Highly efficient heterogeneous V2O5@TiO2 catalyzed the rapid transformation of boronic acids to phenols

Article abstract of DOI:10.1002/ejoc.202100614

A V₂O₅@TiO₂ catalyzed green and efficient protocol for the hydroxylation of boronic acid into phenol has been developed utilizing environmentally benign oxidant hydrogen peroxide. A wide range of electron-donating and the electron-withdrawing group-containing (hetero)aryl boronic acids were transformed into their corresponding phenol. The methodology was also applied successfully to transform various natural and bioactive molecules like tocopherol, amino acids, cinchonidine, vasicinone, menthol, and pharmaceuticals such as ciprofloxacin, ibuprofen, and paracetamol. The other feature of the methodology includes gram-scale synthetic applicability, recyclability, and short reaction time.

Full text of DOI:10.1002/ejoc.202100614

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Highly efficient heterogeneous V2O5@TiO2 catalyzed the
rapid transformation of boronic acids to phenols
Rahul Upadhyay,[a, b] Deepak Singh,[a] and Sushil K. Maurya*[a, b]
aerobic hydroxylation of boronic acid using KOH and DMSO.[27]
A V2O5@TiO2 catalyzed green and efficient protocol for the
Several other transition metal-catalyzed reactions were also
reported, such as palladium-chitosan-CNT core-shell nanohy-
brid-H2O2 system,[5] Ru@imine-nanoSiO2,[6] N-doped-C-encapsu-
lated ultrafine In2O3 nanoparticles,[9] and K-10 supported silver
nanoparticles-H2O2 system.[8] Apart from this, photoredox cata-
lytic systems were also explored for the synthesis of phenols
using Ru(bpy)3,[12] [Ir(OMe)(COD)]2,[13] and other organic photo-
catalysts (COF, QDs, POF). However, most of these photo-
catalysts utilized noble transition metal and a tedious prepara-
tion process. These methodologies also have several associated
shortcomings, such as harsh reaction conditions, prolonged
reaction time, a stoichiometric amount of the base, environ-
mentally hazardous solvents, and limited functional group
tolerance. In recent times, the utilization of heterogeneous
metal catalysts gains considerable attention due to their
advantages over homogeneous catalysts, as they can be
recycled and reused and avoided metal contamination. More-
over, heterogeneous catalysis also enables easy large-scale
production, high selectivity, and high thermal stability. There-
fore, to overcome these shortcomings associated with the
previous methodology and advantages associated with hetero-
geneous catalysts, we previously used heterogenous V2O5@TiO2
to oxidize various scaffolds.[28] Here, we report the heteroge-
neous V2O5@TiO2 catalyzed efficient and rapid hydroxylation of
boronic acid to phenol containing a wide array of function-
alities.
To obtain the optimal reaction condition for boronic acid
hydroxylation into phenol, phenylboronic acid was chosen as a
model substrate. The finding begins with the screening of
active catalysts among five synthesized V2O5@TiO2 catalysts (C-
03, C-05, C-10, C-20, C-30)[28] containing different weight % of
V2O5 using hydrogen peroxide and acetonitrile (Table 1).
Initially, the 10 wt. % of each synthesized catalysts were
evaluated, and among them, C-30, C-20, and C-10 provided the
best results (Table 1, entries 1–5). We choose the C-10 catalyst
for further investigation because it has low V2O5 content
(Table 1, entry 3). A poor yield of the product was obtained
when the reaction was performed in the presence of TiO2 and
V2O5 (Table 1, entries 6 and 7). Also, V2O5 provided a better yield
than TiO2, which confirmed the active participation of vanadium
during the reaction. The obtained result also suggested that the
vanadium efficiency enhanced on coordination with TiO2, as
they provided excellent yield in their combined form
(V2O5@TiO2). A reaction in the absence of a catalyst provided
poor conversion into the product (Table 1, entry 8). Next, the
catalyst loading was screened and found that the 5.0 wt. %
catalyst gave a 99% yield of desired phenol within 5.0 minutes
hydroxylation of boronic acid into phenol has been developed
utilizing environmentally benign oxidant hydrogen peroxide. A
wide range of electron-donating and the electron-withdrawing
group-containing (hetero)aryl boronic acids were transformed
into their corresponding phenol. The methodology was also
applied successfully to transform various natural and bioactive
molecules like tocopherol, amino acids, cinchonidine, vasici-
none, menthol, and pharmaceuticals such as ciprofloxacin,
ibuprofen, and paracetamol. The other feature of the method-
ology includes gram-scale synthetic applicability, recyclability,
and short reaction time.
