Influence of coordinating groups of organotin compounds on the Fries rearrangement of diphenyl carbonate

Article abstract of DOI:10.1039/c9ra05390e

In this paper, the Fries rearrangement of diphenyl carbonate (DPC) catalyzed by organotin compounds with different coordination groups was studied for the first time. The electronic effect and steric hindrance of the coordinating groups were discussed with respect to the reactivity of DPC rearrangement. The results showed that both the electronic effect and steric hindrance of the coordinating groups influenced the acidity of the active tin centers and then affected the catalytic performance of organotin as a Lewis acid for the rearrangement of DPC, and the influence of the electronic effect is greater than that of steric hindrance. The catalytic activity is in the order of BuSnO(OH) > Bu₂SnO > Bu₂Sn(OCOC₁₁H₂₃)₂ > BuSnCl₃ > Bu₃SnOSnBu₃ > Bu₃SnCl, and Bu₂SnO showed the best catalytic activity due to its strong electron absorption effect, small steric hindrance, and good stability. Under the optimum reaction conditions, the conversion of DPC was up to 93%, and the yields of phenyl salicylate (PS) and xanthone (XA) were 62% and 28%, respectively. In addition, a reaction mechanism of DPC rearrangement catalyzed by the organotin compounds was speculated. This research can provide vigorous theoretical data support to control the byproducts produced by DPC rearrangement in the process of DPC synthesis. It also provides a new route for the preparation of PS and XA.

Full text of DOI:10.1039/c9ra05390e

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PAPER
Influence of coordinating groups of organotin
compounds on the Fries rearrangement of diphenyl
carbonate†
Cite this: RSC Adv., 2019, 9, 28112
abc
Xiaoxue Yuan,‡ Gang Zhang, Yi Zeng, Tong Chen *a
c
ab
a
Tao Liu,‡
a
and Gongying Wang
In this paper, the Fries rearrangement of diphenyl carbonate (DPC) catalyzed by organotin compounds with
different coordination groups was studied for the first time. The electronic effect and steric hindrance of the
coordinating groups were discussed with respect to the reactivity of DPC rearrangement. The results
showed that both the electronic effect and steric hindrance of the coordinating groups influenced the
acidity of the active tin centers and then affected the catalytic performance of organotin as a Lewis acid
for the rearrangement of DPC, and the influence of the electronic effect is greater than that of steric
2 2 23 2 3
hindrance. The catalytic activity is in the order of BuSnO(OH) > Bu SnO > Bu Sn(OCOC11H ) > BuSnCl
>
Bu SnOSnBu > Bu SnCl, and Bu SnO showed the best catalytic activity due to its strong electron
3
3
3
2
absorption effect, small steric hindrance, and good stability. Under the optimum reaction conditions, the
conversion of DPC was up to 93%, and the yields of phenyl salicylate (PS) and xanthone (XA) were 62%
and 28%, respectively. In addition, a reaction mechanism of DPC rearrangement catalyzed by the
organotin compounds was speculated. This research can provide vigorous theoretical data support to
control the byproducts produced by DPC rearrangement in the process of DPC synthesis. It also
provides a new route for the preparation of PS and XA.
Received 14th July 2019
Accepted 29th August 2019
DOI: 10.1039/c9ra05390e
rsc.li/rsc-advances
13
work. While DPC as the key raw material of non-phosgene PC,
trace impurities will also seriously affect the quality of PC.
Fries rearrangement is a reaction where, when a phenolic ester is
1
Introduction
14,15
With low toxicity and no pollution to the environment, diphenyl
carbonate (DPC) is an important “green” chemical interme- in a solvent such as nitrobenzene and nitromethane or when the
diate, which is commonly used to synthesize many important phenolic ester is directly heated with the absence of solvents, the
organic compounds and polymer materials in elds such as acyl group is rearranged under the catalysis of Lewis acids to
16,17
medicine, pesticides and plastic industries. Particularly, it is produce ortho- or para-acyl phenols.
Fries rearrangement has
used as the main raw material for the synthesis of poly- been extensively used in the synthesis of organic pharmaceuticals
1,2
18–22
carbonate (PC) by non-phosgene processes. Among the non- and gradually become a research hotspot in recent years.
