Melamine hydrogen peroxide (MHP): Novel and efficient reagent for the chemo- and homoselective and transition metal-free oxidation of thiols and sulfides

Article abstract of DOI:10.1080/10426500902758386

Melamine hydrogen peroxide (MHP) as a novel hydrogen peroxide adduct was easily prepared. Both chemo- and homoselective oxidation of thiols and sulfides to their corresponding disulfides and sulfoxides occurred by using MHP in moderate to excellent yields. AlCl₃ acts as a suitable activator in the oxidation of sulfides with MHP.

Full text of DOI:10.1080/10426500902758386

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Phosphorus, Sulfur, and Silicon and the
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Melamine Hydrogen Peroxide (MHP):
Novel and Efficient Reagent for the
Chemo- and Homoselective and
Transition Metal–Free Oxidation of Thiols
and Sulfides
Gholamabbas Chehardoli a & Mohammad Ali Zolfigol b
a School of Pharmacy , Hamedan University of Medical Sciences ,
Hamedan, Iran
b Faculty of Chemistry , Bu-Ali Sina University , Hamedan, Iran
Published online: 28 Dec 2009.
To cite this article: Gholamabbas Chehardoli & Mohammad Ali Zolfigol (2009) Melamine Hydrogen
Peroxide (MHP): Novel and Efficient Reagent for the Chemo- and Homoselective and Transition
Metal–Free Oxidation of Thiols and Sulfides, Phosphorus, Sulfur, and Silicon and the Related Elements,
185:1, 193-203
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Phosphorus, Sulfur, and Silicon, 185:193–203, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902758386
MELAMINE HYDROGEN PEROXIDE (MHP): NOVEL
AND EFFICIENT REAGENT FOR THE CHEMO-
AND HOMOSELECTIVE AND TRANSITION METAL–FREE
OXIDATION OF THIOLS AND SULFIDES
Gholamabbas Chehardoli1 and Mohammad Ali Zolfigol2
1School of Pharmacy, Hamedan University of Medical Sciences, Hamedan, Iran
2Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
Melamine hydrogen peroxide (MHP) as a novel hydrogen peroxide adduct was easily pre-
pared. Both chemo- and homoselective oxidation of thiols and sulfides to their corresponding
disulfides and sulfoxides occurred by using MHP in moderate to excellent yields. AlCl3 acts
as a suitable activator in the oxidation of sulfides with MHP.
Keywords Disulfides; homoselective oxidation; melamine hydrogen peroxide; sulfoxides;
transition metal–free oxidation
INTRODUCTION
Concentrated H2O2 is very dangerous to handle and not readily available. Hence this
reagent is now replaced by more stable and safe complexes.1 Organic addition compounds
with hydrogen peroxide were prepared first by Tanatar.2 These compounds are formed
for example, with urea, urethane, succinimide, and erythritol. Krepelka and Buksa3a also
prepared addition compounds of hydrogen peroxide with hexamethylene tetramine, di-
acetylhydrazine, quininesulfate, and aminoacetic acid. Of these compounds, the addition
compound of urea with hydrogen peroxide is well known, is commercially available, and
is important in industry.3
Oxidative coupling of thiols to disulfides is of practical importance in synthetic chem-
istry and biochemistry. Disulfides have found industrial applications as vulcanizing agents
and are important synthetic intermediates with many applications in organic synthesis.4
Disulfides are also essential moieties of biologically active compounds for peptide and
protein stabilization.5 The most important protocol for the preparation of disulfides is the
oxidative coupling of thiols, and variety of reagents have been reported for this purpose.6
Received 16 July 2008; accepted 13 January 2009.
Financial support for this work by the Management and Programming Organization of I.R. Iran (special
financial support of the PhD thesis of G. C.) and Center of Excellence in Development of Chemical Methods
(CEDCM), Bu-Ali Sina University, Hamedan, Iran, is gratefully acknowledged.
Address correspondence to Gholamabbas Chehardoli, School of Pharmacy, Hamedan University of Medical
Sciences, 65178 Hamedan, Iran. E-mail: chehardoli@umsha.ac.ir; cheh1002@gmail.com and Mohammad Ali
Zolfigol, Faculty of Chemistry, Bu-Ali Sina University, P. O. Box 4135, Hamedan 6517838683, Iran. E-mail:
zolfi@basu.ac.ir; mzolfigol@yahoo.com
193

