Photo-tunable oxidation of toluene and its derivatives catalyzed by TBATB

Article abstract of DOI:10.1016/j.jphotochem.2021.113301

In this report, tetrabutylammonium tribromide (TBATB) was introduced as an efficient visible light active catalyst to carry out the aerobic oxidation of toluene, its derivatives, and some of methyl arenes to benzaldehydes, benzoic acids and ketones in good to high yields. All the oxidation reactions were performed under mild conditions using oxygen as a green oxidant, a catalytic amount of TBATB under blue (460 nm), royal blue (430 nm), and violet LED (400 nm) irradiation. It was found that the reactions selectivity was significantly affected by changing the solvent (from CH₃CN to EtOAc) and LED wavelength (from blue to violet). In the following, our mechanistic studies revealed that the visible light oxidation of toluenes and methyl arenes over TBATB could be following a benzyl peroxy radical intermediate.

Full text of DOI:10.1016/j.jphotochem.2021.113301

Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
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Journal of Photochemistry & Photobiology, A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photo-tunable oxidation of toluene and its derivatives catalyzed by TBATB
,
Atefeh Mardania, Foad Kazemia b,*, Babak Kaboudina
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, PO Box 45195- 1159, Zanjan, Iran
b Center for Climate and Global Warming (CCGW), Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, Zanjan, 45137-66731, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this report, tetrabutylammonium tribromide (TBATB) was introduced as an efficient visible light active
catalyst to carry out the aerobic oxidation of toluene, its derivatives, and some of methyl arenes to benzalde-
hydes, benzoic acids and ketones in good to high yields. All the oxidation reactions were performed under mild
conditions using oxygen as a green oxidant, a catalytic amount of TBATB under blue (460 nm), royal blue (430
nm), and violet LED (400 nm) irradiation. It was found that the reactions selectivity was significantly affected by
changing the solvent (from CH3CN to EtOAc) and LED wavelength (from blue to violet). In the following, our
mechanistic studies revealed that the visible light oxidation of toluenes and methyl arenes over TBATB could be
following a benzyl peroxy radical intermediate.
Tunable synthesis
Visible light photocatalyst
Photooxidation
Toluene
Tetrabutylammonium tribromide
Benzoic acid
1. Introduction
benzaldehyde [16]. Photooxidation of toluene over TiO2-pillared
montmorillonite under UV light irradiation led to the production of
Tunable synthesis is a powerful and valuable approach to direct a
reaction to a particular product. Tunable synthesis of benzaldehydes and
benzoic acids via oxidation of toluene and its derivatives has great
importance in organic chemistry. In recent years, valuable studies have
been conducted relating to the oxidation of toluene to the more useful
products like benzaldehyde and benzoic acid as essential materials for
organic synthesis, antibacterial, biological and pharmaceutical appli-
cations [1–8].
benzaldehyde and para-cresol as main intermediates [19]. Photooxida-
tion of low amount of toluene in the presence of 2-chloroanthraquinone
as an organophotocatalyst led to the production of high conversion of
toluene to benzoic acid under irradiation of fluorescent lamp (400 nm
<λ < 450 nm) [20].
