Current Research on Synthetic Process
Currently, hundreds of techniques are available for catalytic oxidation of alky arenes,
and they can be divided into three categories: (1) chemical oxidation methods, including
oxygen oxidation,[10 ]
[11 ] hydrogen peroxide oxidation,[12 ]
[13 ] and nitric acid oxidation[14 ]; (2) electrocatalytic oxidation[15 ]; and (3) photocatalytic oxidation.[16 ] The first two are conventional oxidation processes for alkyl arenes while the last
one is a newly developed green oxidation technique. Among other things, [Fig. 1 ] illustrates the conventional oxygenation synthesis of alkylated arenes. Since 1980,
the oxidation of aromatic hydrocarbons using air or oxygen in the liquid phase has
received more and more attention, and it can be classified into three categories:
solvent-free, and acid or alkali liquid-phase oxidation methods. The alkali and solvent-free
method is very promising because of its high selectivity and lack of the drawbacks
of the acid method. The acid method has the advantages of high yields, a simple post-treatment
process, low production costs, and the product precipitated by crystals, while a serious
drawback is the use of organic acid as a solvent affecting the corrosion of the equipment.
Although solvent-free and alkaline methods avoid the problems of the acid procedures,
the selection of suitable catalytic systems with a high selectivity is very challenging.
Fig. 1 Conventional oxidation of methyl-substituted aromatic compounds.
Light is the ideal energy source for green synthetic chemicals due to its accessibility
and safety. As a gentle method, it has been widely used to construct organic molecules.
In 1967,[17 ] Akira Fujishima of the University of Tokyo (known as the “father of photocatalysis”
and the discoverer of “photocatalyst”) and his mentor Kenichi Hondo jointly discovered
that water molecules on the surface of the titanium dioxide (TiO2 ) electrode photolyzed—that is, broke down into hydrogen and oxygen—when exposed to
ultraviolet light.[18 ]
[19 ] This discovery opened a new chapter in photocatalysis research, followed by significant
advances in photoinduced catalytic synthesis of organic compounds over the past few
decades.
[Table 1 ] summarizes the benefits and drawbacks of the main oxidation modes. Additionally,
coupling with flow chemistry is a popular subject, and its use in oxidation reactions
can effectively address the risk issue and provide a safer guarantee for oxidation
reactions.
Table 1
Comparison of main oxidation modes
Oxidation mode
Advantage
Drawback
Oxygen oxidation
Green oxidizer; mature technology
Normally efficient catalyst
Hydrogen peroxide oxidation
Green oxidizer
High production cost
Ozonation
Strong oxidation capacity; fast reaction rate
Damage to reactor; highly expensive treatment costs
Electrooxidation
Less pollution; strong selectivity
High energy consumption
Photooxidation
Great selectivity; mildness; environment-friendly
Low catalytic efficiency
Aromatic Branched-chain Oxidation to Alcohols
In 2022, Long and collogues reported the synthesis of phenols by electrochemical C–H
hydroxylation of arenes in a continuous flow ([Fig. 2 ]).[20 ] The method has a broad scope (compatible with arenes spanning diverse electronic
properties) and is performed under mild conditions without any catalysts or chemical
oxidants. Among them, a graphite anode (exposed electrode area = 10 cm2 ) and a Pt cathode plate are separated by 0.15 mm, at a constant current of 61 mA,
pumping 1-ethyl-4-methoxybenzene at 0.4 mL/min. 1-Methoxy-4-(trifluoromethyl)benzene
was chosen as a model substrate for reaction optimization. The desired phenol was
obtained in 83% yield in less than 22 seconds. They revealed that the substrates with
relatively rich electronic properties afforded good yields. Furthermore, continuous
production of phenol at 1 mol quantity was achieved, further suggesting the synthetic
utility of the procedure.
Fig. 2 Continuous-flow oxidation of 1-methoxy-4-(trifluoromethyl)benzene.
