Iron(III) chloride (FeCl3)-catalyzed electrophilic aromatic substitution of chlorobenzene with thionyl chloride (SOCl2) and the accompanying auto-redox in sulfur to give diaryl sulfides (Ar2S): Comparison to catalysis by aluminum chloride (AlCl3)

Article abstract of DOI:10.1080/10426507.2016.1244206

The Lewis acids MCl₃(M = Fe and Al)-catalyzed electrophilic aromatic (ArH) substitution reactions with thionyl chloride (SOCl₂) have been shown to give diaryl sulfoxide (Ar₂SO) and the reduced diaryl sulfide (Ar₂S). Under various selected conditions, the FeCl₃-catalyzed reactions of chlorobenzene gave substantially much higher percent yields of Ar₂S (Ar = p-ClC₆H₄) than the reactions catalyzed by AlCl₃, showing that FeCl₃facilitates the reduction in the sulfur center of SOCl₂. A d_π?>p_π^*back bond between Fe(III) and the O?S group is thought to be responsible for enhancement of the reduction.

Full text of DOI:10.1080/10426507.2016.1244206

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Iron(III) chloride (FeCl3)-catalyzed electrophilic
aromatic substitution of chlorobenzene with
thionyl chloride (SOCl2) and the accompanying
auto-redox in sulfur to give diaryl sulfides (Ar2S):
Comparison to catalysis by aluminum chloride
(AlCl3)
Xiaoping Sun, David Haas & Chyress Lockhart
To cite this article: Xiaoping Sun, David Haas & Chyress Lockhart (2016): Iron(III) chloride
(FeCl3)-catalyzed electrophilic aromatic substitution of chlorobenzene with thionyl chloride
(SOCl2) and the accompanying auto-redox in sulfur to give diaryl sulfides (Ar2S): Comparison
to catalysis by aluminum chloride (AlCl3), Phosphorus, Sulfur, and Silicon and the Related
Elements, DOI: 10.1080/10426507.2016.1244206
To link to this article: http://dx.doi.org/10.1080/10426507.2016.1244206
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Oct 2016.
Published online: 07 Oct 2016.
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Date: 27 January 2017, At: 04:07