In the
phosgene processes for DPC synthesis, transesterication of synthesis of DPC, organotin, organotitanium and other Lewis acids
dimethyl carbonate with phenol to synthesize DPC is the only are usually used as catalysts, and the pilot-scale amplication
non-phosgene synthetic route with industrial application reaction is carried out under high temperature and high pressure.
3,4
prospect at present. In recent years, researchers have con- As a phenolic ester, DPC was prone to Fries rearrangement in the
ducted in-depth studies on catalysts and process conditions of condition of Lewis acid catalysts under high temperatures and
5
–11
this synthetic approach.
However, rare studies on its pressures, which leads to the decrease of the yield and purity of
12
byproducts (except anisole ) were reported in our previous DPC. As shown in Scheme 1, Fries rearrangement in DPC may
produce byproducts such as phenyl salicylate (PS) and xanthone
(XA). In our research group, the catalysts and process conditions in
a
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu
6
10041, Sichuan, P. R. China. E-mail: chentongw@cioc.ac.cn
b
National Engineering Laboratory for VOCs Pollution Control Material & Technology,
University of Chinese Academy of Sciences, Beijing 101408, P. R. China
c
Sichuan Center for Disease Control and Prevention, Chengdu 610041, Sichuan, P. R.
China
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra05390e
These authors contributed equally to this work.
Scheme 1 Fries rearrangement of DPC to produce PS and XA.
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transesterication synthesis of DPC had been studied for a long analyzed by external standard method on a gas chromatograph
23–27
time.
Such byproducts as PS and XA have also been discovered equipped with a hydrogen ame ionization detector. Analytical
in the product of the pilot-scale reaction. Based on the molecular conditions of gas chromatography were referred to our previous
1
3
À1
structures of PS and XA, the transesterication catalysts and process work: the makeup nitrogen ow rate was 25 mL min . The air
À1
conditions, PS and XA should be the Fries rearrangement products ow rate was 400 mL min . The hydrogen ow rate was 30
of DPC. The boiling points of both PS and XA are high and mL min . The column ow rate was 2.5 mL min . The injection
approximate to that of DPC, so they cannot be easily isolated during volume was 0.4 mL. Samples were injected in a split mode with
À1
À1
ꢀ
purication of DPC and are very prone to remain in DPC to affect a split ratio of 50 : 1. The injection port temperature was 280 C. The
ꢀ
purity of the transesterication product DPC and thus seriously detector temperature was 300 C. Programmed temperature
ꢀ
affect the molecular weight and performance of PC synthesized in increase was performed: the initial temperature was 120 C, and the
28–34
ꢀ
À1
ꢀ
the next step.
Therefore, in this paper, organotin were used as temperature was increased at a rate of 20 C min to 280 C and
ꢀ
catalysts to study the effect of coordination groups on the reactivity held at 280 C for 5 min.
of Fries rearrangement of DPC for the rst time, and the possible
reaction mechanism was proposed in the process. The study can
provide an industrial reference for controlling the byproducts orig-
3
Results and discussion
inated from DPC rearrangement in the DPC synthesis. At the same 3.1 Catalytic performance of organotin compounds for the
time, PS and XA have antiseptic and bactericidal effects in the rearrangement of DPC
intestine. They are oen used as ultraviolet absorbents, plasticizers,
preservatives, and stabilizers for antibacterial disinfectants and
As shown in Scheme 1, DPC can generate two-step rearrangement in
sequence. Fries rearrangement of DPC occurs to produce PS rstly,
and then PS is rearranged to generate XA, that is, the PS is the
intermediate of DPC rearrangement. Fries rearrangement reaction
was usually catalyzed by Lewis acids. In this study, organotin
plastic products, as well as for the synthesis of bactericidal drugs
35–38
and essences.
So this study can also provide a method reference
for the preparation of PS and XA.
compounds with different coordinating groups, including Bu
Bu Sn(OCOC11 , Bu SnOSnBu , BuSnO(OH), Bu SnCl and
BuSnCl , were selected as catalysts for the DPC rearrangement. As
shown in Fig. S1 and S2,† H and C NMR spectra of all the
2
SnO,
2
H )
23 2
3
3
3
2
Experimental
3
2.1 Instruments and reagents
1
13
A GS-0.1 intermittent high-pressure reactor (Weihai Chemical products revealed that DPC rearrangement did generate PS and XA.
