One-pot tandem reactions for direct conversion of thiols and disulfides to sulfonic esters, alcohols to bis(indolyl)methanes and synthesis of pyrroles catalyzed by N-chloro reagents

Article abstract of DOI:10.2174/1570178611310020008

A convenient synthesis of sulfonic esters from thiols and disulfides has been described. In situ preparation of sulfonyl chlorides from thiols is accomplished by oxidation with Chloramin-T, tetra-butylammonium chloride (t-Bu4NCl) and water. The sulfonyl chlorides are then further allowed to react with phenol derivatives in the same reaction vessel. Also, a facile synthesis of bis(indolyl)methanes from alcohols using TCCA/KBr/wet-SiO2, and N-substituted pyrroles by reaction of hexane-2,5-dione with primary amines has been accomplished under mild condition with excellent yields.

Full text of DOI:10.2174/1570178611310020008

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Letters in Organic Chemistry, 2013, 10, 111-117
111
One-pot Tandem Reactions for Direct Conversion of Thiols and Disulfides
to Sulfonic Esters, Alcohols to Bis(indolyl)methanes and Synthesis of
Pyrroles Catalyzed by N-Chloro Reagents
Hojat Veisi,a,b,* Meral Ataee,b Leila Fatolahia and Shahram Lotfia
aDepartment of Chemistry, Payame Noor University, 19395-4697 Tehran, I.R. of Iran
bDepartment of Chemistry, Islamic Azad University-Saveh Branch, P. 39187-366, Saveh, Iran
Received September 16, 2012: Revised November 23, 2012: Accepted January 10, 2013
Abstract: A convenient synthesis of sulfonic esters from thiols and disulfides has been described. In situ preparation of
sulfonyl chlorides from thiols is accomplished by oxidation with Chloramin-T, tetra-butylammonium chloride (t-Bu4NCl)
and water. The sulfonyl chlorides are then further allowed to react with phenol derivatives in the same reaction vessel.
Also, a facile synthesis of bis(indolyl)methanes from alcohols using TCCA/KBr/wet-SiO2, and N-substituted pyrroles by
reaction of hexane- 2,5-dione with primary amines has been accomplished under mild condition with excellent yields.
Keywords: Thiols, sulfonic esters, Bis(indolyl)methanes, pyrroles, N-Chloro regents.
INTRODUCTION
reactions involve the use of excess amounts of acids, hazard-
ous organic solvents, tedious workup, or large amounts of
solid catalysts [27], which may not be the preferred choices
in view of green chemistry. Hence, an efficient and mild
Paal-Knorr condensation is needed for contemporary chemi-
cal synthesis. Also, N-Halo compounds are versatile reagents
and have been employed as potentially reactive intermediates
that are widely used in organic synthesis [28].
Sulfonic esters are a class of sulfonic acid derivatives that
have drawn much attention because they are valuable
intermediates in organic synthesis and are suitable precursors
for sulfonamides [1]. In addition, various sulfonic esters
exhibit interesting biological and pharmacological properties
[2]. Therefore, it is not surprising that the synthesis of
sulfonic esters has always been of great interest to organic
chemists. The most typical method for the preparation of
sulfonic esters is the reaction of sulfonyl chlorides with
alcohols or phenols in the presence of a base [3-10].
However, the preparation of sulfonic esters from phenols has
been much less investigated as compared to the preparation
of sulfonic esters from alcohols.
RESULTS AND DISCUSSION
We would like to present a direct convenient one-pot
synthesis of sulfonic esters from thiols and disulfides, and
phenols using Chloramin-T/t-Bu4NCl /H2O under mild con-
ditions (Scheme 1).
