Continuous-Flow Process for the Synthesis of m-Nitrothioanisole

Article abstract of DOI:10.1021/acs.oprd.6b00023

A continuous-flow process for the preparation of m-nitrothioanisole has been set up. The starting material m-nitroaniline was diazotized to give diazonium chloride, followed by azo-coupling with sodium thiomethoxide to give 1-(methylthio)-2-(3-nitrophenyl)diazene, then dediazoniated to gain m-nitrothioanisole in high yield. The continuous-flow process minimized accumulation of the energetic intermediate diazonium salt and has a better capacity for adapting large-scale production. A solvent was introduced in the azo-coupling section to create a biphasic flow system. Side products were inhibited eminently in this flow process.

Full text of DOI:10.1021/acs.oprd.6b00023

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Full Paper
Continuous-Flow Process for the Synthesis of m-Nitrothioanisole
Zhiqun Yu, Xiaoxuan Xie, Hei Dong, Jiming Liu, and Weike Su
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00023 • Publication Date (Web): 24 Mar 2016
Downloaded from http://pubs.acs.org on March 28, 2016
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ContinuousꢀFlow Process for the Synthesis of
mꢀNitrothioanisole
Zhiqun Yu,† Xiaoxuan Xie,† Hei Dong,† Jiming Liu,‡ and Weike Su*,†,‡
†National Engineering Research Center for Process Development of Active Pharmaceutical Ingredients, Collaborative
Innovation Center of Yangtze River Delta Region Green Pharmaceuticals, Zhejiang University of Technology,
Hangzhou 310014, P.R. China
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‡Key Laboratory for Green Pharmaceutical Technologies and Related Equipment of Ministry of Education, College of
Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou 310014, P.R. China
* Correspondent. Tel: (+86)57188320899. Eꢀmail: pharmlab@zjut.edu.cn.
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Abstract: A continuousꢀflow process for the preparation of mꢀnitrothioanisole has been set up. The
starting material mꢀnitroaniline was diazotized to give diazonium chloride, followed by azoꢀcoupling
with sodium thiomethoxide to give 1ꢀ(methylthio)ꢀ2ꢀ(3ꢀnitrophenyl)diazene, then dediazoniated to gain
mꢀnitrothioanisole in high yield. The continuousꢀflow process minimized accumulation of the energetic
intermediate diazonium salt and has a better capacity for adapting largeꢀscale production. A solvent was
introduced in azoꢀcoupling section to create a biphasic flow system. Side products were inhibited
eminently in this flow process.
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Key Words: Diazotization, continuousꢀflow, azoꢀcoupling, dediazoniation, mꢀnitrothioanisole.
Introduction
An aromatic sulfide has been shown to be a universal pharmacophore in many pharmaceutically
active agents1. The target molecule mꢀnitrothioanisole is an important start material for the synthesis of
tricyclic antidepressant (Thioridazine) and also has been used for the synthesis of SIRT2 inhibitor2 and
antimalarial agents3.
Among several synthetic methods that have been reported, two of them showed potential for applying
to mass production (Scheme 1). Route I1 started with nitrobenzene, treated with chlorosulfonic acid to
give 3ꢀnitrobenzeneꢀ1ꢀsulfonyl chloride, after recrystallization, 3ꢀnitrobenzeneꢀ1ꢀsulfonyl chloride was
added in portions to a mixture of potassium iodide and phosphorous acid in water to gain
1,2ꢀbis(3ꢀnitrophenyl)disulfane. Then the reaction mixture was filtered and the filtrate was reacted with
sodium sulfide and dimethyl sulfate in sequence to give mꢀnitrothioanisole (total yield of 61%). Another
significant method involved diazotization of mꢀnitroaniline followed by azoꢀcoupling and
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dediazoniation of diazonium salt (route II). Prakash et al.4 and Majek et al.5 reported their applications
of this route in lab scale with total yield of 69% and 60%, respectively. These methods still have some
disadvantages need to be solved, such as the first route suffers from long reaction steps and
environmental unfriendly reagents like dimethyl sulfate, phosphorous acid, chlorosulfonic acid and etc.
While the second route, the cost of chemical materials is relative higher and its yield is moderate.
