Quaternary ammonium hydroxide-functionalized g-C3N4 catalyst for aerobic hydroxylation of arylboronic acids to phenols

Article abstract of DOI:10.1002/jccs.202000141

A new and efficient metal-free approach toward the synthesis of phenols via an aerobic hydroxylation of arylboronic acids by using a novel quaternary ammonium hydroxide g-C₃N₄ catalyst has been described. The functionalized quaternary ammonium hydroxide (g-C₃N₄-OH) has been prepared from graphitic carbon nitride (g-C₃N₄) scaffold by converting the residual –NH₂ and –NH groups to quaternary methyl ammonium iodide by performing a methylation reaction with methyl iodide followed by ion-exchange with 0.1 N KOH or anion exchange resin Amberlyst A26 (OH- form) by post-synthetic modification. The resultant g-C₃N₄-OH was characterized by XRD, FTIR, field-emission scanning electron microscope (FESEM), high-resolution transmission electron microscope (HRTEM), N₂ adsorption/desorption isotherms, and acid–base titration. Tested as solid-base catalysts, the g-C₃N₄-OH showed excellent catalytic activity in the aerobic hydroxylation reaction by converting a variety of arylboronic acids to the corresponding phenols in high yields. More importantly, the g-C₃N₄-OH solid-base has been successfully reused four times with the minor loss of initial catalytic activity (10.5percent).

Full text of DOI:10.1002/jccs.202000141

Received: 4 April 2020
Accepted: 7 June 2020
DOI: 10.1002/jccs.202000141
A R T I C L E
Quaternary ammonium hydroxide-functionalized g-C3N4
catalyst for aerobic hydroxylation of arylboronic acids to
phenols
Ibrahim Muhammad1
| Subramaniyan Mannathan2
| Manickam Sasidharan1
1SRM Research Institute and Department
of Chemistry, SRM Institute of Science
and Technology, Kattankulathur,
Tamilnadu, India
2Department of Chemistry, SRM
University-AP, Amaravati, Andhra
Pradesh, India
Abstract
A new and efficient metal-free approach toward the synthesis of phenols via an
aerobic hydroxylation of arylboronic acids by using a novel quaternary ammonium
hydroxide g-C3N4 catalyst has been described. The functionalized quaternary
ammonium hydroxide (g-C3N4-OH) has been prepared from graphitic carbon
nitride (g-C3N4) scaffold by converting the residual –NH2 and –NH groups to qua-
ternary methyl ammonium iodide by performing a methylation reaction with
methyl iodide followed by ion-exchange with 0.1 N KOH or anion exchange resin
Amberlyst A26 (OH- form) by post-synthetic modification. The resultant g-C3N4-
OH was characterized by XRD, FTIR, field-emission scanning electron microscope
(FESEM), high-resolution transmission electron microscope (HRTEM), N2 adsorp-
tion/desorption isotherms, and acid–base titration. Tested as solid-base catalysts,
the g-C3N4-OH showed excellent catalytic activity in the aerobic hydroxylation
reaction by converting a variety of arylboronic acids to the corresponding phenols
in high yields. More importantly, the g-C3N4-OH solid-base has been successfully
reused four times with the minor loss of initial catalytic activity (10.5%).
Correspondence
Manickam Sasidharan, SRM Research
Institute and Department of Chemistry,
SRM Institute of Science and Technology,
Kattankulathur, Tamilnadu, India.
Email: sasidham@srmist.edu.in
Subramaniyan Mannathan, Department
of Chemistry, SRM University-AP,
Amaravati, Andhra Pradesh, India.
Email: mannathan.s@srmap.edu.in
Funding information
Department of Science and Technology
(DST), New Delhi, India, Grant/Award
Number: DST/INSPIRE/04/2015/002987;
Science & Engineering Research Board,
Grant/Award Number: SB/S1/PC-
043/2013
K E Y W O R D S
aerobic hydroxylation, functionalization of g-C3N4, heterogeneous catalyst, phenols, quaternary
ammonium hydroxide
1 | INTRODUCTION
because of its interesting electronic and optical properties,
photo-electrochemical water splitting,[4,5] CO2 reduction,[6,7]
In recent years, metal-free organic polymeric materials have
received great attention as heterogeneous catalysts owing to
their abundance, nontoxicity, chemical/thermal stability,
and readily tailorable surface chemistry.[1–3] Carbon as het-
erogeneous catalysts allows controlling their surface chemis-
try and designing catalytic sites for a specific application. In
this perspective, graphitic carbon nitride (g-C3N4) has
received great attention because of its photocatalytic and
other diverse applications.[1–3] In the last decade, g-C3N4 has
been extensively investigated in the field of photo-catalysis
and photocatalytic degradation of pollutants.[8–11] Later on,
g-C3N4 was also widely studied as versatile support for noble
metals to accomplish photocatalytic hydroxylation of ben-
zene, oxidation and hydrogenation reactions, Suzuki and
Sonogashira couplings, and Knoevenagel reaction.[12–16] In
the context of metal-free catalysis, g-C3N4 has been tethered
with several functional groups such as amines, poly-
ethyleneimine (PEI), and hydrazine either by chemical
grafting or physical impregnation for CO2 capture, CO2
reduction, and water splitting to H2.[17–19] Later on, boronic
© 2020 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2020;1–7.
http://www.jccs.wiley-vch.de
1