On the other hand, photocatalysts of bromo sources such as lithium
bromide [21,22], bromine, hydrobromic acid [23], N-bromosuccini-
mide [24], MgBr2-Et2O [25], AlBr3.6H2O [26], CBr4 [27], and allyl
bromide [28] have shown a high potential for aerobic photooxidation
reactions; the photocatalytic oxidation of toluene and its derivatives
over LiBr under UV irradiation was reported by the Itoh group; benzoic
acids were obtained in good to high yield in the best case [22]. The
aerobic photooxidation of a low amount of toluene over a catalytic
amount of allyl bromide under xenon lamp irradiation was reported by
Sugai and Itoh; benzoic acids were obtained in good to high yield in the
best condition [28]. Also, the photooxidation of low amount of toluene
and its derivatives over CBr4 and triphenylphosphine under fluorescent
lamps irradiation was reported by Sugai and Itoh; benzoic acids were
obtained in good to high yield in the best reaction condition [29]. The
selective photooxidation of methylarenes (1 mmol) to aldehydes over
(0.1 mmol) CBr4 in CH3CN under irradiation of 18 W CFL (380–740 nm)
was reported by Tripathi, et al. [27], however, we reported
photo-tunable oxidation of toluenes (1 mmol) to benzoic acids or
benzaldehydes over lower amount of TBATB as a bromo source
Molecular oxygen as a green oxidant has been widely used in the
oxidation of toluene and its derivatives [9–13]. Photocatalytic reactions
have been considered from both technological and economical view-
points due to providing a clean, short, and direct route [14,15]. Among
different aerobic oxidation processes, photocatalytic oxidation has
gained popularity. Recently, several studies relating to the aerobic
photooxidation of toluene have been reported [1,9,16–20]. Bi2MoO6
nanosheets with different thicknesses were prepared and examined in
the photooxidation of toluene to benzaldehyde under irradiation of
Xenon lamp (300 W): Low toluene conversion and high selectivity to
benzaldehyde were obtained in the best case [1]. Pd/Bi2WO6 was used
for the selective aerobic photooxidation of toluene and its derivatives
under visible light irradiation (λ > 400 nm): Low toluene conversion and
high benzaldehyde selectivity was obtained in the best reaction condi-
tion [9]. H3PW12
O40–TiO2 composites utilized for the selective photo-
oxidation of toluene led to low toluene conversion and high selectivity to
* Corresponding author at: Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava Zang, PO Box 45195- 1159, Zanjan, Iran.
E-mail address: kazemi_f@iasbs.ac.ir (F. Kazemi).
https://doi.org/10.1016/j.jphotochem.2021.113301
Received 7 March 2021; Received in revised form 9 April 2021; Accepted 12 April 2021
Available online 14 April 2021
1010-6030/© 2021 Elsevier B.V. All rights reserved.

A. Mardani et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
photocatalyst (0.02ꢀ 0.04 mmol) in EtOAc and CH3CN under irradiation
of violet, royal blue, and blue LED (400, 430, and 460 nm). Some of the
strategies in the selective aerobic photooxidation of toluene have been
shown in Scheme 1.
2. Experimental
2.1. Materials
Tetrabutylammonium tribromide (TBATB) is a safe, stable, and mild
solid source of bromine which has been used as a brominating agent in
several chemical reactions [30–37]. Recently, several research groups
have tried to cleavage the silyl ethers and oxidize alcohols with a stoi-
chiometric amount of TBATB [38–40], while for the first time we used a
catalytic amount of TBATB as an excellent photocatalyst in metal free
photopolymerization of vinyl monomers [41], selective photooxidation
of benzyl alcohols, and oxidative-desilylation of silyl ethers [42]: 1
mmol of benzyl alcohols or tert-butyldimethylsilyl ethers converted
successfully to the benzaldehyde in high yield (98 %) over 10ꢀ 20 mg
TBATB under air atmosphere and blue LED irradiation (460 nm) in
CH3CN. Also, by changing the LED wavelength from 460 to 400 nm,
benzyl alcohols transformed into the benzoic acids in high yields [42]. In
fact, by changing the LED wavelength, benzaldehyde-benzoic acid syn-
thesis could be tuned.
Tetrabutylammonium tribromide (TBATB) (>98 %) was purchased
from Acros (Acros, 123970250). Ethylbenzene (≥99.5 %), p-nitro-
toluene (99 %), o-nitrotoluene (≥99 %), p-methoxytoluene (99 %), p-
chlorotoluene (98 %), cumene (98 %), fluorene (98 %), o-xylene (≥99
%), p-toluidine (99.6 %), diphenylmethane (99 %), adamantane (≥99
%), and diphenylacetic acid (99 %) were provided from Merck Com-
pany. p-Bromotoluene (98 %), 1-bromo-4-ethylbenzene (97 %), and p-
iodotoluene (99 %) were purchased from Sigma-Aldrich Company. p-
Cresol (≥99 %), and p-toluic acid (98 %) were purchased from Fluka
Company. Ethyl acetate and acetonitrile (Laboratory grade) were pur-
chased from Dr-Mojalali Company (Iran). All of the chemicals and sol-
vents used without further purification.