194
G. CHEHARDOLI AND M. A. ZOLFIGOL
Organic sulfoxides are useful synthetic intermediates for the construction of var-
ious chemically and biologically active molecules. They often play an important role
as therapeutic agents such as antiulcer (proton pump inhibitor), antibacterial, antifungal,
anti-atherosclerotic, anthelmintic, antihypertensive, and cardiotonic agents, as well as psy-
chotonics and vasodilators. The oxidation of sulfides is the most straightforward method
for the synthesis of sulfoxides. There are a lot of reagents available for the oxidation of
sulfides, but many of them cause over-oxidation to sulfones. Therefore, the conditions of the
reaction, that is, time, temperature, and the relative amount of oxidants, must be controlled
to avoid forming side products of the oxidation.7
In continuation of our studies on the application of hydrogen peroxide adducts,6a,6bb,8
we found that melamine, which is widely used as a fire retarder in polymers,3 also forms
an addition compound with hydrogen peroxide. More studies on this subject have shown
us that the thermal behavior of melamine hydrogen peroxide was discussed previously by
Nagaishi et al.,3 but we did not find any reports on its application in organic functional group
transformations. Therefore, we decided to apply this reagent for this purpose. On the basis
of our previous investigations in the oxidation of thiols9 and sulfides,10 in this article we
report melamine hydrogen peroxide [MHP, (I)] as a novel, efficient, and bench top reagent
for the chemo- and homoselective oxidation of thiols and sulfides to their corresponding
disulfides and sulfoxides.
RESULTS AND DISCUSSION
Aromatic thiols were oxidized with MHP (I) in acetonitrile under reflux conditions
and give the corresponding disulfides in moderate to excellent yields (Scheme 1 and Table I).
Scheme 1
The results shown in Table I indicate that the method is applicable for the oxidative
coupling of aromatic thiols and is not suitable for aliphatic analogs (entries 10–12). For
showing the chemoselectivity of the described system, some competitive reactions were
designed. A mixture of equal amounts of aromatic thiols and aliphatic ones was subjected
to oxidation in the presence of MHP under the optimized reaction conditions. Furthermore,
we observed the excellent chemoselectivity between aromatic thiols and other oxidizable
organic functional groups (Scheme 2).

MHP AS A REAGENT FOR OXIDATION OF THIOLS AND SULFIDES
195
Scheme 2
Since MHP could not oxidize the sulfides alone (Scheme 2, Entry b), we decided
to find a suitable activator for the activation of MHP in the oxidation of sulfides. For this
purpose, we studied a number of various activators. The results are presented in Scheme 3
and Table II.
Scheme 3

196
G. CHEHARDOLI AND M. A. ZOLFIGOL
Table I Coupling of thiols (a) to their corresponding disulfides (b) using MHP (I) in acetonitrile
under reflux conditions
Reagent/Substrate
Entry
1
Substrate (a)
Product (b)
(mmol)
Time (h) Yield (%)i
2.2
0.08
0.25
1
82
91
75
99
88
2
3
4
5
2.2
2.2
2.9
2.2
0.25
0.25
6
2.9
0.17
71
7
8
2.9
2.2
0.5
94
85
0.17
9
1.4
2
59ii
19
10
11.5
20
11
12
11.5
1.4
24
1
48
—
13
1.4
1
Sluggish
iIsolated yields.
ii41% Undesired product.
Among the activators that have been used (Table II), we selected AlCl3 as it was
cheap and commercially available due to the best reaction conditions. In the next step, we
checked the effect of various solvents on the progress of reactions. As illustrated in Scheme 4
and Table III, we have found that acetonitrile is the best solvent for the abovementioned
reactions.

