[MoO3(2,2″–bipy)]n catalyzed oxidation of amines and sulfides

Article abstract of DOI:10.1016/j.catcom.2017.09.022

The polymeric catalyst [MoO₃(2,2′–bipy)]_n has been employed in the oxidation of secondary amines to nitrones, and sulfides to sulfoxides or sulfones, using tert–butylhydroperoxide (TBHP) as the stoichiometric oxidant. Whereas the oxi

Full text of DOI:10.1016/j.catcom.2017.09.022

Accepted Manuscript
[MoO3(2,2′–bipy)]n catalyzed oxidation of amines and sulfides
Irene Tosi, Carolina Vurchio, Marta Abrantes, Isabel S.
Gonçalves, Martyn Pillinger, Fabrizio Cavani, Franca M.
Cordero, Alberto Brandi
PII:
DOI:
Reference:
S1566-7367(17)30404-1
doi:10.1016/j.catcom.2017.09.022
CATCOM 5204
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Catalysis Communications
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Accepted date:
8 July 2017
23 September 2017
25 September 2017
Please cite this article as: Irene Tosi, Carolina Vurchio, Marta Abrantes, Isabel S.
Gonçalves, Martyn Pillinger, Fabrizio Cavani, Franca M. Cordero, Alberto Brandi ,
[MoO3(2,2′–bipy)]n catalyzed oxidation of amines and sulfides. The address for the
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ACCEPTED MANUSCRIPT
[MoO3(2,2'–bipy)]n catalyzed oxidation of amines and sulfides
Irene Tosia,b, Carolina Vurchioa, Marta Abrantesc, Isabel S. Gonçalvesd, Martyn
Pillingerd, Fabrizio Cavanib,*, Franca M. Corderoa,*, Alberto Brandia
a Department of Chemistry "Ugo Schiff", University of Florence, via della Lastruccia 13, 50019
Sesto Fiorentino (FI), Italy, and Consorzio C.I.N.M.P.I.S.
b Department of Industrial Chemistry “Toso Montanari”, University of Bologna, viale Risorgimento
4, 40136, Bologna (BO), Italy
c Centro de Química Estrutural, Complexo Interdisciplinar, Instituto Superior Técnico,
Universidade de Lisboa, 1049–001 Lisbon, Portugal
d Department of Chemistry, CICECO – Aveiro Institute of Materials, Campus Universitário de
Santiago, University of Aveiro, 3810–193 Aveiro, Portugal
* Corresponding authors.
E-mail addresses: franca.cordero@unifi.it (F.M. Cordero); fabrizio.cavani@unibo.it (F. Cavani).

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ABSTRACT
The polymeric catalyst [MoO3(2,2'–bipy)]n has been employed in the oxidation of secondary amines
to nitrones, and sulfides to sulfoxides or sulfones, using tert–butylhydroperoxide (TBHP) as the
stoichiometric oxidant. Whereas the oxidation of amines occurs with less proficiency with respect
to other previously studied polymeric Mo catalysts, the oxidation of sulfides occurs in high yield
and selectivity. The catalyst is flexible in terms of substrate scope (aromatic, aliphatic and cyclic
sulfides, thioheterocycles such as dimethyldibenzothiophene, the oxidation of which forms the basis
of oxidation/separation processes for the desulfurization of fuels) and the ability to switch
sulfoxide/sulfone product selectivities by adjusting the reaction conditions.
Keywords: oxidation; molybdenum; hybrid materials; oxidative desulfurization; sulfides; amines

