Sodium hypochlorite-promoted novel synthesis of organic ammonium tribromides and application of phenanthroline hydrotribromide in chemoselective oxidation of organic sulfides by hydrogen peroxide

Article abstract of DOI:10.1246/cl.131192

A novel method of synthesis of organic ammonium tribromides (OATBs) is developed by using an inexpensive and eco-friendly sodium hypochlorite as oxidant for conversion of Br to Br3 . The OATBs thus prepared include both quaternary ammonium tribromides and N-heterocyclic tribromides. A new addition to the family of OATBs is made in the form of phenanthroline hydrotribromide. The efficacy of this new tribromide as catalyst is ascertained in the oxidative transformation of organic sulfides to their corresponding sulfoxides and sulfones by hydrogen peroxide.

Full text of DOI:10.1246/cl.131192

Received: December 22, 2013 | Accepted: January 8, 2014 | Web Released: January 16, 2014
CL-131192
Sodium Hypochlorite-promoted Novel Synthesis of Organic Ammonium Tribromides
and Application of Phenanthroline Hydrotribromide in Chemoselective Oxidation
of Organic Sulfides by Hydrogen Peroxide
Madhudeepa Dey, Rupa R. Dey, and Siddhartha S. Dhar*
Department of Chemistry, National Institute of Technology Silchar, Assam 788010, India
(
E-mail: ssd@nits.ac.in)
A novel method of synthesis of organic ammonium
tribromides (OATBs) is developed by using an inexpensive
and eco-friendly sodium hypochlorite as oxidant for conversion
of Br to Br3 . The OATBs thus prepared include both
quaternary ammonium tribromides and N-heterocyclic tribro-
mides. A new addition to the family of OATBs is made in the
form of phenanthroline hydrotribromide. The efficacy of this
new tribromide as catalyst is ascertained in the oxidative
transformation of organic sulfides to their corresponding
sulfoxides and sulfones by hydrogen peroxide.
first time for conversion of bromide to tribromide leading to the
synthesis of a variety of OATBs including nitrogen-containing
heterocyclic tribromides such as PyHTB, MPHT, BHTB, and
phenanthroline hydrotribromide (PhenHTB). It is also revealed
from exhaustive literature survey that PhenHTB has not yet been
reported to date and thus is added as a new member to the family
of OATBs. The new reagent is expected to be stable, long-lived,
and easy to handle.
¹
¹
Furthermore mention may be made of the chemistry
involving selective oxidation of organic sulfur compounds that
have attracted attention of researchers for many decades. Such
transformations are of immense significance in synthetic and
7
The synthesis and application of organic ammonium
tribromides (OATBs) have garnered overwhelming interest in
the past two decades.1 Quite a few quaternary ammonium
tribromides such as tetrabutylammonium tribromide (TBATB),
tetraethylammonium tribromide (TEATB), and cetyltrimethyl-
ammonium tribromide (CTMATB) have been reported to be
highly useful reagents for a variety of organic functional group
transformations. Moreover, OATBs such as pyridine hydrotri-
bromide (PyHTB), quinolinium tribromide (QHTB), N-methyl-
pyrrolidin-2-one hydrotribromide (MPHT), phenyltrimethylam-
monium tribromide (PTATB), and bipyridine hydrotribromide
bioorganic chemistry.
We wish to report herein a very efficient, cost-effective, and
mild synthetic protocol for a wide variety of organic ammonium
tribromides, and exploration of a new reagent, 1,10-phenanthro-
line hydrotribromide as a catalyst for oxidative transformation of
organic sulfides to sulfoxides and sulfones by hydrogen peroxide.