PHOSPHORUS, SULFUR, AND SILICON
, VOL. , NO. , –
http://dx.doi.org/./..
Iron(III) chloride (FeCl)-catalyzed electrophilic aromatic substitution of
chlorobenzene with thionyl chloride (SOCl) and the accompanying auto-redox in
sulfur to give diaryl sulfides (ArS): Comparison to catalysis by aluminum chloride
(AlCl)
Xiaoping Sun, David Haas, and Chyress Lockhart
Department of Natural Sciences and Mathematics, University of Charleston, Charleston, West Virginia, USA
ABSTRACT
ARTICLE HISTORY
Received  August 
Accepted  September 
The Lewis acids MCl3 (M = Fe and Al)-catalyzed electrophilic aromatic (ArH) substitution reactions with
thionyl chloride (SOCl2) have been shown to give diaryl sulfoxide (Ar2SO) and the reduced diaryl sulfide
(Ar2S). Under various selected conditions, the FeCl3-catalyzed reactions of chlorobenzene gave substantially
KEYWORDS
much higher percent yields of Ar2S (Ar = p-ClC6H4) than the reactions catalyzed by AlCl3, showing that FeCl3
Iron catalysis; auto-redox,
aromatic; CH substituti∗on;
chlorobenzene; dπ -pπ
bonding
∗
=
facilitates the reduction in the sulfur center of SOCl2. A dπ −> pπ back bond between Fe(III) and the O S
group is thought to be responsible for enhancement of the reduction.
GRAPHICAL ABSTRACT
p-ClC6H4, and p-HOC6H4) were produced in different percent
yields depending on reaction conditions without formation of
diphenyl disulfide (Ph2S2) under any conditions.3,4 The forma-
tions of Ar2SO and Ar2S were demonstrated3,4 to take place
via the EAS and auto-redox on sulfur which are analogous to
those involved in the FeCl3-catalyzed EAS of SeOCl2 as shown
in Figure 1. In the present work, we study the FeCl3-catalyzed
reactions of SOCl2 with chlorobenzene (PhCl). We have found
that at the same conditions (temperature and the manner of
mixing starting materials) all the FeCl3-catalyzed reactions gave
substantially much higher percent yields of diaryl sulfide Ar2S
(Ar = 4-ClC6H4) than the reactions catalyzed by AlCl3, show-
ing that FeCl3 facilitates the auto-redox process on the sulfur
center in the reactions of PhCl with SOCl2. The findings are
unique, and we believe it is worthy to report them in the present
article.
Introduction
Iron(III) chloride (FeCl3) is a good Lewis acid and has been
very widely and effectively employed as a catalyst in a variety
types of organic chemical reactions.1 Of these types of reac-
tions, particularly interesting to us are the FeCl3-catalyzed elec-
trophilic aromatic substitution (EAS) reactions.1,2 As part of the
efforts made in this scenario, we have previously carried out
the reaction of benzene with selenyl chloride (SeOCl2) in the
presence of FeCl3 as a catalyst, and the reaction was found to
give a mixture of diphenyl selenide (Ph2Se) and diphenyl dis-
elenide (Ph2Se2) without the formation of diphenyl selenoxide
(Ph2SeO).3 We demonstrated that the formations of Ph2Se and
Ph2Se2 occur via an EAS mechanism which is accompanied by
auto-redox processes on the selenium center facilitated by FeCl3
(Figure 1).3 In this work, we extend the FeCl3-catalyzed EAS
reactions to thionyl chloride (SOCl2), the sulfur analogue of
SeOCl2.
As reported previously by different workers, diaryl sulfox-
ides and sulfides have many important applications in organic
synthesis and biomedical aspects.5–10 Our findings in the role of
FeCl3 in facilitating the auto-redox of sulfur are also significant
and have contributed new aspects to the iron catalysis.
Previously, we have studied aluminum chloride (AlCl3)-
catalyzed EAS reactions of SOCl2, and both diaryl sulfoxides
=
(Ar2SO, Ar C6H5, p-CH3C6H4, o-CH3C6H4, and p-ClC6H4)
=
and diaryl sulfides (Ar2S, Ar C6H5, p-CH3C6H4, o-CH3C6H4,
CONTACT Xiaoping Sun
xiaopingsun@ucwv.edu
Department of Natural Sciences and Mathematics, University of Charleston, Charleston, WV , USA.
©  Taylor & Francis Group, LLC

