TiCl4 hiocarbonyls and oxidation of sulfides in the presence of H2O2

Article abstract of DOI:10.1080/17415993.2011.647915

H₂O₂ in combination with TiCl₄ proved to be a highly reactive reagent system for the desulfurization of thioamide and thioketone derivatives in excellent yields and short reaction times with high purity. Sulfides were also found to undergo oxidation to sulfones under similar reaction conditions. In most cases, these reactions are highly selective, simple, and clean, affording products in high yields and purity.

Full text of DOI:10.1080/17415993.2011.647915

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TiCl4-promoted desulfurization of
thiocarbonyls and oxidation of sulfides
in the presence of H2O2
Kiumars Bahrami a , Mohammad M. Khodaei a , Vida Shakibaian a
Donya Khaledian a & Behrooz H. Yousefi b
,
a Department of Organic Chemistry, Faculty of Chemistry , Razi
University , Kermanshah , 67149-67346 , Iran
b Department of Nuclear Medicine, Klinikum rechts der Isar ,
Technische Universität München , Ismaninger Straße 22, D-81675 ,
Munich , Germany
Published online: 30 Jan 2012.
To cite this article: Kiumars Bahrami , Mohammad M. Khodaei , Vida Shakibaian , Donya
Khaledian & Behrooz H. Yousefi (2012) TiCl4-promoted desulfurization of thiocarbonyls and
oxidation of sulfides in the presence of H2O2 , Journal of Sulfur Chemistry, 33:2, 155-163, DOI:
10.1080/17415993.2011.647915
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Journal of Sulfur Chemistry
Vol. 33, No. 2, April 2012, 155–163
TiCl4-promoted desulfurization of thiocarbonyls and oxidation
of sulfides in the presence of H2O2
Kiumars Bahramia*, Mohammad M. Khodaeia∗, Vida Shakibaiana, Donya Khalediana
and Behrooz H. Yousefib
aDepartment of Organic Chemistry, Faculty of Chemistry, Razi University, Kermanshah 67149-67346, Iran;
bDepartment of Nuclear Medicine, Klinikum rechts der Isar, Technische Universität München, Ismaninger
Straße 22, D-81675 Munich, Germany
(Received 13 October 2011; final version received 5 December 2011)
H2O2 in combination with TiCl4 proved to be a highly reactive reagent system for the desulfurization of
thioamide and thioketone derivatives in excellent yields and short reaction times with high purity. Sulfides
were also found to undergo oxidation to sulfones under similar reaction conditions. In most cases, these
reactions are highly selective, simple, and clean, affording products in high yields and purity.
S
C
O
C
R2
R2
R1
R1
H2O2-TiCl4
16 examples
O
CH3CN, 25 °C
R3
S
R4
R3
S
R4
O
R1 = Alkyl, aryl, aryl-NH, H, Ph-NMe
R2, R3, R4 = Alkyl, aryl
14 examples
Keywords: desulfurization; thioamides; thioketones; sulfones; titanium tetrachloride
1. Introduction
Functional group manipulations are of paramount importance to synthetic organic chemists and
hence the development of novel transformations still remains of great interest (1). The con-
version of thiocarbonyl compounds to their carbonyl compounds has received considerable
attention. Among carbonyl compounds, amides are important commercial and biological com-
pounds. Because amides constitute the backbone of protein molecules, their chemistry is of
extreme importance. The penicillin and cephalosporin antibiotics (Figure 1) are among the best
known products of the pharmaceutical industry (2). In recognition of their importance, several
reagents such as sodium peroxide (3), t-butyl hypochlorite (4), diaryl telluroxide (5), singlet
*Corresponding authors. Emails: kbahrami2@hotmail.com, mmkhoda@razi.ac.ir
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2012 Taylor & Francis
http://dx.doi.org/10.1080/17415993.2011.647915
http://www.tandfonline.com