The pyrrole ring system is a useful structural element in
medicinal chemistry [11] and has found broad application in
drug development for antibacterial, antiviral, anti-
inflammatory, antitumoral and antioxidant [12] activity and
is widely used in materials science [13]. Therefore, prepara-
tion of pyrroles is an important reaction for which a wide
variety of methods has been developed [14]. One of the gen-
eral routes for synthesis of pyrroles is the Paal-Knorr reac-
tion which converts ꢀ-diketone to pyrroles by the interaction
of primary amines in the presence of various promoting
agents such as: montmorillonite KSF [15], microwave irra-
diation [16], Bi(NO3)3.5H2O [17], Sc(OTf)3 [18], layered
zirconium phosphate and zirconium sulfophenyl phospho-
nate [19], Titanium [20], TiCl4/Et3N [21], p-TSA [22] and I2
[15]. Some other methods for synthesis of pyrroles included
conjugate addition reactions [23], annulation reactions [24]
and aza-Wittig reactions [25, 26]. However, some of these
In order to find optimized conditions for the reaction, 4-
methyl-thiophenol and phenol were selected as model sub-
strates in acetonitrile at room temperature. After many opti-
mization experiments with respect to the molar ratio of the
reactants, reaction time, and possible solvents, the best result
was achieved by using the reaction of 4-methyl-thiophenol
(1 equiv), with t-Bu4NCl (3 equiv), H2O (2.5 equiv), and
Chloramin-T (2 equiv) in CH3CN (10 min, rt) with continu-
ous addition of phenol (1.1 equiv, 30 min) and triethylamine
(Et3N, 1.1 equiv) to give the corresponding sulfonic ester in
94% yield (Table 1, Entry 1). Et3N was used as base and HCl
trapper in the reaction medium.
To extend the scope of the reaction and to generalize the
procedure, we then investigated the generality and versatility
of this procedure using a series of structurally different thiols
and phenols (commercially available) under these optimized
conditions. A combinatorial library (parallel format) of sul-
fonic esters was smoothly prepared in good to high yields
whose results are summarized in Table 1.
*Address correspondence to this author at the Department of Chemistry,
Payame Noor University (PNU), 19395-4697 Tehran, I.R. of Iran;
Fax +98(838)4227463; E-mail: hojatveisi@yahoo.com
1875-6255/13 $58.00+.00
© 2013 Bentham Science Publishers

112 Letters in Organic Chemistry, 2013, Vol. 10, No. 2
Veisi et al.
O
S
O
S
R
SH
Chloramin-T
ArOH
R
Cl
R
OAr
Et3N, r.t.
t-Bu4NCl
CH3CN, H2O
R S
S R
O
O
Scheme 1. Convenient one-pot synthesis of sulfonic esters from thiols and disulfides.
Table1. Preparation of Sulfonic Esters from Thiols.
Entry
R
Ar
Yield (%)a
Mp/ºC
Ref.
1
2
3
4
5
6
7
8
4-MeC6H5
4-MeC6H5
4-MeC6H5
4-FC6H5
C6H5
4-MeOC6H5
4-NO2C6H5
C6H5
94
92
80
92
90
90
85
90
92-94
70-71
94-95
oil
[10]
[10]
[8]
[10]
[10]
[7]
Naphtyl
C6H5
90-92
110
4-NO2C6H5
Cyclohexyl
4-NO2C6H5
C6H5
4-MeOC6H5
oil
[10]
[7]
4-MeC6H5
99-100
aProducts were characterized from their physical properties, comparison with authentic samples and by spectroscopic methods.
O
S
Chloramin-T
H3C
t-Bu4NCl
CH3CN, H2O
PhOH
O
H3C
CH3
S
S
O
Scheme 2. Formation of sulfonic ester from the corresponding disulfide.
Table 2. Preparation of Sulfonic Esters from Disulfides.
Entry
Disulfide (R)
Sulfonic ester (Ar)
Yield (%)a
1
2
3
4
5
6
7
4-MeC6H5
4-MeC6H5
4-MeC6H5
4-FC6H5
C6H5
4-MeOC6H5
4-NO2C6H5
C6H5
90
95
92
80
92
90
95
Naphtyl
C6H5
4-NO2C6H5
Cyclohexyl
C6H5
4-MeOC6H5
aProducts were characterized from their physical properties, comparison with authentic samples and by spectroscopic methods.
