A tunable synthesis of either benzaldehyde or benzoic acid through blue-violet LED irradiation using TBATB

Article abstract of DOI:10.1016/j.jphotochem.2019.112220

In this paper, a highly efficient, metal-free, and homogeneous method for the selective aerobic photooxidation of alcohols and photooxidative-desilylation of tert-butyldimethylsilyl ethers (TBDMS) in the presence of tetrabutylammonium tribromide (TBATB) under irradiation of visible light was reported. The light source: blue (460 nm) and violet (400 nm) LED, can control selective oxidation to aldehyde or carboxylic acid.

Full text of DOI:10.1016/j.jphotochem.2019.112220

Journal Pre-proof
A tunable synthesis of either benzaldehyde or benzoic acid through
blue-violet LED irradiation using TBATB
Atefeh Mardani, Marouf Heshami, Yadollah Shariati, Foad Kazemi,
Mazaher Abdollahi Kakroudi, Babak Kaboudin
PII:
S1010-6030(19)31255-9
DOI:
https://doi.org/10.1016/j.jphotochem.2019.112220
JPC 112220
Reference:
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4 November 2019
Please cite this article as: Mardani A, Heshami M, Shariati Y, Kazemi F, Abdollahi Kakroudi M,
Kaboudin B, A tunable synthesis of either benzaldehyde or benzoic acid through blue-violet
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A tunable synthesis of either benzaldehyde or benzoic acid
through blue-violet LED irradiation using TBATB
Atefeh Mardania, Marouf Heshamia, Yadollah Shariatia, Foad Kazemia, b*, Mazaher
Abdollahi Kakroudia, 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
* Corresponding author: Foad Kazemi
E-mail: kazemi_f@iasbs.ac.ir
Graphical abstract
1

Highlights

TBATB an efficient metal-free and homogeneous visible light active
photocatalyst for selective aerobic oxidation

Selective aerobic photooxidation of benzyl alcohols and benzyl tert-
butyldimethylsilyl ethers to benzaldehydes in high yield under blue LED
irradiation


Selective aerobic oxidation of benzyl alcohols to the corresponding benzoic
acids under irradiation of violet LED (400 nm) in the presence of TBATB
Visible light-controlled selectivity to products in aerobic photooxidation
Abstract: In this paper, a highly efficient, metal-free, and homogeneous
method for the selective aerobic photooxidation of alcohols and
photooxidative-desilylation of tert-butyldimethylsilyl ethers (TBDMS) in the
presence of tetrabutylammonium tribromide (TBATB) under irradiation of
visible light was reported. The light source: blue (460 nm) and violet (400
nm) LED, can control selective oxidation to aldehyde or carboxylic acid.
Keywords: "tetrabutylamonium tribromide", "visible light photocatalyst",
"tunable synthesis", "oxidative-desilylation", "metal-free", "benzyl alcohol".
2

1. Introduction
Oxidation of benzyl alcohols and oxidative-desilylation of benzyl silyl ethers are
very important reactions in organic synthesis because related oxidation products
such as benzaldehydes and benzoic acids are versatile intermediates which can
serve as starting materials and precursors in a wide range of areas from industrial
chemistry to biological and pharmaceutical applications [1-5] . On the other hand,
controlled oxidation of benzyl alcohol to benzaldehyde, while avoiding over-
oxidation to benzoic acid, is highly important as considerable research efforts have
been devoted to the selective oxidation of benzyl alcohol to benzaldehyde [6-8].
Several aerobic photocatalytic systems have been reported to support oxidation
reactions [9-13]. Some of these strategies were shown in scheme 1. Aerobic
photooxidation reactions have provided a clean, short, and direct route in organic
synthesis. Photocatalysts of bromo sources have shown high potential for aerobic
photooxidation of alcohols under mild conditions. Itoh et al. used lithium bromide
[14], N-bromosuccinimide [15], hydrobromic acid or bromine [16] for the aerobic
photooxidation of alcohols to the corresponding carboxylic acids under UV
irradiation by a high-pressure mercury lamp.
3