Phenols symbolize a dynamic class of organic compounds
present in various natural products,[1] pharmaceuticals,[2] and
biopolymers.[3] The phenolic compounds are well known for
their antioxidant, anticancer, and antimicrobial activity.[3,4] Due
to their vast existence and enormous utility in the living system,
cost-effective and greener methods for synthesizing phenolic
compounds are highly desired. Generally, the phenols were
synthesized by the nucleophilic substitution reactions over aryl
halides, diazo compound, the potassium salt of aryl trifluoro
borate, and boronic acids. Amongst these, the ipso-hydroxyla-
tion of boronic acid emerges as the most conventional route for
installing the hydroxyl group. In this regard, several methods
were reported for the synthesis of phenols which utilized metal
catalysts (Pd,[5] Ru,[6] Au,[7] Ag,[8] In,[9] Zn,[10] Cu[11]), photocatalysts
(Ru,[12] Ir,[13] Zn,[14] COF,[15] QDs[16] POF[17]), electrochemical[18] and
in metal-free conditions (H2O2,[19] TBHP,[20] m-CPBA,[21] oxone,[22]
N-oxide,[23] peroxodisulfate[24]) (Figure 1). Previously, Olah and
co-workers reported hydroxylation of boronic acid using hydro-
gen peroxide and water systems. Still, the methodology took a
longer reaction time, and a low yield of the desired product
was obtained.[19] Sequentially, further advancement in the
catalytic system has been progressed, and a wide array of the
non-metallic catalytic system has been developed, which
utilized WERSA-H2O2,[25] ascorbic acid-H2O2,[26] TBHP-KOH,[20] and
N-oxide.[23] Recently, He group reported microwave-assisted
[a] R. Upadhyay, D. Singh, Dr. S. K. Maurya
Chemical Technology Division,
CSIR-Institute of Himalayan Bioresource Technology Palampur,
Himachal Pradesh, 176 061, India
[b] R. Upadhyay, Dr. S. K. Maurya
Academy of Scientific and Innovative Research (AcSIR),
Ghaziabad 201 002, India
E-mail: skmaurya@ihbt.res.in
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100614
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Figure 1. Methods for the hydroxylation of boronic acids into phenols.
Table 1. Reaction optimization.[a]
Entry
Catalyst
Oxidant
[2.0 equiv.]
Solvent
[1.0 mL]
Temperature
Yield[b]
°
1
2
3
4
C-30
C-20
C-10
C-05
C-03
TiO2
V2O5
–
C-10
C-10
C-10
C-10
C-10
C-10
C-10
C-10
C-10
C-10
C-10
C-10
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
K2S2O8
H2O
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
ACN
DCM
EtOAc
H2O
H2O:ACN
H2O:ACN
H2O:ACN
25 C
99%
99%
99%
80%
40%
38%
76%
36%
99%
99%
78%
54%
NR
°
25 C
°
25 C
°
25 C
°
5
25 C
6[c]
7[c]
8
25 C
°
°
25 C
°
25 C
9[d]
10[e]
11[f]
12
13
14
15
16
17
18[g]
19[h]
20[i]
25 C
°
°
25 C
°
25 C
°
25 C
°
25 C
°
25 C
NR
°
25 C
48%
62%
82%
99%
99%
46%
°
25 C
°
25 C
°
25 C
°
25 C
°
25 C
°
[a] Reaction conditions: 1 (50 mg, 1.0 equiv.), C-10 (10 wt%), H2O2 (2.0 equiv.), at 25 C for 5.0 min. [b] Isolated yield. [c] 5.0 mol%. [d] 7.0 wt%. [e] 5.0 wt%. [f]
3.0 wt%. [g] H2O:ACN (1:1). [h] H2O:ACN (7:3). [i] H2O2 (1.0 equiv.).