The next year, the team performed highly site-selective monooxygenation of benzylic
C(sp3 )-H bonds using continuous-flow reactors ([Fig. 3 ]). The method, which does not require catalysts or chemical oxidants, separated graphite
anode sheet (exposed electrode area = 10 cm2 ) from Pt cathode sheet by 0.15 mm, at a constant current of 6.1 mA, pumping 1-ethyl-4-methoxybenzene
at 0.4 mL/min. As a result, 1-(4-methoxyphenyl)ethanol was obtained at a residence
time of 22 seconds, with a yield of 86%.[21 ]
Fig. 3 Continuous-flow oxidation of 1-ethyl-4-methoxybenzene.
To prepare the corresponding alcohol compounds, Zhu et al created a photocatalyst
based on g-C3 N4 doped Co ions that can activate molecular oxygen and oxidize the benzylic C(sp3 )-H bond of alkyl aromatic hydrocarbons at ambient temperature.[22 ] The catalyst favors high selective conversion under both UV and visible light conditions
and is suitable for a wide range of light sources ([Fig. 4 ]). The product can be prepared in the batch reaction system without the need for
an extra solvent and free radical initiator. No oxygen atmosphere was needed in continuous-flow
and fixed-bed devices. Using cumene (CM) as a substrate, the selectivity of 2-phenyl-2-propanol
(BP) was improved from 89.3 to 96.8% and 97.7%, respectively, and the reaction time
was reduced from 9 hours to 100 and 165 minutes. This study also illustrates the benefits
of highly active carbon nitride–based photocatalysts for environment-friendly and
sustainable chemistry.
Fig. 4 Continuous-flow oxidation of aromatic hydrocarbons to alcohols. BPR, back pressure
regulator.
Patrick's group reported the creation of a novel mesofluidic flow oxygenation method
for benzylic C–H oxidation ([Fig. 5 ]).[23 ] Activation of the photocatalyst using a UV lamp generates a radical on the substrate's
benzylic position. Singlet oxygen (1 O2 ) was incorporated into the radical intermediate to create ketone or alcohol. This
technique works well and has a variety of substrates. Notably, when oxidation occurs
selectively on benzylic sites without over-oxidizing the heterocyclic atoms, the necessary
carbonyl or hydroxyl derivatives are produced. A flow process makes possible a more
productive and sustainable protocol. By reducing the time and increasing substrate
concentration attained in the flow, it is possible to achieve scalability to reach
chemical levels that could be challenging to produce in the batch. It has also been
successfully applied to the synthesis and late-stage modification of bioactive compounds,
as demonstrated by the production of a single hydroxyl regioisomer (a nonsteroidal
anti-inflammatory analgesic medication) from ibuprofen in a good yield (55%) of oxidation.
Fig. 5 Continuous-flow oxidation of ibuprofen. BPR, back pressure regulator; MFC, mass flow
controller.
The tube-in-tube reactor was also used for biocatalytic production of 3-phenylcatechol
from 2-hydroxybiphenyl, which was catalyzed by 2-hydroxybiphenyl 3-monooxygenase ([Fig. 6 ]). Formate dehydrogenase was added for cofactor recycling to convert sodium formate
to carbon dioxide.[24 ] However, high substrate loadings can be achieved by using an organic liquid feed
containing substrate and an aqueous stream consisting of enzymes, cofactor, and sodium
formate. Under optimized conditions, a productivity of approximately 18 g • L−1 h−1 of the desired catechol was achieved, which is 38 times higher than that of the conventional
batch reactions.
Fig. 6 Continuous-flow oxidation of 2-hydroxybiphenyl. BPR, back pressure regulator; MFC,
mass flow controller.
