Laccase-TEMPO as an efficient catalyst system for metal- and halogen-free aerobic oxidation of thioethers to sulfoxides in aqueous media at ambient conditions

Article abstract of DOI:10.1016/j.catcom.2018.03.032

We present for the first time an eco-friendly procedure for the aerobic oxidation of thioethers to sulfoxides using laccase-TEMPO catalyst system. This catalyst system allows for simpler (easy work-up) and greener methodologies (room temperature, phosphate buffer) while keeping high reaction yields and selectivity. This work is superior to others due to free from any transition metal and halide co-catalysts.

Full text of DOI:10.1016/j.catcom.2018.03.032

Accepted Manuscript
Laccase-TEMPO as an efficient catalyst system for metal- and
halogen-free aerobic oxidation of thioethers to sulfoxides in
aqueous media at ambient conditions
Amin Rostami, Behnaz Mohammadi, Zahra Shokri, Shaghayegh
Saadati
PII:
DOI:
Reference:
S1566-7367(18)30124-9
doi:10.1016/j.catcom.2018.03.032
CATCOM 5373
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
20 October 2017
23 March 2018
28 March 2018
Please cite this article as: Amin Rostami, Behnaz Mohammadi, Zahra Shokri, Shaghayegh
Saadati , Laccase-TEMPO as an efficient catalyst system for metal- and halogen-free
aerobic oxidation of thioethers to sulfoxides in aqueous media at ambient conditions. The
address for the corresponding author was captured as affiliation for all authors. Please
check if appropriate. Catcom(2018), doi:10.1016/j.catcom.2018.03.032
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ACCEPTED MANUSCRIPT
Laccase-TEMPO as an efficient catalyst system for metal- and
halogen-free aerobic oxidation of thioethers to sulfoxides in
aqueous media at ambient conditions
Amin Rostami*a, Behnaz Mohammadia, Zahra Shokria, Shaghayegh Saadatib
aDepartment of Chemistry, Faculty of Science, University of Kurdistan, 66177-15175
Sanandaj, Iran. [Fax: +988716624004, Tel: +989183730910, E-mail: a.rostami@uok.com]
bDepartment of Medicinal Chemistry, School of Pharmacy, Shahid Beheshti University of
Medical Science, Tehran, Iran.
ABSTRACT
We present for the first time an eco-friendly procedure for the aerobic oxidation of thioethers
to sulfoxides using laccase-TEMPO catalyst system. This catalyst system allows for simpler
(easy work-up) and greener methodologies (room temperature, phosphate buffer) while
keeping high reaction yields and selectivity. This work is superior to others due to free from
any transition metal and halide co-catalysts.
Keywords: laccase-mediator, aerobic oxidation, thioethers, sulfoxides
1. Introduction
Green chemistry is a philosophy of chemical research that encourages the design of
green processes employing safe catalysts, minimizing the generation of hazardous
substances, replacing toxic solvents with H2O, and using O2 or air as environmentally
benign oxidant [1]. Green catalysis is one of the most important subchapters of green
chemistry. In this context, the employing of enzymes as safe and biodegradable
catalysts offer many benefits since biocatalytic reactions can be performed under mild
conditions [2]. Laccases, multi-copper-containing oxidoreductase enzymes [3], are
highly attractive biocatalysts in modern organic synthesis, they catalyze the oxidation
of various compounds such as benzenediols, aminophenols, polyphenols, polyamines,
and lignin-related molecules using oxygen as an electron acceptor and producing
water as by-product [4-7]. The most efficient strategy to extend the range of laccase
substrates is the simultaneous use of the enzyme and redox mediators [8-11].
The selective oxidation of thioethers into the corresponding sulfoxides is one of the
most important reactions in organic synthesis since sulfoxides are useful intermediates for