2
X. SUN ET AL.
results demonstrate that by comparison to the AlCl3 catalysis,
utilization of FeCl3 as a catalyst substantively facilitated the auto-
redox process on sulfur so that the diaryl sulfides became the
major products.
In addition to the auto-redox on sulfur to lead to the forma-
tion of (4-ClC6H4)2S, the FeCl3-catalyzed reactions of PhCl with
SOCl2 led to the formations of further chlorinated diaryl sul-
fide products (4-ClC6H4)S(3,4-Cl2C6H3) and (3,4-Cl2C6H3)2S
(Table 1). It is a remarkable feature observed for the reactions.
At an elevated temperature (80°C) (Entries 3 and 4), formations
of (4-ClC6H4)S(3,4-Cl2C6H3) and (3,4-Cl2C6H3)2S became the
major processes. When the molar ratio of PhCl : SOCl2 : FeCl3
= 2:1:1 (Entry 4), the molar percentages (normalized) of (4-
ClC6H4)2S, (4-ClC6H4)S(3,4-Cl2C6H3), and (3,4-Cl2C6H3)2S
in the product were 7%, 52%, and 41%, respectively. When the
molar ratio of PhCl : SOCl2 : FeCl3 became 1:1:1, as the quantity
of PhCl decreased and the quantity of SOCl2 increased relatively,
the percentage of (3,4-Cl2C6H3)2S in the product substantially
increased coincident with the decrease in (4-ClC6H4)S(3,4-
Cl2C6H3), and the molar percentages (normalized) of (4-
ClC6H4)2S, (4-ClC6H4)S(3,4-Cl2C6H3), and (3,4-Cl2C6H3)2S
in the product became 3%, 24%, and 73%, respectively (Entry 3).
Our previous work has shown that the auto-redox on sulfur
in the AlCl3-catalyzed EAS reactions of SOCl2 and the auto-
redox on selenium in the AlCl3- and FeCl3-catalyzed EAS reac-
tions of SeOCl2 gave a byproduct of Cl2.3,4 Presumably and con-
ceivably, the Cl2 byproduct is also formed in the FeCl3-catalyzed
reaction of PhCl with SOCl2. This is strongly evidenced by
the observed chlorination of the aromatic rings (by Cl2) to
lead to the formations of (4-ClC6H4)S(3,4-Cl2C6H3), and (3,4-
Cl2C6H3)2S in all the FeCl3-catalyzed reactions (Table 1).
Comparing to the previously studied AlCl3-catalyzed reac-
tions,3,4 the production of Cl2 in the FeCl3-catalyzed reac-
tions, along with the conversion of S(IV) in SOCl2 to S(II) in
Ar2S, supports a similar, proposed auto-redox mechanism as
Figure . The FeCl-catalyzed reaction of benzene with SeOCl giving PhSe and
PhSe via an EAS reaction and subsequent auto-redox processes on selenium.
Results and discussion
The FeCl3-catalyzed reactions of PhCl with SOCl2 and
comparison to the AlCl3 catalysis
Table 1 shows the product distributions for the MCl3 (M =
Fe and Al)-catalyzed reactions of PhCl with SOCl2 in different
conditions (temperature, manner of mixing the starting materi-
als, and molar ratio of the starting materials). At both 0°C and
25°C (Entries 1 and 2), the reactions catalyzed by FeCl3 gave a
total of five products; two isomers of diaryl sulfoxides and three
reduced diaryl sulfides, with the total percentage (normalized)
of the diaryl sulfides being about 69% and 57% at 0°C and 25°C,
respectively. By contrast, the AlCl3-catalyzed reactions at these
temperatures mainly gave two sulfoxide isomers with only 1%
and 7% (normalized) diaryl sulfide formed at 0°C and 25°C,
respectively. When the temperature was further raised to 80°C
(Entries 3 and 4), the FeCl3-catalyzed reactions only gave the
reduced diaryl sulfide products without formation of sulfoxide,
while the AlCl3-catalyzed reactions mainly gave two isomers of
diaryl sulfoxide with only 6% diaryl sulfide formed. All these
a
Table . Distributions of products formed in the MCl (M = Fe and Al)-catalyzed reactions of PhCl with SOCl
M = Fe
%
%
.%
%
%
%
.%

.%

M = Alb
(Entry 1)
SOCl added dropwise to the PhCl/MCl mixture at 0°C PhCl : SOCl : MCl = :: (molar ratio)
M = Fe
%
%
%
%
%
%
%

%

M = Al
(Entry 2)
MCl added to the PhCl/SOCl mixture at 25°C PhCl : SOCl : MCl = :: (molar ratio)
M = Fe

%

%
%
%
%

%

M = Alc
(Entry 3)
SOCl added dropwise to the PhCl/MCl mixture at 80°C PhCl : SOCl : MCl = :: (molar ratio)
M = Fe

%

%
%

%

%

M = Al
(Entry 4)
SOCl added dropwise to the PhCl/MCl mixture at 80°C PhCl : SOCl : MCl = :: (molar ratio)
aAll the percentages are relative normalized yields (GC yields), determined by comparison of areas of the GC spectra of related components on the basis, as previously
established, that the peak area is proportional to the number of moles of the component.
bEssentially, only the para-isomer of diaryl sulfoxide was formed based on GC-MS analysis. The overall isolated percent yield was determined to be % by comparison
of the actually recovered mass of the product with the theoretical yield of diaryl sulfoxide. Therefore, the absolute conversion yield of the para-isomer should be %
(% × %).
cMainly, the para- and ortho-isomers of diaryl sulfoxide were formed with overall isolated percent yield being %. Therefore, the absolute conversion yields of the para-
isomer and ortho-isomer should be % (% × %) and % (% × %), respectively.