Scheme 1 Strategies to carboxylic acid and aldehyde from alcohol
They believed that the bromine was generated from bromo sources under UV
irradiation. Because of harmful suppressing effects on human beings health, they
used solar radiation in the oxidation of alcohols with a combination of NaBr and
ion-exchange resins Amberlyst 15. The combination of sodium bromide and
Amberlyst 15 resulted in the highest yield of carboxylic acids [17]. they also
found that the aerobic photooxidation of a trace amount of the alcohols (0.3 mmol)
to the corresponding carboxylic acids can carry out in the presence of a catalytic
amount of MgBr2-Et2O under irradiation of visible lights from fluorescent lamps
[18]. Lei et al. reported aerobic photooxidation of benzylic alcohols (0.1 mmol) to
4

the corresponding benzaldehydes over a catalytic amount of AlBr3.6H2O under
visible light irradiation from a high-pressure xenon lamp [19]. Ananthakrishnan et
al. reported aerobic photooxidation of benzyl alcohols (1 mmol) to the
corresponding benzaldehydes in the presence of a catalytic amount of
bromodimethylsulfonium bromide under visible light irradiation. They proposed
that the bromo radicals help to the production of benzylic radicals [20]. Evans et al.
reported direct oxidation of primary tert-butyldimethylsilyl ethers to the
corresponding carboxylic acids with jones reagent [21]. To our knowledge,
there seems to be no report on the oxidative-desilylation of benzyl tert-
butyldimethylsilyl ethers by light.
TBATB, as a stable solid source of bromine, is a considerable reagent due to
mildness, easy to handle and safety in comparison to elemental bromine. It is an
efficient environmentally benign brominating agent [22-26] which has been used
in some reactions such as cleavage of dithioacetals [27], pyranylation-
depyranylation of alcohols [28], diacylation of aldehydes [29], preparation of a
wide variety of flavones and aurones [30], direct condensation of various alcohols
and carboxylic acids [31], etc. Although oxidation of alcohols, diols, and also
cleavage of silyl ethers with TBATB has been reported by a few research groups
[32-34], to our knowledge, selective aerobic photooxidation of benzyl alcohols and
5

photooxidative-desilylation of benzyl tert-butyldimethylsilyl ethers with a catalytic
amount of TBATB has not been investigated previously.
In this work, we found that benzyl alcohols and tert-butyldimethylsilyl ethers
were selectively oxidized into the corresponding benzaldehydes in high yield in the
presence of catalytic amount of TBATB at room temperature under blue LED
irradiation and air atmosphere.
2. Experimental
2.1 Materials and methods
Tetrabutylammonium tribromide (TBATB) was (Acros, 123970250). The other
chemicals and solvents purchased from Merck and Sigma-Aldrich. The reagents
were applied without further purification. Oxidation products were detected by gas
chromatography (GC) using a VARIAN CP3800 model. Identification of products
was performed by 1H NMR and 13C NMR spectroscopies (Bruker 400MHz
AVANCE III spectrometer) in CDCl3. Chemical shifts (δ) were reported in ppm
13
from TMS using the residual solvent resonance (CDCl3: 77.1 ppm for C NMR
1
and 7.3 ppm for H NMR). The blue (460 nm), violet (400 nm), and green (520
nm) LED lamps (Epistar 3W 20-30LM 700mAh) were applied in photooxidation
reactions .The progress of the photocatalytic oxidation reactions was monitored by
TLC on silica gel plates.
6

2.2 General experimental procedure for the Photocatalytic Oxidation of Alcohol:
In a 50 ml Pyrex round-bottom flask, a mixture of alcohol (1 mmol), TBATB
(10-20 mg, 0.02-0.04 mmol) in 10 ml of CH3CN was exposed to blue or violet
light LED irradiation at room temperature under an air atmosphere with stirring.
The progress of the photocatalytic oxidation reaction was monitored by TLC on
silica gel plates. The reaction mixture externally irradiated until the alcohol was
completely consumed.
2.3 General experimental procedure for the photooxidative-desilylation of tert-
butyldimethylsilyl ether:
In a 50 ml Pyrex round-bottom flask, 10 ml of CH3CN was taken. Then TBATB
(20 mg, 0.04 mmol) and tert-butyldimethylsilyl ether (1 mmol) which has been
produced by using a very widely used procedure[35] were added. The solution was
exposed to the blue LED irradiation at room temperature under an air atmosphere
with stirring. The irradiation was continued until the complete photooxidative-
desilylation of tert-butyldimethylsilyl ether in the reaction mixture.
3. Results and Discussion
First, aerobic photooxidation of benzyl alcohol with TBATB was carried out in
acetonitrile under irradiation of blue LEDs (3*3W) as model reaction. Aerobic
photooxidation of 1 mmol benzyl alcohol in the presence of 0.01 mmol TBATB
7