A variety of quaternary ammonium bromides, QABs and N-
heterocyclic compounds were converted to the corresponding
¹
tribromides by using NaOCl as a reagent for oxidizing Br to
¹
Br3 (Schemes 1 and 2). Two equivalents of potassium bromide,
KBr, were used for QABs to transform them into their respective
QATBs while three equivalents of KBr were needed for
transformation of N-heterocycles such as phenanthroline to their
corresponding tribromides, PhenHTB (Scheme 3). It may be
reiterated that NaOCl has never been used for the synthesis of
OATBs and proved to be an excellent and efficient oxidant for
(BHTB) have been well documented in the literature as
alternative sources of bromine that offer the advantages of
2
safety, selectivity, mild reaction conditions, and stability. It is
evident from the available literature that OATBs find increasing
applications as reagents for a variety of organic reactions
including functional group transformations like bromination,
oxidation, epoxidation, acylation, etc.3 For example they act as
vital reagents for preparation of bromo organics, for bromination
such conversion. A small amount of dilute H SO4 is also
2
,1c
necessary to obtain the respective tribromides. It may be further
claimed that PhenHTB is the first reported tribromide. All the
synthesized tribromides were characterized by UV­visible
spectroscopic analysis. The tribromides exhibit a strong absorp-
4
of alcohols, enones, alkenes, and activated aromatics and also
as bifunctional catalyst in oxidative bromination of aromatic
compounds. Thus, looking into the spectrum of uses of OATBs
in synthetic organic chemistry it was considered pragmatic to
design newer synthetic protocols for the synthesis of OATBs.
Traditional methods of synthesis of OATBs utilize liquid
NaOCl + 3 Br- + 2 H+
Br - + NaCl + H O
3 2
¹
¹
Scheme 1. Oxidation of Br to Br3 by NaOCl.
bromine, Br , for their synthesis which severely handicaps their
2
applicability to large-scale industrial processes. Therefore as a
part of our continued research endeavor toward development of
NaOCl, H+
R NBr + 2 KBr
R4NBr3
4
Grinding
newer improved and greener methods for synthesis of OATBs
+
4
and other organic substrates,1
c,1d,5
R N
we report herein for the first
+
1
2
3
4
. = Me4N
time a new transition-metal-free, simple, and efficient method
for the synthesis of OATBs employing an inexpensive and
useful environmentally benign oxidant, sodium hypochlorite,
NaOCl. The use of NaOCl as potent oxidant in synthetic organic
+
. = Et N
4
+
. = n-Bu N
4
+
. = Benzyl-Me N
5. = Cetyl-Me N+
3
3
6
chemistry has been well documented in literature. However, its
application in the synthesis of OATBs is hitherto unknown.
Therefore it has been chosen here as the oxidizing agent for the
Scheme 2. Formation of ammonium tribromides from their
corresponding quaternary ammonium bromides.
Chem. Lett. 2014, 43, 631–633 | doi:10.1246/cl.131192
© 2014 The Chemical Society of Japan | 631

O
S
+
H , NaOCl
PhenHTB (10 mol %)
H O (1 equiv)
+
3 KBr
2
2
R1
r.t.
N
N
N
N
HBr
S
R1
O
S
3
H2O2 (2-3 equiv)
PhenHTB (20-30 mol %)
R1
Scheme 3. A representative example of formation of a tribromide
from N-heterocyclic compound.
O
Room Temperature,
70-94%
Scheme 4. Oxidation of alkyl aryl sulfides to their corresponding
sulfoxides and sulfones.
8
¹
tion band at ca. 270 nm, characteristic of Br3 anion. PhenHTB
was additionally analyzed for its CHN and the total bromine
content was estimated by volumetric method. All the results
obtained are consistent with theoretical values. The analytical
results also suggest that only one nitrogen atom of phenanthro-
line gets protonated thus forming a monotribromide rather than
a ditribromide. A survey of the available literature also points to
Table 1. Oxidation of sulfides to sulfoxides using 30% H2O2
catalyzed by PhenHTB in CH3CN­H2Oa
Time Isolated
Entry Substrate
Sulfoxideb
/
min yield/%
3
b,2c,9
1
2
3
4
5
6
7
8
9
0
PhSCH3
PhSOCH3
30
30
35
25
25
25
26
60
32
30
92
94
91
87
87
80
89
84
70
88
the fact that a monotribromide is formed as the product.
As an example, two recent reports on ditribromides such as
,2-bis(N-pyridinium)ethane ditribromide and ethylenebis(N-
4-MeOPhSCH3
4-MePhSCH3
4-ClPhSCH3
PhSPh
PhCH2SCH2Ph
PhSC2H5
4-NO2PhSCH2Ph 4-NO2PhSOCH2Ph
t-C4H9SCH3
PhSCH2Ph
4-MeOPhSOCH3
4-MePhSOCH3
4-ClPhSOCH3
PhSOPh
PhCH2SOCH2Ph
PhSOC2H5
1b
1
2d
methylimidazolium) ditribromide indicate that stable ditribro-
mides of N-heterocyclic compounds have CH2­CH2 groups
between two N-heterocycles. Tribromides obtained from an N-
heterocyclic compound always lead to the formation of mono-
tribromide. The analytical results suggest that tribromide
obtained from bipyridine and phenanthroline in the present
methodology are also monotribromide consistent with literature.
