Photochemical oxidation of benzylic primary and secondary alcohols utilizing air as the oxidant

Article abstract of DOI:10.1039/c9gc03000j

A mild and green photochemical protocol for the oxidation of alcohols to aldehydes and ketones was developed. Utilizing thioxanthenone as the photocatalyst, molecular oxygen from air as the oxidant and cheap household lamps or sunlight as the light source, a variety of primary and secondary alcohols were converted into the corresponding aldehydes or ketones in low to excellent yields. The reaction mechanism was extensively studied.

Full text of DOI:10.1039/c9gc03000j

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Photochemical oxidation of benzylic primary and
secondary alcohols utilizing air as the oxidant†
Cite this: DOI: 10.1039/c9gc03000j
Nikolaos F. Nikitas, Dimitrios Ioannis Tzaras, Ierasia Triandafillidi and
Christoforos G. Kokotos
*
A mild and green photochemical protocol for the oxidation of alcohols to aldehydes and ketones was
developed. Utilizing thioxanthenone as the photocatalyst, molecular oxygen from air as the oxidant and
cheap household lamps or sunlight as the light source, a variety of primary and secondary alcohols were
converted into the corresponding aldehydes or ketones in low to excellent yields. The reaction mecha-
nism was extensively studied.
Received 25th August 2019,
Accepted 11th December 2019
DOI: 10.1039/c9gc03000j
rsc.li/greenchem
Unfortunately, the use of acetic acid as the solvent can be con-
sidered harsh for sensitive groups and not environmentally
Introduction
Carbonyl compounds are one of the most important function- friendly. Moreover, biocatalysis has also provided significant
alities in pharmaceuticals and their synthesis via alcohol oxi- advances in oxidation reactions.5
dation is a key reaction in chemical science.1 Among the tra-
Over the past two decades, photocatalysis has made a
ditional and most common methods for the oxidation of alco- powerful impact in the activation of molecules and its appli-
hols are the use of stoichiometric amounts of oxidants, such
as MnO2, hypochlorite, permanganate, osmium oxide, acti-
vated dimethylsulfoxide (DMSO), hypervalent iodine reagents,
peroxide-based or salt-based protocols, corresponding to a
number of named reactions in organic synthesis, that nowa-
days are common practice (Scheme 1A).1,2 Unfortunately, these
processes create large volumes of waste that deteriorate the
climate on Earth, while most of them can be also considered
quite toxic. In order to avoid the use of large amounts of toxic
and hazardous reagents, a variety of homogeneous and hetero-
geneous catalytic systems have been reported. In an effort to
make these reactions friendlier to the chemical industry and
more environmentally-friendly, new, green and sustainable
protocols have been developed utilizing air or oxygen as the
oxidant (Scheme 1B).3,4
These catalytic systems use either metal-complexes or metal
nanoparticles or Tempo-like radicals as catalysts which, unfor-
tunately, lead to a variety of byproducts and wastes. For
example, the group of Iwabuchi have elegantly demonstrated
the oxidation of alcohols to carbonyl compounds, utilizing a
balloon of oxygen or air as the oxidant and a synthetic catalyst,
which is not commercially available (TEMPO-like reagent).4d
Laboratory of Organic Chemistry, Department of Chemistry, National and
Kapodistrian University of Athens, Panepistimiopolis, Athens 15771, Greece.
E-mail: ckokotos@chem.uoa.gr
†Electronic supplementary information (ESI) available: Experimental data, com-
pound characterization, UV-Vis absorbance spectra, fluorescence quenching and
mechanistic studies. See DOI: 10.1039/c9gc03000j
Scheme 1 Different approaches for the oxidation of alcohols to carbo-
nyl compounds.
