NiFe2O4 as a magnetically recoverable nanocatalyst for odourless C–S bond formation via the cleavage of C–O bond in the presence of S8 under mild and green conditions

Article abstract of DOI:10.1002/aoc.4691

We present green methodologies for one-pot and odourless syntheses of unsymmetric and symmetric diaryl sulfides via C─O bond activation using NiFe₂O₄ magnetic nanoparticles as a reusable heterogeneous nanocatalyst. The synthesis of unsymmetric sulfides is performed using the cross-coupling reaction of phenolic esters such as acetates, triflates and tosylates with arylboronic acid/S₈ or triphenyltin chloride/S₈ as thiolating agents in the presence of base and NiFe₂O₄ magnetic nanoparticles as a catalyst in poly(ethylene glycol) as solvent at 60–85°C. Also, the synthesis of symmetric diaryl sulfides from phenolic compounds using S₈ as the sulfur source and NiFe₂O₄ as catalyst in dimethylformamide at 120°C is described. Using these protocols, the syntheses of various unsymmetric and symmetric sulfides become easier than using the available protocols due to the use of a magnetically reusable bimetallic nanocatalyst and avoiding the use of thiols and aryl halides.

Full text of DOI:10.1002/aoc.4691

Received: 20 August 2018
DOI: 10.1002/aoc.4691
Revised: 11 October 2018
Accepted: 15 October 2018
F U L L P A P E R
NiFe O as a magnetically recoverable nanocatalyst for
2
4
odourless C–S bond formation via the cleavage of C–O bond
in the presence of S under mild and green conditions
8
1
2
3
1
Bahareh Atashkar | Amin Rostami | Abed Rostami | Mohammad Ali Zolfigol
1
Department of Organic Chemistry,
We present green methodologies for one‐pot and odourless syntheses of
unsymmetric and symmetric diaryl sulfides via C―O bond activation using
NiFe O magnetic nanoparticles as a reusable heterogeneous nanocatalyst. The
Faculty of Chemistry, Bu‐Ali Sina
University, Hamedan 6517838683, Iran
2
Department of Chemistry, Faculty of
2
4
Science, University of Kurdistan, 66177‐
synthesis of unsymmetric sulfides is performed using the cross‐coupling reaction
15175 Sanandaj, Iran
3
of phenolic esters such as acetates, triflates and tosylates with arylboronic acid/S8
Vice Chancellor for Food and Drug,
Kurdistan University of Medical Sciences,
Sanandaj, Iran
or triphenyltin chloride/S as thiolating agents in the presence of base and
8
NiFe O magnetic nanoparticles as a catalyst in poly(ethylene glycol) as solvent
2
4
at 60–85°C. Also, the synthesis of symmetric diaryl sulfides from phenolic
Correspondence
Mohammad Ali Zolfigol, Department of
Organic Chemistry, Faculty of Chemistry,
Bu‐Ali Sina University, Hamedan
compounds using S8 as the sulfur source and NiFe O4 as catalyst in
2
dimethylformamide at 120°C is described. Using these protocols, the syntheses
of various unsymmetric and symmetric sulfides become easier than using the
available protocols due to the use of a magnetically reusable bimetallic
nanocatalyst and avoiding the use of thiols and aryl halides.
6517838683, Iran.
Email: zolfi@basu.ac.ir; mzolfigol@yahoo.
com
Amin Rostami, Department of Chemistry,
Faculty of Science, University of
Kurdistan, 66177‐15175 Sanandaj, Iran.
Email: a.rostami@uok.ac.ir
KEYWORDS
2 4 8
C–O bond activation, C–S bond formation, diaryl sulfides, NiFe O magnetic nanoparticles, S
Funding information
Iran National Science Foundation (INSF),
Grant/Award Number: 940124; University
of Kurdistan Research Councils; Bu‐Ali
Sina University
1
| INTRODUCTION
bond cleavage. Phenol derivatives that are coupling
partners in these reactions, widely found in nature, can
be converted to aryl C―O electrophiles using C―O
bond‐activating agents such as esters, pivalates, acetates,
Carbon–sulfur bond formation reactions leading to aryl
sulfides are most important in organic and medicinal
chemistry, as several bioactive compounds with broad‐
spectrum therapeutic activities for the treatment of some
diseases, such as Alzheimer's disease, Parkinson's disease,
diabetes, HIV, anti‐inflammatory and immune diseases,
[1]
phosphonates,
triflates,
carbamates,
carbonates,
phosphoramides, phosphates, tosylates, mesylates,
[
4]
sulfamates and heteroaryl ethers. Additionally, pheno-
lic compounds are more suitable than aryl halides due
[2]
[5]
contain aryl sulfide moieties.
Also, aryl sulfides
to their higher diversity, abundance and availability.
transform into other useful functional groups such as
sulfoniums, sulfones or sulfoxides.
One of the significant strategies for the synthesis of
aryl sulfides is the formation of C―S bonds via C―O
At first, aryl halides were used for the synthesis of
unsymmetric sulfides using the reaction of an organosulfur
nucleophile with an organic (pseudo) halide in the pres-
[3]
[
6]
ence of a transition metal as catalyst. Arylboronic acids
Appl Organometal Chem. 2019;33:e4691.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 16
https://doi.org/10.1002/aoc.4691