MHP AS A REAGENT FOR OXIDATION OF THIOLS AND SULFIDES
197
Scheme 4
Table II Oxidation of Bn-S-Ph using MHP (I) in the presence of various activators in acetonitrile under
ambient conditions
Entry
Activator
Activator (mmol)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
—
—
17
2
2
0.33
2.5
Immed.
2.5
3.5
1.5
2
0.83
3
3
1.5
2
0.5
0.25
0.33
2
—
7e
ZrCl4
WCl6
AlCl3
3
2
2
4
100
100
100
100
8a
8a
11
Al(HSO4)3
FeCl3
b
4
—
Silica Chloride11
0.4(g)
trace
80
100
70
100
—
—
—
—
100c
100c
100c
—
11
NaHSO4
DCC12
4
6
4
4
12
12
CaCl2.2H2O
ZnCl2
ZnO
MgO
Silica sulfuric acid11
Poly-SO3H11
Succinic anhydride
Maleic anhydride
Phtalic anhydride
KBr
0.1(g)
0.4(g)
6
6
6
8
aConversion.
bMixture of products.
cWith the side product.
Table III Solvent effect in the oxidation of sulfides with MHP in the presence of AlCl3 as an activator
Entry
Solvent
CH2Cl2
CHCl3
EtOAC
Time (h)
Yield (%)a
1
2
3
4
5
6
7
2
2
Trace
Trace
b
0.5
2
2.5
0.17
0.33
—
Acetone
—
—
n-Hexane
Acetic anhydride
CH3CN
b
—
100
aConversion.
bMixture of products.

198
G. CHEHARDOLI AND M. A. ZOLFIGOL
For studying the scope and limitations of the described procedure, the oxidation of
other sulfides was also studied. A good range of substrates such as aryl alkyl, diaryl, and
dialkyl sulfides has been selectively oxidized to their corresponding sulfoxides (Table IV).
An excellent homoselectivity was observed in the oxidation of thianthrene. We wish
to introduce the term homoselectivity10c for the first time in this article. We think that this
keyword is suitable for application in special reactions so that a substrate that has two
or more identical functional groups and one of them reacts in the course of the reaction
selectively and to produce only one product. When the thianthrene was subjected to the
oxidation (entry 4), homoselective monosulfoxidation took place, and surprisingly further
oxidation to disulfoxidation or monosulfonation did not occur (Scheme 5).
Scheme 5
For demonstration of this fact, the 13C NMR of the substrate and obtained product
has been shown in Figures 1 and 2. As can be seen, the obtaining product has only one
plane of symmetry and indicates six distinguished peaks.
Also, as is show in the entry 8 and Scheme 6, there is a chemoselectivity in the
oxidation of 8c. In this substrate, the functional group of sulfide was converted to the
sulfoxide, while the alcoholic hydroxy group did not react with reagents and was intact.
Scheme 6
Unfortunately, several attempts for the further oxidation of sulfoxides to their cor-
responding sulfones using the described system have failed, and the desired sulfones (2)
were produced in low yield with some unidentified byproducts (Scheme 7). Therefore, this
reagent system is not practical and suitable for this purpose.
Scheme 7

MHP AS A REAGENT FOR OXIDATION OF THIOLS AND SULFIDES
199
A plausible mechanism for the oxidation of sulfides to their corresponding sulfox-
ides using MHP/AlCl3 was shown in the Scheme 8 based on the literature7c,8a and our
observations in the course of the reaction.
Scheme 8
Although very recently, aqueous H2O2 has been reported for the oxidation of sulfides
into the corresponding sulfoxides/sulfones in the presence of L-proline,14 silica sulfu-
ric acid,15 and 10,10ꢁ-linked bisflavinium perchlorates,16 but MHP in the same as other
analogs such as PVP-H2O2,6a DABCO-DNOHP,6b,8b UHP,8 sodium perborate13 was used
as a suitable “dry carrier” of the hazardous and unstable hydrogen peroxide, due to easy
preparation, easy handling, safety, and stability at room temperature.
As shown in Table V, various systems for the oxidation of diphenylsulfide (6c)
to diphenylsulfoxide (6d) suffer at least from one of the following disadvantages: long
reaction times, using transition metal, low selectivity between sulfoxides and sulfones,
expensiveness of reagents and catalyst, tedious workup procedure, low yields of products,
etc., whereas the MHP/AlCl3 system does not have the abovementioned disadvantages.
In conclusion, we think that the ability of MHP to generate oxidative species via
in situ releasing of H2O2 in the reaction media will make it a useful reagent in organic
functional group transformations.
EXPERIMENTAL
Chemicals were purchased from Fluka, Merck, and Aldrich chemical companies.
Yields refer to isolated pure products. The oxidation products were characterized by com-
parison of their spectral (IR and 1H NMR) and physical data with those authentic samples,
which were produced by other reported procedures.14–16,30
Figure 1 The 13C NMR of thianthrene (Table II, entry 4).