(Table 1, entries 9–11). During the catalyst loading optimization,
the C-10 catalyst was found better among the various catalyst
tested (Table 1, entries 4, 5 and 10–11), even when used in the
same mol % (see Supporting information, Table S1). Other
oxidants effect was checked and found that no other oxidant
provided phenol efficiently than hydrogen peroxide (Table 1,
entries 12–14). Furthermore, to screen the solvent other than
acetonitrile, the reaction was performed in dichloromethane,
ethyl acetate, and water, the reaction in ethyl acetate and
dichloromethane resulted in poor conversion, whereas reaction
in water gave 82% yield of desired phenol (Table 1, entries 15–
17). An increase in the yield was observed when the reaction
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was performed in the mixture of water and acetonitrile (Table 1,
substituted pyridine skeleton were successfully transformed
(Scheme 1, 2q–2s). A smooth reaction was observed when the
boronic acid derivative of quinoline was subjected to the
optimized reaction conditions (Scheme 1, 2t). An exciting
reaction was observed when benzofuran-3-boronic acid was
attempted to transform into the corresponding phenol. The
process displayed a keto-enol tautomerization to form ketone
with an additional hydroxyl group transfer adjacent to ethereal
oxygen to give 2-hydroxybenzofuran-3(2H)-one (Scheme 1, 2u).
After successfully implementing the methodology over a
diverse range of boronic acid derivatives, the protocol‘s robust-
ness was evaluated over structurally complex bioactive natural
products and pharmaceuticals (Scheme 2). The investigation
began with the boronic acid derivative of the widely used
generic drugs ibuprofen and paracetamol. These pharmaceut-
ical-derived boronic acids were successfully transformed into
their respective phenols (Scheme 2, 2v and 2w). Interestingly,
the boronic acid derivative of antimalarial agent (À ) cinchoni-
dine resulted in the formation of phenol along with the
insertion of oxygen atom over aliphatic bridged nitrogen to
furnish particular N-oxide product in good yield (Scheme 2, 2x).
The boronic acid derivative of the well-known antibacterial
drug ciprofloxacin was successfully converted to the corre-
sponding phenol (Scheme 2, 2y). Vasicinone, a quinazoline
alkaloid used to treat allergic and cardiac disease, effectively
converted to their corresponding phenol (Scheme 2, 2z).
Menthol, a molecule capable of blocking voltage-sensitive
sodium channels, reducing neural activity that may stimulate
entries 18 and 19). As the reaction in the water: acetonitrile
mixture gave a similar result, the solvent system was selected
for further evaluation based on the green chemistry aspects
(Table 1, entries 10 and 19). On decreasing the hydrogen
peroxide quantity, a decrease in the product‘s yield was
observed (Table 1, entry 20). Further, based on TON/TOF data,
C-10 is the best catalyst for this transformation (see Supporting
information, Table S2).
With optimized conditions in hand, we investigate the
substrate compatibility and versatility of the developed proto-
col. The methodology enables the hydroxylation of various
boronic acid derivatives containing electron-donating and
electron-withdrawing groups (Scheme 1, 2a–2p). Initially, the
methyl and methoxy substituted boronic acid were smoothly
converted to desired phenols (Scheme 1, 2b–2d). Halogenated
phenols were synthesized under the developed conditions, and
F, Cl, Br, and trifluoromethyl functional groups were well
tolerated (Scheme 1, 2e–2i). The boronic acid derivatives
containing nitrile, nitro, and amino functionalities were success-
fully converted to their corresponding phenols in good to
excellent yields (Scheme 1, 2j–2l). Boronic acid bearing
carbonyl group (aldehyde and carboxylic acid) were tolerated
well and converted to corresponding phenols in excellent yields
with high selectivity (Scheme 1, 2m and 2n). The boronic acid
derivative of bicyclic and polycyclic arenes provided a good to
an excellent yield of required phenols (Scheme 1, 2o and 2p).