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cations have increased significantly.6 Inspired by the potential
of photocatalysis, several metal-based photocatalysts have been
reported, based on Cu, Ti, Pt, Nb, Pt-functionalized porphyri-
nic MOF, supported nanoparticles, etc. for the oxidation of
alcohols (Scheme 1C).7 Although most of these processes are
very elegant, the requirement of a metal cluster can be con-
sidered as a disadvantage. In an effort to prevent the use of
toxic metallic reagents in oxidation reactions, there is a strong
requirement for alternative metal-free photocatalysts.8
Table 1 Optimization of the reaction conditions for the photochemical
oxidation of benzyl alcohol
Entry
1
Solvent
DMSO
Catalyst
Time (h)
18
Yielda (%)
34
Photoorganocatalysis is
a low-cost and environmentally
friendly alternative.9 Photocatalysis has already been used for
the oxidation of alcohols to carbonyl compounds, utilizing
various oxidants, such as DDQ or TBHP.10 Small organic mole-
cules have also shown their activity towards the activation of
O2 under visible light, and this concept has already shown
powerful applications in organic synthesis.11 A part of these
applications constitutes the oxidation of alcohols to carbonyl
compounds.12
2
DMSO
18
11
3b
4
DMSO
DMSO
Eosin Y 3c
18
18
29
6
5
DMSO
18
12
We have a long interest in photocatalysis13 and in sustain-
able processes for oxidations.14 In an effort to develop a green
catalytic protocol, herein, we report a mild photochemical
system for the selective oxidation of alcohols, without the use
of toxic metal clusters and complexes or the use of stoichio-
metric oxidation. The presented protocol herein, focuses on an
eco-friendly metal-free process, which avoids the use of toxic
and hazardous reagents, minimizing the need for energy con-
sumption (since the reaction can be also performed by sun-
light irradiation) and utilizes air, which is considered by green
metrics the best source, as the oxidant (Scheme 1D).
6
7
8
DMSO
DMSO
DMSO
18
18
18
26
5
4
9
DMSO
DMSO
DMSO
MeCN
H2O
DMF
18
19
14
14
14
14
14
14
61
10
11
12
13
14
15
16
55
85(82)c
33
0
25
0
58
Toluene
MeCN/DMSO
Results and discussion
a All reactions were carried out with 1a (0.20 mmol), catalyst (20 mol%)
and solvent (0.6 mL) in an open-air vessel, under household bulb
irradiation. b 5 mol% catalyst loading. c Isolated yield.
We initiated our study utilizing benzyl alcohol (1a) as the sub-
strate and air as the oxidant. In the optimization of the reac-
tion conditions, a variety of different catalysts and solvents
were tested (Table 1). As shown in Table 1, aromatic ketones,
in the role of the catalyst, such as sodium anthraquinone-2- (Table 1, entry 11). For more details regarding the ratio of over-
sulfonate (3a) or acenaphthenquinone (3b) provided low yields oxidation, see ESI.† We also examined if benzaldehyde could
(Table 1, entries 1 and 2). A common photocatalytic organic act as the photocatalyst (autocatalytic reaction), which gave a
dye, eosin Y (3c), showed similar reactivity (Table 1, entry 3). poor 5% yield (Table 1, entry 8). Since DMSO is still considered
Different type of catalysts, such as acetophenone (3f) or benzo- a practical solvent for chemical industries, but it would be
phenone (3g), which are usually used as photoinitiators, highly desirable if we could substitute it with a greener
afforded the product in low yields (Table 1, entries 4–8). In solvent, our next step was to examine the role of the solvent in
contrast, the use of thioxanthenone (3i) as the photocatalyst, the reaction. Acetonitrile provided the product in 33% yield
provided the desired aldehyde in 61% yield (Table 1, entry 9). (Table 1, entry 12), while other polar solvents, such as water or
Further increase in the reaction time dropped the yield in 55% DMF, afforded lower results (Table 1, entries 13 and 14). When
(Table 1, entry 10). This result was very interesting, and by non-polar solvents, such as toluene, were used no product was
1
monitoring the reaction outcome by H-NMR, it was possible detected (Table 1, entry 15). When a mixture of MeCN/DMSO
to detect the production of benzoic acid. This meant that over- (instead of MeCN alone) was used as the solvent, the yield was
oxidation to the corresponding carboxylic acid was occuring, a increased from 33% to 58%, which indicates that DMSO plays
common problem in the selective oxidation of primary alco- a significant role in the reaction outcome and cannot be
hols.15 Therefore, we decided to reduce the reaction time and replaced (Table 1, entry 16 vs. 12). Trying to determine the role
examine whether we could obtain a higher yield of the of the solvent, it is known that DMSO can stabilize the excited
product. To our delight, when the reaction time dropped to state of diaryl ketones (of similar chemical type with 3i), so it
14 h, we managed to obtain the aldehyde in 85% yield is rational to assume that the excited state of thioxanthene-9-
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one (3i) has a prolonged lifetime in the presence of DMSO as loading to 5 mol%, in order to highlight the green character of
the solvent, making it an important part of the reaction.16
the protocol. In some cases we could purify the desired ketone
Having in hand the optimized reaction conditions, we then just by an ice-water workup-extraction, in order to remove the
explored the substrate scope (Scheme 2). Various mono- or di- DMSO from the reaction mixture. The best results were pro-
substituted benzyl alcohols were used and provided the vided from the oxidation of substituted 1-phenylethan-1-ols.