Machinery Co., Ltd); a 7820A gas chromatograph, equipped with Moreover, as shown in Fig. S3,† the results of the products analysis
a HP-5 column (30 m  0.32 mm  0.25 mm, 5% phenyl methyl and matching analysis of their fragment ion peaks by GC-MS also
siloxane) (Agilent Technologies, USA); an Ascend 400 NMR (Bruker); further conrmed that PS and XA produced by DPC rearrangement.
a Nicolet IS 10 FT-IR (Thermo Scientic); and a HP 6890/5973 GC/ Catalytic performance of organotin compounds for the DPC rear-
MS system (Agilent Technologies, USA), equipped with a HP-5 MS rangement reaction is presented in Table 1. The amounts of orga-
column (30 m  0.32 mm  0.25 mm) (Agilent Technologies, USA) notin compounds are calculated on the basis of tin atoms, and the
were used in this study. PS (GC) was purchased from Alfa Aesar; XA mass ratio of Sn to DPC is 1 : 100. As shown in Table 1, with the
(
GC) was purchased from J&K Scientic Ltd.; DPC (GC) was absence of catalysts, DPC rearrangement was not observed, but
purchased from Nanjing Duly Biotech Co., Ltd.; phenol (GC) was a small quantity of phenol was identied. All organotin compounds
purchased from Aladdin; acetone (HPLC) was purchased from can catalyze the Fries rearrangement reaction of DPC, and the
Scientic; Bu
hydroxide [BuSnO(OH)] (AR) and tributyltin chloride (Bu
were obtained from Merck; bis(tributyl)tin oxide (Bu SnOSnBu ) quantity of phenol was also produced. The origin of phenol was
2
SnO (AR) was obtained from Acros; butyl tin conversion of DPC was all over 40%. The main products were PS
SnCl) (AR) and XA, and the yield of PS is higher than XA. In addition, a small
3
3
3
(
AR), dibutyltin dilaurate [Bu Sn(OCOC H ) ] (AR) and butyl tin analyzed by online thermal desorption-gas chromatography. No
2 11 23 2
trichloride (BuSnCl ) (AR) were obtained from Alfa Aesar. Prior to
3
ꢀ
use, all solid reagents were dried in vacuum at 70 C in a vacuum
ꢀ
Table 1 Catalytic performance of organotin compounds for the Fries
rearrangement of DPC
drier, and all liquid reagents were dried using a 4 A molecular sieve.
a
2.2 Fries rearrangement of DPC
Yield (%)
Fries rearrangement of DPC was carried out in a 100 mL high-
pressure reactor provided with a thermometer, a nitrogen inlet,
and a stirrer. DPC and organotin catalyst were added (the mass ratio
of Sn and DPC is 0.05–15%, and the optimum ratio is 10%) into the
reactor, and then air in the assembly was replaced with high-purity
nitrogen at room temperature. Following pressurized to the
Catalysts
DPC conversion (%)
PS
XA
Phenol
—
7
92
88
82
73
57
41
—
53
51
49
45
35
26
—
31
30
25
24
17
12
6
7
7
7
4
6
3
BuSnO(OH)
Bu
Bu
2
SnO
Sn(OCOC11H )
23 2
2
BuSnCl
3
required pressure (0.10–0.70 MPa), and heated to the desired Bu SnOSnBu3
3
ꢀ
3
Bu SnCl
temperature (140–280 C) under stirring to start timing. Aer the
reaction (2–14 h), the product was dissolved and diluted in acetone,
a
Reaction conditions: DPC: 15 g; mass ratio of Sn to DPC: 1%; reaction
ꢀ
qualitatively analyzed on a GC/MS system and quantitatively time: 8 h; temperature: 220 C; pressure: 0.50 MPa.
