HbA/H2O2: an efficient biomimetic catalytic system for the oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1016/j.tetlet.2007.08.128

Human hemoglobin (HbA) catalyzed chemo-selective oxidation of sulfides to sulfoxides has been reported using hydrogen peroxide as an oxidant in a phosphate buffer. This biomimetic catalytic procedure was found to be simple, environmentally friendly, selec

Full text of DOI:10.1016/j.tetlet.2007.08.128

Tetrahedron Letters 48 (2007) 7857–7860
HbA/H2O2: an efficient biomimetic catalytic system for
the oxidation of sulfides to sulfoxidesI
Atul Kumar* and Akanksha
Medicinal and Process Chemistry Division, Central Drug Research Institute, Lucknow 226 001, India
Received 17 July 2007; revised 23 August 2007; accepted 30 August 2007
Available online 4 September 2007
Abstract—Human hemoglobin (HbA) catalyzed chemo-selective oxidation of sulfides to sulfoxides has been reported using hydro-
gen peroxide as an oxidant in a phosphate buffer. This biomimetic catalytic procedure was found to be simple, environmentally
friendly, selective, and high yielding. Purified human oxyhemoglobin (HbA0), crude human oxyhemoglobin, and ferryl oxyhemo-
globin were used for sulfoxidation. HbA0 and ferryl oxyhemoglobin (Fe+4@O) efficiently catalyze the sulfoxidation reaction without
over oxidation to the sulfone.
Ó 2007 Elsevier Ltd. All rights reserved.
Sulfoxidation reactions have significant importance in
organic chemistry, medicinal chemistry, and drug
metabolism.1 Organic sulfoxides are extensively used
as anti-ulcer2 (omeprazole, proton pump inhibitor),
antibacterial,3 antifungal,4 antiatherosclerotic,5 antihy-
pertensive,6 anthelmintic7 and cardiotonic agents,8 psy-
chotropics,9 vasodilators10 and as CNS/CVZ11
stimulants (Fig. 1).
O
S
O
S
N
S
rac-allicin
rac-esproquin
O
H
O
S
O
O
N
Ph
NH
NH2
N
S
Ph
O
rac-ricobendazole
rac-modafinil
In addition to this, sulfoxides are also valuable synthons
used in synthetic organic reactions for carbon–carbon
bond formation,12 as chiral auxiliaries,13 in Diels–Alder
reactions14 and for metal centered catalysis.15
O
O
H3CO
H3C
OCH3
CH3
N
S
R
S
N
H
N
O
There are several methods reported in the literature for
sulfoxidation using reagents such as TEMPO-linked
metalloporphyrins,16 hypervalent iodine compounds,17
H2O2/ZrCl4,18 transition-metal (Ti, Mo, Fe, V, W, Re,
Thioflavone-S-oxide
Omeprazole
Figure 1. Structures of organic sulfoxides used as drug candidates.
Ru)/H2O2 and halogens/H2O2.20 Unfortunately many
19
or sulfinic acids as byproducts. Another major problem
is over-oxidation to the sulfone. Therefore, there is still a
need for an efficient, inexpensive, selective, and environ-
mentally friendly catalyst for sulfoxidation. Biocatalysts
such as peroxidases21 and myoglobin22 are used for
selective sulfoxidation reactions, but their low opera-
tional stability with hydrogen peroxide limits their appli-
cation. Human hemoglobin is an iron porphyrin
metalloprotein, which acts as an oxygen carrier in
biological systems.
of these processes suffer from the use of toxic and expen-
sive oxidizing systems, complicated and resource-wast-
ing workup procedures, employment of chlorinated
solvents, and the occurrence of side reactions. The use
of halogen-mediated systems, for example, led to the
generation of halogen-substituted sulfides or sulfonic
Keywords: Human hemoglobin (HbA); Oxyhemoglobin; Ferryl
oxyhemoglobin; Sulfoxidation; Chemoselective.
q CDRI Communication No. 7274.
*
Herein we report the biocatalytic activity of human
hemoglobin (HbA) and H2O2 for sulfoxidation. We
Corresponding author. Tel.: +91 522 2612411; fax: +91 522
2623405; e-mail: dratulsax@gmail.com
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.08.128