2.2. Instruments
Because of the valuable results which were achieved in photooxi-
dation of benzyl alcohols and the probable radical mechanism which can
be operated in the photooxidation of toluene, we decided to investigate
aerobic photooxidation of toluene over a catalytic amount of TBATB.
Here, we have reported a tunable aerobic photooxidation of toluene and
its derivatives to the corresponding benzaldehydes and benzoic acids in
good to high yield over TBATB at room temperature under O2 and LED
irradiation. Unlike the other studies in which a low amount of toluene
(0.3 mmol) has been oxidized [20,28,29] or UV lamps was as light
sources [19,22], in this study we oxidized higher amount of toluene
(1ꢀ 2 mmol) to the corresponding oxidation products under mild reac-
tion condition and LED light sources.
The gas chromatography (GC) method was utilized for the detection
of products. GC chromatograms were recorded using a VARIAN CP-3800
Gas Chromatograph instrument. 13C NMR and 1H NMR spectra of syn-
thesized products were recorded with a Bruker 400 MHz AVANCE III
spectrometer. The light sources utilized in the photooxidation reactions
were violet (400 nm), royal blue (430 nm), and blue (460 nm) LED
Epistar 3 W 20ꢀ 30LM 700 mA h light bulbs.
2.3. General experimental procedure for the Photocatalytic Oxidation of
toluene
0.02ꢀ 0.04 mmol of TBATB and 1 mmol of toluene were added to 10
mL solvent (CH3CN or EtOAc) in a round-bottomed flask (Pyrex). Af-
terward, the reaction mixture was exposed to LED lamps irradiation (3
blue or 5 violet LED lamps) under an oxygen atmosphere (O2-balloon)
and stirring at room temperature (Fig. S1). The resulted products were
isolated by silica gel-coated TLC plates in EtOAc and n-hexane (1:9)
solvent.
3. Results and discussion
In order to find the best reaction condition for photooxidation of 1
mmol toluene to benzaldehyde or benzoic acid over TBATB (photo-
catalyst), reactions were performed in the presence of various amount of
TBATB, different solvents, and under irradiation of violet (400 nm) and
blue (460 nm) LED at room temperature. The effect of reaction param-
eters such as TBATB, solvent, LED wavelength, and reaction time on
both of the toluene conversion and selectivity to products was examined.
The obtained results are presented in Table 1.
First, the photooxidation of toluene was examined under blue LED
irradiation (Table 1, entries 1–5); When the toluene photooxidation was
carried out in the presence of 0.02 mmol TBATB under blue LED irra-
diation in CH3CN, 58 % conversion and 50 % benzaldehyde were ob-
tained in 6 h (Table 1, entry 1). Although with increasing the reaction
time from 6 to 8, the conversion of toluene increased to 73 %, the
selectivity to benzaldehyde decreased due to over oxidation of benzal-
dehyde to benzoic acid (Table 1, entry 2). When the photooxidation of
toluene was carried out over 0.04 mmol TBATB instead of 0.02 mmol,
the toluene conversion and yield of benzaldehyde were respectively 61
% and 51 % after 3 h (Table 1, entry 3) which was similar to the obtained
result over 0.02 mmol TBATB in 6 h (Table1, entry 1). It revealed that
the conversion rate of toluene to benzaldehyde in CH3CN was faster in
the presence of higher amount of TBATB (0.04 mmol) as compared to
lower amount of it (0.02 mmol) as demonstrated by comparison of entry
1 with 3 in Table 1. In the following, the toluene photooxidation was
investigated under blue LED irradiation in EtOAc (Table 1, entries 4–5).
The obtained results show that changing the solvent from CH3CN to
Scheme 1. Some of the strategies in the aerobic photooxidation of toluene to
benzaldehyde and benzoic acid under light irradiation.