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MUHAMMAD ET AL.
acid and phenyl groups were also chemically functionalized
on the surface of g-C3N4 scaffold to enhance the photo-
luminescence and sensing properties, which helps in the
detection of glycoprotein under physiological environ-
ments.[20,21] However, g-C3N4 has rarely been utilized in
non-photocatalytic heterogeneous catalysis for organic trans-
formations. Very recently, –SO3H functionality has been
anchored onto the g-C3N4 matrix, which served as an effi-
cient solid-acid catalyst in Biginelli three component cou-
pling reactions.[22] Mesoporous g-C3N4 has also been
scrutinized for thermal reactions involving oxidation, and
hydrogenation reactions without any light illumina-
tion.[23,24] In view of academic and industrial relevance,
phenols constitute an essential class of compounds mainly
due to its importance in biological, pharmaceuticals, and
polymer industries.[25] Among the several methods that
have been developed for phenol synthesis, oxidative
hydroxylation of organoboronic acids represents a simple
and elegant approach to obtain phenols and its derivatives
in an efficient way. Conventionally, peroxides, oxones,
benzoquinones, N-oxides, and hypervalent iodine com-
2 | RESULTS AND DISCUSSION
The crystallinity of pristine g-C3N4, quaternary ammo-
nium salt (g-C3N4-I), and quaternary ammonium hydrox-
ides (g-C3N4-OH) were assessed with powder X-ray
diffraction as shown in Figure 1. The pristine g-C3N4
exhibits an intense peak at 27.4ꢀ corresponding to (002)
reflection, which is ascribed to graphite-like interlayer-
stacking[16]; whereas other weak (100) reflection at 13.1ꢀ
is attributed to in-plane tris-s-triazine motif structural
packing. On treatment with methyl iodide and subse-
quent conversion to g-C3N4-OH, the diffraction peak fea-
tures are well maintained except that the intensity of
(100) peak is rather reduced. These observations suggest
that possible delamination of g-C3N4 layer and subse-
quent restacking upon treatment with CH3I and KOH
(0.1 N KOH). The XRD results indeed confirm the high
stability of g-C3N4 toward functional group modification.
The FTIR spectra of pristine and functionalized sam-
ples are shown in Figure 2a–c. The broad absorption
bands at 2,830–3,450 cm−1 are assigned to N–H
stretching vibration.[34] The bands observed between
1,000 and 1,650 cm−1 corresponds to C=N and C–N het-
erocyclic aromatic ring units, which are characteristic
peaks of tris-s-triazine as observed in the case of XRD.[35]
Upon treatment with methyl iodide, the intensity of N–H
rocking vibration at 703 and 782 cm−1 decreased signifi-
cantly, and the appearance of a new band at 802 cm−1
(O–H out of plane bending vibration) confirms the for-
mation of g-C3N4-OH. However, due to overlapping of –
NH and –OH stretching vibrational frequencies, func-
tional group information could not be drawn in the high-
frequency region between 3,000–3,600 cm−1. We further
investigated the morphological features with electron
microscope (FESEM and HRTEM). Scanning electron
microscope of pristine g-C3N4, g-C3N4-I, and g-C3N4-OH
materials showed rod-like morphology (Figure 3a–c); the
particle size of pristine material was found to be ~2–7 μm
in length with thickness of about 200–300 nm, whereas
the post-synthetically modified g-C3N4-I, and g-C3N4-OH
materials were found to be in the range of 1–3 μm in
length. The rod-like morphology was further observed
pounds were employed to accomplish such
a
transformation.[26–29] However, nowadays, the eco-friendly
molecular oxygen (O2 and air) are conveniently used as an
effective oxidant, albeit in the presence of transition metal
catalysts or under photochemical reaction conditions.[30,31]
Despite these advances, hydroxylation of organoboronic
acids to give phenols with heterogeneous organocatalyst is
scarcely studied.[32] Particularly, methods involving hetero-
geneous organo-catalysts under photochemical free condi-
tions remain underdeveloped. Herein we report an aerobic
hydroxylation of arylboronic acids by using a novel quater-
nary ammonium hydroxide g-C3N4 catalyst.[33] The
functionalized quaternary ammonium hydroxide (g-C3N4-
OH) was prepared from graphitic carbon nitride (g-C3N4)
scaffold by converting the residual –NH2 and –NH groups
to quaternary methyl ammonium iodide by performing a
methylation reaction with methyl iodide followed by ion-
exchange with 0.1 N KOH or anion exchange resin
(Amberlyst-A26) by post-synthetic modification (Scheme 1).
The resultant solid base was thoroughly characterized by
XRD, FTIR, FESEM, HRTEM, and acid–base titration.
(a)
(b)
(c)
SCHEME 1 Schematic
representation of the formation
of g-C3N4-OH(c) from g-C3N4(a)