Oxidation of Aromatic Branched Chains to Aldehydes or Ketones
Recently, Zhang and coworkers systematically studied continuous aerobic oxidation
of ethylbenzene to acetophenone over homogeneous and heterogenous NHPI in a micro-packed
bed reactor.[25 ] The microreactor platform provided an enhanced gas–liquid mass transfer, enabling
multiphase oxidation under kinetic control ([Fig. 7 ]). Compared with the conventional batch reactor, the space-time yield (STY) of oxidation
is increased by 2 orders of magnitude. When the liquid flow rate is 0.1 mL/min, the
gas flow rate is 10 mL/min, the selectivity is 90.1%, the conversion rate is 93%,
and the STY value reaches 2.33 × 105 mol /(L • h • kg).
Fig. 7 Cobalt salt catalyzed continuous-flow oxidation of ethylbenzene. BPR, back pressure
regulator; MFC, mass flow controller.
Continuous-flow reactors can be used in a wide variety of hazardous processes. Chemists
have begun to design flow systems that even allow new biocatalytic reactions to occur.
Chapman et al decomposed H2 O2 in the presence of galactose oxidase to produce a quantitative amount of oxygen for
the oxidation of benzyl alcohol with a yield of 92% at a residence time of 8 minutes
([Fig. 8 ]).[26 ] The advantage of this scheme is that, by using the same flow-through system, incompatible
enzymes can work together, which provides a new research line for catalytic oxidation.
Gutmann et al investigated the oxidation of ethylbenzene with hydrogen peroxide and
molecular oxygen catalyzed by cobalt and bromide ions in acetic acid ([Fig. 9 ]).[27 ] When hydrogen peroxide was used for oxidation, a mixture of products, including
ethylbenzene hydroperoxide, acetophenone, 1-phenylethanol, and 1-phenylethyl acetate,
was produced. Ethylbenzene and reaction intermediates were not fully converted to
acetophenone. However, when atmospheric oxygen was used for oxidation, no catalyst
deactivation was observed. Ethylbenzene was oxidized to acetophenone at 80°C with
74% selectivity at 150 minutes. Reaction conditions are converted with a tubular gas–liquid
reactor to a continuous-flow process, which typically provides superior mass transfer
characteristics and prevents oxygen depletion during the initial stages of rapid oxidation.
A continuous-flow scheme allows the reaction temperature to be increased to 110 to
120°C, thereby reducing the reaction time to only 6 to 7 minutes without affecting
the reaction selectivity.
Fig. 8 Continuous-flow oxidation of benzyl alcohol.
Fig. 9 Oxidation of ethylbenzene to acetophenone in a continuous stream. BPR, back pressure
regulator; MFC, mass flow controller.
A novel method for the catalytic oxidation of o -chlorotoluene to o -chlorobenzaldehyde has also been investigated. In 2020, Yang et al reported the liquid
phase selective oxidation of o -chlorotoluene (OCT) to o -chlorobenzaldehyde (OCBD) using oxygen as an oxidant and cobalt acetate/manganese
acetate/potassium bromide system as catalysis, and the reaction medium was acetic
acid doped with a small amount of water ([Fig. 10 ]).[28 ] The reaction parameters can be easily controlled by performing the reaction in a
microchannel reactor. Under the conditions of 0.88 mol% catalyzing substrate concentration
of 2.39 mol/L, Co/Mn/Br molar ratio of 0.3/0.3/1, oxygen/substrate molar ratio of
5.5, reaction pressure of 0.5 MPa, and reaction temperature 150°C, OCT conversion
was maintained at 10.3% and the selectivity to OCBD was as high as 71.8%.
Fig. 10 Continuous-flow microchannel reactor oxidation of o -chlorotoluene. BPR, back pressure regulator; MFC, mass flow controller.
Yun et al performed selective aerobic oxidation of benzylic sp3 C-H bonds to generate the corresponding ketones under continuous-flow conditions.[29 ] The reaction was driven by N -hydroxyphthalimide (NHPI) and tert -butyl nitrite (TBN) as catalysts under aerobic conditions ([Fig. 11 ]). The residence time of 54 seconds was 466-fold higher than the batch parallel reaction
(7 hours), giving benzophenone in 87.9% yield.