PHOSPHORUS, SULFUR, AND SILICON
3
=
Figure . The dπ –pπ ࢩ back bonding between Fe(III) and S O in the ArSOCl–FeCl
adduct molecule and possible resonance structures of ArSOCl–FeCl. The dπ (dyz)
orbital is perpendicular to the FeCl molecule plane (the xy-plane).
Figure . Mechanism for the FeCl-catalyzed reaction of PhCl with SOCl.
shown in Figure 2.11 The overall process is demonstrated in
Figure 2.
effectively in sideways to form a dπ −> pπ ∗ back bond between
the iron and oxygen atoms in ArSOCl-FeCl3 (Figure 3). As a
result, the dπ electrons in iron flow into the S O pπ ∗ orbital to
Previously, we have identified chlorination of PhOH to form
minor p-ClC6H4OH in the course of an AlCl3-catalyzed reac-
tion of phenol (PhOH) with SOCl2 giving (p-HOC6H4)2S
and the Cl2 by-product as a result of the auto-redox on sul-
fur.3 By comparison of the reaction with the molar ratio of
PhCl : SOCl2 : FeCl3 = 2:1:1 (Entry 4) and the reaction with
the molar ratio of PhCl : SOCl2 : FeCl3 = 1:1:1 (Entry 3)
at 80°C (Table 1), as the relative quantity of SOCl2 increased
and became excess and the relative quantity of PhCl decreased
and became deficit, the extent of chlorination on the aromatic
rings of the initially formed (4-ClC6H4)2S was enhanced. As a
result, the 1:1:1 reaction (Entry 3) gave much higher percentage
(73%) of (3,4-Cl2C6H3)2S than that (41%) of (3,4-Cl2C6H3)2S
produced in the 2:1:1 reaction (Entry 4). As the percentage
of (3,4-Cl2C6H3)2S was getting higher, the percentage of (4-
ClC6H4)S(3,4-Cl2C6H3) in the product was getting correspond-
ingly lower.
=
partially neutralize the positive charge in sulfur decreasing its
electrophilicity. The∗resonance structure (C) in Figure 3 results
from the dπ −> pπ back bonding and further demonstrates
the electron transfer from iron to sulfur. Consequently, the sul-
fur center in ArSOCl-FeCl3 cannot be attacked efficiently by
the nucleophilic aromatic ring in PhCl to make Ar2SO (Ar =
p-ClC6H4). Instead, the intramolecular auto-redox process in
ArSOCl-FeCl3, as shown in Figure 2, becomes more prominent,
much faster than the EAS of ArH with ArSOCl-FeCl3, to give
Ar2S as the major or sole product and the Cl2 by-product. We
believe that this accounts for the role of FeCl3 in facilitating the
auto-redox processes in the FeCl3-catalyzed EAS reactions of
SOCl2. Recently, we have found that by comparison with the
AlCl3 catalysis, the FeCl3-catalyzed reaction of benzene with
SeOCl2, the selenium analogue of SOCl2, generated higher yield
of the reduced Ph2Se2 indicating a similar role of FeCl3 in facil-
itating the analogous auto-redox on selenium in the EAS reac-
tions of SeOCl2.3
The iron(III) d-orbital facilitated intramolecular auto-redox
processes on sulfur in the FeCl3-catalyzed EAS reactions of
SOCl2
Conclusions
The studies of MCl3 (M = Fe and Al)-catalyzed reactions of In this work, the previously established FeCl3-catalyzed EAS
3
PhCl with SOCl2 have shown that under the same conditions, reaction of SeOCl2 has been extended to SOCl2 (the sul-
the FeCl3-catalyzed reactions always produce much higher per- fur analogue) and the results are compared to the cataly-
cent yields of diaryl sulfides than the reactions catalyzed by sis by AlCl3. Under the same conditions, including reaction
AlCl3. Under certain conditions (Table 1 Entries 3 and 4, reac- temperature, the molar ratio of the reactants, and the man-
tions at 80°C), all the products generated from the FeCl3- ner of mixing starting materials, the FeCl3-catalyzed reac-
catalyzed reactions are diaryl sulfides without formation of tions of PhCl with SOCl2 have been found to give sub-
diaryl sulfoxides, while the AlCl3 catalyzed reactions, on the stantially much higher percent yields of diaryl sulfides Ar2S
other hand, give diaryl sulfoxides as major products. This shows (Ar = 4-ClC6H4) than the reactions catalyzed by AlCl3, show-
that the utilization of FeCl3 as a catalyst for the EAS reactions of ing that FeCl3 facilit∗ates the auto-redox in the sulfur center of
SOCl2 greatly facilitates the intramolecular auto-redox on sulfur SOCl2. A dπ −> pπ back bond between the iron and oxygen
(Figure 2) to lead to formation of diaryl sulfides. We believe that atoms in the ArSOCl-FeCl3 intermediate formed in the first
this effect is accomplished via the iron(III) d-orbital participa- step of EAS reaction is proposed to account for the enhance-
tion in the overall reactions.
ment of the auto-redox by FeCl3. The dπ∗electrons in iron can
The symmetry of the empty pπ ∗ orbital (the antibonding π∗ be transferred into the empty S O pπ orbital to decrease
=
=
orbital) in the S O bond domain of the ArSOCl-FeCl3 inter- the electrophilicity in sulfur. As a result, the sulfur center in
mediate (Figure 2) matches the symmetry of the filled dπ orbital ArSOCl-FeCl3 becomes much less reactive towards an aromatic
in the iron atom. Therefore, the dπ and pπ ∗ orbitals can overlap ring. Instead, the intramolecular auto-redox in ArSOCl-FeCl3