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ATASHKAR ET AL.
have also been used as aryl sources in the synthesis of
thioetherification reaction of phenolic esters (including
triflates, tosylates, acetates and phosphonates) with alkyl
halides or alkyl tosylates using thiourea as the sulfur
unsymmetric sulfides via cleavage of their C―B bonds
[7]
catalysed by copper salts. After that, phenolic esters were
introduced as a new coupling partner in C―S bond forma-
source in wet poly(ethylene glycol) (Scheme 1, route
[8]
[18]
tion reactions and the synthesis of aryl sulfides. Zheng
et al. reported the palladium‐catalysed thioetherification
of aryl triflates in the presence of bidentate phosphine
ligands such as BINAP or Tol‐BINAP (Scheme 1, route
j).
Although these efforts have improved reaction
conditions, most of these reported methods were
performed under homogeneous and non‐recoverable
[
6i–k, 7b, 12b, 13, 18–20]
reaction conditions.
Over the past
[8]
a). Perrio and co‐workers have described the synthesis
of butylphenyl sulfide using phenyl triflate as the substrate
under palladium‐catalysed reaction in the presence of
few years, from both economic and industrial points of
view, heterogeneous catalysts have attracted the attention
of researchers as compared to homogeneous cata-
[
9]
[19,20]
xantphos as a ligand at 140°C (Scheme 1, route b).
lysts.
But these systems suffer from low activity. In
Pd/ligand system has also been reported to be effective
for the reaction of aryl triflates with thiols (Scheme 1,
order to overcome this problem, recently magnetic nano-
particles (MNPs) have been introduced into the field of
catalysis. Such MNPs couple the advantages of homoge-
neous systems (high activity and reproducibility) and het-
erogeneous systems (easy separation and excellent
reusability), which make them promising materials for
[6b]
routes c and d).
Utilizing indium tri(organothiolate) as
the nucleophile for reaction with aryl triflates was reported
[6h]
under palladium catalysis (Scheme 1, route e).
Sodium
benzenethiolates reacted with aryl mesylates in the pres-
[
21]
ence of in situ generated Ni(0) using zinc metal (Scheme 1,
practical and large‐scale applications.
NiFe O MNPs are air‐stable, easily made, low‐cost
[6a]
route f). The major problems associated with these pro-
cedures are the use of expensive, volatile and foul‐smelling
thiols as the starting materials.
2
4
and bimetallic nanoparticles displaying very good
[
22]
catalytic activity in various organic reactions.
Such
To avoid the use of thiols as thiolating agents for the
thioetherification of aryl halides, arylboronic acids and
phenolic esters with various sulfur sources have been
properties can stem from synergetic electronic interac-
tions between two different metal atoms in an individ-
ual nanoparticle. These nanoparticles can be separated
conveniently using an external magnetic field and
reused several times, which is the main benefit of these
systems.
[10]
[11]
[14]
[17]
employed such as thiourea,
potassium thioacetate,
[12]
[13]
metal sulfides,
carbon disulfide,
ethyl xanthogenate,
thiocyanate,
[15]
[16]
S8
and sodium thiosulfate.
Firouzabadi et al. reported the copper‐catalysed
Recently, we have reported odourless methods for the
synthesis of unsymmetric sulfides using the reaction of
arylboronic acid/S (Scheme 2, route a) or triphenyltin
8
chloride/S8 (Scheme 2, route b) systems as thiolating
[
23]
agents with aryl/alkyl halides,
nitroarenes via C―N
[
24]
bond activation
cleavage (Scheme 1, route a)
salts. Also, the synthesis of symmetric diaryl sulfides
and phenolic esters via C―O bond
[
25]
in the presence of Cu
[
23b]
[25]
using the reaction of aryl halides
or phenolic esters
with S8 has been investigated (Scheme 2, route c).
Although these protocols represent considerable progress
in C―S bond formation, recycling and reuse of the cata-
lysts under the homogeneous conditions are difficult
and large amounts of catalysts are lost during work‐up
of the reactions. Moreover, in some cases, toxic solvents,
long reaction times and high reaction temperatures are
required for completion of the reactions.
In order to overcome these drawbacks and expand
these methods, we report for the first time the application
of NiFe O4 MNPs as
a highly active recyclable
2
heterogeneous catalyst for one‐pot and non‐thiolic
thioetherification of phenolic esters with triphenyltin
chloride or arylboronic acids in the presence of S in a
8
SCHEME 1 Various methods for the synthesis of aryl/alkyl and
diaryl sulfides using phenolic esters as starting materials
green solvent. Also reported is the synthesis of symmetric
sulfides from homocoupling reaction of phenolic esters in