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3
phenol was detected from DPC, indicating phenol was not a product the steric hindrance of Bu SnCl is slightly greater than that of
of thermal degradation or thermal rearrangement of DPC but Bu SnOSnBu , the electronegativity of oxygen atoms coordinating
3
3
possibly a product of DPC hydrogenolysis, and the amount of with tin atoms in Bu SnOSnBu is stronger than that of chlorine
3
3
phenol was independent of the catalyst. The catalytic activity of atoms coordinating with tin atoms in Bu SnCl, and the number of
3
organotin is in the order of BuSnO(OH) > Bu
Sn(OCOC11 > BuSnCl > Bu SnOSnBu > Bu SnCl.
When organotin compounds are used as Lewis acid to catalyze active centers in Bu
the Fries rearrangement of DPC, tin atoms play the role of active that it is more catalytically active than Bu
2
SnO > Bu
2
-
tin atoms at the catalytic active centers in Bu
than that in Bu SnCl. Therefore, the Lewis activity of tin atoms as
SnOSnBu is stronger than that in Bu SnCl so
SnCl. This also veries
3 3
SnOSnBu is larger
H )
23 2
3
3
3
3
3
3
3
3
3
centers. The greater the steric hindrance of the groups coordinating that the electronic effect of coordinating groups is stronger than
with tin atoms is, the more coordinating chelation of tin atoms with steric hindrance with respect to the impact on the catalytic activity
the carbonyl oxygen atoms in DPC is disfavored, and thus the more of organotin compounds.
unfavorable the rearrangement reaction is. Additionally, the elec-
It thus can be seen from the above results that the catalytic
tronic effect of coordinating groups will affect the acidity of tin activity of organotin compounds for DPC rearrangement is affected
atoms, thereby exerting inuence on the catalytic performance. As by the steric hindrance and electronic effect of groups coordinating
an active center, each tin atom in BuSnO(OH) or Bu SnO has three with tin atoms. The smaller the steric hindrance of coordinating
2
coordinating groups, among which two groups are identical (i.e. groups is, the more favorable for the tin atom to contact with the
butyl group and oxygen) and the third groups are hydroxyl group carbonyl oxygen on DPC is. The greater the electronegativity of
and butyl group respectively in the two compounds. Since the steric coordinating atoms of tin atoms is, the stronger the electron
hindrance of hydroxyl groups is slightly weaker than that of butyl absorption effect is, the stronger the activity of tin atoms is, and the
groups while the electron-withdrawing ability of hydroxyl groups is greater the catalytic activity for DPC rearrangement is. The impact of
stronger than that of butyl groups, the acidity of tin atoms in BuS- the electronic effect of coordinating groups is greater than that of
nO(OH) is stronger than that in Bu
more catalytically active than Bu SnO. Coordinating atoms con- ported on organotin compounds. This is also the reason why the
necting tin atoms in Bu Sn(OCOC11 and Bu SnO are identical, steric hindrance of Bu SnCl is small but its catalytic activity is the
2
SnO; therefore, BuSnO(OH) is steric hindrance. The results are in accordance with previously re-
39
2
2
H )
23 2
2
3
but the steric hindrance of groups coordinating with tin atoms is lowest among the series of organotin compounds. Both BuSnO(OH)
greater in Bu Sn(OCOC H ) than in Bu SnO, so Bu Sn(OCOC - and Bu SnO have demonstrated good catalytic performance in the
2
11 23 2
2
2
11
2
H ) is not as active as Bu SnO. The steric hindrance of groups rearrangement of DPC. The DPC conversion could reach over 85%,
23 2
2
coordinating with tin atoms in Bu Sn(OCOC H ) is far greater and the total yield of PS and XA could reach over 80%. However,
2
11 23 2
4
0,41
than in BuSnCl
effect, groups coordinating with tin atoms in Bu
are oxygen and carbon atoms, and each tin atom in Bu
Sn(OCOC11 is coordinated by two oxygen atoms and two
carbon atoms, whereas each tin atom in BuSnCl is coordinated by
three chlorine atoms and one butyl group; as the electronegativity of
3
. However, from the point of view of electronic BuSnO(OH) is unstable and easily degradable.
Therefore,
2
Sn(OCOC11 Bu SnO is a better catalyst for the Fries rearrangement of DPC.