4
X. SUN ET AL.
takes place predominately to give Ar2S as the major or a sole to ensure that the organic product had been fully extracted into
product. the ether phase. The ether and water phases were separated. The
Iron catalysis has been shown to play very important roles in water phase was extracted by diethyl ether (20 mL) again. Then
organic chemistry.1,2 We believe that our present research has all the ether solutions were combined and dried by anhydrous
made contributions of new and interesting aspects to this sig- sodium sulfate. The dried ether solution was filtered off and left
nificant area.
in the fumehood. Eventually, all the diethyl ether solvent evap-
orated, giving the final reaction product. The product was then
characterized by GC-MS, and the results are shown in Table 1
(Entry 1).
Experimental
Approach 2: The reactions were conducted in the fumehood
at 25°C with the molar ratio of PhCl : SOCl2 : MCl3 being
1:1:1 and usage of SOCl2 approximately 10 mmol (as above).
For the FeCl3-catalyzed reactions, powdery FeCl3 was added
Chemical reagents
Thionyl chloride (SOCl2) (purified) from J. T. Baker Chemical
Company was used. Chlorobenzene (PhCl) (99%) were obtained
from Sigma Aldrich. Granular anhydrous aluminum chloride
(AlCl3) (99%, extra pure) was obtained from Acros Organics.
Powdery iron(III) chloride (FeCl3) (98%), diethyl ether (sol-
vent), dichloromethane (99.9%, the solvent for GC-MS sam-
ples), and anhydrous sodium sulfate (99.5%, drying agent) were
purchased from Fisher Scientific Company.
–
in 10 aliquots to the PhCl SOCl2 mixture in a large test tube
(80 mL). The reaction took place quickly, indicated by bub-
bling as above. After all the FeCl3 was added, the reaction went
to completion, indicated by the cessation of bubbling. Then
the aqueous work-up was performed as above. The final prod-
uct was characterized by GC-MS, and the results are shown in
Table 1 (Entry 2). For the AlCl3-catalyzed reactions, granular
–
AlCl3 was added piecewise to the PhCl SOCl2 mixture in a large
The GC-MS measurements
test tube (80 mL) as follows: First, one piece of AlCl3 was added
and gas (HCl) bubbles started to form immediately. The piece
of AlCl3 was crushed using a stirring rod. When most of the
pieces of AlCl3 had dissolved and bubbling occurred very slowly,
another piece of AlCl3 was added. All the AlCl3 granules were
Reaction products were identified using a Varian CP-3800 gas
chromatograph-Varian Saturn 2200 mass spectrometer. The
sample of each product was prepared in an approximate 1%
(m/m) dilution in dichloromethane. One-microliter of the solu-
tion was injected onto a Chrompack CP-SIL 8-CB 30 m ×
0.25 mm capillary column using a He carrier gas of 1.0 mL/min,
a 1/100 injection split ratio, and a temperature ramp of 150°C
to 240°C (10°C/min). All chemical compounds were eluted in
10 min. The individual compounds contained in each prod-
uct, including Ar2SO (Ar = p-ClC6H4), ArSOArꢀ (Ar, Arꢀ = p-
ClC6H4, o-ClC6H4), Ar2S (Ar = 4-ClC6H4, and 3,4-Cl2C6H3),
and ArSOArꢀ (Ar, Arꢀ = 4-ClC6H4, 3,4-Cl2C6H3), were identi-