2

A. Mardani et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
Table 1
Aerobic visible light oxidation of toluene over TBATB.
Entry
TBATB (mmol)
Solvent
LED
Time (h)
A (%)d
B (%)d
C (%)d
Conversion (%)d
1
0.02
0.02
0.04
0.02
0.04
0.02
0.04
0.02
0.02
0.04
0.01
0.02
0.02
0
CH3CN
CH3CN
CH3CN
EtOAc
EtOAc
CH3CN
CH3CN
EtOAc
EtOAc
EtOAc
EtOAc
CH2Cl2
H2O
Blue
6
8
3
8
8
8
3
8
10
8
8
8
8
8
8
8
8
8
5
50
51
51
19
24
51
54
10
10
4
3
58
73
61
74
68
83
84
87
87
80
36
30
76
0
2
Blue
8
14
1
3
Blue
9
4
Blue
3
52
41
23
22
75
75
74
25
6
5
Blue
3
6
Violet
Violet
Violet
Violet
Violet
Violet
Violet
Violet
Violet
–
9
7
8
8
2
9
2
10
11
12
13
14
15a
16b
17c
18c
2
0
11
17
15
0
7
35
0
26
0
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
0.02
0.02
0.02
0.04
0
0
0
0
Violet
Violet
Violet
0
0
0
0
0
16
0
40
60
56
74
14
Photooxidation of toluene.
a
In a dark room.
b
Under an argon atmosphere.
c
d
2 mmol toluene.
GC yield.
EtOAc resulted in no significant effect on toluene conversion under
irradiation of blue LED, but the selectivity to benzoic acid was further in
EtOAc as compared to CH3CN as demonstrated by comparison of entry 2
with entry 4 in Table 1. Increasing the amount of TBATB from 0.02 to
0.04 mmol did not offer considerable progress on the toluene conversion
under irradiation of blue LED in EtOAc (Table 1, entries 4 and 5).
Therefore, in the photooxidation of toluene under blue LED irradiation
in EtOAc, the best result (74 % toluene conversion and 52 % benzoic
acid) was obtained over 0.02 mmol TBATB in 8 h (Table 1, entry 4). A
comparison of the results obtained in the photooxidation of toluene over
TBATB under blue irradiation indicated that the EtOAc was the best
solvent to produce benzoic acid, and CH3CN was the best one to produce
benzaldehyde (Table 1, entries 1–5).
shorter than the blue ones. When the photocatalytic oxidation of toluene
over TBATB under violet LED irradiation was examined in EtOAc
(Table 1, entries 8–10), it was found that toluene can be oxidized to
benzoic acid more efficiently in EtOAc under violet LED irradiation
compare to blue LED irradiation as demonstrated by comparison of entry
4 with 8 in Table 1 (Fig. 1) [21,22,25].
With increasing the reaction time from 8 to 10 h, no change was
observed in both of the toluene conversion and selectivity of products
(Table 1, entry 9). On the other hand, photooxidation of toluene over
0.01 mmol TBATB led to a decrease in the toluene conversion (Table 1,
entry 11). The best result (87 % toluene conversion and 75 % benzoic
acid) was obtained over 0.02 mmol TBATB in EtOAc under violet LED
irradiation after 8 h (Table 1, entry 8). Therefore, 0.02 mmol TBATB,
EtOAc, and violet LED were selected as the optimized conditions for the
photooxidation of toluene to benzoic acid (Table 1, entry 8). It was
found that all of the factors (such as wavelength, type of solvent, amount
of TBATB, and reaction time) together influence the tunable photooxi-
dation of toluene. By adjusting these factors, the reactions conduct to
Next, toluene photooxidation was examined under violet LED irra-
diation in CH3CN (Table 1, entries 6–7). The violet LED irradiation
compared to blue LED irradiation has a more accelerating effect on the
conversion rate of toluene, as demonstrated by comparison of entry 2
with 6 in Table 1, because the wavelengths of the violet LED were
Fig. 1. Aerobic Photooxidation of 1 mmol toluene over 0.02 mmol TBATB in 10 mL CH3CN or EtOAc under irradiation of A) violet or B) blue LED in 8 h.