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1. Introduction
The chemistry of Mo(VI) assumes a key role in organic synthesis and in industrial processes,
where Mo(VI) catalysts find valuable applications in a large variety of different reactions like
oxidation, hydrocarbon dehydrogenation, ammoxidation, and metathesis [1-2].
Complexes containing the cis-dioxomolybdenum(VI) structural unit represent the vast majority
of the complexes studied as catalysts or catalyst precursors [3-5]. Among the potential applications,
catalytic oxidative desulfurization (ODS) of liquid fuels is one of the most important, since it is
seen as a viable alternative to conventional hydrodesulfurization (HDS) for the near-complete
removal of sulfur-containing compounds from oils [6,7]. ODS is based on the oxidation of the
organosulfur compounds to the corresponding sulfoxides and/or sulfones which can then be
removed by adsorption, distillation, or solvent extraction. The oxidation and removal of
benzothiophene derivatives from a model oil by using the complex [MoO2Cl2(4,4'-di-tert-butyl-2,2'-
bipyridine)] as a (pre)catalyst was recently reported [8].
One of the main goals in catalytic sulfide oxidation is to obtain both sulfoxide and sulfone with
high selectivity by using the same system. Although many methods are known for the selective
oxidation of sulfides [7,9-12] only a few studies are available for obtaining sulfoxides and sulfones
with the same system, under adjusted reaction conditions [13-18]. Selectivity is very high, mainly
for sulfoxides that are usually obtained in high yields [14]. Reaction conditions are mild for the
formation of both sulfoxides and sulfones, in contrast to other methods that need a higher amount of
oxidant to obtain high yields of sulfones [16]. Molybdenum complexes have already been studied as
catalysts for the oxidation of sulfides. Molybdenum oxo–peroxo complexes have shown good
results for the formation of sulfoxides [19]. Molybdenum carbonyls [18] and oxides [15] have been
able to catalyze both selective oxidations, to sulfoxides and to sulfones, by changing the reaction
conditions.
In the course of the exploration of metal oxide-based hybrid materials for their potential

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properties in the fields of catalysis, sorption, electrical conductivity, magnetism, and optics, some of
us have been employing soft inorganic chemistry processes to obtain molybdenum(VI)
oxide/organic hybrid materials [20,21]. One of these compounds, a molybdenum oxide/2,2'-
bipyridine hybrid material with the formula {[MoO3(2,2'-bipy)][MoO3(H2O)]}n, was tested in the
oxidation of secondary amines to nitrones using either urea hydrogen peroxide (UHP) adduct or
TBHP as the oxidant, and proved to be a good water-tolerant catalyst for the oxidation of secondary
amines under mild conditions [22].
Continuing the study of molybdenum(VI) oxide inorganic-organic hybrid materials in oxidation
reactions we now focus on a new catalyst, polymeric [MoO3(2,2'-bipy)]n (1) (Fig. 1) [20,23-25], for
its stability, synthetic availability, and recycling possibilities. Structurally, 1 can be considered as a
polymeric version of [MoO2X2(L)] complexes such as the dichloro complex mentioned above.
Fig. 1. Schematic representation of the structure of 1.
The catalytic potential of 1 for oxidation reactions was apparent after obtaining interesting
results for the epoxidation of olefins using TBHP as oxidant [20,24]. In the present work, the
oxidation power of this catalyst was tested for the oxidation of secondary amines to nitrones to
compare its reactivity with that of other catalysts [22]. The study was then extended to the oxidation
of sulfides to sulfoxides and sulfones. All these oxidation products, by-the-way, are versatile
synthons in organic chemistry [26,27].
Mo(VI) catalysts have only rarely been examined in parallel as catalysts for the oxidation of
sulfides and amines [28]. Although much less studied, the performance of ODS catalysts in the