Scheme 1. Synthetic routes of m-nitrothioanisole
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We chose the second approach to modify for its convenience in terms of operation and showed
promising potential for improving yield. Aqueous sodium nitrite was selected as diazotization agent,
along with sodium thiomethoxide as methanethiolate donator (Scheme 2). In this route, the energetic
intermediates 3 and 4 are very unstable, in a condition of relatively high temperature or high pH value,
they may decompose while releasing nitrogen, and that may cause some safety issues, especially in large
scale manufacture6. Due to the high reactivity of diazonium salt, insufficient mixing in large vessel may
lead to undesired coupling product or other byproducts (Scheme 2). Obviously, side reactions are mainly
consecutive reactions to diazotization, and applying flow reaction technology is an effective way to
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avoid consecutive side reactions. Thus, we decided to adapt this process to continuousꢀflow to address
the safety concerns and improve the yield by reducing byproducts.
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Scheme 2. Reaction mechanism of m-nitrothioanisole from m-nitroaniline
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Compared to batch technology, continuousꢀflow technology has some remarkable advantages7. The
precise control of reaction parameters (including reaction temperature, residence time and stoichiometry,
etc.) is easier; the selectivity is improved; high reproducibility; easier to scaleꢀup; and often better
reaction yields. Plus, the volume of reactor is smaller, so that the hazard is minimized and the heat
transfer efficiency is improved, so that the thermal unstable energetic intermediates are prevented to
decomposition8. in batch manner，the generated diazonium salt is usually added to another material
slowly，yet by using flow reaction technology, there is no need to wait to the whole batch is completed
which the energetic intermediate can be directly treated by following process continuously. Thus we
turned our interest to develop a continuousꢀflow process which is not only for diazotization, but also
include subsequent steps.
Results and Discussion
Adapting the Batch Process to Continuous-Flow.
For a great many industrial processes covered, the diazotization of aromatic amine has been well
studied9. In this century, continuous diazotization microreactors followed by iododeamination,
chlorodeamination, azo dyes, and chlorosulfonylation have been reported10. All these reaction processes
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are well performed, but microreactor has its limitation of scaleꢀup. Compared to micrometer scale
reactor, millimeter scale reactor, due to its relatively higher productivity, is easier to promote in
industrial production. Continuousꢀflow processes of diazotization using millimeter scale reactors has
been studied in our previously works11. We herein aimed to apply this technology to the synthesis of 1.
As shown in Scheme 3, the original thought was two streams (mꢀnitroaniline in aqueous
hydrochloride and aqueous sodium nitrite) pumped into a reaction tube for diazotization in Loop I, and
then introduced another stream of aqueous sodium thiomethoxide, after dediazotization in Loop II to
gain mꢀnitrothioanisole.
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Scheme 3. Original design of continuous flow process
Before we started adapting, trails in batch chemistry were done to conduct the method and their
yields approximately 60%. In these trials, the diazotization of 2 showed a promising potential to adapt to
continuousꢀflow, but the intermediate 1ꢀ(methylthio)ꢀ2ꢀ(3ꢀnitrophenyl)diazene (4) is insoluble in
reaction system, and very sticky. In the consideration of the reaction tube may be fouled and clogged,
we decided to adapt the diazotization process and following processes separately.
Primitive Continuous-flow Process
The primitive continuousꢀflow setꢀup was shown in Figure 1. Commercially available arylamine 2
was combined with hydrochloric acid as one feed stream (material A) and sodium nitrite in water as a
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separate stream (material B). These two streams were pumped into a loop reactor (Hastelloy, 3 mm i.d.,
5 mm o.d.) via a Tꢀjoint (Hastelloy, 3 mm i.d.) by two peristaltic pumps (P1, P2, Baoding Longer, China),
respectively, and the molar flow ratio between aniline hydrochloride and sodium nitrite was set
according to their equivalents in batch process. The reaction tube was submerged by water bath in a
thermostat. The reactants were directly flowed into a stirring collection vessel containing aqueous
sodium thiomethoxide and sodium hydroxide. In the collection vessel, reaction mixture was stirred for
0.5 h at 40 oC, and then heated the collection vessel to 70 oC for 2 h. Then the reddish brown oil layer
was separated by gravity and the samples were analyzed by GC.
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Figure 1. Primitive Continuousꢀflow Process. Material A is 1.29 M mꢀnitroaniline in 2.6 equiv. of
hydrochloric acid with a flow rate of 112.8 mL/min, material B is 4.24 M aqueous sodium nitrite.
Material C is a mixture of 1.2 equiv. of sodium thiomethoxide (2.05M) and 0.58equiv. of aqueous
sodium hydroxide which was added in collection vessel before the operation.