fied by comparison of their mass spectra with those on the avail-
able NIST database. Their molar percentages (normalized) in
the product were estimated, as previously established,3 by com-
paring the related GC peak areas on such a basis that the peak
area is proportional to the number of moles of the related com-
pound.
–
eventually added into the PhCl SOCl2 mixture piece-by-piece.
The reaction went to completion, indicated by the cessation of
bubbling after all the AlCl3 had been added. Then an aque-
ous work-up was performed as above. The final product was
characterized by GC-MS, and the results are shown in Table 1
(Entry 2).
Approach 3: The reactions were conducted in the fumehood
at 80°C with the molar ratio of PhCl : SOCl2 : MCl3 being 1:1:1
and usage of SOCl2 approximately 10 mmol (as above). For each
–
reaction, SOCl2 was added dropwise to the PhCl MCl3 (M = Fe
or Al) mixture in a large test tube with constant stirring (For the
AlCl3-catalyzed reaction, SOCl2 was added after the granular
AlCl3 had been mostly crushed). The procedure and observa-
tions were the same as those for the above reactions conducted
at 0°C. The final product was characterized by GC-MS, and the
results are shown in Table 1 (Entry 3).
The MCl3 (M = Fe and Al)-catalyzed reactions of PhCl with
Approach 4: The reactions were conducted in the fume-
hood at 80°C with the molar ratio of PhCl : SOCl2 : MCl3
being 2:1:1 and usage of SOCl2 approximately 10 mmol [PhCl
(2.26 g, 20.1 mmol), SOCl2 (1.19 g, 10.0 mmol), and AlCl3
(1.33 g, 10.0 mmol) or FeCl3 (1.62 g, 10.0 mmol)]. For each reac-
SOCl2
Approach 1: The reactions were conducted in the fumehood at
0°C with the molar ratio of PhCl : SOCl2 : MCl3 being 1:1:1
and usage of SOCl2 approximately 10 mmol [PhCl (1.13 g,
10.0 mmol), SOCl2 (1.19 g, 10.0 mmol), and AlCl3 (1.33 g,
10.0 mmol) or FeCl3 (1.62 g, 10.0 mmol)]. For each reaction,
–
tion, SOCl2 was added dropwise to the PhCl MCl3 (M = Fe or
Al) mixture in a large test tube with constant stirring (For the
AlCl3-catalyzed reaction, SOCl2 was added after the granular
AlCl3 had been mostly crushed). The procedure and observa-
tions were the same as those for the above 1:1:1 reactions at 80°C
(Approach 3). The final product was characterized by GC-MS,
and the results are shown in Table 1 (Entry 4).
–
SOCl2 was added dropwise to the PhCl MCl3 (M = Fe or Al)
mixture in a large test tube (80 mL) with constant stirring (For
the AlCl3-catalyzed reaction, SOCl2 was added after the gran-
ular AlCl3 had been mostly crushed). The reaction took place
quickly, indicated by bubbling (formation of HCl) as above.
After all the SOCl2 was added, the reaction went to completion
(in about 2 h), indicated by the cessation of bubbling. Then iced
water (40 mL) was poured into the reaction mixture. This was
followed by the addition of diethyl ether (20 mL). All the con-
Funding
We would like to thank University of Charleston and its Chemistry Program
tents were transferred into a separatory funnel and shaken well for financial support.

PHOSPHORUS, SULFUR, AND SILICON
5
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