Surfactant mediated oxygen reuptake in water for green aerobic oxidation: Mass-spectrometric determination of discrete intermediates to correlate oxygen uptake with oxidation efficiency

Article abstract of DOI:10.1039/c0cc03166f

A novel strategy of catalytic green aerobic oxidation by surfactant-mediated oxygen reuptake in water offers a new dimension to the applications of surfactants to look beyond as solubility aids and a conceptual advancement in understanding the role of surfactants in aquatic organic reactions through mass spectrometry guided identification of discrete intermediates.

Full text of DOI:10.1039/c0cc03166f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Surfactant mediated oxygen reuptake in water for green aerobic
oxidation: mass-spectrometric determination of discrete intermediates
to correlate oxygen uptake with oxidation efficiencyw
Naisargee Parikh, Dinesh Kumar, Sudipta Raha Roy and Asit K. Chakraborti*
Received 10th August 2010, Accepted 19th November 2010
DOI: 10.1039/c0cc03166f
A novel strategy of catalytic green aerobic oxidation by
surfactant-mediated oxygen reuptake in water offers a new
dimension to the applications of surfactants to look beyond as
solubility aids and a conceptual advancement in understanding
the role of surfactants in aquatic organic reactions through mass
spectrometry guided identification of discrete intermediates.
deoxycholate (SDC) were found to be the next best effective
catalysts and exhibited 3a : 3b selectivities of 98 : 2 and 95 : 5
and afforded 3a in 91 and 87% yields, respectively. Further
studies on the optimization of the critical amount of SDOSS
and the reaction time, revealed that the use of 5 mol% of
SDOSS afforded 100% conversion to 3a after 1 h at rt. The use
of lesser quantities of SDOSS resulted in accumulation of some
8
Aerobic oxidation offers environmentally benign chemistries
as toxic and corrosive stoichiometric oxidants (e.g., peracids,
iodoxyarenes, hydroperoxides or metallic oxidants such as lead
tetraacetate, ceric ammonium nitrate etc.) can be replaced by
amount (4–15%) of 3b. Elaborative GCMS studies on the
SDOSS-catalysed reaction of 1 and 2 revealed that the use of
water has two beneficial effects: (i) acceleration of the rate of
cyclocondensation of 1 with 2 to form 3b and (ii) enhancement
of the rate of oxidation of 3b to 3a and that there is
enhancement of oxygen transporter ability of water through
oxygen uptake/reuptake mediated by SDOSS in accelerating
1
dioxygen directly. However, this needs dioxygen activation by
2
various chemical models and has a long-standing interest. A
non-heme and transition-metal free model of dioxygen
activation for green aerobic oxidation processes remains
unexplored and is a challenging task. The adverse effect of
chemical processes on the environment has put a thrust on
8
the oxidative conversion of benzothiazoline to benzothiazole.
To establish oxygen reuptake/activation as a specific role of
the surfactant we measured the oxygen uptake property/ability
of the surfactants to find any correlation between the oxygen
3
sustainable development with major focus on the use of green
4
reaction media. Water is most preferred on the basis of solvent
8
uptake and the observed 3a : 3b selectivities (Fig. 1). z
5
selection guide of the pharmaceutical industry and its use in
The oxygen content/uptake was found to be in higher
amounts in the presence of SDOSS, SDS and SDC than that
6
organic reactions has gained momentum. Yet its use is
8
of the others and followed the order SDOSS > SDS > SDC
currently limited due to poor aqueous solubility of organic
compounds and efforts are made to overcome the problem by
7
using surfactants. However, the general assumption that ions
which is in parallelism with the 3a : 3b selectivities observed in
carrying out the cyclocondensation of 1 with 2 in water in the
presence of these surfactants. A good correlation was also
observed between the optimal oxygen content/uptake and the
effective concentration/amount of SDOSS (Fig. 2). Maximum
oxygen count/uptake was observed with 5 mol% of SDOSS
which is evidently the critical micellar concentration to obtain
the maximum conversion to benzothiazole.
in dissolved water have a strong effect on the bulk properties of
liquid water led us to consider the role of surfactants beyond a
solubility aid. Herein we report surfactant-induced direct
fixation/activation of aerial oxygen by oxygen reuptake in
water for an efficient strategy for green oxidation
demonstrating that the role of surfactants extends beyond
the solubility enhancing property.