3

A. Mardani et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
benzaldehyde or benzoic acid. It seems that the toluene photooxidation
tunability over TBATB depends on the rate of oxidation: Parameters
such as wavelength, type of solvent, amount of TBATB, and reaction
time effect on the rate of transformation of toluene to benzaldehyde or
benzoic acid. The longer wavelengths (blue LED irradiation, 460 nm),
CH3CN, higher amount of photocatalyst (0.04 mmol), and short reaction
time (3 h) led to tunable photooxidation of toluene to benzaldehyde
(Table 1, entry 3). On the other hand, the shorter wavelengths (violet
LED irradiation, 400 nm), EtOAc, lower amount of photocatalyst (0.02
mmol), and long reaction time (8 h) led to over oxidation of toluene to
benzoic acid (Table 1, entry 8).
under mild condition (LED irradiation) has great importance. In the
photooxidation of p-toluic acid, 40 % terephthalic acid was produced
after 21ꢀ 60 h under method A (Table 2, entries 14 and 15). This report
has great importance compare to the reports in which terephthalic acid
was obtained from oxidation of p-toluic acid over metal catalysts at high
temperature [46,47]. In the photooxidation of o-xylene, 97 % o-toluic
acid was generated with a high TON 50 after 48 h under method A
(Table 2, entry 16). 44 % o- tolualdehyde was obtained after 23 h under
method B (Table 2, entry 17). Diphenyl acetic acid converted into
benzophenone in high yields 96 % with high TONs (50 and 25) under
methods A, B, and C (Table 2, entries 19–21). p-Iodotoluene was
oxidized to the p-iodobenzoic acid (89 %) after 45 h under method A
(Table 2, entry 22). On the other hand, photooxidation of p-iodotoluene
led to the production of only 5% p-iodotoluene under method C after 24
and 48 h (Table 2, entries 23 and 24). It seems that the photooxidation of
p-iodotoluene was not proceeding well in CH3CN. In the photooxidation
of p-bromotoluene, a good yield (43 %) of p-bromobenzoic acid was
obtained after 12 h under method A (Table 2, entry 25). The produced
p-bromobenzoic acid increased from 43 to 50 %, with an increase in the
reaction time from 12 to 28 h (Table 2, entry 26). Also, 67 % of p-bro-
mobenzaldehyde was achieved after 24 h under method C (Table 2,
entry 27).
Subsequently, the effect of solvents such as CH2Cl2 and H2O on the
aerobic photooxidation of toluene was examined in the optimized re-
action condition (Table 1, entries 12 and 13); the photooxidation of
toluene in CH2Cl2 resulted in a low conversion of toluene (Table 1, entry
12). A mixture of benzyl alcohol, benzaldehyde, and benzoic acid was
obtained in the photooxidation of toluene in H2O (Table 1, entry 13). On
the other hand, No oxidation product was obtained in the absence of
TBATB as a photocatalyst, LED as light source, and oxygen as oxidant
(Table 1, entries 14–16); it can be concluded that the TBATB, LED, and
oxygen play essential roles in the toluene oxidation.
When the photooxidation was conducted for 2 mmol toluene using
the optimum reaction condition, 56 % toluene conversion and 40 %
benzoic acid were obtained (Table 1, entry 17). On the other hand, 2
mmol of toluene was successfully oxidized in high conversion (74 %)
and high yield to benzoic acid
p-Chlorotoluene compare to the p-bromotoluene exhibited a lower
tendency to oxidize over TBATB: a mixture of p-chlorobenzoic acid (12
%) and p-chlorobenzaldehyde (6%) was obtained in the photooxidation
of p-chlorotoluene under violet LED irradiation in EtOAc after 24 h
(Table 2, entry 28). It can be concluded that the presence of electron-
withdrawing groups on toluene ring leads to lower reactivity of sub-
strate and a lower product yield. In the photooxidation of p-chlor-
otoluene 21 and 33 % p-bromobenzaldehyde was respectively generated
under methods B and C in 18 h (Table 2, entries 29 and 30). The
presence of the nitro group at the para or ortho position of the toluene
ring resulted in a decelerating influence on the photooxidation reaction
(Table 2, entries 31–33).