In a typical synthesis of OATBs, a solution of 20 mmol
of KBr in 10 mL of 2 M H2SO4 was added to 10 mmol of
quaternary ammonium bromide. Sodium hypochlorite (10 mL of
t-C4H9SOCH3
PhSOCH2Ph
1
aAll reactions were carried out at room temperature using
oxidant-to-substrate molar ratio of 1:1 and 10 mol % PhenHTB
b
used as catalyst in CH3CN­H2O as the solvent. All products
4
%) was then added to the resulting solution and stirred for
were identified by their IR and NMR spectral data and
comparison of their mp with published data.
1
0
ca. six to ten minutes to obtain bright orange-yellow crystalline
tribromide compound. The synthesized tribromide compound
thus formed was isolated by suction filtration and washed with
water (10 mL) three to four times and dried under vacuum over
anhydrous CaCl2 to get orange crystals of quaternary ammonium
tribromides in very good to excellent yields.
N-Heterocyclic tribromides were prepared by adding an
amount of 30 mmol of KBr in 10 mL of 2 M H2SO4 to 10 mmol
of N-heterocyclic compound. To the resulting solution 10 mL of
carried out by taking catalytic amount of PhenHTB (10 mol %)
and 30% H2O2 (1 equiv) as oxidant under acetonitrile­water as
solvent (Scheme 4). A controlled reaction carried out without
the catalyst either shows no product formation or in some cases
formation of products in extremely poor yields. Also it was
observed that in absence of peroxide the reaction did not take
place at all using 10 mol % of PhenHTB indicating that the
tribromide does not act as oxidant at this concentration.
Therefore, the main oxidant in the reported protocol is PhenHTB
acts as catalyst only. It may be further mentioned that no
significant overoxidation of sulfide to sulfone was observed even
on prolonging the reaction time indicating that sulfoxide is the
main product under the given reaction conditions. The same
protocol was adopted to transform sulfides to sulfones by
increasing the concentration of catalyst to 20­30 mol % and
oxidant (30% H2O2) to 20 mmol, respectively.
In the oxidation of organic sulfides to sulfoxides, a solution
of organic sulfide (10 mmol) and PhenHTB (10 mol %) in
acetonitrile­water (10 mL, 1:1) was stirred and 30% hydrogen
peroxide was added to the resulting solution drop wise
(10 mmol) at room temperature for an appropriate time (30­
70 min) (Table 1). After completion of the reaction as monitored
by TLC, the reaction mixture was poured in water and the
excess hydrogen peroxide was destroyed by the addition of
hydrogensulfite (aq) followed by the filtration through a small
Buckner funnel. After filtration the organic products were
extracted with ether. The ether layer was washed with water
(2 mL) and dried over Na2SO4. The organic solvent was
4
% sodium hypochlorite was added and stirred for ca. four to
five minutes. The bright orange-yellow crystalline compound of
tribromide thus formed was separated by suction filtration and
washed with water (5 mL) three to four times and dried under
vacuum over anhydrous CaCl2 to obtain orange crystals of the
tribromide in excellent yields.
This new tribromide PhenHTB is characterized by CHN
1
analysis, UV­vis, FT-IR, and H NMR spectroscopy. UV­vis
(
­max): 269 nm; FT-IR (KBr): 3390, 3048, 1535, 1184,
¹
1 1
713 cm ; H NMR (400 MHz, CDCl ): ¤ 9.33 (s, 1H), 8.8 (d,
3
J = 8.4 Hz, 2H), 8.1 (d, J = 11.2 Hz, 2H), 8.0 (d, J = 1.6 Hz,
2H), 7.2 (t, J = 13.4 Hz, 2H); Anal Calcd for C12H9N2Br3: C,
34.24; H, 2.16; N, 6.65; Br, 56.95%. Found: C, 33.5; H, 2.0; N,
6.4; Br, 57.02%.