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the construction of important organic compounds. Also many biologically important
molecules have a sulfoxide functional group [12-13]. For these reasons, a large number of
catalysts (homogeneous and heterogeneous catalysts) and stoichiometric amounts of
oxidants have been reported for the synthesis of chiral and achiral sulfoxides [14-19].
Disadvantages of these methods include poor atom efficiency, utilization of environmentally
unfavorable reagents, solvents, and catalysts, and the formation of large amounts of toxic
waste.
There is substantial interest towards the use of molecular oxygen or air as the highly atom-
economical, environmentally benign, and abundant oxidant in the oxidation of thioethers to
the sulfoxides in the presence of catalysts [20-22]. Although these methods have been
developed considerably, these procedures suffer from some problems such as the preparation
of complex catalysts, removal or recovery of the expensive catalyst, and the use of toxic
organic solvents. In addition, they may possibly leave toxic traces of heavy metals in the
products. Therefore, the development of green and non-metallic catalysts such as organic
oxidizing agents [23-24] for the aerobic oxidation of sulfides is still desirable and is in
demand. Of particular interest in this area is the application of the stable nitroxyl radical
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in combination with various types of non-
metallic, stoichiometric co-oxidants such as m-chloroperbenzoic acid, sodium hypochlorite or
sodium periodate as an alternative to metal-based oxidants [26-27]. Accordingly, the use of
TEMPO-based catalyst systems in combination with transition metals as co-catalysts
has been well documented for the oxidation of thioethers to sulfoxides using molecular
oxygen as terminal oxidant. Typical examples include i) the use of MnII-CoII dual
metal catalysts and TEMPO as a co-catalyst under relative harsh conditions [28]
(Scheme 1, eqn (a)) ii) the use of TEMPO as the redox mediator under visible-light-
induced photoredox catalysis of dye-sensitized TiO2 (Scheme 1, eqn (b)) [29].
Although these TEMPO-based catalyst presented an interesting breakthrough in the
field of aerobic oxidation of thioethers, these methods still require transition metals as
co-catalysts. It is vital to eliminate the catalyst (especially in pharmaceutical industry)
because metal contamination is highly regulated. A potential approach to overcome
this problem is to change the transition metal catalysts to safe catalysts such as
biocatalysts for the aerobic oxidation of thioethers.

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Previous works
O
S
TEMPO , O2, Mn(NO3)2, Co(NO3)2
(a)
(b)
S
R1
R2
R1
R2
O
AcOH, 40 oC
S
S
ASR-TiO2, TEMPO
300 W Xe lamp
O2
HCHO +
+
+ CH3OH
+
H2O
R
R
This work
O
S
S
Laccase -TEMPO, air
R1
R2
(c)
NaPBS-Buffer [pH=5, THF (4%)], r.t
R1
R2
R1, R2 = Ar, benzylic, linear, cyclic
Scheme 1. Approaches to aerobic oxidation of thioethers
2. Experimental
The activity assays of commercial laccase from T. versicolor: The activity of laccase was
determined
spectrophotometrically
with
2,2’-azino-bis-(3-ethylbenzthiazoline-6-sulfonic
-1
acid) (ABTS, ε= 36 000 M-1.cm ) as substrate (5 mM, 100 µL) in acetate buffer (100
mM, pH=5.0). The absorbance change was observed at 420 nm for 5 min at room
temperature [30]. One unit of activity is defined as the amount of enzyme required to
oxidize 1 µmol of ABTS per minute. The activity of laccase enzyme batch applied in this
investigation was evaluated at 0.87 U/mg.
General Procedure for the aerobic oxidation of thioethers to sulfoxides catalyzed by
laccase-TEMPO system
Thioether (1 mmol) and TEMPO (62.5 mg, 0.4 mmol) were added to a solution of
laccase (17.4 mg, 20 U) in phosphate buffer (0.1 M, 5 mL, pH=5) and THF (4 vol%).
The reaction mixture was stirred under air at room temperature for an appropriate time
(see table 2). The progress was monitored by TLC (n-hexane/EtOAC, 4:1). After the
completion of the reaction, the product was extracted with EtOAc (3 × 10 mL) and dried
over anhydrous Na2SO4. Evaporation of the solvent under reduced pressure and purification
by column chromatography (n-hexane/EtOAc) gave the desired sulfoxide. All products were
known and were identified by comparison their spectra and physical data with literature
values (see supplementary material)