during 5, 15, and 20 h was examined. Increasing the reaction time from 5 to 20 h
did not offer significant advantages on the conversion of benzyl alcohol (Table 1,
entries 1-3). Increasing the amount of TBATB from 0.01 to 0.02 mmol led to a
significant increase in the conversion of benzyl alcohol (Table 1, entry 4).
Increasing the amount of TBATB from 0.02 to 0.03 and 0.04 mmol resulted in a
decrease in benzaldehyde formation from 98% to 92% and 85%, respectively,
due to the over-oxidation of benzyl alcohol to benzoic acid (Table 1, entries 4-6).
Therefore, 0.02 mmol of TBATB and 5 h were chosen as optimized amount of
catalyst and time, respectively (Table 1, entry 4).
In the following, the effect of irradiation of LED with various wavelengths on the
photooxidation conversion of benzyl alcohol and the product yield was
investigated. Photocatalytic aerobic oxidation over 0.02 mmol TBATB under green
LED lamps resulted in low conversions 7% during 5 h, (Table 1, entry 7).
Photocatalytic aerobic oxidation of benzyl alcohol over 0.02 mmol of TBATB
under violet LED irradiation resulted in both complete conversion (100%) and
good yield (87%) to benzaldehyde in 5 h (Table 1, entry 8). Therefore, the
photocatalytic activity of the TBATB is strictly affected by the wavelength of the
light. TBATB exhibited lower activity under green light irradiation and higher
activity under violet light irradiation. With increasing the amount of TBATB to
8

0.04 mmol, 100% conversion to benzoic acid was obtained under irradiation of
violet LED in 8 h (Table 1, entry 9).
The photocatalytic behavior of the oxidation of benzyl alcohol was confirmed by
the fact that the negligible conversion of benzyl alcohol was observed when the
reaction was performed in the absence of TBATB or LED irradiation (Table 1,
entries 10-11). The non-thermal and aerobic properties of the oxidation of benzyl
alcohol was considered through the formation of benzaldehyde in low yields of 2%
and 7%, respectively, when the reaction was carried out under an argon
atmosphere and under reflux condition without LED irradiation (Table 1, entries
12-13). Therefore, the LED irradiation, TBATB, and air have a crucial role in the
photooxidation transformation of benzyl alcohol.
Subsequently, for the purpose of comparison, the same photooxidation reaction
was also studied with Br2 (Table 1, entries 14-18). Complete conversion of benzyl
alcohol to benzaldehyde (85%) and benzoic acid (15%) was observed in
photooxidation of benzyl alcohol in the presence of Br2 (Table 1, entry 14). It can
be concluded that TBATB is more selective than elemental bromine. Also, the
probability of the formation of bromine from MgBr2, AlBr3, TBAB
(tetrabutylammonium bromide), and NBS as bromide sources was examined. The
result revealed that bromine radical generated from bromine in TBATB, NBS and
Br2 is an active species in photooxidation of benzyl alcohol, while the other
9