The role of the surfactant for oxygen activation/reuptake
can be visualised by the formation of the non-covalent adducts
A and B, as exemplified in the case of SDOSS and SDS,
respectively, (Fig. 3).
We followed the cyclocondensation of benzaldehyde 1
(
2.5 mmol) with 2-aminothiophenol 2 (2.5 mmol) in water in
8
the presence and absence of surfactant by GCMS (Scheme 1). z
Sodium dioctyl sulfosuccinate (SDOSS) showed 100%
conversion to the desired 2-phenylbenzothiazole 3a (96%
isolated yield after purification) and was found to be the best
promoter. Other surfactants afforded a mixture of 3a and the
9
The oxygen activation/fixation occurs via a cooperative
hydrogen bond network involving (i) one hydrogen of water
and the oxyanionic site of sulfonate/sulfate group of SDOSS/
SDS due to a labile hydrogen bond network at the surface of
1
0
2
-phenylbenzothiazoline 3b with the 3a : 3b selectivity varying
water and the propensity of the anion of alkali metal salts to
1
1
from 41 : 59 to 98 : 2 (3a was obtained in 32–91% yield after
form hydrogen bonds with water molecules, (ii) the other
hydrogen of the water molecule and one of the oxygen atoms
of OQO, akin to proton assisted O–O bond scission in
purification). Sodium dodecyl sulfate (SDS) and sodium
Department of Medicinal Chemistry, National Institute of
Pharmaceutical Education and Research (NIPER), Sector 67,
S. A. S. Nagar, Punjab 160 062, India.
E-mail: akchakraborti@niper.ac.in; Fax: +91 (0)172-2214692
w Electronic supplementary information (ESI) available: Detailed
experimental procedures and scanned spectra. See DOI: 10.1039/
c0cc03166f
Scheme 1 Surfactant-induced reaction of 1 with 2 in water.
Chem. Commun., 2011, 47, 1797–1799 1797
This journal is c The Royal Society of Chemistry 2011

View Article Online
(
ESI MS) has the ability to efficiently generate ions of non-
1
5
covalent species in the gas phase and is the frontline
biochemical research tool to study non-covalent
1
interactions. We recently demonstrated ‘ion fishing’ by
6
MALDI and ESI MS to detect/identify non-covalently
1
formed supramolecular assemblies of small molecules.
7
The total ion chromatogram (TIC) under (+ve) ESI MS (on
linear ion trap) of aliquots of samples from a stock solution of
5
mol% (with respect to the aldehyde used for reaction)
of SDOSS exhibited ion peaks at m/z 495.57, 494.58, 493.56
+
+
+
8
as [A + H] , [A] and [A ꢀ H] , respectively. Further
structural proof of the ion of m/z 494.57 was obtained by the
2
+
MS–MS (MS ) that resulted in daughter ions [A ꢀ H] ,
+
+
Fig. 1 Oxygen uptake by different surfactants.
[A ꢀ C H ] and [A ꢀ 2C H + H] at m/z 493.52, 381.36
8
17
8
17
8
and 269.24, respectively. The characteristic ion peaks observed
in the TIC of similar MS experiments on SDS indicated the
8
involvement of B. More convincing evidences were obtained
from the +ve ESI HRMS (on Q-TOF). The TIC (Fig. 4a)
exhibited ion peaks at m/z 495.2469, 494.3898 and 493.3865
+
+
+
corresponding to [A + H] , [A] and [A ꢀ H] , respectively.
2
The MS (Fig. 4b) of the ion of m/z 494.3898 resulted in daughter
+
ions at m/z 493.3865, 467.2043 [SDOSS + Na] , 381.2597
+
17
]
+
[
A ꢀ C
8
H
and 355.0814 [SDOSS + Na ꢀ C
8
H17 + H] .
The poor catalytic ability of other surfactants can be
accounted for their inefficiency to form the oxygen adduct
and was demonstrated by the fact that when an aqueous soln
Fig. 2 Oxygen uptake with varying amounts of SDOSS.
(
(
5 mol%) of Tween 80 and cetyl trimethyl ammonium bromide
CTAB), as representative surfactants that afforded poor
conversion to benzothiazole, were subjected to (+ve) ESI
MS, no ion peak corresponding to the respective oxygen
adduct could be detected in the TIC. Additionally, these
surfactants also showed inferior oxygen uptake, as measured
with the microelectrode.