ATASHKAR ET AL.
3 of 16
2
.2 | Procedure for preparation of
phenolic esters (acetates, triflates,
tosylates)
Phenolic esters (acetates, triflates, tosylates) were synthe-
[
25]
sized according to the literature.
2
.3 | General procedure for synthesis of
unsymmetric diaryl sulfides using
arylboronic acid and phenolic esters
A one‐necked flask was charged with NiFe O (30 mg,
2
4
0.15 mmol), sodium tert‐butoxide (376 mg, 4.0 mmol),
sulfur (48 mg, 1.5 mmol), phenolic ester (1 mmol),
arylboronic acid (1.1 mmol) and poly(ethylene glycol)
(
PEG200; 2 ml). The reaction mixture was magnetically
stirred and heated at 90°C for the appropriate reaction
time. After completion of the reaction, the reaction mix-
ture was cooled to room temperature and the nanocatalyst
was separated using an external magnet. Water (4 ml) was
added and the product was extracted with EtOAc (3 × 4 ml)
and dried over anhydrous Na SO . Evaporation of the
2
4
solvent and purification by column chromatography on
silica gel (n‐hexane–EtOAc) gave the desired unsymmetric
diaryl sulfides in 72–94% yields.
SCHEME 2 Our previous (a–c) and present (d–f) strategies for
the synthesis of symmetric and unsymmetric diaryl sulfides
the presence of S and NiFe O as catalyst via C―O bond
8
2 4
activation (Scheme 2, routes d–f).
2
.4 | General procedure for C―S bond
formation using triphenyltin chloride and
phenolic esters
2
2
| EXPERIMENTAL
A one‐necked flask was charged with NiFe O (30 mg,
2
4
0.15 mmol), potassium carbonate (414 mg, 3.0 mmol),
2
.1 | Procedure for preparation of NiFe O4
sulfur (48 mg, 1.5 mmol), KF (180 mg, 3 mmol), phenolic
ester (1 mmol), triphenyltin chloride (0.35 mmol) and
PEG200 (1.5 ml). The mixture was magnetically stirred
and heated at 85°C for the appropriate reaction time.
After completion of the reaction, the reaction mixture
was cooled to room temperature and the nanocatalyst
was separated using an external magnet. Water (4 ml)
was added and the product was extracted with EtOAc
Nickel ferrite nanoparticles were synthesized using a
chemical co‐precipitation technique similar to that in
the literature,
basic solution. The synthesis procedure for NiFe O was
as follows. In a two‐necked flask, FeCl ⋅6H O (2.160 g,
8
[26]
using FeCl ⋅6H O and NiCl ⋅6H O in
3
2
2
2
2
4
3
2
mmol) and NiCl ⋅6H O (0.950 g, 4 mmol) were
2
2
dissolved in 75 ml of distilled water and stirred at room
temperate under a nitrogen atmosphere for 20 min.
Thereafter, under very rapid mechanical stirring, a
solution of NaOH (3 g, 75 mmol) in water (15 ml) was
added slowly dropwise during 15 min and then resultant
mixture with brown precipitate was heated to 90°C for
(
3 × 4 ml) and dried over anhydrous Na SO . Evaporation
2 4
of the solvent and purification by column chromatogra-
phy on silica gel (n‐hexane–EtOAc) gave the desired
phenylaryl sulfides in 79–94% yields.
5
h. Subsequently, the reaction mixture was cooled to
2.5 | General procedure for synthesis of
symmetric diaryl sulfides
room temperature, and the MNPs were separated by
magnetic decantation and washed with deionized water
(4 × 5 ml) and ethanol (2 × 5 ml) and dried under
A one‐necked flask was charged with NiFe O (10 mg,
2
4
vacuum at room temperature. Finally, the NiFe O MNPs
0.05 mmol), sodium tert‐butoxide (376 mg, 4 mmol), sul-
2
4
were calcined at 600°C for 5 h.
fur (16 mg, 0.5 mmol), phenolic ester (1 mmol) and dry

4
of 16
ATASHKAR ET AL.
dimethylformamide (DMF; 2 ml) under inert atmosphere.
The mixture was magnetically stirred and heated at
gradient field magnetometry (see supporting informa-
tion). In order to optimize the reaction conditions, we
considered the reaction of phenyl acetate with
phenylboronic acid as a model reaction in the presence
of S and NiFe O catalyst in PEG200 (Table 1). We first
1
20°C for the appropriate reaction time. After completion
of the reaction, the reaction mixture was cooled to room
temperature and the nanocatalyst was separated using
an external magnet. Water (4 ml) was added and the
product was extracted with EtOAc (3 × 4 ml) and dried
over anhydrous Na SO . Evaporation of the solvent and
8
2 4
performed the reaction in the presence of NiFe O , S
8
2
4
and KOH in PEG200 (Table 1, entry 1). Under these con-
ditions, the reaction did not produce the desired product
and the phenyl acetate converted to phenol. Using
Cs CO as base in this reaction, very low yield of product
2
4
purification by column chromatography on silica gel
n‐hexane–EtOAc) gave the desired symmetric diaryl
sulfides in 71–94% yields.
(
2
3
was observed (Table 1, entry 2). Organic bases such as
1
,4‐diazabicyclo[2.2.2]octane (DABCO) and Et N were
3
also used and no products were obtained (Table 1, entries
3 and 4). Notable conversion of phenyl acetate to product
was observed by changing the base to NaO Bu (Table 1,
3
| RESULTS AND DISCUSSION
t
NiFe O4 MNPs were prepared through a chemical
entry 5). In order to study the effect of solvent, we
2
co‐precipitation method according to the literature and
characterized using scanning electron microscopy, X‐ray
diffraction, Fourier transform infrared (FT‐IR) spectros-
copy, energy‐dispersive X‐ray analysis and alternating
conducted the reaction in PEG200, CH CN, DMF and
3
dioxane (Table 1, entries 5–8). Among the solvents, the
best yield was found with PEG200 (Table 1, entry 5).
We tested other sulfur sources such as thiourea and
a
TABLE 1 Optimization of the reaction conditions for the reaction of phenyl acetate and phenylboronic acid
Entry
S source
Base
Catalyst
Solvent
GC yield(%)
1
2
3
4
S
S
S
S
8
8
8
8
KOH
NiFe
NiFe
NiFe
NiFe
2
2
2
2
O
O
O
O
4
4
4
4
PEG200
PEG200
PEG200
PEG200
PEG200
—
14
—
—
100
24
95
42
—
53
68
78
59
84
21
88
87
Cs
DABCO
Et
2
CO
3
3
N
b
t
5
S
8
NaO Bu
2 4
NiFe O
t
6
7
8
9
S
8
S
8
S
8
NaO Bu
NiFe
NiFe
NiFe
NiFe
NiFe
NiFe
NiFe
NiFe
2
O
O
O
O
O
O
O
O
4
3
CH CN
t
NaO Bu
2
2
2
2
2
2
2
4
4
4
4
4
4
4
DMF
t
NaO Bu
Dioxane
PEG200
PEG200
PEG200
PEG200
PEG200
PEG200
PEG200
PEG200
PEG200
t
Thiourea
Na
NaO Bu
t
10
11
12
13
14
15
16
17
2
S
NaO Bu
c
d
e
t
S
8
S
8
S
8
S
8
S
8
S
8
S
8
NaO Bu
t
NaO Bu
t
NaO Bu
t
NaO Bu
CuFe
Fe
NiCl
NiFe
2 4
O
t
NaO Bu
3
O
4
t
NaO Bu
2
f
t
NaO Bu
2 4
O
a
2 4
Reaction conditions: phenyl acetate (1 mmol), phenylboronic acid (1.1 mmol), sulfur (1.5 mmol), NiFe O (15 mol%), base (4 mmol) and solvent (2 ml).
b
c
Most effective reaction conditions.
Sulfur (1 mmol) was used.
d
t
NaO Bu (3 mmol) was used.
e
f
2 4
NiFe O (10 mol%) was used.
The reaction carried out at 75°C.

