tel: +86-21-
31249190
Scheme 2 The synthetic procedure of AQ-PHEMA and its application
† Electronic supplementary information (ESI) available: Experimental procedures,
and characterization data for all compounds. See DOI: 10.1039/d0ra00176g
in photooxidative hydroxylation of boronic acids.
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electron transfer could coordinate with the boron atom, then the catalyzed photooxidative hydroxylation of boronic acids. The
reaction undergoes rearrangement and hydrolysis to produce the initial attempt was carried out using i-Pr2NEt as the electron
nal phenol or alcohol.33,34 Small molecule photosensitizers has donor, CH3CN as the solvent at rt under air atmosphere. An
the signicant disadvantage as a homogenous catalyst, as they are 86% NMR yield of desired product 4-methoxyphenol (2a) was
difficult to be recycled or reused. With our continuous interest in formed aer irradiation under a purple LED for 35 hours
the synthesis and application of PHEMA,35 we proposed to (entry 1, Table 1). With this result in hand, a survey of solvents
synthesize a novel macromolecular photocatalyst by introducing was carried out (entries 2–16, Table 1). Reactions in ether,
anthraquinone (AQ) group of anthraquinone-2-carboxylic acid toluene, or dichloromethane gave lower yield (entries 2–4,
(AQ-2-COOH)36–38 into polymeric chains of PHEMA (Scheme 2). Table 1). While methyl tert-butyl ether (MTBE), acetone,
This polymeric catalytic material can be easily recovered and CH3NO2, or THF resulted in slightly higher yield (entries 5–8,
reused aer catalyzing photoreaction. Herein, we wish to report Table 1). When methyl acetate, EtOH or CH3OH was chosen as
our recent research in aerobic oxidative hydroxylation of boronic the solvent, the yield of 2a was remarkably increased (entries
acids employing AQ-PHEMA (for synthesis and characterization of 9–11, Table 1). Reactions in propyl acetate or isopropyl acetate
AQ-PHEMA, see ESI†) as catalyst.
resulted in similar excellent yields (entries 12 and 13, Table 1).
When ethyl acetate, dimethyl carbonate (DMC) or 1,4-dioxane
was employed as the solvent, the NMR yield of 2a was
increased to 99% (entries 14–16, Table 1). Considering the
reaction time, 1,4-dioxane was chosen as the optimal solvent.
Notably, AQ-PHEMA is insoluble in all the above tested
solvents and can only be dispersed in the solvent, so it can be
Results and discussion
Optimization and scope investigation
In the beginning, (4-methoxyphenyl)boronic acid (1a) was
chosen as the model substrate to optimize the AQ-PHEMA-
Table 1 Optimization of the reaction conditionsa
Entry
Solvent
Catalyst (mol%)
e-Donor (equiv.)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
CH3CN
Et2O
Toluene
DCM
MTBE
Acetone
CH3NO2
THF
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
1
—
3
3
3
—
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
NH3$H2O (4 mL)
DBU
35
48
48
48
48
48
37
42
38
33
32
36
36
29
34
27
27
27
27
27
27
27
27
27
27
27
12
27
86
34
46
51
67
68
74
78
84
89
92
95
96
99
99
99
9
Methyl acetate
EtOH
MeOH
Propyl acetate
Ispropyl acetate
Ethyl acetate
DMC
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25e
26e,f
27g
28h
45 (34)c
69
82
84
Dicyclohexylamine
NEt3
i-Pr2NEt (1)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
i-Pr2NEt (2)
81 (14)c
99 (97)d
83 (11)c
11 (89)c
0 (99)c
0 (99)c
99
10 (90)c
a
The reaction were carried out using 1a (1 mmol) in solvent (5 mL), irradiated by purple LED under air atmosphere at rt. (Based on AQ anchored on
b
PHEMA, the mass of 5 mol% AQ-PHEMA is 17 mg; the mass of 3 mol% AQ-PHEMA is 10 mg; the mass of 1 mol% AQ-PHEMA is 3 mg). Yield
determined by 1H NMR analysis using CH2Br2 (1 mmol) as internal standard. Recovered yield of 1a determined by 1H NMR analysis using
c
d
e
f
CH2Br2 (1 mmol) as internal standard. Isolated yield of 2a. The reaction was carried out without light. The reaction was carried out at
80 ꢀC. g The reaction was carried out using 3 mol% AQ as catalyst. The reaction was carried out in the presence of 4 mg of PHEMA.