156 K. Bahrami et al.
HO
O
N
H
S
O
R
N
R
N
O
O
S
O
OH
HN
Cephalosporin
Figure 1. Structures of organic amides used as drug candidates.
R'
O
Penicillin
oxygen (6), benzeneseleninic anhydride (7), thiophosgene (8), dimethyl sulfoxide/iodine (9),
m-chloroperbenzoic acid (10), soft NO+ species (11), trifluoroacetic anhydride (12), clay-
supported ferric nitrate (13), manganese dioxide (14), and bismuth(III) nitrate (15) are available
for the oxidation of thioamides to their oxygen analogs.
However, the reported methods rarely offer the ideal combination of simplicity of procedure,
fast and selective reactions, and high yields of products and often suffer from a lack of generality
and economic applicability. As a consequence, the introduction of new methods and/or further
work on technical improvements to overcome the limitations is still an important experimental
challenge.
Titanium tetrachloride is a valuable reagent promoting various synthetic reactions, both in the
cases requiring a stoichiometric amount of TiC14 and in those requiring a catalytic amount of
TiCl4 together with other metal salts (16).
Aqueous hydrogen peroxide (30%) is an ideal oxidant in view of its high effective oxygen
content, its eco-friendly byproduct (water), its relative safety in storage and operation, and its
comparatively low cost of production and transportation (17). These advantages inspired us to
explore the potential of H2O2 in combination with TiC14 for the desulfurization of thioamides.
As part of an ongoing program directed at the development of efficient reagents for use under
mild conditions and following our latest interest in the use of hydrogen peroxide in organic
synthesis(18), weenvisagedthatacombinationofH2O2 withTiC14 couldbringabouttheselective
desulfurization of thioamides. To our knowledge, there are no literature reports on the conversion
of thioamides to the corresponding amides by the use of H2O2–TiC14 system (Scheme 1).
O
S
30% H2O2-TiCl4
CH3CN, 25 °C
R1
C
1
R2
R1
C
2
R2
Scheme 1. Desulfurization of thiocarbonyls.
2. Results and discussion
To evaluate the solvent effect, the reaction of N-(4-bromophenyl)thioacetamide as the model com-
pound was carried out under similar reaction conditions using various organic solvents. Among
the various solvents such as chloroform, dichloromethane, toluene, acetonitrile, and ethanol used
for this transformation, acetonitrile was the solvent of choice as the best results were obtained
with it.
To arrive at an optimum stoichiometry, when N-(4-bromophenyl)thioacetamide (1 mmol) was
allowed to react with 0, 1, 2, and 3 equivalent of H2O2 in the presence of different amounts of
TiCl4 at 25◦C, the yields of the product obtained were 5%, 50%, 80%, 96%, and 95%, respectively.
As a result, higher amounts of H2O2 neither increased the yield nor lowered the reaction time,
and TiCl4 was an effective agent only in the presence of H2O2. Therefore, the best result in 96%