Heterocyclic boronic acid compounds containing pyridine and
°
Scheme 1. Scope of the synthesis of phenols. [a] Reaction conditions: 1 (100 mg, 1.0 equiv.), C-10 (5.0 wt%), H2O2 (2.0 equiv.), H2O:ACN (2.0 mL, 7:3), at 25 C
for 5–10 min. [b] Isolated yield.
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Scheme 2. Scope of phenol synthesis from natural products and pharmaceutical-derived boronic acids. [a] Reaction conditions: Boronic acid (50 mg,
°
1.0 equiv.), C-10 (5 wt%), H2O2 (2.0 equiv.), H2O:ACN (2.0 mL, 7:3), at 25 C for 10 min. [b] Isolated yields.
muscles, and increasing GABAergic transmission in PAG
neurons, provided respective phenol utilizing their correspond-
ing boronic acid derivative (Scheme 2, 2aa). Amino acids are an
integral part of our living system and play an important role in
various biological processes. Boronic acid derivatives derived
from the different amino acids such as glycine, L-phenyl alanine,
and L-valine were tested for their compatibility with the
developed protocol and resulted in corresponding phenols in
good yield (Scheme 2, 2ab–2ad). The boronic acid derivative of
α-tocopherol, a form of vitamin E, efficiently yielded the desired
product (Scheme 2, 2ae).
Furthermore, phenol synthesis was successfully achieved in
good to excellent yield utilizing other organoborane com-
pounds rather than boronic acids. In this regard, the organo-
borane compounds such as potassium phenyltrifluoroborate,
phenylboronic acid pinacol ester, 2,4,6-triphenylboroxin pro-
vided phenol under optimized reaction conditions (Scheme 3A).
To demonstrate the synthetic applicability at the multi-gram
scale, we performed the gram-scale synthesis of phenol and
obtained excellent yield (Scheme 3B). The catalyst used for the
transformation was recycled up to 6.0 cycles without any
significant loss in catalytic activity. After each cycle, the catalyst
was recycled by centrifugation or filtration, and then the
residual catalyst was washed with acetone, dried in an oven for
°
2.0 h at 100 C, and again used for the next cycle
(Scheme 3C).[28] These obtained results further extended the
robustness and versatility of the developed protocol.
The methodology was applied to synthesize the potential
therapeutic candidate and the key intermediate for drug
molecules to demonstrate the applicability of the developed
protocol in medicinal chemistry. The synthesis of 3-hydroxy-2,5-
dimethoxy pyridine (2r) was done in 88% yield, which is used
to prepare an active antitubercular agent. The previous
methodology utilized multiple reagents, required
a long
reaction time, and hazardous chemicals such as peracetic acid
(Figure 2A).[29] The benzofuranone (2u), an essential antimicro-
bial agent used in various active pharmaceuticals, was synthe-
sized rapidly from the boronic acid derivative of benzofurane in
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Scheme 3. (A) Scope of the synthesis of phenol from potassium phenyltrifluoroborate, phenylboronic acid pinacol ester, and 2,4,6-triphenylboroxin (B) Gram-
scale synthesis (C) Recyclability experiment.
Figure 2. Application in medicinal chemistry and comparison studies.
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Table 2. Comparison with the recent catalytic system for ipso-hydroxylation of boronic acids.
°
Entry
Catalyst
Oxidant
Oxidant load
Solvent
Time
Temp. [ C]
Yield [%]
Ref.
1
2
3
4
–
H2O2
1.0 eq.
–
2.5 eq.
–
H2O
ACN
MeOH:H2O
DMSO
Several hours
4.0 h
3.0 h
RT
RT
80
100
MW
(300 w)
RT
85
99
98
98
19
15
24
27
COFÀ P-3Ph/Visible light
O2/TEA (additive)
(NH4)2S2O8
Air
–
–
5.0 min
5
6
Graphene oxide
V2O5@TiO2
H2O2
H2O2
4.0 eq.
2.0 eq.