h
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Table
2
The oxidative hydroxylation of arylboronic acid under
easily recovered by simple ltration. Next, the screening of
electron donors was carried out. Ammonia gave a poor yield
and the DBU did not work very well (entries 17 and 18, Table
1). Secondary amine (dicyclohexylamine) and tertiary amine
(NEt3) gave decreased yields (entries 19 and 20, Table 1).
Decreasing the amount of i-Pr2NEt to 1 equivalent resulted in
a decreased in yield (entry 21, Table 1). Next, optimization of
catalyst loading were conducted (entries 22 and 23, Table 1).
The results showed that 3 mol% catalytic is suitable (entry 22,
Table 1). Control experiments were also performed which
demonstrated the necessity of both the catalyst and light
(entries 24 and 25, Table 1). Furthermore, a reaction carried
out at 80 ꢀC without light did not lead to any conversion,
eliminating the possibility of the thermal effect of the LED
light (entry 26, Table 1). Next, the oxidation using AQ as
catalyst was conducted as comparasion. The reaction was
completed in 12 hours in an excellent yield (entry 27, Table 1).
However, AQ was difficult to recover in 1,4-dioxane. Finally,
the oxidation using PHEMA as catalyst was investigated and no
reaction took place (entry 28, Table 1). The above results
clearly demonstrated that AQ was the key catalyst in this
transformation. We also tried other different wavelengths for
LED lights (Table S1†). The results showed that reactions at
other wavelengths were similar to background reactions and
purple LED is the only working light source in this reaction.
Thus, Condition A (3 mol% of AQ-PHEMA, i-Pr2NEt (2 equiv.),
1,4-dioxane, purple LED, air (1 atm), and room temperature)
was considered as the optimized conditions for further
studies.
condition Aaa
With the optimal conditions in hand, we explored the
scope of this oxidative hydroxylation (Table 2). Firstly, the
electronic effect of the arylboronic acids was examined.
Substrates with strong electron donating group, like ortho-,
meta- and para-methoxy, showed nice reactivity with excellent
yields (2a–2c). Para-aminophenol was also obtained in an
excellent yield (2d). Weak electron donating group such as
ortho-, meta- and para-methyl (2e–2g), ethyl (2h), and phenyl
(2i) were also conducted and generated in similar excellent
yields. Phenol could be obtained from phenylboronic acid
with an excellent yield (2j). Nice reactivity was also acquired
with halogen bearing arylboronic acids (2k–2n). Substrates
with strong electron withdrawing groups (EWGs), including
nitro (2o), ortho-, meta- and para-(triuoromethyl) (2p–2r),
and ortho-, meta- and para-nitrile (2s–2u) also showed high
reactivities as well as excellent yields. Reactions with other
electron withdrawing groups, like formyl (2v), acetyl (2w),
methoxy carbonyl (2x) and carboxyl (2y) were also showed
excellent reactivities in excellent yields. Further naphthyl
groups were employed, and both a- and b-naphthol (2z and
2aa) could obtain excellent yields. Next, we tested some het-
eroaromatic boronic acids. Unfortunately, the resulting
reaction mixtures were complicated (2ab–2af).
a
All reactions were carried out using 1 (1 mmol), AQ-PHEMA (3 mol%),
i-Pr2NEt (2 equiv.), in 1,4-dioxane (5 mL) irradiated by a purple LED light
at rt under air atmosphere. Isolated yield was reported.
boronic acid ester (3d and 3e) also gave excellent yields of
phenol (2j) and phenylmethanol (4e). The above results showed
high functional group tolerance and very good substrate scope.