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oxidation of nitrogen-containing compounds (or in sulfoxidation in the presence of nitrogen-
containing compounds) is important since the removal of such compounds from oils is highly
desirable, and there is interest in the development of processes for simultaneous desulfurization and
denitrogenation [29].
2. Experimental
2.1. Catalyst synthesis
[MoO3(2,2'-bipy)]n (1) was prepared as described previously by the oxidative decarbonylation
of the tetracarbonyl complex cis-[Mo(CO)4(2,2'-bipy)] with TBHP under mild conditions [20].
2.2. General procedure for amine oxidation
To a 0.5 M solution of amine in anhydrous CH2Cl2 (1 mL) was added the catalyst (5 mol%)
and TBHP (5.5 M solution in decane, 3 mol equiv.). The reaction mixture was stirred at 40 °C for
63–118 h and then dried over anhydrous Na2SO4, and filtered. The solution was concentrated under
reduced pressure and the crude residue purified by flash column chromatography (silica gel).
2.3. General procedure for sulfide oxidation
To a 0.5 M solution of sulfide in CH2Cl2 was added TBHP (5.5 M solution in decane, 1.5 or 3
mol equiv.) and the catalyst (3 mol%). The suspension was stirred at 20 °C or 40 °C for 1–20 h and
then filtered through a short pad of Celite®. The solution was concentrated under reduced pressure
and the crude residue purified by flash column chromatography (silica gel). Spectroscopic data of
isolated compounds are provided in the Supplementary Data.
3. Results and discussion
3.1. Oxidation of amines

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Dibenzylamine 2 was used as a model substrate for preliminary screening and optimization of
the reaction conditions for the oxidation of secondary amines to nitrones catalyzed by polymeric
[MoO3(2,2’–bipy)]n (1) (Table 1). Reactions were initially run with dibenzylamine (0.5 mmol),
TBHP (1.5 equiv.), and catalyst 1 in CDCl3 at 40 ºC to check the reactivity of the catalyst by NMR
in forming nitrone 3 (Fig. 2).
Fig. 2. Oxidation of dibenzylamine with different amounts of catalyst 1.
The catalyst amount affects intensely the reaction rate and the best choice (5 mol%) was a
compromise between the very long reaction times required when using 2 mol% of catalyst and the
somewhat shorter times, but less practical in terms of costs, required when 10 mol% of catalyst was
employed.
Reactions were then repeated at different temperatures with a view to testing the stability of
1
nitrones under the reaction conditions. Conversions and yields were calculated from the H NMR
spectra with 1,4–dioxane as internal standard, and measured by recovery of the amine and the
nitrone after purification (Table 1). A slight increase of the temperature from 40 to 55 °C provides a
higher conversion and much shorter reaction time (25 vs 95 h); however, as expected, the yield of

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the nitrone, which is temperature sensitive, is reduced. As already observed, the change of the
reaction solvent from CDCl3 to CH2Cl2 does not affect the results [22]. UHP was also tested as
oxidant under the same reaction conditions except for the solvent because UHP requires polar
solvents, such as MeOH. UHP is able to increase the reaction rate (23 hours), but the nitrone is less
stable in a protic solvent and a higher amount of benzaldehyde and other decomposition products,
such as benzyloxime, were present in the final reaction mixture. Accordingly, the use of TBHP in
CH2Cl2 was preferred because the reaction can be more readily carried out under anhydrous
conditions.
Table 1 Oxidation of dibenzylamine 2 to C-phenyl,N-benzyl nitrone 3a
T (°C) Oxidant
Solvent Time (h) Conv. (%) Yield 3 (%)
40
40
55
55
40
TBHP
TBHP
TBHP
TBHP
UHP
CDCl3
CH2Cl2
CDCl3
CDCl3
MeOH
95
94
25
24
23
90b
93c
89b
75c
67b
54c
26c
>99b
>99c
96
aReaction conditions: dibenzylamine (0.5 mmol), TBHP (3 mol equiv.), catalyst 1 (5 mol %).
bConversions and yields calculated by using internal standard method (1,4-dioxane as internal
standard).
cConversions and yields based on the recovered amine and nitrone after purification.
At the end of the reactions, the catalyst can be recovered unchanged by centrifugation of the
reaction mixture and subsequent washing. The recovered catalyst shows the same activity in a
second run (conversion of substrate 90%, yield to 3 87%, for the reaction carried out at 40°C and 90
h reaction time, with TBHP oxidant).