Fig. 11 Continuous-flow oxidation of diphenylmethane. BPR, back pressure regulator; MFC,
mass flow controller.
Notably, recovery of catalysts and solvents (92.6 and 94.5%) and scale-up experiments
(0.87 g/h run for 28 hours) of the oxidation of 1,2,3,4-tetrahydronaphthalene to α-tetralone
proved the versatility and applicability of the scheme for large-scale production
with a certain degree of cost control.
Quite recently, Bannon et al proposed a continuous-flow method for the aerobic photo-oxidation
of benzyl substrates to ketones and aldehydes.[30 ] The process exploits UV-A LEDs (375 nm) in combination with a Corning AFR reactor
that ensures effective gas–liquid mixing ([Fig. 12 ]). With an airflow rate of 24.8 mL/min, solvent flow rate of 1 mL/min, temperature
of 50°C, and pressure of 14.4 bar, the conversion rate of benzophenone was 98% in
1 minute. Under the optimized conditions, the substrates were screened, obtaining
a wide range of ketones. Overall, this continuous-flow approach offers several improvements
over alternative oxidation methods due to the combined use of air as an oxidant and
sodium anthraquinone-2 sulfonate (SAS) as a water-soluble photocatalyst. The use of
greener and safer conditions as well as process-strengthening principles make this
flow method attractive for further industrial applications.
Fig. 12 Continuous-flow photo-oxidations of benzylic. MFC, mass flow controller.
Gary et al used a specially designed continuous-flow photochemical reactor to selectively
photooxidize alkyl benzenes.[31 ] The reactor was fitted with a low-power UV light source and a fine bubble generator
as an oxidizing agent in conjunction with sodium anthraquinone sulfonate, a water-soluble
catalyst ([Fig. 13 ]). The creation of small bubbles accelerates the reaction and significantly increases
the two phases' interaction efficiency. The efficiency of the air-based slug-flow
system with fine bubbles was 1.4 times at a lower feed-flow rate of 2 mL/min, and
1.8 times at a higher feed-flow rate of 5 mL/min. Ethylbenzene was selectively oxidized
to acetophenone in continuous flow at room temperature during a brief residence period
of 5 minutes, with a conversion of ethylbenzene of 90% and 92% selectivity to acetophenone.
Compressed air can be used as an oxidant instead of pure O2 because of improved mass transfer and increased efficiency, thus alleviating the
potential safety concerns and making the process amenable for scale-up.
Fig. 13 Continuous-flow oxidation of ethylbenzene. BPR, back pressure regulator; FB, fine
bubble; MFC, mass flow controller; PEEK, a three-way mixer of polyether ether ketone
polymer material.
Greene et al developed a continuous-flow process to establish a homogeneous Cu(I)/TEMPO
catalyst system for the aerobic oxidation of primary alcohols to aldehydes ([Fig. 14 ]).[32 ] When 4-nitrobenzylic alcohol was used as a substrate, 4-nitrobenzaldehyde was achieved
in 99% yield in the presentence of 5 mol% of Cu(OTf)/bpy, 10 mol% of N -methylimidazole (NMI), and 5 mol% of TEMPO in a 5-minute residence period. Additionally,
a 100-g scale scaling experiment was conducted using benzyl alcohol as the model,
resulting in 99% quantitative formation of benzaldehyde after 5 minutes of residence
time, which was successfully allowed to remain for 24 hours. The findings offer a
crucial basis for the application of flow-based, large-scale aerobic oxidation processes
in pharmaceutical process chemistry.
Fig. 14 Continuous-flow oxidation of phenyl ethanol. BPR, back pressure regulator; MFC, mass
flow controller.