An investigation into the mechanistic aspects of oxidative
chlorination of thiols showed the corresponding disulfide as
the main intermediate in this transformation. In order to fur-
ther verify the mediation of the disulfides in the oxidative
chlorination of thiols, reactions were repeated with a range
of symmetrical disulfides for the synthesis of sulfonic esters
(Scheme 1, 2). After optimizing the reaction in order to iden-
tify conditions that consistently produced excellent yields of
sulfonic esters, we found that the best reaction conditions
required the presence of Chloramin-T (1.2 equiv), t-Bu4NCl
(1.5 equiv), H2O (1.5 equiv) and disulfide (1 mmol) in ace-
tonitrile at room temperature. The generality and the scope
of the reaction were investigated and the results of the study
are summarized in Table 2. As shown, all reactions resulted
in the formation of the corresponding sulfonic esters in ex-
cellent yields with high purity. This shows that successive
oxidation of the sulfur atom, followed by S–S bond cleavage
and subsequent chlorination, occurs during the direct conver-
sion of thiols into the corresponding sulfonyl chlorides and
subsequently to sulfonic esters.
A possible mechanism for this transformation is shown in
Scheme 3 [29i]. Molecular chlorine generated from Chlora-
min-T and t-Bu4NCl affects the oxidative chlorination. It is
reasonable to assume that the thiol will chlorinates in the
presence of chlorine. Therefore, the mechanism proceeding

One-pot Tandem Reactions for Direct Conversion
Letters in Organic Chemistry, 2013, Vol. 10, No. 2
113
O
S
RSH
t-BuNCl
Cl
RS Cl
-HCl
Cl2
H3C
N
-
CH3CN
-H2O
Na+
O
H2O
O
RS OH I
H3C
S
NH2
H2O
O
HCl
-Cl
Cl
Cl2
RSH
-H2O
-HCl
RS OH2
RS SR
R
S
SR
II
-OH
Cl
O
S
O
O
S
O
R
S
SR
-HCl
Cl2
R
SR
R
S
SR
R
S
R
O
H
III
O
IV
-RSH HCl
O
O
S
PheOH
Et3N
R
S
Cl
R
OPh
VI
O
O
V
Scheme 3. The possible mechanism for this transformation.
require only slightly more time than the more basic amino
compounds, and both lead to high yields of the pyrrole prod-
ucts.
through hydroxylation of thiol leads to the formation of
sulfenic acid (I), which gives the corresponding symmetric
disulfide (II). Then the successive oxidation of both sulfur
atoms of the disulfide molecule by chlorine produces the
intermediate (III) that undergoes rapid isomerization to the
thiosulfonate (IV), which can easily furnish sulfonyl chloride
(V). Then, the sulfonyl chloride (V) reacts with phenol to
form the corresponding sulfonic esters (VI).
As shown in Table 3, aromatic amines with electron do-
nating groups (Table 3, entries 6, 7, 10) or electron-
withdrawing group (Table 3, entries 8, 9) are both effective
in the Paal-Knorr reaction. Due to the good results obtained,
we found that, the present protocol that was also applied to
less nucleophilic aromatic amines.
In continuation of our works, we wish to report our pre-
liminary results on the synthesis of pyrroles from a ꢀ-
diketone and primary amines with Chloramin-T as catalyst
under mild condition with excellent yields (Scheme 4).
Having these results in hand, several amines including
monocyclic, bicyclic aromatic, aliphatic, benzylic, diamines
and triamine, have been subjected to the above-mentioned
optimized conditions, and the results are also presented in
Table 3. It is evident that our methodology is reasonably
general and can be applied to several amines. The other start-
ing material hexane- 2,5-dione, was commercially available
(Scheme 4). The yields of products are also shown in Table
3.
O
Chloramin-T
CH3CN, r.t.
RNH2
+
N
R
O
R: alkyl and aryl
Scheme 4. Synthesis of pyrroles from ꢀ-diketone and primary
amines.
As shown in Table 3, a series of aromatic amines bearing
either electron-withdrawing or electron releasing groups on
the aromatic ring was investigated. The substituent group on
the phenyl ring did not make any difference in the Paal-
Knorr reaction. Morever, we also examined the Paal-Knorr
reaction of aliphatic amines with hexane- 2,5-dione (Table 3,
Entries 14-17). Similarly, the corresponding products were
obtained in excellent yield.