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The optimized conditions were then applied to different substrates to verify the scope of the
reaction (see Supplementary Data, Table S1, Scheme S1 and S2). The study confirmed the
moderate reactivity of the catalyst. A test of oxidation of 1,2,3,4–tetrahydroisoquinoline conducted
in the presence of the reactive dipolarophile N–Ph maleimide showed the formation of the amine
Michael addition product besides minor amounts of the cycloaddition product. Indeed, the Michael
addition is preferred over the cycloaddition of 3,4–dihydroisoquinoline N–oxide, because of the
very slow formation of the nitrone (see Supplementary Data).
A plausible oxidation mechanism could involve the coordination of the stoichiometric oxidant
by the metal without changing the metal oxidation number. In analogy with the hypothesized
mechanism [30-32] for catalytic alkene epoxidation, the oxygen is transferred only from the
hydroperoxide and not from the catalyst that is found unchanged at the end of the reaction. The
proposed mechanism involves a rate determining nucleophilic attack of the amine to an oxygen of
the coordinated hydroperoxide giving the corresponding hydroxylamine. A further faster oxidation
followed by water elimination results in the nitrone.
3.2. Oxidation of sulfides
The catalyst [MoO3(2,2'–bipy)]n showed a higher selectivity in the oxidation of sulfides than
other molybdenum compounds previously studied. It seems to have a similar reactivity to MoO2Cl2:
this compound, previously studied for the oxidation of sulfides to sulfoxides or sulfones [15],
showed high efficacy and selectivity without affecting other functional groups present in the
molecule. The extension of the study to [MoO3(2,2'–bipy)]n can bring several advantages because of
its stability, availability and possibilities of recycling.
Thioanisole 4 was chosen as a model substrate to check the best reaction conditions. At the
beginning the same conditions as used for the oxidation of amines were used. The [MoO3(2,2'–
bipy)]n catalyst showed much higher efficiency in the oxidation of sulfides than amines. The
sulfoxide 5 was recovered after an easy filtration of the reaction mixture in good yield (Table 2).

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Using 5 mol% of catalyst 1 and 3 mol equiv. of TBHP at room temperature, the thioanisole
consumption was complete after just two hours (Table 2, entry 1). The catalyst in these conditions
showed high selectivity: although an excess of oxidant (3 mol equiv. of TBHP) was used, the ratio
between sulfoxide 5 and sulfone 6, the product of over–oxidation, was 22:1 in the final crude
1
mixture, calculated from H–NMR spectra (see spectroscopic data for sulfides and sulfones in the
Supplementary Data). Using a stoichiometric amount of oxidant (1 mol equiv. of TBHP) (Table 2,
entry 2), the reaction was slower: the thioanisole consumption was complete after 24 hours, the
ratio between 5 and 6 was slightly higher (26:1), and the isolated yield of sulfoxide 5 high (88%).
Table 2 Oxidation of thioanisole 4 to sulfoxide 5 and sulfone 6 with TBHP and [MoO3(2,2'-bipy)]n
catalyst.
Entry
TBHP
Cat. 1
T
Time
1H-NMR
Ratio
Isolated
(mol eq.)
(mol%)
(°C) (h)
Yield (%)
5/6
5
6
-
1
2
3
4
5
6
3
5
5
3
1
3
5
20
20
20
20
20
40
2
22:1
26:1
81:1
39:1
70:1
-
-
1
24
2
88
-
3
97 traces
94 traces
93 traces
3
4
1.5
3
3
16
-
97
The impact of catalyst amount was also evaluated. Use of 3 mol% of 1 with 3 equiv. of oxidant
affords 5 in excellent yield and selectivity after 2 hours (Table 2, entry 3). When the catalyst
amount was reduced to 1 mol% a lower selectivity is observed and a longer reaction is required for
the complete conversion of the reagent (Table 2, entry 4). Finally the best conditions from a
practical point of view turned out to be the use of 3 mol% of 1 with 1.5 mol equiv. of TBHP at 20