H )
23 2
2
2
-
23 2
H )
3.2 Effect of the amount of Bu
2
SnO on the rearrangement of
3
DPC
oxygen atoms is stronger than that of chlorine atoms, the total The effect of the amount of Bu SnO on the rearrangement of
2
electron-withdrawing effect of Bu Sn(OCOC H ) is stronger than DPC was investigated. The amount of the catalyst was based on
2
11 23 2
that of BuSnCl
3
. Therefore, the Lewis activity of tin atoms in Bu
is stronger than that in BuSnCl , and Bu
is more catalytically active than BuSnCl
2
-
2
-
the mass ratio of Bu SnO to DPC. As shown in Fig. 1, the DPC
conversion increased gradually with the increase of catalyst
2
23
Sn(OCOC11H )
H
2
3
Sn(OCOC11
23
)
2
3
. The dosage and reached its peak (66%) when the amount of the
analytical results indicate the electronic effect of coordinating catalyst was 2%. However, the DPC conversion did not change
groups plays a more important role than steric hindrance in aer 2%. The PS yield rst increased and then decreased with
deciding the activity of organotin catalysts, which has contributed to increases in the amount of the catalyst, reaching its peak (45%)
the pronounced activity of Bu Sn(OCOC H ) so that, with it as the when the amount of the catalyst was 2% and beginning to
2
11 23 2
catalyst, the yield of the DPC rearrangement product is only slightly decrease with further increases in the amount of the catalyst
lower than that with BuSnO(OH) and Bu SnO having relatively due to rearrangement of PS into XA. The XA yield showed
weaker steric hindrance. Regarding BuSnCl and Bu SnOSnBu a continually rising trend with the increase of the amount of the
though the electronegativity of chlorine atoms coordinating with tin catalyst and reached its peak (28%) when the amount of the
atoms in BuSnCl is weaker than that of oxygen atoms in Bu catalyst was 10%. However, the XA yield kept unchanged with
SnOSnBu , there are three chlorine atoms coordinating with each further increases in the amount of the catalyst. This is because
2
3
3
3
,
3
3
-
3
tin atom in BuSnCl but only 0.5 oxygen atom coordinating with the generated PS will be further rearranged and converted into
3
each oxygen atom in Bu SnOSnBu , so the total electronegativity of XA. Additionally, with increases in the amount of the catalyst,
3
3
atoms coordinating with tin atoms as active centers in BuSnCl is the number of acids in the catalytic system will increase, which
3
still greater than that in Bu
of groups coordinating with tin atoms in BuSnCl
than that in Bu SnOSnBu . Therefore, the Lewis activity of tin atoms will keep unchanged with further increases in the amount of the
as active centers in BuSnCl is stronger than that in Bu SnOSnBu catalyst as the PS rearrangement reaction to produce XA has
so that it has greater catalytic activity than Bu SnOSnBu . Though reached equilibrium. Overall, when the amount of the catalyst is
3
SnOSnBu
3
; besides, the steric hindrance will further promote rearrangement of PS into XA. When the
3
is much weaker amount of the catalyst is more than 10%, yields of PS and XA
3
3
3
3
3
3
3
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Fig. 1 Effect of the amount of Bu
2
SnO on the Fries rearrangement of Fig. 2 Effect of the reaction time on the Fries rearrangement of DPC.
2
DPC. Reaction conditions: DPC: 15 g; reaction time: 8 h; temperature: Reaction conditions: DPC: 15 g; mass ratio of Bu SnO to DPC: 2%;
ꢀ
ꢀ
temperature: 200 C; pressure: 0.40 MPa.
2
00 C; pressure: 0.40 MPa.
0.05%, a small quantity of DPC can be converted into PS and XA. 3.4 Effect of reaction temperature on the rearrangement of
When the amount of the catalyst is between 0.05% and 2%, the DPC
yields of PS and XA will rise continually with increases in the
Normally, high temperatures favor Fries rearrangement reac-
amount of the catalyst; when the amount of the catalyst is
between 2% and 10%, increases in the amount of the catalyst
will favor further rearrangement of PS and conversion into XA.
When the amount of the catalyst is 10%, the tandem rear-
rangement reaction of DPC will reach equilibrium. The more
acidic the catalyst, the more complete the rearrangement. The
16,17
tions.