Journal of Sulfur Chemistry 157
Table 1. Effect of increasing amounts of H2O2 and TiCl4 on the
desulfurization of N-(4-bromophenyl)thioacetamide.a
S
O
H3C
H3C
HN
Br
HN
Br
Entry
TiCl4 (mmol)
30% H2O2 (mmol)
Yield%b
1
2
3
4
5
1
0
1
2
2
3
5
50
80
96
95
0.8
0.8
1
1
Notes: aReaction conditions: The reactions were performed with N-(4-
bromophenyl)thioacetamide (1 mmol) for 2 min at 25◦C.
bIsolated yield.
yield was obtained by carrying out the reaction with 1:2 mol ratios of thioamide to H2O2 in the
presence of 1 mmol TiC14 for 2 min (Table 1).
Under these optimized reaction conditions, the generality and scope of this new desulfurization
protocol were then explored. A range of structurally diverse thioamides were subjected under
optimized reaction conditions to produce the corresponding amide derivatives. The results are
given in Table 2.Aromatic thioamides having electron-donating and electron-withdrawing groups
in the aromatic ring afforded excellent yields of products irrespective of the electronic effects.
Notably, primary, secondary, and tertiary thioamides underwent this reaction with equal effi-
ciency. For example, benzamide (Table 2, Entry 13) was produced in 95% yield, N-phenyl
benzamide (Table 2, Entry 1) in 96% yield, and N-methyl-4-nitro-N-phenylbenzamide (Table 2,
Entry 9) in 93% yield. Furthermore, it worked well with a variety of thioketones, and a repre-
sentative list of the compounds used to test the above reaction is given in Table 2 along with
the excellent yields and the melting points of the corresponding oxo compounds thus obtained.
Steric effects did not influence the yield significantly; for example, thiocamphor (Table 2, Entry
14) and fluorene-9-thione (Table 2, Entry 16) gave the desired ketones in 92% and 93% yields,
respectively. The functional group tolerance of this method is evident from Entry 15 because the
sulfur group was unaffected under the reaction conditions.
Since the oxidation of thioamides and thioketones proved the potential of TiC14 as an effective
and mild promoter agent, the oxidation of sulfides to the corresponding sulfones was studied.
Sulfones are useful synthetic intermediates for the construction of various chemically and bio-
logically significant molecules (19). Despite a number of alternative methods available for the
synthesis of sulfones, the oxidation of sulfides to sulfones is much less investigated compared
with the oxidation of sulfides to sulfoxides. However, the oxidation of sulfides to sulfones is the
most favored method. The popularity of this method is due to the availability of a wide variety of
sulfides that can be utilized in the oxidation of sulfides to the corresponding sulfones.
A survey of the literature revealed that several methods have been reported for the oxidation of
sulfides to sulfones (20–22). Many of these are not practical in some respects, such as prolonged
reaction times, low yields, undesired side reactions occurring on other functional groups, not
readily available reagents, and harsh reaction conditions. Therefore, a need for the development
of an easy and efficient reagent for the conversion of sulfides to sulfones still exists. In this paper,
we describe the successful use of the H2O2–TiC14 system as an efficient method for this purpose
(Scheme 2).
The optimum molar ratio of sulfide to H2O2 to TiC14 (1:4:1) was found to be ideal for the
complete conversion of sulfide into sulfone in acetonitrile at room temperature, while with lesser
amounts (e.g. 1:4:0.75 and 1:3:1), the reaction remained incomplete and increased sulfoxide
contamination, thereby reducing the selectivity of the oxidation.

158 K. Bahrami et al.
Table 2. Selective desulfurization of thiocarbonyls.a
S
C
O
C
R1
R2
R1
R2
Entry
product
Yield/%b
(t/min)
Mp/◦C (Lit. Mp.)
References
O
1
2
3
4
96 (2)
91 (2)
93 (2)
90 (2)
162–163 (158–162)
(18p)
HN
O
148 (149)
(18e)
(18e)
(18e)
Et
HN
O
196–198 (195–197)
209–210 (208)
HN
NO2
O
O2N
HN
O
5
6
95 (2)
95 (2)
200–201 (200–202)
160 (157–158)
(18e)
(18e)
O2N
HN
Me
O
O2N
HN
Cl
O
7
8
93 (2)
90 (2)
103–105 (105–106)
164 (164–165)
(23)
(24)
HN
O
HN
O
N
9
93 (2)
105–107 (104)
(18p)
O2N
H3C
O
10
11
96 (2)
92 (2)
165–167 (168)
121–123 (121)
(18p)
(18e)
H3C
O2N
HN
Br
O
N
O
12
13
14
94 (2)
95 (2)
92 (2)
144–145 (147)
129 (129)
(25)
(26)
(26)
HN
O
NH2
180–181 (180–182)
O
O
S
15
16
93 (2)
93 (2)
210 (209)
(18e)
(18p)
83–84 (80)
O
Notes: aThe products were characterized by comparison of their spectroscopic and physical data with
those of authentic samples synthesized by the reported procedures.
bYields refer to pure isolated products.