Finally, a gram–scale reaction using 1x under condition A
afforded 2x in 96% (Scheme 3), demonstrating the scalability
and practicality of the current reaction.
Furthermore, some alkyl and alkenyl boronic acid, as well as
aryl boronic acid esters were studied (Table 3). Under condition
A, cyclohexylboronic acid (3a) gave a good yield of cyclohexanol
(4a). Alkenyl boronic (3b and 3c) gave similar good yields of 4b
and 4c. In this case, aldehyde products were formed. Aryl
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Table 3 The oxidative hydroxylation of 3a–e under condition Aaa
Fig. 2 1H NMR spectra for the recovered AQ-PHEMA (after 21 cycles)
and the original AQ-PHEMA in d6-DMSO.
a
All reactions were carried out using 1 (1 mmol), AQ-PHEMA (3 mol%),
i-Pr2NEt (2 equiv.), in 1,4-dioxane (5 mL) irradiated by a purple LED light
at rt under air atomsphere. Isolated yield was reported.
Scheme 3 Gram scale reaction.
Fig. 3 The GPC traces of the original AQ-PHEMA and the recovered
Recycling experiments
AQ-PHEMA (after 21 cycles).
To investigate the recyclability of AQ-PHEMA in photooxidative
hydroxylation of boronic acids. 4-Methoxycarbonylbenzenebor-
onic acid (1x) was selected as the model substrate for the AQ-
PHEMA recycling experiments under condition A. 1x was
completely consumed aer 27 hours and the NMR yield of the
product 4-methoxycarbonylphenol (2x) was 99% in the rst cycle
of the photocayalytic reaction. Aer the rst cycle, the catalyst AQ-
PHEMA could be easily separated and recovered by simple ltra-
tion and directly used for the next cycle under the same procedure.
Finally, we found that AQ-PHEMA could be reused more than 20
times without signicant loss of the catalytic activity (Fig. 1).
Aer the recycling experiments (21 cycles), the recovered AQ-
PHEMA was characterized by 1H NMR and GPC. As shown in
Fig. 2, no signicant changes were observed on the 1H NMR spectra
compared to the original AQ-PHEMA. The molecular weight Mn,GPC
and molecular distribution Mw/Mn of the recovered AQ-PHEMA
(aer 21 cycles) were very close to the original AQ-PHEMA
(Fig. 3). These results clearly indicated that the structure of AQ-
PHEMA did not change during the whole catalytic procedure.
Mechanism studies
To gain insights into the reaction mechanism, some control
experiments were carried out (Scheme 4). When 2 equivalents of
9,10-dimethylanthracene (5) was added to the reaction system as
an organic singlet oxygen quencher,39 severe inhibition was
observed. Bis((dibutylcarbamothioyl)th-io) nickel(II) (6) was also
tried as an inorganic singlet oxygen quencher.40 Similarly, severe
inhibition was also observed with 2 equivalents of 6 (Scheme 4a).
N-tert-Butyl-1-phenylmethanimine oxide (7), a superoxide radical
anion quencher,41–43 was used, the reaction was efficiently
inhibited when 2 equivalents of 7 was added (Scheme 4b). The
results above suggested that both singlet oxygen and superoxide
radical anion were likely involved in this reaction. Cyclic vol-
tammogram of AQ-PHEMA was also recorded under irradiation.
(see Fig. S2†) The redox potential validated the single electron
oxidation of AQ-PHEMA by oxygen might occur under irradiation.
Proposed mechanism
Based on the above control experiments and literature
reports,36–38,44–48 a possible reaction mechanism was described
as shown in Fig. 4. Firstly, AQ-PHEMA was excited under photo
Fig. 1 Recycling experiments of the photocatalytic reaction of 1x.
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