bromide sources such as TBAB, AlBr3 and MgBr2 shown lack or low ability to
perform oxidation of benzyl alcohol (Table 1, entries 16-18).
Table 1 Aerobic photooxidation of benzyl alcohol and photooxidative-desilylation of benzyl
tert-butyldimethylsilyl ether using TBATB
entry
TBATB
(mmol)
0.01
0.01
0.01
0.02
0.03
0.04
0.02
0.02
0.04
0
time (h)
A (%)y
B (%)y
conversion (%)y
1
2
3
4
5
5
15
20
5
3
3
5
5
8
24
24
24
24
5
5
5
5
5
31
32
35
98
92
85
7
87
0
2
0
0
0
0
8
15
0
13
100
0
0
0
0
15
6
0
0
0
4
55
12
31
32
35
98
100
100
7
100
100
2
6
7a
8b
9b
10c
11d
12e
13f
14
15
16
17
18
19i
20i
21i
22i
0.02
0.02
0.02
2
2
7
2
2
7
g
Br2
85
89
0
18
13
13
45
88
83
100
95
0
18
13
17
100
100
100
NBSg
TBABh
g
AlBr3
MgBr2
g
0.02
0.02
0.04
0.04
9
12
5
9
17
b
Photooxidation a under irradiation of green LED lamps (520 nm, 3*3W), violet LED lamps (400 nm, 3*3W), in
the absence of TBATB, in the absence of LED lamps irradiation (dark room), under argon atmosphere, under
reflux condition, g 0.02 mmol, h 0.04 mmol, i photooxidative-desilylation of 1 mmol of benzyl tert-butyldimethylsilyl
ether. y GC yield.
c
d
e
f
Next, direct aerobic photooxidative-desilylation of 1 mmol benzyl tert-
butyldimethylsilyl ether to benzaldehyde was carried out in the presence of
TBATB in CH3CN under blue LED irradiation (Table 1, entries 19-22). In the
presence of 0.02 mmol of TBATB, the conversion of benzyl tert-butyldimethylsilyl
10

ether was 17% in 9 h (Table 1, entry 19). Increasing the time of reaction from 9 to
12 h encouraged oxidation of benzyl tert-butyldimethylsilyl ether and production
of benzoic acid (Table 1, entry 20). Increasing the amount of TBATB from 0.02 to
0.04 mmol led to the complete conversion of benzyl tert-butyldimethylsilyl ether
and production of 88% benzaldehyde in 5 h (Table 1, entry 21). Also, increasing
the reaction time to 9 h led to the more over-oxidation of benzyl alcohol to benzoic
acid (Table 1, entry 22). Therefore, 0.04 mmol of TBATB and 5 h reaction time
were selected as the optimized condition for photooxidative- desilylation of benzyl
tert-butyldimethylsilyl ether (Table 1, entry 22).
Subsequently, the effect of solvent on the photocatalytic aerobic oxidation of
benzyl alcohol was investigated in the optimized reaction condition (0.02 mmol
TBATB, 10 ml CH3CN, blue LED irradiation and 5 h). The results were shown in
Figure 1.
100
100
80
100
100
100
98
95
84
40
34
3
CH3CN
EtOAc
THF
DCM
DMF
H2O
Conversion (%)
Selectivity to benzaldehyde (%)
Figure 1 Photooxidation of 1 mmol benzyl alcohol in various solvents using 0.02 mmol TBATB
in 5 h under blue LED irradiation
11

Among CH3CN, EtOAc, THF, DCM, DMF, and H2O, the lowest conversion (3%)
was observed when using H2O indicating a non-radical and non-catalytic
mechanism for the conversion of benzyl alcohol into the benzaldehyde [36].
CH3CN, as a common solvent for oxidation purposes [37], provided the highest
conversion (98%) of benzyl alcohol to benzaldehyde.
Our investigation was extended to the photooxidation of substituted benzyl
alcohols, allylic, and aliphatic alcohols (1a-19a) (Table 2).
Table 2 Photooxidation of various alcohols
entry
substrate
method
A
time (h)
5
product
conversion (%)
98
GC yield (%)
98
isolated yield (%)
96
1a
B
A
8
7
100
98
100
98
98
96
2a
3a
B
A
15
10
100
90
98
90
95
87
B
24
100
100
98
A
B
A
3
8
3
100
100
100
100
100
100
97
98
97
4a
5a
B
6
100
100
95
12