The operation of this diazotization process is relatively convenient, once the parameters was set
and corrected. Started the peristaltic pumps, let materials pumped into the loop reactor respectively, and
synthesized 3 was flowed into collection vessel for following process. The related parameters of
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diazotization in continuousꢀflow process are residence time τ1 and reaction temperature T1, therefore sets
of reaction conditions involved τ1 and T1 were tested to find an appropriate reaction condition. The
results were shown in Figure 2, the yield was increased to a constant value with prolonged residence
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time at low reaction temperature (less than 10 C), while it was increased to a peak value followed by a
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decrease with prolonged residence time at relative high reaction temperature (excess than 20 C). The
yield was not monotonic rise over time at the same temperature, and that’s because the reaction rate of
diazotization was overwhelmed by the reaction rate of decomposition after peak. The maximum yield
(68%) of 1 was achieved at about T1 = 30 oC, τ1 = 30 s.
Figure 2. Effect of temperature (T1) and residence time (τ1) on the yield of mꢀnitrothioanisole.
After reaction temperature (T1) and residence time (τ1) were settled, the reaction system was run
with different molar flow ratio to optimize the usage of sodium nitrite. The results were shown in Figure
3. It turned out the molar flow ratio between aniline hydrochloride and sodium nitrite were 1 : 1.1 as we
adopted in previously trials.
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Figure 3. Effect of molar flow ratio of sodium nitrite : mꢀnitroaniline on the yield of mꢀnitrothioanisole.
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Advanced Continuous-Flow Process
The primitive continuousꢀflow process of diazotization was successfully functioning, however, the
adapting of batch chemistry to continuousꢀflow only halfꢀway through. The yield was still lower than
our expectation and byproducts were analyzed and identified by GCꢀMS. As shown in Scheme 4,
reduction byproducts 6 (12%), 7 (~1%), and 8 (~1%), were found in crude product. According to the
result, byproducts are caused by reduction. Of all the chemicals in this process, sodium thiomethoxide is
the most likely reducing agent. Therefore, we tried to separate 4 from reaction mixture and heated it to
dediazoniation, we found this method is very dangerous since 4 is extremely explosive in dry condition.
So introducing a coꢀsolvent to extract 4 and created a biphasic system came out. This method could
prevent the intermediate 4 from being reduced, and also solve the transportation problem.
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Scheme 4. The reduction side-reactions
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Table 1. Introduction of different solvents
Entry
Solvent
1,2ꢀdichloroethane
toluene
Purity (%)a
Yield (%)b
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petroleum etherc
aPurity was determined by GC. bYield was corrected on purity. cBoiling point was 90ꢀ120 oC.
Reactions were run in the same continuousꢀflow setꢀup as shown in Figure 1, different inert
solvents, toluene (aromatic), 1,2ꢀdichloroethane (halogenated hydrocarbon) and petroleum ether (ether)
were tested to perform in this design. Adding 6 mL solvent to every 1 g of 2 within aqueous sodium
thiomethoxide and sodium hydroxide in collection vessel, and the results were showed in Table 1. As we
expected, the introduction of solvent showed an eminent improvement on yield, and the amount of main
reduction byproduct 6 was reduced to about 8%, byproducts 7 and 8 were less than 1%. Since the
influence of different species to result was negligible, we didn’t expend the candidates, and in the
consideration of recycling rate of the solvent, toluene was selected.
After the solvent toluene was selected, this process was run several times to determine the
appropriate ratio of the volume of solvent to the mass of mꢀnitroaniline. The different usages of solvent
would lead to different concentrations, therefore we conducted these trials with different reaction times
which depend on the outgassing of the each trial. As shown in Figure 4, the volumeꢀmass ratio had little
effect on the yield.
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Figure 4. Effect of volumeꢀmass ratio of toluene to mꢀnitroaniline.