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°C since this combines lower amounts of catalyst and oxidant with efficiency and short reaction
times (Table 2, entry 5). It should be stressed here the role of the catalyst. The same reaction of
Table 2, entry 5 run without the presence of catalyst 1 leads after 16 h to only a 37% conversion of
sulphide 4 to sulfoxide 5.
Interestingly, by heating 4 in the presence of 3 mol equiv. of TBHP and 5 mol% of catalyst at
40 °C for 16 hours only sulfone 6 was obtained and isolated in excellent yield with opposite
selectivity of that observed at room temperature (Table 2, entries 6 and 1).
The fast sulfide oxidation to sulfoxide and the slower oxidation to sulfone, which is the key for
the observed selectivity, can be appreciated from Fig. 3. These results open new interesting paths
for selective oxidation of sulfides, because catalyst 1 shows high efficiency and selectivity, and
prompted us to study the scope of the oxidation by polymeric catalyst 1.
Fig. 3. Kinetics of thioanisole (4) oxidation. Reaction conditions: 1 (3 mol%), TBHP (3 mol
equiv.), CH2Cl2 or CDCl3, 40 °C.
Allylmethylsulfide 7 was selectively oxidized to the sulfoxide 8 under the common reaction
conditions (Scheme 1). Only traces of sulfone 9 were observed in the crude mixture. The lack of
reactivity of the allylic double bond with this reagent is noteworthy.

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Oxidation of dithiane 10 turned out to be strongly affected by the amount of the oxidant TBHP
used (Table 3). Using an almost stoichiometric amount of TBHP and 3 mol% of the catalyst,
monosulfoxide 11 was selectively obtained in good yield (86%) after 4 hours (Table 3, entry 1).
When pure 11 was treated with 1.5 mol equiv. of TBHP in the presence of the catalyst, 1,3–dioxides
12 and 1,1,3–trioxide 13 were obtained in a 1.6:1 ratio and quantitative overall yield (Table 3, entry
2). Oxidation of 10 with 3 mol equiv. of TBHP afforded a higher 12/13 ratio (3.3:1) (Table 3, entry
3). The thermodynamically more stable disulfoxide trans–12 [33] was obtained as the major
product (45–46% yield) in both the last two experiments. The combined yields of cis–12 and 13
were 52–54% in entries 2–3 (Table 3) suggesting that trioxide 13 derives almost exclusively from
cis–12 under the reported reaction conditions.
Scheme 1. Oxidation of allylmethylsulfide.
Table 3 Oxidation of dithiane 10a
Yield (%)b
Entry SMc TBHP (mol equiv) Time (h) 11 trans-12 cis-12 13
1
2
3
10
11
10
1.1
1.5
3
4
14
17
86 traces
traces
16
-
38
22
-
-
46
45
30
areaction conditions: [MoO3(2,2'-bipy)]n (3 mol%), TBHP (5.5 M in decane), CH2Cl2, 20 °C.
bYield of isolated compounds
cSM = starting material.
A competition experiment on the ability of catalyst 1 to oxidize sulfides in the presence of
amines was carried out. A 1:1 mixture of thioanisole 4 and 2–methylpiperidine was treated with 3