It was reasonably conceived that the catalytic efficiency of
SDOSS should correspond to the amount/concentration of
the oxygen adduct formed/present during the course of the
reaction. Hence the estimation of the oxygen adduct was
performed by measuring the ion current (represented by peak
Fig. 3 The oxygen adducts of SDOSS (A) and SDS (B).
1
2
cytochrome P450, followed by (iii) charge–charge interaction
between the electron deficient (due to hydrogen bond
formation of the other oxygen atom with the water molecule)
oxygen atom of OQO and the oxygen lone pair of electrons in
the SQO of SDOSS/SDS. The oxygen atom of one of the SQO
1
8
area) of ions at m/z 492.5–496.5 using a fixed amount (10 mL)
1
3
groups forms a bridgehead through the bifurcated hydrogen
bond with water and provides rigidity to the structure. The
absence of such hydrogen bonds and charge–charge
interaction network in organic solvents explains the poor
conversion and low 3a : 3b selectivity observed in these
solvents. Although a clear solution of SDOSS is formed in
these solvents they are not conducive for oxygen uptake/
reuptake due to the absence of the desired co-operative non-
covalent hydrogen bond interactions. This further suggests
that the role of SDOSS in aqueous medium extends beyond
as its function as a solubility enhancer. These structures mimic
the protein environment of water-assisted dioxygen activation
1
4
for heme metabolism and constitute a non-heme and
transition metal-free model.
In support of oxygen activation/uptake by SDOSS we
planned to detect/identify the actual catalytic species (oxygen
adduct A). Electrospray ionisation mass spectrometry
Fig. 4 (a) (+ve) ESI HRMS TIC of sample of 5 mol% SDOSS in 1 : 1
2
MeCN–water. (b) MS of ion at m/z 494.3898 (TIC Fig. 4a).
1
798 Chem. Commun., 2011, 47, 1797–1799
This journal is c The Royal Society of Chemistry 2011

View Article Online
with EtOAc (5 mL). An aliquot portion (500 mL) of the supernatant
EtOAc layer was taken out and subjected to GCMS to observe the
benzothiazoline : benzothiazole selectivity of 2 : 98. Determination
of oxygen uptake by surfactants: In a blank experiment ultrapure
water (1 mL) (27 1C, 18.2 O) was taken into the cuvette of the
Oxygraph and the oxygen content was recorded (0–5 min) at 70 rpm.
Following a similar procedure the oxygen content/uptake of 1 mL
freshly prepared 0.01 M solution of the surfactant in ultrapure water
(
determined by subtracting the blank reading from the corresponding
27 1C, 18.2 O) was recorded. The actual oxygen content was
ꢀ1
reading of the analyte solution at 5 min and normalised to nmol mL
.
Determination of ion current of the oxygen adduct of SDOSS: An
aliquot portion (10 mL) of a solution of SDOSS (0.022 g) in 5 mL of
water–acetonitrile (1 : 1) was subjected to (+ve) ESI MS (linear ion
trap) and the ion current was determined by measuring the area of the
ion peak corresponding to the oxygen adduct of SDOSS.
Fig. 5 Ion current of the species at m/z 492.5–496.5 measured using
different amounts of SDOSS.
8
of stock solutions containing various amounts of SDOSS. z A
1
Special issue on dioxygen activation by metalloenzymes and
models, Acc. Chem. Res., 2007, 40, 465; J. M. Bollinger and
C. Krebs, Curr. Opin. Chem. Biol., 2007, 11, 151.
Fe (II) complexes: S. Hong, Y.-M. Lee, W. Shin, S. Fukuzumi and
2
W. Nam, J. Am. Chem. Soc., 2009, 131, 13910; Au–TiO /gold
cluster cations: S. M. Lang, T. M. Bernhardt, R. N. Barnett,
B. Yoon and U. Landman, J. Am. Chem. Soc., 2009, 131, 8939;
Pd species: J. M. Keith and W. A. Goddard III, J. Am. Chem. Soc.,
good correlation was observed between the ion current and the
critical amounts of SDOSS required for the formation of the
benzothiazole (Fig. 5). An increasing trend of the ion current
was observed with an increase in the concentration of SDOSS
from 1–4 mol%. A sharp increase in the ion current was
observed in changing the amount of SDOSS from 4 to
2
2
009, 131, 1416; Multi-Ru-substituted polyoxymetalates:
5
mol%. An optimum value of the ion current was obtained
A. E. Kuznetsov, Y. V. Geletii, C. L. Hill, K. Morokuma and
D. G. Musaev, J. Am. Chem. Soc., 2009, 131, 6844; Cu-catalyst:
B. Zhang and N. Jiao, J. Am. Chem. Soc., 2010, 132, 28.
M. Poliakoff and P. Licence, Nature, 2007, 450, 810.
for the sample of the 5 mol% solution that also corresponds to
the critical concentration/amount of SDOSS required to
obtain the best conversion (100%) to 3a.