Although the ideal result didn’t came out, yet the original problem of set up a continuousꢀflow
process to conduct diazotization and following process was solved. By introducing a solvent to extract 4
while it was being formed in reactor could prevent reaction tube clogged. However, due to the
dediazoniation is a timeꢀconsuming and outgassing process, the reaction tube needs to be extremely
long to give adequate residence time, and in that case, the pressure drop was beyond our acceptance
range. Therefore, dediazoniation was rearranged in collection vessel. As shown in Figure 5, the device
used two peristaltic pumps (P1, P2) to introduce the feed streams of combination of 2 and hydrochloric
acid (material A) and aqueous sodium nitrite (material B). A Tꢀjoint (Hastelloy, 3 mm i.d.) was used as a
mixer, which was connected with reaction tube (Hastelloy, 3 mm i.d., 5 mm o.d.). The reaction mixture
from diazotization tube then entered into a cross joint and mixed with two other streams of combination
of aqueous sodium thiomethoxide and sodium hydroxide (material C) and toluene (material D) which
were pumped by pumps (P3, P4), respectively. Then the mixture was entered into another tube reactor
(SS316L, 6 mm i.d., 8 mm o.d.) for azoꢀcoupling. The mixture was collected in a vessel, heated to
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dediazoniation at 70 C for 2 h, and then separated. The solvent was removed from organic phase to
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afford the crude product.
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Figure 5. Continuousꢀflow process of diazotization & azoꢀcoupling. Material A is 1.29 M mꢀnitroaniline
in 2.6 equiv. of aqueous hydrochloric acid with a flow rate of 112.8 mL/min. Material B is 4.24 M
aqueous sodium nitrite with a flow rate of 37.8 mL/min. Material C is 2.09 M sodium thiomethoxide in
aqueous sodium hydroxide with a flow rate of 71.7 mL/min. Material D is toluene with a flow rate of
80.3 mL/min. Diazotization coil is Hastelloy coil with 3 mm i.d. and 5 mm o.d.. Azoꢀcoupling coil is
SS316L coil with 6 mm i.d. and 8 mm o.d..
Then, tests were run to determine the temperature (T2) and residence time (τ2) of azoꢀcoupling
reaction. The flow rate of material C was 83.6 mL/min. The residence time was varied by adjusting tube
length, and the results were shown in Figure 6. Lower temperature resulted in slow conversion rate,
while higher temperature led to a decrease of yield and the decline rate increased with the rising
temperature. That’s because during the generation of 4 from diazonium chloride 3, there was a side
reaction of 3 decomposed while release nitrogen and became phenolic product. This side reaction rate
raised along with reaction T2 rose, thus, yield decreased at higher T2. Therefore, a maximum yield of
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84.5% was obtained when T2 of 30 oC and τ2 of 150 s was selected.
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Figure 6. Effect of T2 and τ2 on the yield of mꢀnitrothioanisole.
On top of that, with better control of reaction parameters in continuousꢀflow reactor, the usage of
sodium thiomethoxide may reduce (which is 1.2 equiv. in batch chemistry). Since the sodium
thiomethoxide could lead to reduction side products, and as the advantages of continuousꢀflow
technology that was mentioned previously. The molar flow ratio of sodium thiomethoxide : 2 was
optimized by adjusting the flow rate of material C. As shown in Figure 7, the molar equivalent of
sodium thiomethoxide was reduced to 1.03 with a slight increase of yield12. A maximum yield of 89%
was achieved, and the purity of crude product was 91%. Further purification can be fulfilled by
distillation.
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Figure 7. Effect of molar flow ratio of sodium thiomethoxide : 2 on the yield of mꢀnitrothioanisole.
To underline the advantages of the proposed flow synthesis, a comparison with the reaction
performed in a batch manner is summarized. Figure 8 shows the comparison of byproducts in different
operate manner. The yields of different processes are listed on Table 2 to make a comparison. It is very
obviously that advanced continuousꢀflow process performed better than others.
Figure 8. Content of byproducts of different operate manner.
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Table 2. Comparison of different operate manner.
operate manner
purity (%)a
yield (%)b
Batch
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Advanced Continuous Flow Process
aPurity was determined by GC. bYield was corrected on purity.
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Conclusion
We have described a continuousꢀflow process allowing the save handling of the energy
intermediates, in this case, diazonium salt. Using flow technology to solve problems caused by active
intermediates has become an effective measure. But in this process particularly, we faced another
difficulty. In standard diazotization model, side reaction is a consecutive reaction, which means we can
apply flow chemistry technology by setting appropriate residence time to keep reaction stays in main
reaction. Yet, azoꢀcoupling, on the other hand, due to the nucleophilicity and reductivity, side reaction is
a parallel reaction, and that is hard to solve by normal flow chemistry technic. Therefore, we introduced
a solvent to create a liquidꢀliquid biphasic system to reduce the exposure of sodium nitrite in aqueous
phase and products (including azoꢀcoupling product and final product). This method was proved
successful for its yield was raised to 89% and the byproducts were reduced eminently. We thought this
could be a new way to deal with similar situations in continuousꢀflow process.