MUHAMMAD ET AL.
3
with high-resolution TEM, which revealed nano-rods of
size approximately ~1–2 μm in length as shown in
Figure 3d–f. The textural features were evaluated with N2
sorption at liquid nitrogen temperature, 77 K (Figure 4a–c).
Both pristine g-C3N4 as well as modified g-C3N4-I, and g-
C3N4-OH samples showed irregular large mesopores of size
ranging 2–12 nm. The large pores could have originated
from restacking and refolding of inter-layers and rod-like
particles. The BET surface of all the materials was found to
be between 38–87 m2/g−1, while the pore volume was
found to be 161 and 208 ml/g−1 for g-C3N4 and g-C3N4-I,
respectively. However, pore volume of g-C3N4-OH was
found to be lower ~61 ml/g−1, possibly due to restacking of
inter-layers after treatment with KOH during anion
exchange. The concentration of OH− estimated by acid–
base titration was found to be 1.6 mmol/g of g-C3N4-OH.
After characterizing g-C3N4–OH, the efficacy of the
solid base for the hydroxylation of phenylboronic acid
(1a) to phenol (2a) was investigated (Table 1). Prelimi-
nary studies on solvent screening indicated that the
reactions with water, acetonitrile, t-butanol, toluene,
DMF, and dichloroethane as solvents are totally ineffec-
tive at 100ꢀC with g-C3N4–OH (20 wt%). However,
changing the solvent to THF, and 1,4-dioxane gave 2a in
61 and 82% yields, respectively (Table 1, entries 1–2).
Notably, binary solvent system comprising THF as co-
solvent, significantly improved the desired product
yield. Among them, solvent systems such as H2O/THF,
acetonitrile/THF, and t-butanol/THF, and toluene/THF
in the ratio of 4:1 (2 ml) provided 2a in 96, 92, and 95%
yields, respectively (entries 3–5). As shown in Table 1,
lowering the reaction temperature, catalyst loading, and
reaction duration led to an incomplete conversion
(entries 7–13). The g-C3N4 serves as functional support
for anchoring quaternary ammonium hydroxide (≡N+
OH−) and moreover, no reaction was observed in the
absence of catalyst (entry 14) and also in the presence of
either pristine g-C3N4 or quaternary ammonium salt (g-
C3N4-I). Similarly, no hydroxylation was realized for the
reaction carried out in an inert atmosphere, which
FIGURE 1 Powder X-ray diffraction pattern of g-C3N4,
g-C3N4-I and g-C3N4-OH
FIGURE 2 FTIR spectra of
g-C3N4, g-C3N4-I, and g-C3N4-
OH: (a) enlarged in the region of
700–900 cm−1; (b) enlarged in
the region of 1,100–1,800 cm−1
and (c) FTIR spectra in
500–4,000 cm−1