Bu
Therefore, the effect of the reaction temperature on
2
SnO-catalyzed rearrangement of DPC was investigated with
ꢀ
temperatures ranging between 140 and 280 C. As can be known
from Fig. 3, the DPC conversion gradually increased with rises
in the reaction temperature. When the reaction temperature
ꢀ
was 140 C, the XA yield was 11.0% and reached its peak (89%)
42,43
result is consistent with those reported in literature.
ꢀ
at 220 C. However, the DPC conversion kept unchanged with
further rises in the reaction temperature. The PS yield rst
increased and then decreased with rises in the reaction
3.3 Effect of reaction time on the rearrangement of DPC
temperature, reaching its peak (59%) when the temperature was
The Fries rearrangement of DPC consists of two sequential
rearrangements. So the reaction time has a great inuence on
the distribution of rearrangement products. Fig. 2 illustrates
ꢀ
220 C and gradually decreasing with further temperature rises.
Fries rearrangement of DPC is an endothermic reaction. Aer
ꢀ
the reaction temperature has reached 220 C, the reaction
2
the effect of the reaction time on Bu SnO-catalyzed rearrange-
equilibrium will move towards the endothermic direction so
that the PS yield has shown some decreases. When the reaction
ment of DPC. With the prolongation of the reaction time, the
DPC conversion increased gradually. When the reaction time
was 10 h, the DPC conversion reached its peak (79%). The DPC
conversion did not change signicantly aer 10 h. The yield of
PS increased gradually with the prolongation of the reaction
time. When the reaction time was 2 h, the PS yield was 22%;
when the reaction time was 8 h, the PS yield reached its peak
ꢀ
temperature has reached 260 C, the rearrangement reaction
has reached equilibrium so the PS yield will keep unchanged.
(51%). The PS yield began to decrease aer 10 h. The PS yield
kept unchanged at 51% aer 12 h. The XA yield increased
gradually with the prolongation of the reaction time. When the
reaction time was 2 h, the XA yield was 11% and reached its
peak (35%) at 12 h. This is mainly because the rst step rear-
rangement is synchronized with the second step rearrange-
ment. PS converted to XA slowly before 10 h, while aer 10 h, the

rst step was no longer occurred, and only the second step went
on, so PS converted to XA quickly. Aer 12 h, the yield of neither
PS nor XA showed signicant change. Because the rearrange-
ment reaction reached equilibrium. Between 2 h and 12 h, the
longer the reaction time is, the more the tandem rearrangement
of DPC is favored. In addition, the PS yield was more than XA in
the whole reaction, indicating that the rearrangement in the
Fig. 3 Effect of the reaction temperature on the Fries rearrangement
of DPC. Reaction conditions: DPC: 15 g; mass ratio of Bu SnO to DPC:
2

rst step was more likely to occur than that in the second step. 2%; reaction time: 10 h; pressure: 0.40 MPa.
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The XA yield continually increased with rises in the reaction parallel, and PS will never be completely converted into XA. The
temperature, and the increasing rate quickened aer the XA yield continually increased with rises in the reaction pres-
ꢀ
temperature has risen to 240 C. PS rearrangement is also an sure and reached its peak (32%) when the reaction pressure was
endothermic reaction, and the reaction equilibrium will move 0.60 MPa. The PS rearrangement reaction has reached equilib-
towards the endothermic direction with further temperature rium, so the XA yield will keep unchanged when the reaction
ꢀ
rises aer the reaction temperature has reached 240 C so that pressure was greater than 0.60 MPa. It can also be known from
PS is quickly rearranged and converted into XA. The XA yield Fig. 4 that, within 0.6 MPa, the higher the reaction pressure is,
ꢀ
reached its peak (41%) at 260 C. The PS rearrangement reac- the more the tandem rearrangement of DPC is favored.
tion has reached equilibrium so the XA yield will keep
ꢀ
ꢀ
ꢀ
unchanged aer 260 C. Between 140 C and 260 C, the higher
the reaction temperature is, the more the DPC rearrangement
reaction is favored.