desired aldehydes 2a–2c in good yields. Furthermore, ortho- para-Substituted alcohols, with either electron-donating or
substituted benzyl alcohols afforded the desired aldehydes 2d electron-withdrawing groups, provided the desired ketones
and 2e in lower yields, probably due to steric hindrance close 2k–2s in moderate to excellent yields. In the case of 2l, it was
to the center of the reaction. The electronics on the aromatic possible to isolate the desired product by simple solvent evap-
ring plays a significant role in the reaction outcome, since pro- oration, without the need of column chromatography. This
ducts 2f–i were obtained in various yields (from 14 to 70%). also applies in all cases where the reaction reached completion
Unfortunately, non-benzylic primary alcohols, like 3-phenyl- (for example, compounds 2m–p). ortho-Substituted alcohols,
propanol and 1-hexanol, did not afford the oxidation. When like 1r, provided the desired product 2r in 39% yield, presum-
the photochemical protocol was applied in an allylic alcohol ably due to steric hindrance close to the center of the reaction.
(cinnamyl alcohol), the desired α,β-unsaturated aldehyde 2j Double and triple bond-bearing alcohols also afforded the
was formed in 36% yield. In the case of benzyl secondary desired products 2u and 2w in very good yields, highlighting
alcohols, we determined that we could lower the catalyst the chemoselectivity of the method. Aromatic substituted alco-
hols, such as benzhydrol, and xanthydrol, provided ketones 2t
and 2aa in excellent yields. In order to expand the substrate
scope, we attempted the oxidation of cyclic secondary alcohols,
which gave the ketones 2ac–2ag in very good yields. Aliphatic
secondary alcohols, such as 2-pentanol, provided the carbonyl
compound 2ab in 49% yield, due to volatility issues. In order
to examine the chemoselectivity of the method, we chose to
oxidize an 1,3-diol to the corresponding β-hydroxy ketone
(2ah), as the only product in 70% yield, showing the chemo-
selectivity of the method to the oxidation of the benzylic posi-
tion versus the aliphatic one. In order to apply the oxidation in
more complicated real-life examples, cholesterol (1ai) was
employed as the substrate and the desired ketone 2ai was
obtained in 63% yield.
After substrate scope evaluation, we became interested in
determining the actual role of the catalyst and the light source.
Performing control experiments for the oxidation reaction of
benzyl alcohol (1a) to benzaldehyde (2a), we extracted some
very important results, in order to understand the different
parameters of the reaction (Tables 2 and 3). These experiments
showed that no product was obtained under dark (Table 2,
entry 2) and thermal conditions (Table 2, entry 3), indicating a
Table 2 Control experiments for the photochemical aerobic oxidation
of benzyl alcohol
Entry
Reaction parameters
Yielda (%)
1
2
3
4
5
Standard conditions
No light
Heating, no light
No catalyst
85
0
0
0
0
Ar atmsphere
a All reactions were carried out with 1a (0.20 mmol), 3h (20 mol%),
solvent (0.6 mL), air, under 2 × 80 W household lamps irradiation.
Yields were determined by GC-MS.
Scheme 2 Substrate scope for the photochemical oxidation of benzylic
and secondary alcohols.
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Table 3 Quenching experiments for the photochemical aerobic oxi-
dation of benzyl alcohol
Yielda
(%)
Entry
Quencher (equiv.)
Note
1
2
3
4
5
Tempo (1)
BHT (1)
NaN3 (1)
DABCO (1)
2,2,6,6-Tetramethyl-
piperidine (1)
Radical scavenger
Radical scavenger
Singlet oxygen scavenger
Singlet oxygen scavenger
Singlet oxygen scavenger
0
0
35
6
5
a Yields were determined by GC-MS.