200
G. CHEHARDOLI AND M. A. ZOLFIGOL
Table IV Chemoselective oxidation of sulfides (c) to their corresponding sulfoxide (d) using MHP (I) in the
presence of AlCl3 (II) as an activator in CH3CN at room temperature
Reagent/
Substrate(mmol)
Entry
1
Substrate (c)
Product (d)
I
II
Time (h)
Yield (%)a
84
1.78
1.25
0.33
2
3
1.71
1.71
1.2
1.2
1.25
0.67
92
92
4
5
1.42
1.71
1
1.25
90
b
—
1.2
2
—
6
7
8
1.71
1.71
1.71
1.2
1.2
1.2
1.5
97
99
52
0.67
0.67
9
1.57
1.57
1.1
1.1
Immed.
97
82
10
0.67
aIsolated yields.
bUndesired products.
Preparation of MHP (I)
In a round-bottomed flask (100 mL) equipped with a magnetic stirrer, a suspension
of melamine (12.6 g, 10 mmol) and H2O2 (30%, 10 mL) was stirred at 60◦C for 30
min. Then reaction mixture was cooled and transferred to a crystallizing dish for slow

MHP AS A REAGENT FOR OXIDATION OF THIOLS AND SULFIDES
201
Figure 2 The 13C NMR resulting from oxidation of thianthrene (Table II, entry 4).
evaporation of water under a mild air flow. After 3 days, H2O2-melamine as a white solid was
obtained.3
General Procedure for the Oxidative Coupling of Thiols
A suspension of thiol (1.425 mmol) and MHP (I) in CH3CN (6 mL) was stirred under
reflux condition for the specified time (for time and molar ratio see Table I). The progress
Table V Various systems for the oxidation of diphenylsulfide (6c)
Entry
Oxidizing systems
Time (h)
Yields(Ref.)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
H2O2-Melamine/AlCl3/acetonitrilea
H2O2/ manganese (III) Schiff-base complex
H2O2/hexafluoro-2-propanol
H2O2/dodecyl hydrogen sulfate
H2O2/Na2WO4, C6H5PO3H2 and PTC
H2O2/ZrCl4
H2O2/Silica sulfuric acid
H2O2/Phenole
H2O2/TiCl3
H2O2 or t-BuOOH/titanium derivatives supported on silicab
NaBrO3/Mg(HSO4)2
H2O2/ silica-based tungstate interphase catalyst
NaClO2/manganese(III) acetylacetonate
Ce(BrO3)3
Silica sulfuric acid/NaBrO3
NaIO4/wet SiO2
1.5
0.5
0.08
2
2
1
97
9117
9918
956d
9619
967e
9515
9920
10021
9522
4523
807c
9524
9925
7526
8527
8528
3
0.07
0.3
24
24
6.5
0.7
3
4
2
0.5
10
Trichloroisocyanuric acid/C5H5N/H2O
KMnO4/CuSO4. 5H2Oc
29
—
aOur obtained result by using the described procedure in this article.
b2% Sulfone.
c92% Sulfone.

202
G. CHEHARDOLI AND M. A. ZOLFIGOL
of the reaction was monitored by TLC. After completion the reaction, the suspension was
cooled. Then CH3CN was evaporated, and the residue was dissolved in boiling chloroform
(10 mL) and filtered. The crude products were obtained after evaporation of solvent.
General Procedure for the Oxidation of Sulfides
A mixture of sulfides (1.425 mmol), MHP (I), and AlCl3 in acetonitrile (6 mL) was
vigorously stirred at room temperature (for time and molar ratio see Table I). The progress
of the reaction was monitored by TLC. When the reaction was completed, CH3CN was
evaporated. The residue was dissolved in boiling chloroform (10 mL) and filtered off.
CHCl3 was removed by simple distillation, and crude products were obtained with high
purity.
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