ATASHKAR ET AL.
5 of 16
a
8 2 4
TABLE 2 C―S bond formation of phenolic esters with arylboronic acids using S catalysed by NiFe O MNPs
b
c
Entry
ArZ
ArB(OH)
2
Product
Time (h)
Yield (%)
1
Z = OAc
OTs
OTf
19
13
11
90
80
82
2
Z = OAc
OTs
OTf
20
14
12
91
78
80
3
4
5
6
7
Z = OAc
OTs
OTf
24
18
15
90
79
86
Z = OAc
OTs
OTf
26
21
18
89
76
79
Z = OAc
OTs
OTf
25
19
17
88
78
84
Z = OAc
OTs
OTf
32
22
20
86
78
80
Z = OA
OTs
OTf
21
17
12
94
79
87
8
9
Z = OAc
OTs
OTf
25
18
15
90
75
83
Z = OAc
OTs
OTf
27
22
18
87
72
78
(Continues)

6
of 16
ATASHKAR ET AL.
TABLE 2 (Continued)
b
c
Entry
ArZ
ArB(OH)
2
Product
Time (h)
Yield (%)
10
11
12
13
14
Z = OAc
OTs
OTf
11
7
5
90
79
86
Z = OAc
OTs
OTf
15
9
6
88
77
81
Z = OAc
OTs
OTf
11
7
4
92
80
88
Z = OAc
OTs
OTf
14
7
5
90
80
82
Z = OAc
OTs
OTf
20
14
10
94
83
88
1
1
5
6
Z = OAc
OTs
OTf
21
14
12
90
80
84
Z = OAc
OTs
OTf
19
13
10
84
74
79
1
1
7
8
Z = OAc
OTs
OTf
21
15
12
91
80
85
Z = OAc
OTs
OTf
14
9
6
87
68
75
(Continues)

ATASHKAR ET AL.
7 of 16
TABLE 2 (Continued)
b
c
Entry
ArZ
ArB(OH)
2
Product
Time (h)
Yield (%)
1
9
Z = OAc
OTs
OTf
14
9
7
90
76
79
a
t
Reaction conditions: phenolic ester (1 mmol), arylboronic acid (1.1 mmol), NiFe
2
O
4
(15 mol %), sulfur (1.5 mmol), NaO Bu (4 mmol), PEG200 (2 ml), 90°C.
[25]
b
1
13
All products are known and were characterized by FT‐IR, H NMR, C NMR analyses.
c
Yield after column chromatographic separation.
Na S. Thiourea failed to react (Table 1, entries 9 and 10),
To gain an understanding of the reaction mechanism,
we conducted a good range of experiments to explore the
roles of living group, thiolating agent, catalyst and other
aspects of the reaction (Scheme 3). Under the standard
conditions without the use of living group, 1‐naphthol
(1a) was not converted to the corresponding product
(1e), leaving the starting material intact and diphenyl
disulfide (1c) was not obtained (Scheme 3I). In the
absence of phenolic ester, the reaction of phenylboronic
2
whereas Na S was an active sulfur source giving the
2
desired product in moderate yield (Table 1, entry 10).
t
The amounts of S , NaO Bu and catalyst were also
8
optimized (Table 1, entries 11–13). CuFe O4 MNPs,
2
Fe O MNPs and NiCl were also used as catalysts for
3
4
2
the reaction (Table 1, entries 14–16) and the best result
was obtained using NiFe O MNPs (Table 1, entry 5).
2
4
On decreasing the reaction temperature to 75°C, the
conversion of phenyl acetate into the product was
decreased (Table 1, entry 17).
Under the optimized reaction conditions (Table 1,
entry 5), the coupling reaction of various phenolic esters
acid or triphenyltin chloride with S produced diphenyl
8
sulfide in quantitative yield detected by GC (Scheme 3II,
III). This result indicated that produced organothiolate
reacts with phenylboronic acid or triphenyltin chloride
to generate diphenyl sulfide. Additionally, in a control
experiment in the absence of naphthalen‐1‐yl acetate sub-
strates, thiophenol (2a) underwent dimerization to afford
(acetates, triflates, tosylates) with arylboronic acid was
studied (Table 2). The products were obtained in
moderate to high yields irrespective of the position of sub-
stituents on the phenolic esters (Table 2, entries 2–13).
The phenolic esters with electron‐releasing groups
reacted slower than those with electron‐withdrawing
groups (Table 2, entries 2–13). Likewise, arylboronic acids
containing electron‐donating groups showed less reactiv-
ity (entries 1–19). Interestingly, phenyl acetates which
are relatively unreactive substrates converted to the
corresponding unsymmetric sulfides in good to excellent
yields under the same reaction conditions (Table 2,
entries 1–19). The successful reaction of sterically
demanding substrates to give the desired products in
good yields is another important aspect of this method
(Table 2, entry 9). Para‐substituted phenolic esters
required shorter reaction times and gave relatively higher
product yields than ortho‐substituted phenolic esters
(Table 2, entries 7–9). The reaction of 2‐pyridine ester
(acetate, triflate, tosylates) as a heteroaromatic compound
with arylboronic acid gave products in excellent yields
Table 2, entries 18 and 19). Among phenolic esters, the
most reactive are aryl trifltes (Table 2, entries 1–19).
(
SCHEME 3 Control experiments