4
MUHAMMAD ET AL.
FIGURE 3 FESEM images
of g-C3N4 (a); g-C3N4-I (b); g-
C3N4-OH (c); and HRTEM
images of g-C3N4 (d); g-C3N4-I
(e); g-C3N4-OH (f)
FIGURE 4 N2 adsorption/
desorption isotherms of g-C3N4
(a); g-C3N4-I (b); and g-C3N4-
OH (c)
indeed signifies the importance of oxygen in aerobic
hydroxylation (entry 15). Thus, the 20 wt% catalyst and
H2O/THF solvent system (4:1, 2 ml) at 100ꢀC for 12 hr
was chosen as the final optimized conditions (Table 1)
for further investigation.
With the above standard reaction conditions, we
investigated the scope of the reaction using different
kinds of substituted phenylboronic acids (Table 2). All
substituted arylboronic acids proceeded smoothly and
afforded the corresponding phenolic compounds in good
to excellent yields (66–96%). Both electron-donating and
electron-withdrawing substituents like methyl, methoxy,
fluoro, chloro, bromo, trifluoromethyl, cyano, carbonyl,
and nitro containing aryl boronic acids were progressed
well and gave the corresponding phenolic compounds in
good to high yields. The nature of substituents on the
phenyl ring at para-, or ortho positions on phenylboronic
acids has no major effect on the yield (Table 2). In order
to investigate the reusability of solid base, we recovered
the catalyst by centrifugation, washed thoroughly with
ethanol and water, and vacuum dried for overnight. Then
the reaction was performed under identical conditions
and the catalyst furnished 86% yield after four successive
recycles with loss of 10.5% initial activity.
Based on the literature reports,[36–39] a plausible mecha-
nism is proposed as shown in Scheme 2. The reaction may
be initiated by forming superoxide radical ion I via single
electron transfer from OH− ion to O2. The, superoxide

MUHAMMAD ET AL.
5
TABLE 1
Optimization of
reaction conditions for aerobic
hydroxylation of phenylboronic acida
Entry
1
Solvents
Temp. (ꢀC)
100
Cat. loading (Wt%)
Yield (%)b
THF
20
61
82
96
92
95
56
74
31
18
43
74
79
63
—
—
2
1,4-dioxane
H2O/THF
CH3CN/THF
t-butanol/THF
Toluene/THF
H2O/THF
H2O/THF
H2O/THF
H2O/THF
H2O/THF
H2O/THF
H2O/THF
H2O/THF
H2O/THF
100
20
3
100
20
4
100
20
5
100
20
6
100
20
7
90
20
8
80
20
9
100
5
10
11
12c
13d
14
15e
100
10
100
15
100
20
100
20
100
No catalyst
20
100
aAll the reactions were carried out using phenylboronic acid 1a (1.0 mmol), H2O/THF (4:1 ratio,
2 ml), g-C3N4-OH (20 wt%) at 100ꢀC for 12 hr.
bIsolated yields.
cReaction time 10 hr.
dReaction time 8 hr.
eReaction was carried out in an inert atmosphere.
radical anion reacts with 1a to give the intermediate II and
further protonation gives III. The hydroxyl ion was
obtained back during 1, 2-aryl shift of the intermediate III
and the final phenol products 2a was obtained after the
hydrolysis of IV. To have an insight on the mechanism of
the reaction, we carried out control reactions using a radi-
cal scavenger (TEMPO). Thus, the reaction of 1a with 1.0
equivalent of TEMPO yielded 18% of 2a. The reduction in
the yield of 2a is ascribed to the scavenging of radical inter-
mediate during the course of reaction. Notably, with 2.5
equivalent of TEMPO, no hydroxylation reaction was
observed which in turn confirms that the reaction may pro-
ceed via a radical pathway mechanism.
under vacuum using a rotary evaporator and the resultant
white solid was dried in an oven at 80ꢀC for 24 hr under
air. The dried white solid was transferred to a crucible and
heated in a tubular furnace under nitrogen at 500ꢀC for
4 hr with heating rate of 3ꢀC/min. The resultant pale yel-
low material with yield of 2.30 g was collected and ground
well using mortar and pestle. Finally, the pore forming
nanosilica template was removed by stirring with 15.0 ml
of 5% dil. HF per gram of material for 24 hr at room tem-
perature. Then, the sample was thoroughly washed with
water until the filtrate attains a neutral pH. The final mate-
rial was dried in an oven at 80ꢀC for overnight and the
yield of the final product was found to be 1.7 g.
3 | EXPERIMENTAL
3.1 | Synthesis of g-C3N4
3.2 | Conversion of g-C3N4 to g-C3N4-I
and g-C3N4-OH
In a typical reaction, 0.5 g of g-C3N4 was suspended in a
mixture of 4.0 ml methanol and 1.8 ml methyl iodide in a
50 ml round bottom flask and covered with a septum to
avoid vaporization of solvent as well as the methyl iodide.
The content was then stirred for 24 hr at 25ꢀC and then
Nanosilica (0.5 g) (10 nm) and 4.0 g melamine was dis-
persed by stirring in 5 ml ethanol in a round bottom flask
(50 ml). Then the content was refluxed at 70ꢀC for 6 hr
with continuous stirring. Then, the solvent was removed