After the successful synthesis of PhenHTB, it was consid-
ered worthwhile to investigate its efficacy as catalyst for
selective oxidative transformation of organic sulfides to corre-
sponding sulfoxides and sulfones. The oxidation of sulfides was
carried out in acetonitrile­water solvent system and 30% H2O2
as the oxidant. For preliminary studies, methyl phenyl sulfide
was chosen as a representative substrate. The oxidation was then
632 | Chem. Lett. 2014, 43, 631–633 | doi:10.1246/cl.131192
© 2014 The Chemical Society of Japan

Table 2. Oxidation of sulfides to sulfones using 30% H2O2
catalyzed by PhenHTB in CH3CN­H2Oa
In conclusion, the present work demonstrates a highly
efficient yet mild and environmentally acceptable protocol for
synthesis of organic ammonium tribromides and also to examine
the potential of PhenHTB as a catalyst for selective trans-
formation of organic sufides. Despite of availability of quite a
few of methods for such transformation, the present method-
ology for synthesis of tribromides can certainly be claimed as
one of the most efficient and eco-friendly. As a part of our
further objective to assess the potential application of the new
tribromide reagent, viz., PhenHTB, we have used this reagent as
catalyst for oxidation of organic sulfides to sulfoxides and
sulfones. The results obtained are quite satisfactory indicating
that methodology can be used as a new and useful alternative to
Time Isolated
Entry Substrate
Sulfoneb
/
min yield/%
1
2
3
4
5
6
7
8
9
0
PhSCH3
PhSO2CH3
95
37
62
25
25
30
32
72
30
35
88
86
88
88
88
82
77
70
85
81
4-MeOPhSCH3
4-MePhSCH3
4-ClPhSCH3
PhSPh
PhCH2SCH2Ph
PhSC2H5
4-NO2PhSCH2Ph 4-NO2PhSO2CH2Ph
t-C4H9SCH3
PhSCH2Ph
4-MeOPhSO2CH3
4-MePhSO2CH3
4-ClPhSO2CH3
PhSO2Ph
PhCH2SO2CH2Ph
PhSO2C2H5
t-C4H9SO2CH3
PhSO2CH2Ph
11
1
the existing procedures for similar transformation.
aAll reactions were carried out at room temperature using
oxidant-to-substrate molar ratio of 2:1 and 20­30 mol %
PhenHTB used as catalyst in CH3CN­H2O as the solvent. All
products were identified by their IR and NMR spectral data and
comparison of their mp with published data.10
This paper is dedicated to Professor Teruaki
Mukaiyama in celebration of the 40th anniversary of the
Mukaiyama aldol reaction.
b
References and Notes
1
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+
Br2
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N
R
N
N
N
HBr
HBr3
Br
1
2
1
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Br
R
S
+ Br2
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S R
.
3
4
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H2O2, H2O
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9, 8163.
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O
S
OH
O
S
-
H2O
1
2
1
2
1
2
R
R
R
S R
OOH
HBr + H2O2
+
2 HBr
R
R
O
2
Br2 + 2H2O
2
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Scheme 5. Proposed mechanism for oxidation of sufides to
sulfoxides and sulfones by H2O2 catalysed by PhenHTB.
5
6
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removed under reduced pressure to give the corresponding
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Evaporation of the solvent yielded the corresponding sulfoxides.
Similar procedure was adopted for the formation of sulfones
from sulfides (Table 2). But in this case the catalyst amount was
increased to 20­30 mol % and also the amount of 30% H2O2 was
increased to 20 mmol.
From a mechanistic point of view, the oxidation of sulfides
to sulfoxides and sulfones is presumed to be achieved by the
reaction of water and Br2 formed in situ from PhenHTB via the
intermediate 1 (Scheme 5). Intermediate 1 produces sulfoxide by
reacting with water and sulfone by reaction with H2O2 and
water. In both these reactions, HBr is produced which gets
7
8
8
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9
1
0
converted into bromine (1 mol of Br is equivalent to 1 mol of
2
PhenHTB) by H2O2. Thus, the real oxidant is H2O2 and Br2
released by PhenHTB is only the catalyst since it is generated at
the end of the reactions.
11 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2014, 43, 631–633 | doi:10.1246/cl.131192
© 2014 The Chemical Society of Japan | 633