8
of 16
ATASHKAR ET AL.
disulfide (1c) in 100% yield (Scheme 3IV). The reaction of
naphthalen‐1‐yl acetate (1d) with thiophenol or diphenyl
is depicted in Scheme 4. We believe that the reaction
involves a heterogeneous process and the catalysis may
occur on the surface of the NiFe O MNPs. It is hypothe-
disulfide (instead of the phenylboronic acid/S system) in
8
2 4
t
the reaction gave the desired product (1e) in 39 and 0%
yield, respectively (Scheme 3VVI). All of these results
described above suggested that neither thiophenol nor
diphenyl disulfide was a possible intermediate in this
chemical process. On the basis of the above experimental
results, previous studies of the application of NiFe O as a
sized that NaO Bu reacts with S to produce sodium
8
[
16b]
disulfide.
Then, stable, NiS is formed from the reac-
tion of sodium disulfide with NiFe O MNPs. In the next
2
4
step, NiS reacts with arylboronic acid via oxidative addi-
tion reaction to give intermediate B which converts to
intermediate B, subsequently. Intermediate B reacts with
phenolic ester via C―O bond cleavage to provide the key
intermediate C. The desired product may be obtained
from reductive elimination of the key intermediate C.
The investigations described above prompted us to
explore the thioetherification of phenolic esters using
triphenyltin chloride as phenyl group source in the
presence of S and catalytic amount of NiFe O MNPs
2
4
[22]
source of Ni(II) in organic reactions
and reported
mechanisms for C―S bond formation reactions using
[23,24]
arylboronic acid/S8 as thiolating agent,
phenolic
ester as coupling partner in cross‐coupling reactions and
[25]
transition metal as catalyst,
the proposed mechanism
8
2 4
to synthesize phenylaryl sulfides. Inspired by previous
[
25]
results,
initially, we conducted the reaction of
p‐methoxyphenyl acetate with triphenyltin chloride in
the presence of K CO as base and S /KF in PEG200 at
2
3
8
60°C. Under these conditions, the desired product was
obtained in 79% yield (Table 3, entry 1). On increasing
the reaction temperature to 85°C, p‐methoxyphenyl ace-
tate was converted to the desired product completely
(
Table 3, entry 2). In order to study the effect of solvents,
we conducted the reaction in PEG200, DMF, dioxane and
CH CN (Table 3, entries 2–5). The most effective solvent
SCHEME 4 Proposed mechanism for C―S bond formation using
3
the reaction of arylboronic acid with phenolic esters in the presence
for this reaction was found to be PEG200 (Table 3, entry
2). Apart from NiFe O , CuFe O MNPs and CuNiFe O
2 4
8 2 4
of S catalysed by NiFe O
2
4
2
4
a
TABLE 3 Optimization of reaction conditions for coupling reaction of p‐methoxyphenyl acetate with triphenyltin chloride
Entry
Catalysts
NiFe
NiFe
Base
CO
CO
Solvent
GC yield (%)
b
1
2
O
4
K
2
3
PEG200
PEG200
DMF
79
100
88
71
60
79
c
2
2
O
4
K
2
3
3
4
5
6
7
NiFe
NiFe
NiFe
2
O
O
O
4
K
K
K
K
K
K
K
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
3
2
2
4
4
3
Dioxane
3
3
3
3
3
3
CH CN
CuFe
2
O
4
PEG200
PEG200
PEG200
PEG200
CuNiFe
2
O
4
90
d
—
8
NiFe
2
2
O
4
e
9
NiFe
O
4
52
a
Reaction conditions: p‐methoxyphenyl acetate (1 mmol), triphenyltin chloride (0.35 mmol), sulfur (1.5 mmol), catalyst (15 mol%), KF (3 mmol), base (3 mmol),
solvent (1.5 ml) and 85°C.
b
Reaction performed at 60°C.
c
Most effective reaction conditions.
d
Without KF.
e
3 8
Reaction performed using diphenyl disulfide instead of Ph SnCl/S for 48 h.