Accordingly, treatment of 1,4-diketone with aniline in the
presence of a catalytic amount of Chloramin-T afforded 2,5-
dimethyl-N-phenylpyrrole in 80% yield under solvent-free
condition (Scheme 4). In continuation, we investigated the
effect of solvents on this reaction. Several solvents including
acetonitrile, dichloromethane, chloroform, water and ethanol
were investigated during the course of this study. These ex-
periments show that CH3CN is a better solvent than the rest
ones tested (97% yield).
We found this method is selective for preparation of pyr-
role containing aromatic rings with functional groups such as
chlorine (Table 3, entry 5), and carboxylic acids (Table 3,
entry 9).
These results promoted us to investigate the scope and
the generality of these new protocols for various amines (ali-
phatic and aromatic) under optimized conditions. In the same
manner, a variety of amines were coupled with 1,4-diketone
in the presence of a catalytic amount of Chloramin-T at room
temperature to give the corresponding pyrroles in good to
excellent yields (Table 3). The less basic aromatic amines
Our experiments also indicated that Chloramin-T is a re-
usable catalyst and after three runs, the catalytic activity of
the catalyst was almost the same as that of fresh catalyst.
Also, to compare this method with previously published

114 Letters in Organic Chemistry, 2013, Vol. 10, No. 2
Veisi et al.
Table 3. Pyrroles Derivatives Produced from Paal-Knorr Condensation with Chloramin-T at Room Temperature.
Entry
Amine (1)
Time
Yield (%)a
1
2
C6H5-NH2
2 h
5 min.
10 min.
10 min.
2.5 h
90
98
96
92
80
85
90
85
70
85
80
82
75
96
96
92
96
C6H5-CH2NH2
4-MeO-C6H4-CH2NH2
3
4
H2NCH2-C6H4-CH2NH2 (p)
4-Cl-C6H4-NH2
5
6
4-OMe-C6H4-NH2
4-Me-C6H4-NH2
3 h
7
2 h
8
3-CF3-C6H4-NH2
3 h
9
4-COOH-C6H4-NH2
H2N-C6H4-NH2 (p)
H2N-C6H4-NH2 (m)
Naphthalen-1-amine
Naphthalene-1,5-diamine
Cyclopentanamine
H2N-CH2CH2-NH2
HN-(CH2CH2-NH2)2
N-(CH2CH2-NH2)3
4 h
10
11
12
13
14
15
16
17
45 h
5.5 h
3..5 h
5 h
15 min.
20 min.
25 min.
1.5 min.
a Products were characterized on the basis of their physical properties, comparison with authentic samples and by spectroscopic methods.
Table 4. Reaction Times and Yield for Previously Published Methods.
Substrate
Conditions
Reaction time
Yield (%)
Benzylamine
Benzylamine
Montmorillonite, KSF
I2
10 h
0.5 h
10 h
0.5 min
2 h
95 [15]
92 [15]
95 [17]
90 [16]
78 [19]
90 [18]
83 [15]
85 [15]
83 [17]
70 [17]
70 [15]
78 [15]
Benzylamine
Bi(NO3)3.5H2O
Microwave
Benzylamine
Benzylamine
ꢀ-Zr(KPO4)2
Sc(OTf)3
1-Naphthylamine
1-Naphthylamine
1-Naphthylamine
1-Naphthylamine
2-aminopyridine
2-aminopyridine
2-aminopyridine
40 min
11 h
1 h
Montmorillonite, KSF
I2
Bi(NO3)3.5H2O
Bi(NO3)3.5H2O
Montmorillonite, KSF
I2
11 h
25 h
25 h
1 h
methods for the synthesis of N-substituted pyrroles with ben-
zylamine, naphthylamine and 2-aminopyridine, we carried
out the following studies, as shown in Table 4. These results
clearly demonstrate that this procedure is a good system for
the preparation of N-alkyl and N-aryl-2, 5-dimethylpyrroles.
Since Chloramin-T contains a halogen atom that is at-
tached to the nitrogen atoms, it is quite possible that Cl+ re-
leased in situ can act as catalyst in the reaction medium. In
Scheme 5, the suggested mechanism for synthesis of pyr-
roles with Chloramin-T is shown.