(60 %) in the presence of 0.04 mmol TBATB in 8 h (Table 1, entry
18).
The results revealed that CH3CN, 0.04 mmol TBATB, and blue (460
nm) LED irradiation (method B) were the best conditions for the se-
lective photooxidation of toluene to benzaldehyde. On the other hand,
EtOAc, 0.02 mmol TBATB, and violet (400 nm) LED irradiation (method
A) were the best conditions for the selective photooxidation of toluene to
benzoic acid. Therefore, methods A and B were separately used to
convert toluene derivatives to benzoic acid and benzaldehyde, respec-
tively (Table 2).
o-Nitrotoluene, compared to p-nitrotoluene, exhibited no reactivity
in photooxidation reaction. Photooxidation of o-nitrotoluene under
methods A, B, and C resulted in no conversion to oxidation products
(Table 2, entries 34–36); it can be caused by the steric hindrance of nitro
substitution near the active site. Photooxidation of p-cresol and p-tolu-
idine over TBATB resulted in no conversion (Table 2, entries 37–40); it
seems that reactions between the phenolic or aniline compounds and
bromine lead to inactivation of bromine [42]. Photooxidation of
diphenylmethane over TBATB resulted in high yield of benzophenone
(Table 2, entries 41–42).
High yield of p-methoxybenzoic acid (95 %) with a turnover number
(TON) of 50 was obtained in the photooxidation of p-methoxytoluene
under method A (Table 2, entry 1). On the other hand, 50 % p-
methoxybenzaldehyde with a TON of 12.5 was generated in 3 h under
method B (Table 2, entry 2). In order to increase the transformation of
p-methoxytoluene into p-methoxybenzaldehyde, method C (CH3CN,
0.04 mmol TBATB, and royal blue LED irradiation with 430 nm wave-
length) was examined. Royal blue LED (430 nm) was used instead of
blue LED in method C. Remarkably, 75 % p-methoxybenzaldehyde
without any production of p-methoxybenzoic acid and with a TON of
18.8 was obtained in 3 h under method C (Table 2, entry 3). In the
photooxidation of 1 mmol ethylbenzene, high yield of acetophenone (95
%) was obtained after 10 h under method A (Table 2, entry 4).
Replacement of the violet LED to blue and royal blue LED resulted in a
decrease in photooxidation of ethyl benzene; 72 and 79 % acetophenone
was obtained after 25 h, respectively, under method B and C (Table 2,
entries 5 and 6). Photooxidation of 2 mmol ethylbenzene resulted in 85
% acetophenone with a TON of 45 in 25 h under method A (Table 2,
entry 7). 1-Bromo-4-ethylbenzene was converted into the p-bromoace-
tophenone in high yield (89 and 96 %) with high TONs (23.3, 25, and
50) under method B, C, and A (Table 2, entries 8–10). Fluorene was
converted into fluorenone in high yields 80 and 65 % after 47 h under
method A and B, respectively (Table 2, entries 12 and 13). It should be
mentioned that the fluorene was oxidized to the fluorenone with a high
TON of 44 under method A (Table 2, entry 12). There were reports in
which fluorene was converted into the fluorenone over the high amount
of metal catalyst Co-Cu (100 mg) at 110 ◦C [43], V2O5-Fe2O3 at high
temperature (350 ◦C) [44], and metal catalyst CoCu-H (50 mg) at 90 ◦C
[45]; therefore the photooxidation of fluorene to fluorenone in high
yield (80 %) over a low amount of TBATB as a metal-free photocatalyst
Then photooxidation of adamantane as an aliphatic hydrocarbon
was investigated over TBATB. Only 13 % 1-adamantole was obtained in
the photooxidation of adamantane after 48 h under method A and C
(Table 2, entries 43 and 44). It can be concluded that compared to ar-
omatic hydrocarbons, aliphatic hydrocarbons show a low tendency to
oxidize.