ATASHKAR ET AL.
9 of 16
a
TABLE 4 C―S bond formation of phenolic esters with Ph
3
8 2 4
SnCl acid using S catalysed by NiFe O MNPs
b
c
Entry
ArZ
Product
Time (h)
Yield (%)
1
Z = OAc
OTs
OTf
9
4
3
91
87
88
2
3
4
Z = OAc
OTs
OTf
14
9
7
87
80
81
Z = OAc
OTs
OTf
16
12
10
88
81
87
Z = OAc
OTs
OTf
14
9
7
94
78
84
5
Z = OAc
OTs
OTf
16
11
8
86
75
79
6
7
8
Z = OAc
OTs
OTf
4
2
1
90
85
86
Z = OAc
OTs
OTf
3
1
0.3
89
87
93
Z = OAc
OTs
OTf
12
8
5
88
80
82
9
Z = OAc
OTs
OTf
9
8
6
87
79
81
(Continues)

10 of 16
ATASHKAR ET AL.
TABLE 4 (Continued)
b
c
Entry
ArZ
Product
Time (h)
Yield (%)
1
0
Z = OAc
OTs
OTf
6
3
2
90
80
80
a
2 4 2 3
Standard conditions: phenolic ester (1 mmol), triphenyltin chloride (0.35 mmol), sulfur (1.5 mmol), NiFe O (15 mol%), KF (3 mmol), K CO (3 mmol),
PEG200 (1.5 ml), 85°C.
b
1
13
[25]
All products are known and were characterized by FT‐IR, H NMR, C NMR analyses.
c
Yield after column chromatographic separation.
MNPs were also active catalysts for the synthesis of
6–7) was subjected to reaction with triphenyltin chloride
in PEG200 as the solvent. The reactions proceeded well
to produce the corresponding phenylaryl sulfides in mod-
erate to excellent yields (Table 4). It was observed that
phenolic esters with electron‐donating groups show lower
activities and those having electron‐withdrawing groups
show greater activities (Table 4, entries 2–7).
desired product in high yields (Table 3, entries 6–7). In
the absence of KF, no product was observed (Table 3,
entry 8). Only 52% of diphenyl sulfide was obtained when
triphenyltin chloride/S8 was replaced with diphenyl
disulfide (Table 3, entry 9).
With optimal reaction conditions in hand (Table 3,
entry 2), a series of structurally diverse phenolic esters
bearing both electron‐donating groups (Table 4, entries
In another effort, we applied a catalytic amount of
NiFe O MNPs for the reaction of phenolic esters and S
8
2
4
1–5) and electron‐withdrawing groups (Table 4, entries
to produce symmetric diaryl sulfides. We optimized the
a
TABLE 5 Optimization of reaction conditions for conversion of phenyl acetate to diphenyl sulfide using S
8
a
Entry
Catalyst
Solvent
Temp. (°C)
GC yield(%)
GC yield(%)
1
2
3
4
5
NiFe
NiFe
NiFe
NiFe
NiFe
2
2
2
2
2
O
O
O
O
O
4
4
4
4
4
PEG200
PEG200
DMF
80
—
100
80
49
12
—
—
—
—
—
—
—
—
—
9
100
100
120
120
120
12o
120
120
120
120
DMF
43
85
100
60
100
86
80
75
80
DMF (dry)
DMF (dry)
b
6
2 4
NiFe O
b
7
NiFe
NiFe
CuFe
2
O
4
3
CH CN(dry)
b,c
8
2
O
4
Solvent‐free
DMF (dry)
DMF (dry)
DMF (dry)
DMF (dry)
b
9
2 4
O
b
1
0
2 4
CuNiFe O
b,d
1
2
NiFe
NiFe
2
2
O
4
b,e
1
3
O
4
120
a
t
Reaction conditions: phenyl acetate (1 mmol), sulfur (0.5 mmol), NiFe
2
O
4
(5 mol%), NaO Bu (4 mmol) and solvent (2 ml).
b
Reaction performed under an inert atmosphere.
NBr (1 mmol) for 13 h.
c
Reaction carried out in the presence of nBu
4
d
t
NaO Bu (3 mmol).
(2.5 mol%) was used.
e
2 4
NiFe O

ATASHKAR ET AL.
11 of 16
a
TABLE 6 Reaction of various phenolic ester compounds with S
8
for the synthesis of symmetric diaryl sulfides
b
c
Entry
ArZ
Product
Time (h)
Yield (%)
1
Z = OAc
OTs
OTf
11
6
3
88
82
85
2
3
4
Z = OAc
OTs
OTf
17
9
7
88
78
81
Z = OAc
OTs
OTf
19
13
10
87
79
85
Z = OAc
OTs
OTf
19
13
10
84
74
80
5
Z = OAc
OTs
OTf
16
9
6
90
77
82
6
7
8
Z = OAc
OTs
OTf
7
4
2
90
81
87
Z = OAc
OTs
OTf
6
2
0.5
88
80
87
Z = OAc
OTs
OTf
9
4
3
90
71
84
9
Z = OAc
OTs
OTf
15
9
7
90
78
84
(Continues)