H2O
H2O: ACN
5.0 min
5.0 min
99
99
33
This work
RT
86% yield. Earlier reported protocol for the synthesis of
benzofuranone (9u) utilizes a stoichiometric amount of catalyst,
required elevated temperature, and resulted in a relatively
lower yield (Figure 2B).[30] The synthesis of a protein disulfide
isomerase inhibitor derivative of menthol (2aa) was accom-
plished in 86% yield by using the developed protocol, whereas
the previously developed protocol utilized UV light irradiation
over 24 h in the presence of molecular O2 and provided
comparatively lower yield (69%) of the desired product (Fig-
ure 2C).[31,32]
We further calculated the green metrics parameter to
determine the developed protocol efficiency and applicability
over existing methodologies. These parameters for three
medicinally important molecules (Figure 2A–C), provided the
efficiency compared to existing methodology in terms of
carbon (For A, 88>57; B, 86>57; C, 87>69), reaction mass (For
A, 54>41; B, 56>50; C, 63>45), atom efficiency (For A, 62>
53; B, 65>50; C, 70>45), and optimum efficiency (For A, 76>
69; B, 73>71; C, 78>69). Similarly, the other green parameter
such as atom economy, E-factor, mass productivity, productive
mass intensity, solvent intensity, and water intensity strengthen
the integrity of the developed reaction condition over the
existing methodology (see Supporting information, Table S3).
Additionally, the comparison of the recently developed meth-
odology and our methodology were also listed to demonstrate
the developed protocol‘s advantages (Table 2). As method-
ologies listed in Table 2, the previous hydroxylation of boronic
acid into sole hydrogen peroxide gave 85% yield and required
a longer reaction duration (Table 2, entry 1). The photocatalyzed
methodology for the hydroxylation of boronic acid using
COFÀ P-3Ph utilizing O2 requires 4.0 hours (Table 2, entry 2). The
elevated temperature treatment either by heating or microwave
resulted in phenol (Table 2, entries 3 and 4). The graphene
oxide catalyzed hydroxylation of boronic acid to phenol using
hydrogen peroxide in water was developed (Table 2, entry 5). In
comparison, our methodology enables hydroxylation of boronic
acid into phenol in good to excellent yield utilizing heteroge-
neous V2O5@TiO2 and hydrogen peroxide system in water:
acetonitrile (7:3) solvent (Table 2, entry 6).
Figure 3. (A) Quenching experiments. (B) The UV-visible spectrum of differ-
ent components used in the reaction.
with catalyst was studied over UV-visible spectrometry (Fig-
ure 3B); the data analysis of peroxide in acetonitrile with and
without catalyst indicated the interaction between catalyst and
peroxide. The concentration of the peroxide was diminished
after 1.0 minutes of the addition of the catalyst. In the
mechanistic investigation of boronic acid hydroxylation into
phenol, it was observed that the radical scavenger does not
affect the yield of the reaction, which vanished the radical
pathway. A series of radical scavengers such as TEMPO,
benzoquinone, BHT, and DABCO was used in this study and
resulted in the respective phenol formation (Figure 3A). The
study indicated that the reaction does not involve any possible
hydrogen peroxide radicals, such as hydroxyl radical, peroxy
hydroxyl radical, and superoxide radical.[34] A plausible mecha-
nism for the reaction has been proposed based on the
previously reported literature.[32] The transformation of boronic
acid II to phenol V was accelerated rapidly by forming boronic
acid adduct III with activated hydrogen peroxide complex I,
which further undergoes rearrangement to give intermediate
compound IV and final hydrolysis yielded phenol V (see
Supporting information, Figure S1).[34]
The previously reported mechanistic study of the vanadium
oxide complexes in various oxidants demonstrates the vana-
dium metal ability to form the active species responsible for the
catalytic cycles.[33] In this regard, to check the oxidant‘s
modification after contacting the catalyst, the control experi-
ments were performed (Figure 3). The interaction of peroxide
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: Boronic acid
Hydroxylation · Pharmaceuticals · Phenol
·
Heterogeneous catalysis
·
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Manuscript received: May 20, 2021
Revised manuscript received: July 4, 2021
Accepted manuscript online: July 6, 2021
Eur. J. Org. Chem. 2021, 3925–3931
www.eurjoc.org
3931
© 2021 Wiley-VCH GmbH