A
A
B
5
6
90
90
100
72
88
97
70
6a
100
100
24
A
B
B
10
6
82
72
82
82
72
82
79
70
78
7a
24
A
B
A
A
2
100
100
5
100
90
5
96
86
2
8a
10
10
10
9a
10a
0
0
0
11a
12a
B
A
24
5
100
100
100
100
99
83
13a
14a
A
A
A
10
24
24
30
2
30
2
24
1
10
10
5
15a
B
B
72
24
70
10
70
10
63
6
16a
17a
B
A
B
A
72
14
72
20
100
5
100
5
95
4
18a
19a
55
0
55
0
48
0
B
72
65
65
59
13

Both the electron donating (–OCH3) and withdrawing (–Cl, –NO2) groups at the
para position of benzyl alcohol ring enhanced its photocatalytic reactivity: In 4a
and 5a, nitro and chloro as electron-withdrawing groups at the para position of
benzyl alcohol ring exhibited an accelerating influence on the photooxidation
reaction, 100% conversions and 100% selectivities to the corresponding
benzaldehydes could be obtained after 3 h. Also, methoxy as an electron-donating
group at the para position of benzyl alcohol ring showed an accelerating effect on
the photooxidation reaction. Photooxidation of 4-methoxybenzyl alcohol (8a)
led to complete conversion and production of 4-methoxybenzaldehyde after 2 h.
Compared to p-nitrobenzyl alcohol (4a), the o-nitro (2a) and m-nitro (3a) benzyl
alcohols exhibited low activity in this reaction but with increasing reaction times,
the high conversions could be obtained. Therefore, the photocatalytic reactivity of
substituted benzyl alcohols can be affected by the position of the substituents on
the aromatic ring of benzyl alcohol. Also, compared to 2,4-dichlorobenzyl alcohol
(6a), the 2,6-dichlorobenzyl alcohol (7a) required to long reaction time (10 h) to
reach high conversion. It may be caused by the steric hindrance of chloro
substitutions near the active site. Photooxidation of 9a and 10a resulted in a very
low and no conversions, respectively. It seems that reactions between the phenolic
compounds and bromine led to inactivation of bromine. Photooxidation of
14

cinnamyl alcohol (13a) as allylic alcohol resulted in the production of only 30%
cinnamaldehyde after 10 h. It seems that TBATB acts as a brominating agent in the
presence of carbon–carbon double bond of cinnamyl alcohol.
The scope of photooxidation reaction with TBATB was also expended to
aliphatic alcohols (14a-19a). Compared to benzyl alcohols, aliphatic alcohols
showed low activity in this reaction. 15a and 18a were oxidized into the
corresponding ketones in 10% and 5% conversions, respectively, after 24 and 14 h
under method A. Also, in 19a, no conversion of 1-octanole to the corresponding
aldehyde and acid observed after 20 h. With increasing the reaction time to 72 h,
the conversion of 15a, 18a and 19a increased to 70, 55 and 65%, respectively
under method B.
To produce carboxylic acids through over-oxidation of alcohols, three violet
LED lamps and 0.04 mmol TBATB (method B) were used instead of three blue
LED lamps and 0.02 mmol TBATB (method A). Obtained results were shown in
Table 2. In general, benzylic and secondary alcohols converted into the
corresponding carboxylic acids and ketones, respectively, in high yield under
irradiation of violet LED; however, in 6a and 7a, corresponding benzaldehydes
were the main oxidation product due to the steric hindrance of chloro substitutions
(in these cases, low over-oxidation to carboxylic acids was observed).
15

Also, the aliphatic alcohols (16a-17a) exhibited a lower activity than benzyl
alcohols to undergo photooxidation.
Subsequently, our study was extended to the photooxidative-desilylation of
several tert-butyldimethylsilyl ethers (1A-8A) over 0.04 mmol of TBATB under
blue LED irradiation (Table 3). High conversion and yield for the photooxidative-
desilylation of 4-nitro (3A) and 4-chloro (4A) benzyl tert-butyldimethylsilyl ethers
were obtained. In 2A, the nitro group at the ortho position of benzyl tert-
butyldimethylsilyl ether showed a decelerating effect on the photooxidation
reaction, 38% conversion was obtained after 20 h.
Table 3 Photooxidative-desilylation of various tert- butyldimethylsilyl ethers
time
(h)
isolated
yield (%)
isolated
yield (%)
entry
substrate
conversion
100
product
product
5
20
6
85
12
0
10
23
98
1A
2A
3A
38
100
4A
5A
5
8
100
100
93
0
2
96
6A
7A
15
20
80
15
70
0
4
10
16