3
.6 Reusability of Bu
methods
Bu SnO is a homogeneous catalyst for the rearrangement
2
SnO and comparison of synthetic
2
reaction, but it can be isolated following sodium hydroxide
precipitation, ltration, washing with water, liquid separation
3.5 Effect of reaction pressure on the rearrangement of DPC
Fries rearrangement reaction is easy to occur under pressurized and drying from the product. The obtained Bu
conditions,
2
SnO can be
and with the process conditions of trans- directly tested for reusability in the reaction, and the results are
esterication for the synthesis of DPC taken into account, the shown in Fig. 5. Following repeated use of Bu SnO for 12 times,
44,45
2
effect of the reaction pressure on Bu SnO-catalyzed DPC rear- the yields of both PS and XA could be maintained above 50%
2
rangement was investigated with pressures ranging between and 20% with only a slight decrease, respectively, indicating tin
0
2
.10 and 0.70 MPa. As can be known from Fig. 4, the reaction atoms as catalytic active centers in Bu SnO have practically not
pressure signicantly affects the rearrangement reaction. The been lost or inactivated during the rearrangement reaction. The
DPC conversion gradually increased with rises in the reaction performance of the catalyst aer used repeatedly for multiple
pressure and reached its peak (93%) when the reaction pressure times is still stable for the tandem rearrangement of DPC. As
was 0.50 MPa; however, the DPC conversion stopped changing indicated in Fig. S4,† the peak position and the intensity of FT-
signicantly with further rises in the reaction pressure. When IR for the fresh and spent Bu
the reaction pressure was 0.10 MPa, the rearrangement prod- conrmed the nature of Bu SnO unchanged during the catalyst
ucts PS and XA were generated, and the yields were 35% and recycling. Furthermore, we also compared this preparation
2%, respectively. The PS yield rst increased and then method of PS and XA with those previously reported methods.
decreased with rises in the reaction pressure, reaching its peak As listed in Table S1,† although the yields of PS and XA are
62%) when the reaction pressure was 0.50 MPa and gradually almost lower than that of other synthetic methods. The method
SnO were all the same, which
2
2
1
(
decreasing with further pressure rises. When the reaction in our work can simultaneous synthesis of PS and XA by DPC
pressure was greater than 0.60 MPa, the PS yield stopped rearrangement, the process is simple, the reaction time is short,
changing. PS rearrangement is also a reaction with spatial the raw material and the catalyst are low cost and easy to obtain,
volume reduction, so, with further rises in the reaction pres- and the use of raw materials, catalysts, and solvents with high
sure, the reaction will proceed towards the direction with the toxicity and corrosivity are avoided. In addition, our preparation
smaller spatial volume (i.e. XA) so that PS can be converted to XA method has few by-products, no waste acid, and waste gas in the
in large quantities. Nevertheless, during the tandem rear- reaction process. It is also environmentally friendly, the
rangement reactions, PS and XA will persistently exist in
2
Fig. 4 Effect of the pressure on the Fries rearrangement of DPC. Fig. 5 Reusability of Bu SnO for the Fries rearrangement of DPC.
Reaction conditions: DPC: 15 g; mass ratio of Bu
reaction time: 10 h; temperature: 220 C.
2
SnO to DPC: 2%; Reaction conditions: DPC: 15 g; mass ratio of Bu
2
SnO to DPC: 2%;
ꢀ
ꢀ
reaction time: 10 h; temperature: 220 C; pressure: 0.5 MPa.
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products are easy to separate, the catalyst can be recycled, and activity of the catalysts is. For the steric hindrance effect, the
has good economic benets.
larger the volumes of groups coordinating with tin atoms are,
the more they will hinder tin atoms from contacting carbonyl
oxygen atoms in DPC, the less easily carbonyl oxygen in DPC
molecules and lone pairs on carbonyl oxygen atoms can be
activated, and the lower the catalytic activity is.
3.7 Possible mechanism of DPC rearrangement catalyzed by
organotin compounds
During the Fries rearrangement of DPC, organotin compounds
may react with DPC to form a class of transition metal
complexes, which are soluble Lewis acid catalysts.
46,47
Catalytic 4 Conclusions
active centers of organotin compounds are tin atoms, and DPC
is the only material involved in the rearrangement reaction, so
the Fries rearrangement of DPC is an electrophilic substitution
reaction. It thus can be deduced that the possible reaction
mechanism of organotin-catalyzed DPC rearrangement is that
shown in Scheme 2: the carbonyl oxygen atoms in DPC have
lone pairs, and the electronegativity of carbonyl oxygen atoms is
greater than that of carbon atoms, so the electron cloud density
tends to be higher around the former than around the latter.