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mol% of the catalyst 1 and 1.5 mol equiv. of TBHP at 20 °C in CH2Cl2. After 3 hours reaction, a
1
mixture of thioanisole, its sulfoxide, and piperidine in a 1:0.7:0.8 ratio (calculated by H NMR) was
obtained along with traces of the nitrone. Accordingly, complex 1 is able to catalyze a selective
oxidation of the sulfide in the presence of the amine. However, the oxidation rate of sulfide was
significantly slowed down (see Table 2, entry 5). During the reaction a solubilization of the catalyst
was observed [34], suggesting a coordinative interaction of the amine with Mo that causes a
reduction of the oxidative power of the Mo complex. This result might also explain the sluggishness
of the catalyzed oxidation of secondary amines (see Table S1, Supplementary Data).
2–Methylbenzothiazole, which contains nitrogen and sulfur in an aromatic ring, was inert to
oxidation under the standard reaction conditions.
Another thio–heterocycle, 4,6–dimethyldibenzo[b,d]thiophene (DMDBT, 14), was then
considered due to its importance in the petrochemical industry and diesel fuel production. In fact,
the removal of this compound from diesel fuel is a practice to improve the quality of the fuel,
mainly regarding pollution by exhaust fumes, and oxidative desulfurization is one of the foremost
possibilities of removing such compounds from liquid fuels by oxidation/extraction [6,35].
Under treatment of DMDBT with 1.5 mol equiv. of TBHP in the presence of 3 mol% of 1 at
room temperature, the conversion after 24 h is only moderate (40%), but only the sulfoxide 15 can
be observed. Increasing the temperature to 40 °C boosts the conversion to over 80%, but a mixture
of sulfoxide 15 and sulfone 16 is formed in a 2.8:1 ratio. More practically, 3 mol equiv. of TBHP in
the presence of 3 mol% of 1 at 40 °C affords a complete conversion after 20 h into a 1.8:1 mixture
of sulfoxide 15 and sulfone 16, that can be isolated in 56% and 30% yield, respectively, after
chromatography.
These last results show that the species [MoO3(bipy)]n is an active catalyst for the selective
oxidation of different types of sulfides, extending the existing technologies [6,7] using a
heterogeneous, recyclable and easy to prepare system.

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3. Conclusion
The one–dimensional inorganic/organic hybrid polymer [MoO3(2,2'–bipy)]n (1) is one member
of a very large family of molybdenum oxide/organonitrogen hybrid materials that has only recently
started to be explored as a source of useful catalysts for oxidation reactions. Some advantages of
these materials are that they can be prepared through convenient soft chemistry routes and are
crystalline, stable, storable forms of [MoO2X2(L)] complexes, for eventual use as either
heterogeneous catalysts or sources of soluble organonitrogen-coordinated MoVI active species.
In the present work, the scope of reactions catalyzed by 1 has been extended to include the
oxidation of amines and sulfides using TBHP as oxidant under mild conditions. Mo-based catalysts
have very rarely been concurrently examined for these two reactions. However, both
transformations are important in the context of developing new oxidative processes for the synthesis
of valuable synthons such as nitrones, sulfoxides and sulfones as well as the production of clean
transportation fuel oils. The findings obtained in the present work suggest that the catalyst 1 may
have potential in this regard and furthermore can be used in procedures for the switchable synthesis
of sulfoxide or sulfone via the oxidation of sulfides with the same oxidant by changing a reaction
parameter.
Although the results are in line with other literature results for oxidation of this substrate [9,
10], only few studies are available for the obtainment of sulfoxides and sulfones with the same
system [13-18]. We point out the remarkable selectivity observed with our polymeric catalyst and
the mild reaction conditions.
Acknowledgment
Part of this work was developed in the scope of the project CICECO–Aveiro Institute of
Materials, POCI–01–0145–FEDER–007679 [FCT (Fundação para a Ciência e a Tecnologia) ref.
UID/CTM/50011/2013], financed by national funds through the FCT/MEC and when applicable

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co–financed by FEDER (Fundo Europeu de Desenvolvimento Regional) under the PT2020
Partnership Agreement.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.catcom.2017.xx.xxx.
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Graphical abstract

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Highlights
 [MoO3(2,2'–bipy)]n is active in oxidation of amines and sulfides with t-BuOOH.
 The oxidation of sulfides occurs in high yield and selectivity.
 Selectivity to sulfones and sulfoxide can be tuned by changing reaction conditions.