6
MUHAMMAD ET AL.
TABLE 2
Substrate scopea
aAll the reactions were carried out with 1a-o (1.0 mmol), g-C3N4-
OH (20 wt%) in H2O/THF (4:1; 2.0 ml) at 100ꢀC for 12 hr unless
otherwise mentioned.
SCHEME 2 A plausible mechanism for the aerobic
hydroxylation of arylboronic acid to phenol
washed with ethanol thoroughly to remove the excess
methyl iodide to get g-C3N4-I. The material was dried in
an oven at 40ꢀC for 24 hr. To obtain g-C3N4-OH, 1 g of g-
C3N4-I was treated with either 10 ml KOH (0.1 M) or
Amberlyst-A26 anion exchange resin at room tempera-
ture for 2 hr. Then the resultant solid was washed with
copious water and then dried under vacuum at 40ꢀC for
overnight and stored in a desiccator.
3.4 | Hydroxylation of phenylboronic
acids
In typical hydroxylation of phenylboronic acid to phenol,
phenylboronic acid (1.0 mmol) and mixture of H2O/THF
(2 ml, 4:1 ratio) were added to a 50 ml round bottom
(RB) flask. The RB flask fitted with a condenser was placed
in a preheated oil bath and stirred at 100ꢀC for 12 hr. The
reaction progress was monitored by thin layer chromatog-
raphy (TLC) and after completion; the reaction mixture
was allowed to cool down to ambiance temperature. The
reaction mixture was extracted with ethyl acetate (3 ml × 3),
dried with anhydrous Na2SO4, and concentrated under vac-
uum. Column chromatography was used to isolate the
product from the concentrated crude reaction mixture.
3.3 | Acid–base titration of g-C3N4-OH
Back titration procedure has been used to estimate the
concentration of OH− liberated by the g-C3N4-OH in
20 ml distilled water. In a typical experiment, 0.5 g of g-
C3N4-OH was suspended in a mixture containing 20 ml
distilled water, 0.5 g NaCl, and 10 ml of HCl (0.1 M) and
stirred at room temperature for 24 hr for completion of
neutralization of [OH−], which was released from the cat-
alyst (g-C3N4-OH).[22,40] Then, two drops of phenolphtha-
lein indicator were added to the aqueous solution and
the excess [H+] was titrated with 0.1 M NaOH until the
appearance of pink color.
4 | CONCLUSIONS
In summary, we have demonstrated a new and efficient
protocol for the hydroxylation of arylboronic acids to
phenols by using quaternary ammonium hydroxide
functionalized graphitic carbon nitride (g-C3N4). The

MUHAMMAD ET AL.
7
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ACKNOWLEDGMENTS
M.S. thanks Science & Engineering Research Board for
their research support (No. SB/S1/PC-043/2013). S.M.
thanks Department of Science and Technology (DST), New
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ORCID
Ibrahim Muhammad https://orcid.org/0000-0002-7254-
6446
Subramaniyan Mannathan https://orcid.org/0000-
0002-3114-8265
Manickam Sasidharan https://orcid.org/0000-0002-
0795-7062
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How to cite this article: Muhammad I,
Mannathan S, Sasidharan M. Quaternary
ammonium hydroxide-functionalized g-C3N4
catalyst for aerobic hydroxylation of arylboronic
acids to phenols. J Chin Chem Soc. 2020;1–7.
https://doi.org/10.1002/jccs.202000141