12 of 16
ATASHKAR ET AL.
TABLE 6 (Continued)
b
c
Entry
ArZ
Product
Time (h)
Yield (%)
1
0
Z = OAc
OTs
OTf
13
10
9
90
81
87
a
t
2 4
Reaction conditions: phenolic ester (1 mmol), NiFe O MNPs (5 mol %), sulfur (0.5 mmol), NaO Bu (4 mmol), dry DMF (2 ml), 120°C.
b
1
13
[25]
All products are known and were characterized by FT‐IR, H NMR, C NMR analyses.
c
Yield after column chromatographic separation.
reaction conditions by employing phenyl acetate as
starting material (Table 5). Inspired by successful results
described above, when we performed the reaction in the
the ability to easily recycle the catalyst is highly desirable.
Thus, the recovery and recyclability of NiFe O MNPs
were studied in the reaction of 4‐bromophenyl acetate
2
4
t
presence of NiFe O , S and NaO Bu in PEG200 at
with phenylboronic acid or triphenyltin chloride in the
2
4
8
8
0°C, the reaction failed (Table 5, entry 1). On increasing
presence of S under optimized conditions. After the first
8
the reaction temperature to 100°C, the complete conver-
sion of phenyl acetate to phenol was observed (Table 5,
entry 2). Using DMF as a solvent afforded the desired
product in a yield of 9% as detected by GC (Table 5, entry
run, the catalyst was separated easily with the application
of an external magnetic field, washed with diethyl ether,
dried under vacuum and subjected to the next run. The
results show that the catalytic activity was preserved
without any significant changes in the yield and the reac-
tion time up to five runs (Figure 1).
3
). When increasing the temperature to 120°C, the GC
yield dropped to 43% (Table 5, entry 4). Using dry DMF
and under inert atmosphere, the full conversion of phenyl
acetate to the desired product was achieved (Table 5,
entries 5 and 6). In order to study the effect of solvent,
Two main reasons for the loss in activity of the catalyst
after six consecutive runs are (1) agglomeration of NiFe O
2
4
nanoparticles in each exposure (this agglomeration is fur-
ther increasing upon extended recycling) and (2) poisoning
of the catalyst by sulfur (the sulfur atoms with lone pairs
we conducted the model reaction in CH CN and
3
solvent‐free conditions (Table 5, entries 7 and 8). In addi-
tion to DMF, solvent‐free conditions in the presence of
phase transfer agent were also suitable for this reaction
(
Table 5, entry 8). CuFe O MNPs and CuNiFe O MNPs
2 4 2 4
were also effective catalysts for the model reaction (Table
5
, entries 10 and 11).
Under the optimized reaction conditions (Table 5,
entry 6), the homocoupling reactions of structurally
diverse phenolic esters in the presence of S were studied,
8
and the corresponding symmetric diaryl sulfides were
obtained in good to high yields (Table 6). Phenolic esters
with electron‐donating groups showed less reactivity
(Table 6, entries 2–7). The reaction of o‐methylphenolic
esters as models for hindered phenolic compounds with
S8 resulted in the desired products in good yields
(Table 6, entry 4).
The reaction of 2‐pyridine esters as heteroaromatic
compounds with S gave the corresponding products in
8
good to high yields (Table 6, entry 8).
At this time the exact mechanism is not clear and the
actual role of NiFe O MNPs in this reaction should be
further studied in detail. For many catalytic processes,
2 4
FIGURE 1 Recyclability of NiFe O MNPs in the reaction of 4‐
bromophenyl acetate with (a) phenylboronic acid and (b)
triphenyltin chloride
2
4

ATASHKAR ET AL.
13 of 16
TABLE 7 Comparison of various methods with respect to required reaction conditions and applied catalyst type, sulfur sources/thiolating
agents and starting materials/coupling partners
Catalyst and
reaction
conditions
Sulfur
source/thiolating
agent
Starting
materials/coupling
partners
Entry
Disadvantages
Ref.
[28]
1
CuO NP(5 mol%),
KOH (1.5 mmol),
Thiophenol
ArX
X = I, Br, Cl
Foul smell of thiols, production
of toxic halogen and use of
toxic organic solvents
DMSO, N
2
, 80°C
[
29]
2
3
4
CuO@GO
Thiophenol
ArX
X = I, Br
Foul smell of thiols and
production of toxic halogen
(0.28 mol%),
TEA (2 mmol),
neat, 90°C
[
[
29]
29]
CuO@GO
Thiophenol
Thiourea
ArCl
Foul smell of thiols, production
of toxic halogen and use of
toxic organic solvents
(0.28 mol%),
Cs CO (2.0 mmol),
2
3
DMSO, 110°C
CuO@GO
ArX
X = I, Br, Cl
Production of toxic halogen
and use of toxic organic
solvents
(0.38 mol%),
Cs CO (2 mmol),
2
3
DMSO, 110°C
[
[
30]
31]
5
6
CuO NP (5 mol%),
Thiourea
ArX
X = I, Br
Production of toxic halogen
and use of toxic organic
solvents
Cs
DMSO, N
2
CO
3
(2 mmol),
, 110°C
2
CuFe MNPs
2
O
4
Thiophenol
ArX
X = I, Br, Cl
Foul smell of thiols, production
of toxic halogen and use
of toxic organic solvents
(
(
5 mol%), Cs
1 mmol),
2
CO
3
DMSO, N
2
, 100°C
[31]
7
CuFe MNPs
2
O
4
Diphenyl
disulfide
ArI
Production of toxic halogen
and use of toxic organic
solvents
(5 mol%), Cs
1 mmol),
2 3
CO
(
DMSO, N
00°C
2
,
1
[
[
[
32]
27]
27]
8
9
CuFe
2
O
4
MNPs
Thiourea
ArX
X = I, Br, Cl
Foul smell of thiols, production
of toxic halogen and use
of toxic organic solvents
(
(
10 mol%), K
3 mmol),
2
CO
3
DMF, 120°C
CuFe MNPs
2
O
4
PhB(OH)
2
/S
8
ArX
X = I, Br, Cl
Production of toxic halogen
(
(
25 mol%), KOH
6 mmol), PEG,
8
0°C
CuFe
17 mol%),
Cs CO
4 mmol),
1
0
2
O
4
MNPs
KF/S
8
ArX
X = I, Br, Cl
Production of toxic halogen
(
2
3
(
PEG, 120°C
[
[
8]
9]
1
1
1
2
Pd (10 mol%),
L, NaO Bu
n‐BuSH
ArOTf
ArOTf
Expensive and non‐recyclable
catalyst, foul smell of thiols
and use of toxic organic solvents
t
(1.4 eq.),
toluene, 80°C
Pd (dba)
Xantphos
0.08 mmol),
2
3
/
AlkylSH
Expensive and non‐recyclable
catalyst, foul smell of thiols
and use of toxic organic
solvents
(
t
NaO Bu
(Continues)