Naik et al reports a straightforward and safe continuous-flow oxidation procedure
that selectively converts primary and secondary alcohols into the corresponding ketone
and aldehyde compounds using catalytic quantities of TEMPO in a biphasic solvent system
in the presence of sodium hypochlorite and sodium bromide ([Fig. 15 ]).[33 ] The entire experimental apparatus was supplied by two peristaltic pumps, and the
reaction was performed through a coil with a static mixer at 0.32 equiv. NaOCl, 20°C,
and 7.5 minutes of retention time. For online quenching of the reaction 10% Na2 S2 O3 was used. In this case, a trifluoromethylated oxazole building block and a precursor
to the anti-HIV drug maraviroc were scaled up, and the desired phenylpropyl aldehyde
was synthesized in 93% isolated yield.
Fig. 15 Continuous-flow oxidation of 3-phenyl-1-propanol. BPR, back pressure regulator.
Using supported photocatalysts on silica , Blanchard et al combined a continuous-flow reactor with a high-performance LED array
to achieve a uniform distribution and the most efficient use of light energy ([Fig. 16 ]).[34 ] Compared with the reactor reaction mode, the reaction does not require expensive
high-pressure equipment and reduces light energy loss and carbon emission for the
photooxidation reaction of 1,5-dihydroxynaphthalene. The continuous-flow mode improves
the reaction efficiency by 24 times with an STY value of 5.35 g/(L • h).
Fig. 16 Continuous-flow photo-oxidation of 1,5-dihydroxynaphthalene to juglone. MFC, mass
flow controller.
Aromatic Branched Chain Oxidation to Acid
A metal-free-catalyzed oxidation process of 4-(methylsulfonyl)-2-nitrotoluene for
the production of 4-(methylsulfonyl)-2-nitrobenzoic acid has been developed by Su's
group in 2022 ([Fig. 17 ]).[35 ] The process uses molecular oxygen as an oxidant, nitric acid as a promoter, and
N ,N ′,N ″-trihydroxyisocyanuric acid (THICA) as a catalyst. The residence time, reaction temperature,
and oxygen/substrate molar flow ratio were optimized to give a reaction conversion
of 90%, a selectivity of 97%, and a residence time of 50 minutes. The use of continuous-flow
technology gives a decent yield and selectivity and is safer and more environment
friendly than the batch oxidation method.
Fig. 17 Continuous-flow oxidation of 4-(methylsulfonyl)-2-nitrotoluene. BPR, back pressure
regulator; MFC, mass flow controller.
In the following year, the team established a continuous-flow process from OCT to
o -chlorobenzoic acid (OCBA), using pure oxygen as the oxidant, acetic acid as a cosolvent,
and CoBr2 /MnBr2 as a catalyst; by creating a slug flow, the reaction was rapidly stimulated ([Fig. 18 ]).[36 ] The reaction parameters can be easily controlled using the continuous-flow reactor,
and 90% conversion of OCBA was achieved at a residence time of 15 minutes. The isolated
yield of OCBA was up to 94%. Shorter residence times, higher product yields, and operational
safety can be achieved using a simple continuous-flow system, as compared with conventional
batch processes.
Fig. 18 Continuous-flow oxidation of o -chlorotoluene. BPR, back pressure regulator; MFC, mass flow controller.
Prieschl et al presented a continuous-flow reactor for the oxidation of aldehydes
to carboxylic acids using an in situ –generated performic acid ([Fig. 19 ]).[37 ] This low-molecular-weight, high-performance acid is an environment-friendly and
inexpensive oxidizer that can be easily produced in situ from formic acid and hydrogen peroxide. This eliminates the safety hazards associated
with handling this potentially explosive reagent, and the product is easily separated
after the reaction by simple decompression distillation. The reactor has an effective
volume of 12 mL, a backup pressure of 5 bar, a residence time of 20 minutes, and a
temperature of 100°C. Both the yield and conversion rate are up to 99%.
Fig. 19 Continuous-flow oxidation of phenylacetone. BPR, back pressure regulator.