Journal of Sulfur Chemistry 159
O
S
O
5
30% H2O2-TiCl4
CH3CN, 25 °C
R3
R4
R3
S
R4
3
Scheme 2. Oxidation of sulfides to sulfones.
The choice of the organic solvent is of particular importance. Acetonitrile and 1,4-dioxane
were found to be suitable, giving rise to a relatively fast reaction rate at room temperature. If
the reactions are carried out in other less polar solvents such as dichloromethane, chloroform, or
some ethers, they become quite sluggish and usually do not reach completion.
Under the optimized conditions, the oxidation of several kinds of sulfides was carried out. The
results are given in Table 3. The aromatic and aliphatic and cyclic and acyclic sulfides can be
oxidized to sulfones in good to excellent yields. Both electron-rich and electron-deficient aryl
sulfides worked pretty well, mostly leading to excellent yields of products. In terms of reactivity,
aryl sulfides including electron-poor groups react slowly than those with electron-rich groups.
For example, the reaction of 4-nitrobenzyl phenyl sulfide remains incomplete even after a long
reaction time.
The chemoselectivity is noteworthy. Under such conditions, the sulfide function is highly reac-
tive, and various other functional groups are tolerable. For example, 2-hydroxyethyl phenyl sulfide
(Table 3, Entry 9) was oxidized to the sulfone products without dehydrogenation of the alco-
hol function. Oxidation of 2-(phenylthio)acetic acid (Table 3, Entry 10) proceeded smoothly to
produce the corresponding sulfone without interference from the carboxyl functional group.
Acid-sensitive sulfides such as 2-[(benzylthio)methyl] furan (Table 3, Entry 8) and methyl 2-
(phenylthio)acetate (Table 3, Entry 11) worked well without the formation of any side products,
whicharenormallyobservedinthepresenceofeitherproticorLewisacids.Allylicsulfide(Table 3,
Entry 12) yielded the corresponding sulfone without affecting the C9C bond. Also, the protocol
worked efficiently in oxidizing 2-(methylthio)-1H-benzo[d]imidazole (Table 3, Entry 7) to afford
the corresponding sulfone.
To further determine the scope of the reaction, a mixed reaction was carried out with benzyl
phenyl sulfide and 4-methylbenzaldehyde.After 2 min, the complete selective oxidation of sulfide
to sulfone occurred and 4-methylbenzaldehyde remained unreacted. Prolonging the reaction time
to 2 h did not result in any change.
The possible mechanism of the reaction is outlined in Scheme 3. Initially, an orange-red solution
was obtained, which may be due to the formation of complex (6) between titanium tetrachloride
and hydrogen peroxide (31). The oxygen atom in this complex is more electrophilic. Therefore,
the mechanism proceeds probably through the nucleophilic attack of sulfur atom in thioamide (1)
and sulfide (3) on this complex as shown in Scheme 3. In the case of sulfone formation, sulfoxide
(4) further reacts with the intermediate (7) to form sulfone (5) and leads to the eventual generation
of TiO2.
3. Conclusion
In summary, a specific selective oxidative protocol using the H2O2–TiCl4 reagent system was
found to be very useful for the desulfurization of thioamides and thioketones to their corresponding
amides and ketones in excellent yields and short reaction times. This reagent system was also
found to be a mild and efficient oxidizing agent for the oxidation of sulfides to the corresponding
sulfones in good to excellent yields. The reaction was highly selective, simple, and clean in most
cases.

160 K. Bahrami et al.
Table 3. Selective oxidation of sulfides into sulfones.a
Yield/%b
(t/min)
Entry
1
Sulfone
Mp/◦C (Lit. Mp.)
147 (146–147)
References
O
S
95 (2)
(18q)
O
O
S
2
90 (3)
157 (156–157)
(27a)
(27b)
Me
Me
Me
O
O
S
3d
80 (20)
180–182 (180–181)
NO2
O
O
4
5
91 (2)
95 (2)
150 (150–151)
124 (125–126)
(27c)
(18q)
S
O
O
S
O
O
6c
95 (2)
187 (186–188)
(18q)
S
O
N
O
O
S
O
7d
8
81 (5)
199–200 (202)
84–85 (84–85)
(28)
Me
N
H
O
S
O
O
90 (10)
(18q)
O
S
O
9d
85 (5)
Oil (Oil)
(29)
OH
OH
O
S
O
10d
11
83 (5)
91 (5)
110–111 (111)
Oil (Oil)
(18q)
(30)
O
O
S
O
CH3
O
O
O
12
90 (10)
Oil (Oil)
(18q)
S
O
O
13
14
92 (2)
90 (5)
Oil (Oil)
(27c)
(29)
S
O
O
S
45–46 (46–46.5)
O
Notes: aThe purified products were characterized by melting point and 1H and 13C NMR.
bYields refer to pure isolated products.
c1,4-Dioxane was used as the solvent.
dIn these cases, the sulfone products were accompanied by 10–15% of their corresponding sulfoxides.