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3. Results and discussions
In continuation of our research interest in the development of green and sustainable methods
[31-33], herein, we present for the first time a simple and efficient method for the aerobic
oxidation of a wide range of thioethers to the corresponding sulfoxides in the presence of
laccase-TEMPO catalyst system in an aqueous media at ambient temperature (Scheme 1, eqn
(c)). We started with the oxidation of the methyl phenyl thioether as a model reaction under
air in phosphate buffer (Table 1). No product formation was observed when methyl phenyl
thioether was reacted with 20 U of laccase under specified conditions (Table 1, entry 1). This
result confirms that the laccase-catalyzed oxidation of the model compound require a
mediator. It is also notable that methyl phenyl sulfoxide was obtained in low yield in the
presence of the bare TEMPO (Table 1, entry 2). Therefore, in order to improve the yields of
the laccase-catalyzed oxidation, the reaction requires the cooperation between laccase and
TEMPO. Gentili and Co-worker tested laccase-TEMPO (10U/6 mM) catalyst system on
oxidation of methyl phenyl thioether (20 mM) at room temperature for 24h. Under these
conditions, the reaction failed.This study was performed to mechanistic survey of the
oxidation of alcohols and etherswith Laccaseas catalyst and TEMPO as mediator [34]. The
result indicated that the used amounts of the Laccase and specially TEMPO are important.
The effect of different amounts of TEMPO and laccase was investigated on yield and time
(Table 1, entries 3-6). The effect of air auto-oxidation was also studied for 48 h. This
conditions did not result in the desired product, leaving methyl phenyl sulfide intact (Table 1,
entry 7). In term of the amount of laccase and TEMPO, better results were obtained when the
laccase-catalyzed oxidation of the methyl phenyl thioether was done with 20 U of laccase and
0.4 mmol of TEMPO in phosphate buffer (0.1 M, 5 mL, pH=5) and THF (4 vol %).

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Table 1. Optimization of the laccase-TEMPO-catalyzed oxidation of methyl phenyl
thioether (1 mmol)a
O
S
S
Laccase-TEMPO , air
NaPBS-Buffer [0.1 M, 5 mL, pH=5, THF (4%)], r.t
GC Yield
(%)
Entry
Laccase (U)
TEMPO (mmol)
Time (h)
1
2
3
4
5
6
7
20
0
0
24
48
48
48
24
48
48
0
0.4
0.4
0.3
0.5
0.3
0
50
100
60
100
60
0
20
20
20
40
0
aAll reactions were done under aerial O2 in phosphate buffer (0.1 M, 5 mL, pH=5) and
THF (4 vol%).
Under the optimized conditions (Table 1, entry 3), the oxidation of a series of structurally
diverse thioethers was examined (Table 2). The summarized results in Table 2 depict that a
variety of thioethers including aryl alkyl (Table 2, entry 1), benzyl alkyl (Table 2, entries 2-
7) and dialkyl (Table 2, entries 8-12) thioethers were successfully used to synthesize the
corresponding sulfoxides. We were pleased to find that, the sulfur group was oxidized
chemoselectively in the presence of C=C as an oxidation-prone functional group (Table 2,
entry 13) and ester group in the substrate remain intact under reaction conditions (Table 2,
entry 14). Noteworthy to mention is that because of the mild conditions of the method,
sulfoxides were obtained in high yields and any over-oxidation to sulfones during the course
of the reaction was not observed.
To demonstrate the greenness, Sheldon´s E-value (kg waste per kg product) was calculated
for the oxidation of the methyl phenyl thioether, the amount of 2.27 kg kg−1 was obtained.
This value compares well with the E-factors of other synthetic methods for the synthesis of
methyl phenyl sulfoxide (Table 3) [14-16]. The TON and TOF values of the procces confirm
the catalytic efficiency of the method (Table 2). The calculation of the amounts of E-factors,
TONs and TOFs have been provided in the supplementary material.