One-pot Tandem Reactions for Direct Conversion
Letters in Organic Chemistry, 2013, Vol. 10, No. 2
115
O
O
O
S
Cl
2 H3C
N
-
Na+
O
O
O
Cl
O
Cl
H C
S
N
2
3
-
+
Na+
2
HOCl
O
RNH2
O
S
H C
N
2
3
-
Na+
O
+
R: alkyl and aryl
H
H
ClO
OCl
N
N
R
R
Scheme 5. The suggested mechanism for synthesis of pyrroles with TCCA.
O
Cl
O
Cl
O
N
N
1)
N
OH
Cl
HN
NH
R1
R2
KBr, wet-SiO2, r.t., 5 min., grinding
2) indole, r.t., grinding
R1
R2
R1: alkyl, aryl
R2: H, alkyl
Scheme 6. Conversion of alcohols to bis(indolyl)methanes in one-pot.
In continuation of our interest in the application of N-
halo reagents in organic synthesis [28], we interested to find
a good way to conversion of alcohols to bis(indolyl)
methanes in one-pot. Therefore, we report the use of
trichloroisocyanuric acid (TCCA)/KBr/wet-SiO2/indole as an
catalytic system to oxidation conversion of alcohols to
bis(indolyl)methanes under solid-state conditions by pulveri-
zation-activation method at room temperature with excellent
yields (Scheme 6).
the use of trichloroisocyanuric acid in the presence of water.
The oxidation of bromide ion by HOCl would give hypo-
bromous acid and subsequent oxidation of alcohols by ꢀ-
elimination of HCl affords aldehydes and ketones. Then,
This aldehyde or ketone reacts with indole in the presence of
HCl to form a bis(indolyl)methane.
CONCLUSION
In conclusion, we have developed a facile and efficient
methodology using one-pot tandem reactions for the direct
conversion of thiols and disulfides to sulfonic esters, alco-
hols to bis(indolyl)methanes, and synthesis of N-substituted
pyrroles using N-chloro regents. The advantages of the
method include (i) short reaction times, (ii) high yields, and
(iii) easy work-up.
For our initial optimization studies, benzyl alcohol was
chosen as the model substrate. A mixture of benzyl alcohol
(1mmol), KBr (0.12 mmol) and wet SiO2 (0.2 g) was stirred
for 5 min., followed by addition of indole (2 mmol) which
afforded the corresponding bis(indolyl)methane in 90% yield
under solid state conditions with pestle in a mortar (Table 5,
entriy 1).
These results promoted us to investigate the scope and
the generality of this new protocol for various alcohols under
optimized conditions. As shown in Table 6, a series of aro-
matic, aliphatic and heterocyclic alcohols underwent oxida-
tion and electrophilic substitution reaction with indole
smoothly to afford a wide range of substituted bis(indolyl)
methanes in good to excellent yields.
EXPERIMENTAL
General Procedures for the Conversion of Thiols to Sul-
fonic Esters
To a stirred mixture of of thiol (1 mmol), t-Bu4NCl (3
equiv), and water (2.5 mmol) in CH3CN (5 mL) at 0 °C,
Chloramin-T (2 equiv) was added in portions over 1-2 min.
After 30 min, a solution of phenol (1.1 mmol) in triethy-
lamine (1.1 mmol) was added to the reaction mixture over 1-
A plausible mechanism for the oxidation is shown in
Scheme 7 based on our observation and obtained results,
catalytic activity of KBr and insitu generation of HOCl by

116 Letters in Organic Chemistry, 2013, Vol. 10, No. 2
Veisi et al.
Table 6. Synthesis of bis(indolyl)methanes from alcohols.