Tin atoms with Lewis acidity in organotin compounds have
vacant orbitals and are prone to chelate with carbonyl oxygen
atoms in DPC to form transition metal complexes. The C–O
single bond will crack to form phenolic tin compounds and acyl
cations due to the weakening of the bond energy between the
acyl group and the oxygen atom in the complexes. Acyl cations
will react electrophilically with ortho positions with higher
active sites in phenolic tin compounds to produce PS. PS can
continue to undergo similar rearrangement and etherication
cyclization reaction to convert to XA.
Organotin compounds are Lewis acids, and tin atoms are the
catalytic active centers, which form transition metal complexes
by activating carbonyl oxygen atoms in DPC and their own lone
pairs. For the electronic effect, the greater the electronegativity
of coordinating groups connecting with tin atoms is, the
stronger the Lewis acidity of tin atoms is, and the more easily
carbonyl oxygen in DPC molecules and lone pairs on carbonyl
oxygen atoms can be activated; the lower the electronegativity is,
the less easily carbonyl oxygen and lone pairs on carbonyl
oxygen atoms can be activated, and the less easily transition
metal complexes can be formed, and the lower the catalytic
Fries rearrangement of DPC catalyzed by organotin was realized
for the rst time. To research the effect of coordination groups
on the catalytic activity of organotin revealed that the stronger
the electron absorption effect of the coordination group was,
the stronger the Lewis acidity of the active center tin atom was,
and the better the catalytic performance of the corresponding
organotin was. The small steric hindrance of the coordination
group was benecial to the activation of DPC by tin atom, and to
boost the rearrangement of DPC. Among a series of organotin
compounds, Bu SnO showed the best catalytic activity because
2
of the strong electron effect and small steric hindrance of the
coordination group. Under the optimal conditions, with 2% of
ꢀ
m(Bu
2
SnO)/(DPC), 10 h of reaction time, 220 C of reaction
temperature and 0.50 MPa of reaction pressure, the conversion
of DPC was 93%, and the yield of PS and XA was up to 62% and
28%, respectively. The research also showed that high pressure,
high temperature, increasing the amount of catalyst and pro-
longing the reaction time was benecial to the rearrangement
of DPC. In the process of DPC rearrangement, the two-step
rearrangement products PS and XA existed at the same time,
owing to PS could not be completely converted into XA. The
yield of PS was always greater than XA, and the highest selec-
tivity of XA was only 47%. Furthermore, the rearrangement
reaction mechanism over organotin compounds as Lewis acid
was proposed. At rst, the tin atom coordinated with the oxygen
over the carbonyl group of DPC to form transition state Sn
complexes, which underwent nucleophilic and electrophilic
substitution attack to form PS. PS proceeded with Fries rear-
rangement and etherication cyclization to form XA. This study
can provide a theoretical and data basis for controlling the
byproducts produced by DPC rearrangement in the synthesis of
DPC, and also provide a new and effective route for the
synthesis of PS and XA.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Financial support for this work from the Major Demonstration
Program of Innovation Academy for Green Manufacture,
Chinese Academy of Sciences (No. IAGM-2019-A10), the
National High-tech R&D Program of China (863 Program, No.
2
2
013AA031703), the National Key R&D Program of China (No.
016YFB0301900), the Science and Technology Support
Program of Sichuan Province (No. 2018JY0615), and the Science
and Technology Innovation Program for Youth Team of Sichuan
Scheme 2 Possible reaction mechanism for the Fries rearrangement
of DPC.
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Province (No. 2013TD0010). The authors would like to thank 25 S. L. Wang, C. G. Li, Z. L. Xiao, T. Chen and G. Y. Wang, J.
Shiyanjia Lab (http:\\www.shiyanjia.com) for the FT-IR, H NMR
and C NMR analysis.
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26 Z. L. Xiao, H. Yang, H. Zhang, T. Chen and G. Y. Wang, Chem.
Pap., 2018, 72, 2347–2352.
2
7 S. L. Wang, H. Y. Liu, J. J. Wang, T. Chen and G. Y. Wang, J.
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