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Table 2. The selective aerobic oxidation of thioethers to sulfoxides by using Laccase-TEMPO catalyst system in
PBS (0.1 M, 5 mL, pH=5) and 4 vol % THF at room temperaturea
Entry
Thioether
Sulfoxideb
Time (h) Isolated yield (%)
TON[TOF (h-1)]b
1
48
27
27
96
90
94
5352 (111)
2
3
5017(186)
5240(194)
4
5
6
24
25
20
90
92
93
5017(209)
5129(205)
5185(259)
7
27
18
10
10
12
15
48
18
95
90
82
90
87
93
87
84
5296(196)
5017(279)
4571(457)
5017(502)
4850(404)
5185(346)
4850(101)
4683(260)
8
9
10
11
12
13
14
S
S
O
aReaction conditions: sulfide (1 mmol), laccase (20 U), TEMPO (0.4 mmol), phosphate buffer (0.1 M, 5 mL,
pH=5) and THF (4 vol %), r.t.
bTurnover number = number of moles of product per mol of catalyst precursor; TOF = TON per hour (values in
brackets)

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Table 3. Comparison of E-factor of Laccase-TEMPO catalyst system with some of the reported catalysts for
the oxidation of the methyl phenyl thioether
E-factor
Entry
1
Substrate
Catalyst
Oxidant
H2O2
Yield (%)
98
Ref.
[14]
(kg.kg-1)
6.06
Merrifield resin supported
peroxomolybdenum (VI)
Ionic liquid compounds based
on Kepleratepolyoxometalates
2
3
4
H2O2
99
83
96
22.12
82.89
2.27
[15]
[16]
Fe3O4/salen of Cu(II)
Laccase-TEMPO
H2O2
This
work
aerobic oxidation
As shown in Table 2, in the case of methyl phenyl thioethers longer reaction time was
required. Therefore, we used this procedure for the selective aerobic oxidation of
benzyl propyl thioether (as model for benzyl alkyl thioethers) in the presence of
methyl phenyl thioether (as model for phenyl alkyl thioether). The result shows that
the method has excellent selectivity and benzyl propyl thioether was completely
converted to the corresponding sulfoxide, while trace conversion for methyl phenyl
thioether was detected (Scheme 2a). Similarly, the excellent selectivity was observed
for the oxidation of methyl phenyl thioether in the presence of diphenyl thioether
(Scheme 2b).
Scheme 2. Selective reactions of different binary mixtures

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Although, it is well-known that TEMPO-oxoammonium ion is generated from the reaction of
Laccase and TEMPO in the presence of O2 or air and it is actual oxidant in the reaction [35],
however, at this time the exact mechanism of this reaction is not clear and the actual role of
TEMPO-oxoammonium in the oxidation of sulfide to sulfoxide should be further studied in
detail.
4. Conclusion
In conclusion, a simple, practical, highly efficient and sustainable protocol for the
aerobic oxidation of thioethers using laccase-TEMPO catalyst system under toxic
transition-metal free reaction conditions has been developed. We report the first
example of biocatalytic aerobic oxidation of thioethers that can replace traditional
oxidation processes. The merits of the current catalytic system include: (a) the use of
laccase as green biocatalyst in due to its ecological advantages (b) the use of air as
greenest, most abundant and inexpensive oxidant and producing water as an only by-
(c) mild reaction conditions including atmospheric pressure, room temperature,
phosphate buffer as the green solvent (d) efficient for the selective oxidation of
various thioethers in high yields (e) other functional groups are tolerable and
compatible with the present oxidative system. This method is easier and more eco-
friendly and practical than the available protocols for the synthesis of sulfoxide and
can be applied in pharmaceutical and other sensitive synthetic procedures.
Acknowledgments
We thank University of Kurdistan and Iran National Science Foundation (INSF, Grant
Number: 95844125) for financial support to our research groups.
Appendix A. Supplementary data
Supplementary dat to this article can be found online at......
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Graphical Abstract
Laccase-TEMPO as an efficient catalyst system for metal- and halogen-free aerobic oxidation
of thioethers to sulfoxides in aqueous media at ambient conditions
Amin Rostami, Behnaz Mohammadi, Zahra Shokri

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Highlights
1. The first example of a Laccase-TEMPOcatalyzed aerobic oxidation of sulfides was
reported.
2. The use of air as eco-friendly, most abundant and inexpensive oxidant.
3. The reactions are occur in phosphate buffer as the green solvent at room temperature.
4. This system allows for greener methodology while keeping high reaction yields and
selectivity.
5. Free from metal and halide make this method more eco-friendlythan the available
protocols