Entry
Carbonyl Compound
Time (min)
Yield (%)a
Mp °C
1
2
3
4
5
6
7
8
9
Benzyl alcohol
4-methyl-Benzyl alcohol
2-methoxy-Benzyl alcohol
4-methoxy-Benzyl alcohol
2-bromo-Benzyl alcohol
4-chloro-Benzyl alcohol
2-chloro-Benzyl alcohol
furan-2-yl-methanol
10
12
8
90
90
92
90
85
89
85
80
90
86
80
75
85
80
75
90
89
90
124-125
94-95
134-136
190-192
110-112
76-78
8
12
10
12
10
15
15
15
12
10
12
15
10
10
12
70-72
>300
4-hydroxy-Benzyl alcohol
4-nitro-Benzyl alcohol
3-nitro-Benzyl alcohol
4-(N,N-dimethyl)-Benzyl alcohol
Cinnamyl alcohol
123-125
218-220
258-260
224-226
98-100
151-154
121-123
165–167
190-192
115-116
10
11
12
13
14
15
16
17
18
thiophen-2-yl-methanol
Pentanol
1-phenylethanol
1-(4-nitrophenyl)ethanol
cyclohexanol
a Products were characterized from their physical properties, comparison with authentic samples and by spectroscopic methods.
O
Wet-SiO2
O
Cl
O
Cl
O
H2O
Grinding
O
N
N
2
HOCl
R1
R2
N
N
H
Br
Cl
HCl
H
O
Br
H
O
N
N
O
N
H
R1
R2
HN
NH
H
Br
Cl
R1
R2
OH
Cl
OH
+
R1
R2
Scheme 7. Plausible mechanism.
2 min. The resulting mixture was stirred at room temperature
for 60 min. until TLC showed complete disappearance of
starting material (Table 2). The reaction mixture was then
diluted with water (10 mL) and extracted with EtOAc (3X10
mL). The combined ethyl acetate extracts were dried with
MgSO4 and concentrated under reduced pressure to give the
corresponding sulfonic ester as the only product.
2 min. The resulting mixture was stirred at room temperature
for 90 min. until TLC showed complete disappearance of
starting material (Table 2). The reaction mixture was then
diluted with water (10 mL) and extracted with EtOAc (3X10
mL). The combined ethyl acetate extracts were dried with
MgSO4 and concentrated under reduced pressure to give the
corresponding sulfonic ester as the only product.
General Procedure for the Synthesis of Pyrroles with
Chloramin-T
General Procedures for the Conversion of Disulfides to
Sulfonic Esters
To a solution of amine 1 (1 mmol) and 2,5-hexanedione 2
(1 mmol) in CH3CN (3 mL) Chloramin-T (0.05 mmol) was
added at room temperature. The mixture was allowed to stir
at this temperature for the period time specified in Table 3.
The reaction was monitored by TLC (3:1 n-hexane/acetone).
To a stirred mixture of thiol (1 mmol), t-Bu4NCl (3
equiv), and water (2.5 mmol) in CH3CN (10 mL) at 0 °C,
Chloramin-T (1.2 equiv) was added in portions over 1-2 min.
After 30 min, a solution of phenol (1.1 mmol) in triethy-
lamine (1.1 mmol) was added to the reaction mixture over 1-

One-pot Tandem Reactions for Direct Conversion
Letters in Organic Chemistry, 2013, Vol. 10, No. 2
117
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After completion of the reaction, the solvent was evaporated.
Then CH2Cl2 (10 mL) was added and the catalyst was re-
moved by filtration. Evaporation of the solvent under re-
duced pressure gave the products. Further purification was
achieved by thin layer chromatography using n-hexane/ ace-
tone (70: 30) as the solvent system to afford the pyrroles.
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Synthesis of bis(indolyl)methanes from Alcohols
A mixture of KBr (0.12 mmol), benzylalcohole (1
mmol), TCCA (0.4 mmol) and wet SiO2 (0.2 g) was taken
and ground with pestle in a mortar for an appropriate time at
room temperature, followed by addition of 2 mmol indole.
After completion of the reaction (monitored by TLC, hex-
ane/acetone 4:1), the reaction mixture was washed with
CH2Cl2 (20 mL). Anhyd Na2SO4 (2g) was added to the fil-
trate and was filtered off after 20 min. CH2Cl2 was removed.
The resulting crude material was purified by recrystalization
from ethanol-water to afford pure products.
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The author(s) confirm that this article content has no con-
flict of interest.
ACKNOWLEDGEMENTS
Financial support for this work by Payame Noor Univer-
sity (PNU), Iran, and Islamic Azad University of Saveh,
Iran, is gratefully acknowledged.
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