8A
20
88
80
5
In 6A, due to the steric hindrance of chloro substitutions, low oxidation to the
corresponding benzaldehyde was observed (2,6-dichlorobenzyl alcohol was the
main oxidation product). Secondary and allylic tert-butyldimethylsilyl ethers were
found to be less reactive than other benzylic tert-butyldimethylsilyl ethers
examined. 2-Octyl tert-butyldimethylsilyl ether (8A) as a secondary tert-
butyldimethylsilyl ether exhibited low activity in photooxidative-desilylation to the
corresponding aldehyde. (2-octanol was the main oxidation product after 20 h).
Also, cinnamyl tert-butyldimethylsilyl ether (7A) as an allylic tert-
butyldimethylsilyl ether afforded the cinnamaldehyde in low yield (10%) after 20
h. It should be noted that oxidation of 1A resulted in no oxidation products in the
absence of TBATB, and only 4% benzyl alcohol in the absence of LED lamps
(dark room) after 24 h. Therefore TBATB and LED lights play crucial role in
photooxidative-desilylation of tert-butyldimethylsilyl ethers.
Finally, based on the obtained results and previous literature, a probable
mechanism was proposed for the present aerobic photooxidation of benzyl alcohol
by TBATB in Scheme 2. The yellow coloration of the solution containing TBATB
suggests that initially Br2 is formed from TBATB. Also, immediately color fading
of this solution after the introduction of phenolic compounds which resulted in a
17

very low and no conversion in the photooxidation reaction (Table 2, 9a and 10a)
verify this suggestion.
Benzyl alcohol can react with a bromo radical, generated via photodissociation of
bromine to afford benzylic radical (species 1) and HBr, which subsequently can be
converted into the bromo radical upon aerobic photooxidation [18]. Then the
benzylic radical can trap O2 to afford the peroxy radical (species 2), which
abstracts hydrogen to afford hydroperoxide (species 3) [14, 15, 17, 19, 20, 38].
Species 3converts into benzaldehyde with the selective C–O bond cleavage [19].
The aldehyde C-H bond in benzaldehyde can be cleaved by free radicals [39] (such
as peroxy radicals which can be produced during the photocatalytic oxidation
reaction), therefore species 4 can be generated. The abstraction of molecular
oxygen by species 4 leads to the formation of radical species 5, which
subsequently transforms to perbenzoic acid (species 6) via hydrogen abstraction
[39]. Finally, perbenzoic acid converts into benzoic acid by non-radical Baeyer–
Villiger-type oxidation [39].
18

Scheme 2 Possible reaction pathway
3. Conclusion
In this work, we report an efficient metal-free visible light active photocatalytic
system that can be used to convert benzyl alcohols and benzyl silyl ethers into their
corresponding carbonyl compounds in high yield under mild conditions, using a
catalytic amount of TBATB (0.02-0.04 mmol) as photocatalyst, air and blue (460
nm) or violet LED (400 nm) as light sources. We found the oxidation products
selectivity can be affected and controlled by changing the irradiation wavelengths
in this photocatalytic system.
Utilizing the air as a green oxidant, LED as visible light source, and TBATB as a
metal-free photocatalyst provided an advantage approach over protocols that
19

needed toxic and expensive metal catalysts, corrosive oxidant, elevated reaction
temperature, and time-consuming intermediate separation.
Conflict of Interest and Authorship Conformation Form
Please check the following as appropriate:
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All authors have participated in (a) conception and design, or analysis and
interpretation of the data; (b) drafting the article or revising it critically for important
intellectual content; and (c) approval of the final version.
o
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o
This manuscript has not been submitted to, nor is under review at, another journal or
other publishing venue.
The authors have no affiliation with any organization with a direct or indirect
financial interest in the subject matter discussed in the manuscript
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Acknowledgement
We gratefully acknowledge the support provided by the Institute for Advanced
Studies in Basic Sciences (IASBS). Also, we gratefully appreciate Asha Zarr for a
great deal of guidance to language editing this paper.
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