“clean” photochemical reaction, while in the absence of cata-
lyst, no product was obtained (Table 2, entry 4). In addition,
when the reaction was performed under argon atmosphere, no
product was observed, which highlighted the importance of
oxygen in the reaction (Table 2, entry 5). When Tempo was
added in the reaction mixture as a radical trap, no oxidation
product was formed (Table 3, entry 1). The intermediate com-
pound formed between Tempo and alcohol 1k was observed by
Scheme 3 Stern–Volmer plots for the fluorescence quenching of
thioxanthenone (1 mM in DMSO) with A. Benzyl alcohol, B. Benzhydrol.
compounds (Eð*redÞ thioxanthenone ¼ þ1:52 eV vs. SCE,6e while
E(ox) benzyl alcohol = +2.16 eV vs. SCE21). Furthermore, bub-
bling oxygen in a cuvette with a solution of thioxanthenone
led to diminished fluorescence, validating that air quenches
the catalyst.17 Further control fluorescence quenching experi-
ments with the quenchers of Table 3 were also performed.17
Based on literature, Orfanopoulos and coworkers per-
formed a series of experiments, in order to distinguish
between SET or HAT processes for the oxidation of alcohols.22
In that work, a control experiment utilizing 1-(4-methoxyphe-
nyl)-2,2-dimethyl-1-propanol (1aj) as the substrate was demon-
strated. Due to its substitution pattern, when a SET process is
occurring in the Cα–H of the hydroxyl group, a Cα–Cβ bond
cleavage follows, leading to the formation of the corres-
ponding aldehyde and carboxylic acid. On the other hand, if
the photooxidation of this substrate proceeds through a HAT
event, then, the reaction furnishes the tert-butyl ketone (2aj),
as the sole product.22 Applying this test reaction in our
method, we examined the photooxidation of 1-(4-methoxyphe-
nyl)-2,2-dimethyl-1-propanol (Scheme 4, 1aj). In our case, after
18 h of irradiation, the starting alcohol was converted comple-
tely in a mixture of 2aj : benzaldehyde : benzoic acid in a ratio
of 68 : 3 : 29, respectively. As we expected, the major product is
ketone 2aj, which indicates that our method does not involve a
SET process, but mainly a HAT event (Scheme 4). Moreover,
GC-MS, hinting the generation of
a
benzyl radical.17
Furthermore, quenching reagents were used, in order to recog-
nize the oxygen species that take part in the reaction mecha-
nism (Table 3). As an oxygen species quencher, NaN3 was used
and partially inhibited the process, which as a fact shows that
the presence of singlet oxygen species in the reaction mecha-
nism is strong (Table 3, entry 3). An additional experiment, that
strongly indicates the association of singlet oxygen, is the use of
anthracene and its derivatives as singlet oxygen traps, which
was introduced in literature by Fukuzumi.18 The addition of
anthracene in the standard reaction mixture furnished the
corresponding anthraquinone in 90% yield, which indicates the
presence of singlet oxygen. The same conclusion can be drawn
with the use of DABCO (Table 3, entry 4). Additionally, utilizing
2,2,6,6-tetramethylpiperidine, which is a known singlet oxygen
quenching reagent,19 no product was formed (Table 3, entry 5).
Another tool that contributes to the understanding of the
reaction mechanism, is the determination of the quantum
yield (Φ) of the photocatalytic reaction. Based on literature,20
the quantum yield (Φ) of the photocatalytic reaction was calcu-
lated: [Φ = 12 (Φ > 1)], which hints that a radical propagation
mechanism is taking place.
In order to gather further information about the reaction
mechanism, fluorescence quenching studies took place. Stern–
Volmer plots for the fluorescence quenching experiments
revealed that the excited photocatalyst does not interact
directly with benzyl alcohol or benzhydrol, since the slope of
the quenching is minimal (Scheme 3).8e Further support for
this notion, that a direct single electron transfer between
excited thioxanthenone and benzyl alcohol is not possible, is
given by the comparison of the reduction potentials of the two Scheme 4 Mechanistic test reaction for SET or HAT processes.
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similar conclusions derived from quenching experiments with
1,3,5-trimethoxybenzene (TMB).17 TMB has a relative low oxi-
dation potential (E(ox) TMB = +1.12 eV vs. SCE)22 and sub-
sequently can act as an electron donor against the excited
form of thioxathenone, quenching the excited state and as a
result the termination of the radical procedure. We performed
the oxidation of benzyl alcohol 1a to aldehyde 2a in the pres-
ence of TMB. The result of the reaction was, as we assumed,
the oxidation of the alcohol to aldehyde, verifying that the
reaction does not proceed through a SET pathway.22
Scheme 6 Test reactions for the oxidant source.