Journal of Sulfur Chemistry 161
H2O2
S
Cl3Ti
Cl
HO
O
TiCl3
+
-
HCl
O
6
S
H
OH
..
HO
O
TiCl2
Cl
+
.
+ N
.
N
-
TiCl2
6
H
Cl-
-
1
O
S
S
O+
O
N
N
N
H
H
-
H+
1/8 S8
-
H
H
2
+
R3R4S
3
4
.
+
HO
O
TiCl2
Cl
OH
.
R R S
-
O
TiCl2
Cl-
3
6
-
+
R3R4S
O
+
H
R3R4S
O
-
HCl
4
H2O2
O
TiCl2
HO
O
TiOCl
OH
-
HCl
7
..
R3R4S
O
+
O
R3R4S
HO
O
Ti
O
+
-TiO2
- Cl-
4
7
Cl
H
O
O
R3R4SO2
R3R4S
H+
+
-
5
Scheme 3. Proposed mechanism for the oxidation of thioamides and sulfides.
4. Experimental section
Thiocarbonyls and sulfides either were commercially available or were prepared as follows:
thioamides from the reaction of the corresponding amides with P4S10 (32), thioketones from
the reaction of the corresponding ketones with Lawesson’s reagent (33), and sulfides from
S-alkylation of thiols (34). Melting points were determined in a capillary tube and are not cor-
rected. 1H NMR and 13C NMR spectra were recorded on a Bruker-200 NMR spectrometer using
TMS as the internal standard.
4.1. General procedure for the desulfurization of thioamides and thioketones
A mixture of thiocarbonyl compound (1 mmol), 30% H2O2 (2 mmol, 0.2 ml), and TiC14 (1 mmol,
0.11 ml) was stirred in CH3CN at room temperature for 2 min (Table 2). A yellow solid (TiO2 and
elemental sulfur) immediately precipitated. After completion of the reaction as indicated by TLC,
the reaction mixture was filtered to give TiO2 and elemental sulfur. The filtrate was poured into
water (10 ml) and extracted with ethyl acetate (4 × 5 ml), and the extract was dried with anhydrous
MgSO4. The filtrate was evaporated under vacuum to afford the analytically pure product.
4.2. General procedure for the preparation of sulfones
To the mixture of sulfide (1 mmol) and 30% H2O2 (4 mmol, 0.4 ml) in acetonitrile (5 ml), TiC14
(1 mmol, 0.11 ml) was added. The mixture was stirred at room temperature for an appropriate
period of time (Table 3). A white solid (TiO2) immediately precipitated. After the complete

162 K. Bahrami et al.
consumption of the starting material as observed by TLC, the reaction mixture was filtered to
give TiO2. The filtrate was poured into water (10 ml). The residue was then extracted with EtOAc
(4 × 5 ml) and the combined extracts were dried (MgSO4). The filtrate was evaporated and the
corresponding sulfone was obtained as the only product. In the case of sulfones 3, 7, 9, and 10,
the crude product after evaporation of the solvent was purified by column chromatography on
silica gel using a mixture of n-hexane and ethyl acetate as eluent (90:10).
All the products are known compounds and were characterized easily by comparison with
authentic samples (1H and 13C NMR, and melting point).
Acknowledgement
We are thankful to the Razi University Research Council for partial support to this work.
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