Finally, a plausible mechanism was suggested for the aerobic visible
light oxidation of toluene over TBATB (Scheme 2). The yellow colora-
tion of the solution containing TBATB offers that bromine is initially
generated from TBATB [21]. Also, instantly color disappearing of the
TBATB solution after the addition of cresols as phenolic compound
confirms this hypothesis. Toluene can react with bromine radicals pro-
duced via photodissociation of Br2 to afford benzyl radical 1 and hy-
drobromic acid (HBr) [22]. HBr subsequently can be transformed into
the bromo radical upon an aerobic photooxidation process [22,25,48].
Benzyl radical 1 can abstract molecular oxygen (exothermic by ΔH298
= ꢀ 93.4 ± 2.5 kJ mol-1) to form peroxy radical 2 [22,49–52]. The
peroxy radical 2 abstracts hydrogen to produce benzyl hydroperoxide 3
[22]. At higher concentrations of benzyl hydroperoxide 3, the
self-reaction (R1) is considerably more probable than other reactions
[52]. Therefore, hydroperoxybenzyl radical 4 and benzyloxy radical 5
are generated which subsequently transform into benzaldehyde (R2)
4

A. Mardani et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
Table 2
Photooxidation of toluenes derivatives over TBATB.
Entry
1
Substrate
Method
Product
Time (h)
Conversion (%)
Product GC yield (%)
Isolated yield (%)
Selectivity
(%)
TONd
A
B
C
12
3
100
50
100
50
95
45
70
100
100
100
50
2
3
12.5
18.8
3
75
75
4
A
B
C
A
10
25
25
25
100
78
100
78
95
72
79
85
100
100
100
100
50
5
19.5
21.25
45
6
85
85
7a
90
90
8
A
B
C
23
23
23
100
93
100
93
96
89
96
100
100
100
50
9
23.3
25
10
100
100
11
12
13
A
A
B
18
47
47
62
88
73
62
88
73
55
80
65
100
100
100
31
44
18.3
14
15
A
A
21
60
–
–
–
–
40
40
100
100
20*
20*
16
17
18b
19
20
21
A
B
C
A
B
C
48
23
23
18
23
23
100
44
100
44
97
40
33
96
96
96
100
100
80
50
11
9.3
50
25
25
46
37
100
100
100
100
100
100
100
100
100
22
23
24
25
A
C
C
A
45
24
48
12
89
10
10
50
89
10
10
50
80
5
100
100
100
100
44.5
2.5
2.5
25
5
43
26
27
A
C
28
24
57
67
57
67
50
60
100
100
28.5
16.8
(continued on next page)
5

A. Mardani et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
Table 2 (continued)
Entry
28c
Substrate
Method
Product
Time (h)
Conversion (%)
Product GC yield (%)
Isolated yield (%)
Selectivity
(%)
TONd
A
B
24
18
18
21
12
21
–
67
6
29
30
17
100
5.3
C
18
33
33
29
100
8.3
31
32
33
A
B
C
24
48
48
10
7
10
7
2
–
6
100
100
100
5
1.8
3.5
14
14
34
35
36
37
38
39
A
B
C
A
B
A
–
–
–
–
–
–
18
18
18
24
24
24
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
–
–
–
–
–
–
0
0
0
0
0
0
40
B
–
24
0
0
0
–
0
41
A
12
100
100
98
100
50
42
43
C
A
C
12
48
48
100
13
100
13
98
–
100
100
100
25
6.5
3.3
44
a
13
13
–
2 mmol substrate, 0.04 mmol TBATB.
9 % of o-toluic acid was produced.
b
c
6 % of p-chlorobenzaldehyde was produced.
d
*
TON = mmol of products (achieved by GC) divided by mmol of TBATB.
mmol of isolated products divided by mmol of TBATB.