7858
A. Kumar, Akanksha / Tetrahedron Letters 48 (2007) 7857–7860
preferred hydrogen peroxide as the oxidant over other
available oxidizing agents since it is cheap, operationally
safe, environmentally friendly, and produces only
water as a byproduct, which reduces purification
requirements.23
corresponding sulfoxide in high yield, but this procedure
needed extra purification. Therefore, we turned our
attention to the in situ generated crude oxyhemoglobin
via dithionite (method II), but this procedure required
a longer time for the sulfoxidation reaction and also
initiated other side reactions. Method III was found to
be best as HbA with 30% of H2O2 in a slightly alkaline
phosphate buffer (pH 7.6) gave a high yield of the sulf-
oxide within a short time. This procedure was also
found to be clean and no further purification was
required.
The commercially available human hemoglobin (HbA)
contains approximately 80% of the methemoglobin
(Met Hb). Methemoglobin (Met Hb) is an oxygen carry-
ing protein hemoglobin having Fe3+ in the heme group
and is unable to transfer oxygen. Therefore, our initial
strategy was the conversion of methemoglobin into
oxyhemoglobin, which can efficiently transfer oxygen.
Hence, we converted commercial human hemoglobin
into oxyhemoglobin by treatment with dithionite in a
phosphate buffer and subsequently eluted it through a
Sephadex G-25 column to obtain the purified form of
oxyhemoglobin (HbA0).24 The percentage of oxy-Hb
of the total HbA can be determined from the absor-
bance (A) at 540 nm and at 576 nm. This equation is
based on reported molar absorptivities of oxy-Hb and
met-Hb.24
In order to check the feasibility of the HbA/H2O2 cata-
lyzed system in an organic medium, we performed the
sulfoxidation in methanol, acetonitrile, methanol, and
DCM (3:1). However in organic medium, hemoglobin
forms an emulsion and its work-up procedure was quite
tedious. Thus an organic medium was not of choice for
this catalytic enzymatic system. The HbA/H2O2 system
worked quite efficiently in 15% acetonitrile in a phos-
phate buffer.
Encouraged by these results, we turned our attention to
explore the generality and scope of the HbA/H2O2 cat-
alyzed sulfoxidation using various sulfides (Scheme 1).
70A540 ꢀ 117A576
% oxy Hb ¼
ꢁ 100
169A576 ꢀ 236A540
Using a similar protocol to that in method III, cyclic as
well as acyclic sulfides formed sulfoxides in high yields
(Table 1). No reaction could be detected when either
hemoglobin or hydrogen peroxide was used alone. This
procedure was also found to be chemoselective, other
functional groups like CH2, CO, CN and as olefin was
unaffected.
We utilized this purified oxyhemoglobin for sulfoxida-
tion (method I). Besides this we also tried two other
methods for sulfoxidation (methods II and III). In
Method II, oxyhemoglobin was generated in situ by
the reaction of dithionite in a phosphate buffer (pH
7.6) from human hemoglobin (HbA) and this crude oxy-
hemoglobin was used for sulfoxidation without any
purification. In method (III) human hemoglobin was
reacted with hydrogen peroxide in a phosphate buffer
(pH 7.6) and converted to [Met Hb–OOH] which finally
yielded ferryl oxyhemoglobin and was used for sulfoxi-
dation.25 In our preliminary experiments, methyl phenyl
sulfide was chosen as a model substrate and the reaction
was studied using 30% aqueous hydrogen peroxide with
a catalytic amount of purified oxy-Hb, in situ generated
oxyhemoglobin and with ferryl oxyhemoglobin (Fig. 2).
We observed that the purified oxyhemoglobin oxidizes
methyl phenyl sulfide quite efficiently and yielded the
O
S
S
H2O2 (30% w/v)
HbA
Phosphate buffer stir, r.t.
PH-7.6
R
R
1
2
Sulfide
Sulfoxide
Scheme 1.
H2O
2 (30% w/v)
[M etHb-OOH]
greenish coloured
Oxy Hb
brown coloured
HbA
Phosphate buffer
pH-7.6
Method III
Na2S2O4, stir, r.t.
Phosphate buffer
pH-6.5
H2O2
Ferryl oxy Hb
Unpurified
Oxy Hb
Crude Oxy Hb
Method II
Method I
(1)
(2)
SephadexG-25 column
S
O
(1)
(2)
1
S
HbAo
Purified Oxy Hb
H2O2
2
Figure 2. HbA/H2O2; a biomimetic catalytic procedure for sulfoxidation.

A. Kumar, Akanksha / Tetrahedron Letters 48 (2007) 7857–7860
7859
Table 1. HbA/H2O2 catalyzed biomimetic oxidation of sulfide to sulfoxide in phosphate buffer (PH 7.6)a,b,c
Entry
Sulfide (1)
Time (h)
Sulfoxide(2)b
Yieldsc (%)
Mp. (°C)/(lit mp.)Ref
O
SMe
30–35/ (30–30.5)18,23
1.
0.5
2a
2b
96
S
Me
O
S
S
2.
1.0
1.2
92
90
(71–72)/(72–73)23
3.a
2c
2d
204–205/ (205–207)18,23
S
S
O
O
O
S
S
4.
1.5
88
96–100
S
O
O
S
Oil17,20
CN
5.
6.
7.
1.8
1.5
1.2
2e
2f
73
69
92
CN
O
S
S
S
Oil
O
S
2g
120–122/(122–124)18
Reaction conditions: Sulfide (1 mmol), HbA (0.5–0.7 mmol), H2O2 (30% w/v, 2 mmol) in phosphate buffer (pH 7.6, 3 mL), stirred at room
temperature.
a 1 mL of 15% acetonitrile used as solvent.
b Products were characterized by 1H NMR, 13C NMR, IR, and MS spectroscopy.
c Isolated yield.
All the compounds reported in Table 1 were character-
In conclusion, we have reported an efficient and versatile
biomimetic procedure for the oxidation of sulfides to
sulfoxides catalyzed by HbA/H2O2 in a 15% acetonitrile
phosphate buffer system at room temperature. This
methodology is efficient, simple, chemoselective, and
high yielding.
1
13
ized by IR, H, and C NMR.26,27
Many reagents are available for the oxidation of sulfides
but most cause over-oxidation to sulfones. Therefore,
we examined reaction conditions such as time, tempera-
ture and the relative amount of oxidant that can cause
over-oxidation to sulfones. For this purpose, we stirred
the reaction overnight in the presence of HbA/H2O2
(50% w/v) in a phosphate buffer but we did not obtain
any sulfone, even when using an excess of hydrogen per-
oxide (Scheme 2).
Acknowledgment
We are thankful to CSIR–CDRI, for financial support.
References and notes
O
O
S
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7860
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2985, 2942, 2909, 1698, 1589, 1447, 1338, 1294, 1185, 1054,
786, 1H NMR (CDCl3, 300 MHz): d 2.90 (1H, m), 3.44
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7.91 (1H, d, J = 7.6 Hz), 8.10 (1H, d, J = 7.6 Hz), 13C
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5.55–5.70 (1H m), 7.50–7.53 (3H, m), 7.57–7.63 (2H, m),
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