EXPERIMENTAL SECTION
All chemicals were purchased from commercial sources and were used without further purification. Gas
chromatography (GC) analysis was carried out on an Agilent 6890 gas chromatograph. GC conditions:
HPꢀ5 column 30 m × 0.32 mm × 0.25 ꢁm, carrier gas: gas: helium (1.2 mL/min), injection temp.:
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270 °C, detector temp.: 290 °C, oven: 150 °C (5 min hold) → 260 °C (250 °C/min, 10 min hold). H
NMR spectra were recorded in CDCl3 with tetramethylsilane (TMS, δ = 0) as an internal standard at
ambient temperature on a Varian 400 MHz spectrometer.
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Batch Experiment. mꢀNitroaniline (13.8 g, 0.1 mol) was placed in an 100 mL glass jar then added
hydrochloric acid (31%, 38.4g, 0.26 mol) and 40.9 mL water with stirring. The mixture was heated to 80
oC for 30 min, and then cooled gradually while stirring. Aqueous sodium nitrite was prepared from
sodium nitrite (7.6 g, 0.11 mol) and 22.8 mL water at room temperature. After aniline hydrochloride was
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cooled to 0 C, the aqueous sodium nitrite was added. Diazotization was initiated by the slow addition
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of the aqueous sodium nitrite, and the temperature was kept below 0 C. After addition completed, the
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mixture continued to stir for 0.5 h while the temperature of reaction mixture was maintained at 0 C.
The mixture was then added to a 500 mL flask with wellꢀmixed sodium thiomethoxide (20%, 42.1 g,
0.12 mol), sodium hydroxide (2.32 g, 0.058 mol) and 20 mL water in it at 40 oC. After addition, the
mixture maintained this temperature for 0.5 h before heated it to 70 oC. After 2 h, separated the mixture,
the brownish solid was obtained. 15.6 g of crude product in 92.3% yield and 65.7% purity was obtained.
Sample for characterization was gained by column chromatography with petroleum ether and ethyl
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acetate as eluent. H NMR (400 MHz, CDCl3) δ/ppm: 8.01 (t, 1H, J = 2.0 Hz), 7.92 (m, 1H), 7.50 (m,
1H), 7.41 (t, 1H. J = 8.0 Hz), 2.55 (s, 3H). Literature data5: 8.00 (t, J = 2.0 Hz, 1H), 7.91 (ddd, J = 8.1
Hz, J = 2.0 Hz, J = 1.0 Hz, 1H), 7.48 (ddd, J = 7.9 Hz, J = 1.8 Hz, J = 1.0 Hz, 1H), 7.39 (t, J = 8.0 Hz,
1H), 2.51 (s, 3H).
Advanced Continuous-flow Process. As shown in Figure 5, a mixture of mꢀnitroaniline (138 g, 1
mol), hydrochloric acid (31%, 390g, 2.6 mol) and 409 mL water was prepared. Aqueous sodium nitrite
was prepared from sodium nitrite (75.9 g, 1.1 mol) and 163 mL water. The aniline hydrochloride and
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aqueous sodium nitrite were pump into the tube reactor (Hastelloy, 3 mm i.d., 5 mm o.d.), which was
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immersed in a thermostat (30 C), via a Tꢀjoint by P1 and P2 at flow rates of 112.8 mL/min and 37.8
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mL/min, respectively. After a residence time of 30 s, a mixture of sodium thiomethoxide (20%, 360.5 g,
1.03 mol) and sodium hydroxide (26.4 g, 0.66 mol) in 170 mL water and toluene were pumped into the
tubular react system via a cross mixer by P3 and P4 at flow rates of 71.7 mL/min and 80.3 mL/min and
converged with diazonium salt. Then, the mixture flowed into the second tube reactor which was
immersed in a thermostat (30 oC), after a residence time of 150 s, the mixture flowed through the outlet
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and accumulated in the stirring collection vessel which had been maintain at 80 C. The mixture was
stirred in the vessel for 100 min, and then the mixture was separated. Collected the organic phase and
distilled to removed solvent, the yellowish solid was obtained. About 165 g of crude product in 97.6%
yield and 91.4% purity was obtained. Further purification can be fulfilled by distillation on commercial
demand.
ACKNOWLEDGMENT
We are grateful to the National Natural Science Foundation of China (No. 21406203) for financial
support.
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(12) The optimized condition was run at the primitive process, and the yield was not improved.
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