–
and benzyl alcohol, respectively (R1 + R2 are exothermic reactions by
ΔH298 = ꢀ 55.7 kJ molꢀ 1 and ΔG = ꢀ 144.3 kJ mol-1) [20,52,53]. Benzyl
alcohol can react with a bromo radical to generate benzylic radical 6 and
HBr. Then benzylic radical 6 can abstract oxygen to produce peroxy
radical 7 which traps hydrogen to generate hydroperoxide 8 [21,24,26,
48,49,54]. The hydroperoxide 8 transforms into benzaldehyde with the
To support the possible dissociation of the toluene C H bond,
trapping O2 by the radical 1, and generation of benzyl hydroperoxide 3,
photocatalytic oxidation of cumene in the presence of TBATB in EtOAc
under violet LED irradiation was examined in 3 h (Scheme 3). The for-
mation of cumene hydroperoxide was confirmed by 1H NMR and 13C
NMR analysis (SI).
–
–
selective CO bond cleavage [26]. The aldehyde C H bond in benz-
aldehyde can be dissociated by free radicals [55], which can be gener-
ated in the photooxidation reaction; therefore, 9 can be produced. The
abstraction of O2 by 9 resulted in the generation of 10, which subse-
quently converts into perbenzoic acid 11 through hydrogen trapping
[55]. Finally, benzoic acid can be produced via non-radical
Baeyer–Villiger-type oxidation reaction [55].
4. Conclusion
Herein, we reported on an efficient visible light active photocatalytic
system which can be utilized to transform toluene, its derivatives, and
methyl arenes into the corresponding benzaldehydes, benzoic acids and
ketones in good to high yield. It was found that the selectivity of the
oxidation products in this photocatalytic system can be influenced by
changing the irradiation wavelengths (from blue (460 nm) to violet (400
nm)) and solvents (from CH3CN to EtOAc). Toluene was successfully
converted into the benzoic acid in high conversion (87 %), and high
selectivity (86 %) under violet LED irradiation (400 nm) in EtOAc.
However, toluene was oxidized to benzaldehyde as in good conversion
(61 %), and high selectivity (84 %) under blue LED irradiation (460 nm)
in CH3CN. EtOAc was introduced as a suitable solvent for the conversion
of toluene to benzoic acid. CH3CN was considered to be an appropriate
solvent in the transformation of toluene to benzaldehyde under blue LED
In several researches in which bromo sources were used as photo-
catalyst, EtOAc was introduced as the best solvent to conduct photo-
oxidation of toluenes and benzyl alcohols to the benzoic acids [21,25,28,
48]. On the other hand, CH3CN known as the best solvent to conduct
photooxidation of toluenes and benzyl alcohols to benzaldehydes [27,
54], but there are no explanation about this phenomenon. It may be
caused by higher tendency of free radicals to hydrogen atom abstraction
–
from C H bonds of benzaldehyde in EtOAc compare to CH3CN [56].
Therefore, radical species 9 was easily formed in EtOAc compare to
CH3CN.
6

A. Mardani et al.
Journal of Photochemistry & Photobiology, A: Chemistry 414 (2021) 113301
Scheme 2. Probable mechanism for aerobic photooxidation of toluene.
Data availability
Data will be made available on request.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgement
Scheme 3. Photocatalytic aerobic oxidation of cumene.
irradiation. All of the LED wavelength radiation, dosage of TBATB, type
of solvent, and the reaction time could be effected the reaction con-
version and products selectivity. Finally, dissociation of the toluene
We gratefully acknowledge the support provided by the Institute for
Advanced Studies in Basic Sciences (IASBS).
–
C
H bond, trapping O2 by the obtained radical species, and generation
Appendix A. Supplementary data
of benzyl hydroperoxide species was confirmed via photocatalytic
oxidation of cumene over TBATB. Our mechanistic studies revealed that
the selective visible light oxidation of toluene could be following a
benzyl peroxy radical intermediate.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2021.
113301.
Author statement
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8