Guo et al reported a continuous flow process for the preparation of 2,4-dichloro-5-fluorobenzoic
acid (BA).[38 ] They chose 2,4-dichloro-5-fluoroacetophenone (AP) as the starting material and acetic
acid as cosolvent, and achieved excellent results in the continuous-flow oxidization
system ([Fig. 20 ]). BA nitric acid oxidation is a violent exothermic reaction. However, a continuous-flow
system with advantages such as good mass and heat transfer ensures the safety of the
reaction. With the optimal reaction conditions, 100% yield was obtained at 70°C for
9 minutes. Compared with conventional batch reactions, a continuous-flow system achieves
lower nitric acid consumption, higher product yield, shorter reaction time, environment-friendliness,
and process continuity, thus ensuring higher operational safety.
Fig. 20 Continuous-flow oxidation of 2,4-dichloro-5-fluorophenone. BPR, back pressure regulator;
MFC, mass flow controller.
Vanoye et al reported a safe, straightforward, and atom-economic approach for the
oxidation of aliphatic aldehydes to the corresponding carboxylic acids within a continuous-flow
reactor, using pure oxygen as the reaction's oxygen source.[39 ] The reaction is typically performed at room temperature using 5 bar oxygen in PFA
tubing without additional catalysts or radical initiators. A steady Taylor flow is
produced by varying the rate ratio of the two materials. The reaction was examined
in real time using online GC analysis ([Fig. 21 ]). The residence time was 17.4 minutes with 95% conversion and 98% selectivity. A
catalytic quantity of an Mn(II) catalyst is occasionally added. The benefits given
by continuous-flow technology make the procedure an effective substitute for conventionally
catalyzed aerobic processes.
Fig. 21 Continuous-flow oxidation of benzaldehyde. BPR, back pressure regulator; MFC, mass
flow controller.
Photosensitizer 2-tert -butylanthraquinone (2-t -Bu-AQN) was used to oxidize aromatics in a glass continuous-flow microreactor built
by Itoh's group ([Fig. 22 ]).[40 ] By evenly applying a particular wavelength of light on the market's surface—a glass
chip microreactor—a particular chemical can be oxidized. There is something special
about the glass continuous-flow microreactor. Internal circulation within each slug
accelerates the mixing of the liquid and gas phases, increasing the response pace
in slug flow. Benzoic acid was synthesized with a yield of 48% in 2 hours and p -tert -butylbenzoic acid with a yield of 83% yield in 2 hours under the conditions of 5
μL/min, 0.1 MPa of oxygen flow, and a 375-nm LED. The photosensitizer used can be
synthesized and recycled, resulting in a certain cost savings.
Fig. 22 Aromatics were oxidized in glass continuous-flow microreactor. MFC, mass flow controller.
The Aromatic Branch Chain Oxidizes to Other Compounds
Kabeshov et al described an atom-economy continuous-flow electrooxidation approach
to benzamide without further reagents.[41 ] The substrate and electrolyte were added to the solvent, and they were moved at
a rate of 0.5 mL/min to the coil's electrolytic cell. The reaction was monitored by
in-line UV. After adding methanesulfonic acid, N -(4-(tert -butyl)benzyl)acetamide was obtained from p -tert -butyltoluene with a yield of 72% ([Fig. 23 ]). Investigations on the stability of the process revealed that the process yielded
9.12 g of amide per day.
Fig. 23 Continuous-flow electrooxidation of aromatics.
According to an overview of the role of flow technology in preparing indole derivatives
provided by Luisi's group, the derivatization of indole rings in the field of photosynthesis
is achieved by connecting two reaction units in series.[42 ] There is a 34-W LED light in the center of the reaction unit ([Fig. 24 ]), which is encircled by fluorinated ethylene propylene (FEP). When exposed to light
with methylene blue as a photosensitizer and oxygen as an oxidant, indoles were obtained
in 60 to 99%. This process transforms various indole derivatives in 1 to 3 hours as
opposed to the 24 hours required by the batch technique.
Fig. 24 Continuous-flow photooxidation of tricyclo-1,4-benzoxazines.