14 of 16
ATASHKAR ET AL.
TABLE 7 (Continued)
Catalyst and
reaction
Sulfur
source/thiolating
agent
Starting
materials/coupling
partners
Entry
conditions
0.5 mmol),
xylene, 140°C
Pd (OAc) /L
2 mol%),
NaOtBu
1.1 eq.),
DME, 110°C
Pd (dba) /Xantphos
2.5 mol%),
i‐Pr Net (2 eq.),
,2‐dioxane, reflux
Pd (OAc) /Xantphos
4.2 mol%),
DMF, 100°C
NiCl (10 mol%),
Disadvantages
Ref.
(
1
3
4
2
2
RSH
R = alkyl, aryl
ArOTf
Expensive and non‐recyclable
catalyst, foul smell of thiols
and use of toxic organic solvents
6b
(
(
2
1
1
2
3
Ar SH
Ar OTf
Expensive and non‐recyclable
catalyst, foul smell of thiols and
use of toxic organic solvents
6b
(
2
1
2
1
15
16
17
2
In(SAr )
3
Ar OTf
Expensive and non‐recyclable
catalyst and use of toxic
organic solvents
6h
6a
(
2
1
2
Ar SNa
Ar OTf
Use of toxic organic solvents
and non‐recyclable catalyst
L, Zn (1 eq.),
THF, 80°C
[18]
CuI (5 mol%),
N
2
H
4
C¼S
ArZ
Use of non‐recyclable
homogeneous catalyst
K
2
CO
AlkylX,
PEG200/H
0°C
3
(4 mmol),
Z = OAc, OTs, OTf
2
O,
8
[
[
25]
25]
1
1
8
9
CuI (15 mol%),
NaO Bu (4 mmol),
PEG200, 80°C
PhB(OH)
2
/S
8
ArZ
Use of non‐recyclable
homogeneous catalyst
t
Z = OAc, OTs, OTf
Cu(OAc)
CO
2
(15 mol%),
(3 mmol),
Ph
3
SnCl/S
8
ArZ
Use of non‐recyclable
homogeneous catalyst
K
2
3
Z = OAc, OTs, OTf
KF (3 mmol),
PEG200, 80°C
2
0
1
NiFe
NaO Bu (4 mmol),
PEG200, 90°C
2
O
4
(15 mol%),
PhB(OH)
2
/S
8
ArZ
Use of strong base
This work
This work
t
Z = OAc, OTs, OTf
2
NiFe
CO
2
O
4
(15 mol%),
(3 mmol),
Ph
3
SnCl/S
8
ArZ
Use of toxic thiolating agent
K
2
3
Z = OAc, OTs, OTf
KF (3 mmol),
PEG200, 85°C
often have the ability to adsorb to metal surfaces, thus
prohibiting access to catalyst sites).
, entries 11–19), toxic organic solvents (Table 7, entries 8,
11–15) and aryl halides as environmental pollutants
(Table 7, entries 1–10).
[27]
We compared the described methods in the present
work with the best of the well‐known methods from the
literature with respect to the required reaction conditions
and applied catalyst type, sulfur sources or thiolating
agents and starting materials or coupling partners for
the synthesis of diaryl sulfide (Table 7). Most of the
reported methodologies suffer from one or more disad-
vantages such as the use of foul‐smelling thiols (Table 7,
entries 1–3, 6, 8, 11–14), expensive ligands and catalysts
4 | CONCLUSIONS
In summary, NiFe O MNPs as a bimetallic and magnet-
2
4
ically reusable nanocatalyst was used for the first time in
the synthesis of diaryl sulfides using the reaction of
arylboronic acid/S , triphenyltin chloride/S and phenolic
8
8
(Table 7, entries 11–15), homogeneous catalysts (Table 7
esters/S8 as thiolating agent with phenolic esters as

ATASHKAR ET AL.
15 of 16
coupling partners. Structurally diverse symmetric and
unsymmetric sulfides were synthesized using simple pro-
cedures (easy work‐up) in good to high yields in green
solvent. The methods are superior to others due to their
being free from ligands, unpleasant‐smelling thiols, aryl
halides and Cu and Pd catalysts. Because of total removal
of the metal‐based catalyst from the reaction media, these
methods can be applied in pharmaceutical and other sen-
sitive synthetic procedures.
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