Utilizing the above mechanistic studies, we can propose a
plausible mechanism for the photochemical oxidation
(Scheme 5). The first step is the excitation of the photocatalyst,
thioxanthene-9-one, by light. Next, based on the results of the
fluorescent quenching studies, excited thioxanthene-9-one
interacts with molecular oxygen, generating singlet oxygen and
returning thioxanthene-9-one to its ground state. This is in
agreement with literature precedent, regarding excited thiox-
anthenone as singlet oxygen generator.23 Then, singlet oxygen,
as an activated form of oxygen, interacts with benzyl alcohol,
either through a two-step mechanism, in which the first step is
the abstraction of a hydrogen atom, leading to the formation
of the benzylic radical I followed by a fast radical recombina-
tion,24 leading, to the formation of peroxy hemiacetal II or
through an one-step insertion in the C–H bond of the benzylic
position, leading again to II. This activated compound decom-
poses with further irradiation, leading to benzaldehyde and
hydroxy radicals, which can lead to propagation or can decom-
pose to hydrogen peroxide. Indication for the presence of
hydrogen peroxide in the reaction is the formation the corres-
ponding dimethylsulfone, as a byproduct, the product of oxi-
dation of the solvent (Scheme 6). In order to be sure for the
origin of the oxidant source, we examined the power of hydro-
gen peroxide as the oxidation reagent, firstly for the oxidation
of dimethylsulfoxide to dimethylsulfone and secondly for the
oxidation of 1-phenylethanol (1k) to acetophenone (2k)
Scheme 7 Photochemical oxidation utilizing sunlight.
(Scheme 6). DMSO can be oxidized either by aqueous hydrogen
peroxide solution itself or hydrogen peroxide-derived from
singlet oxygen,23b,25 the latter can be formed by thioxanthe-
none through a photocatalytic process (Scheme 6A). On the
other hand, aqueous hydrogen peroxide solution cannot
oxidize alcohol 1k, confirming the hypothesis that singlet
oxygen is the main oxidant source (Scheme 6B). Finally, the
use of alternative energy source in our photochemical process
was envisaged, in order to have a significant industrial applica-
bility. Therefore, a variety of primary and secondary alcohols
was tested as substrates, utilizing sunlight as the irradiation
source, instead of CFL lamps, and the desired carbonyl com-
pounds were formed in very good yields (Scheme 7). Secondary
benzylic alcohol, like 1k and 1t afforded the desired ketones in
similar yields as the household lamps. Unfortunately, the oxi-
dation of 1a under sunlight led to diminished yield (48%),
since overoxidation to benzoic acid could not be controlled.
Conclusions
In conclusion, a green and sustainable photochemical protocol
for the oxidation of alcohols was developed. Since a plethora of
stoichiometric and catalytic systems have been used for this
reaction, our approach was to find an environmentally friendly
way to promote this transformation, by utilizing air as the
oxidant and a cheap and commercially available organic mole-
cule as the photocatalyst. After an extensive study, a variety of
benzylic and secondary alcohols were transformed to the corres-
ponding carbonyl compounds and mechanistic studies were
performed in order to better understand our catalytic system.
Scheme 5 Proposed reaction mechanism.
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Experimental
General procedure for the oxidation of alcohols
In a glass vial containing thioxanthene-9-one (3i) (8.5 mg,
0.04 mmol) in DMSO (0.6 mL), benzyl alcohol or secondary
alcohol (0.20 mmol) was added. The vial was left open in air
and left stirring under household bulb irradiation (2 × 80 W
household lamps) for 14–18 h. The desired product was iso-
lated via purification of the crude mixture by flash column
chromatography or just by ice-water workup-extractions.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the John Latsis
Foundation for the financial support through the program
‘EPISTHMONIKES MELETES 2015’ (PhotoOrganocatalysis:
Development of new environmentally-friendly methods for the
synthesis of compounds for the pharmaceutical and chemical
industry). Also, the authors gratefully acknowledge COST
Action C–H Activation in Organic Synthesis (CHAOS) CA15016
for helpful discussions. N. F. N. would like to thank the State
Scholarships Foundation (IKY) for financial support through a
PhD fellowship through the “Strengthening of Human
Resources through Doctoral Research” (MIS-5000432) program
of the Operational Program “Human Resource Development,
Education and Lifelong Learning” 2014-2020, co-financed by
the European Union (European Social Fund ESF) and Greek
national funds. I. T. would like to thank the Hellenic
Foundation for Research and Innovation (HFRI) and the
General Secretariat for Research and Technology (GSRT), for a
HFRI PhD Fellowship grant (GA14476).
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