3
4
C. Capello, U. Fischer and K. Hungerbu
9
¨
hler, Green Chem., 2007, 9,
In conclusion, the novel findings of catalytic aerial oxygen
reuptake in aqueous medium offers a new dimension to the
chemistry of surfactants to look beyond them as simply
solubility aids. The identification of the non-covalent adduct
as a discrete species in dioxygen activation and estimation/
correlation of oxygen activation/uptake as a function of the
ion current of the catalytic species makes the basis for rational
selection of a surfactant for aerobic oxidation in aqueous
medium and provides a non-heme and transition metal-free
model for dioxygen activation under ambient conditions.
Further implication of these findings is that it would provide
insight into the microenvironment of reverse micelles that
enable them to carry out various reactions in confinement,
and the microscopic origin of the role of surfactants as
solubility aids, and as to whether gaseous uptake is the
prerequisite in imparting the solubility enhancing property of
these materials!
27.
5 K. Alfonsi, J. Colberg, P. J. Dunn, T. Fevig, S. Jennings,
T. A. Johnson, H. P. Kleine, C. Knight, M. A. Nagy,
D. A. Perry and M. Stefaniak, Green Chem., 2008, 10, 31.
6
Organic Synthesis in Water, ed. P. A. Grieco, Blackie Academic and
Professional, London, 1998; C. J. Li and T. H. Chang, in Organic
Reactions in Aqueous Media, Wiley, New York, 1997; N. Azizi and
M. R. Saidi, Org. Lett., 2005, 7, 3649; S. V. Chankeshwara and
A. K. Chakraborti, Org. Lett., 2006, 8, 3259; G. L. Khatik,
R. Kumar and A. K. Chakraborti, Org. Lett., 2006, 8, 2433;
A. K. Chakraborti, S. Rudrawar, K. B. Jadhav, G. Kaur and
S. V. Chankeshwara, Green Chem., 2007, 9, 1335, and references
therein.
7
G. Sharma, R. Kumar and A. K. Chakraborti, Tetrahedron Lett.,
2
008, 49, 4269, and references therein.
8 See supporting information (ESIw).
9
The polar head group of a charged alkyl surfactant is oriented at
the air/water interface. D. K. Hore, D. K. Beaman and
G. L. Richmond, J. Am. Chem. Soc., 2005, 127, 9356.
10 N. Galamba and B. J. Costa Cabral, J. Am. Chem. Soc., 2008, 130,
17955.
1
1
1 I. A. Heisler and S. R. Meech, Science, 2010, 327, 857.
2 A. R. Groenhof, A. W. Ehlers and K. Lammertsma, J. Am. Chem.
Soc., 2007, 129, 6204.
Financial support from DST, New Delhi, India (no. SR/S1/
OC-33/2008) is gratefully acknowledged. The authors D. K.
and S. R. R. thank CSIR, New Delhi, India for senior research
fellowships.
13 T. Steiner, Angew. Chem., Int. Ed., 2002, 41, 48.
14 T. Kamachi and K. Yoshizawa, J. Am. Chem. Soc., 2005, 127,
0686.
15 J. B. Fenn, M. Mann, C. K. Meng, S. F. Wong and
1
Notes and references
C. M. Whitehouse, Science, 1989, 246, 64.
z Typical procedures. Determination of benzothiazoline : benzothiazole
16 G. Siuzdak, Proc. Natl. Acad. Sci. U. S. A., 1994, 91, 11290.
17 A. K. Chakraborti and S. Raha Roy, Org. Lett., 2010, 12, 3866;
A. K. Chakraborti and S. Raha Roy, J. Am. Chem. Soc., 2009, 131,
6902; A. K. Chakraborti, S. Raha Roy, D. Kumar and P. Chopra,
Green Chem., 2008, 10, 1111.
selectivity during cyclocondensation of benzaldehyde with
2-aminothiophenol in the presence of surfactant: To a magnetically
stirred suspension of SDOSS (0.044 g, 5 mol%) in water (5 mL) was
added benzaldehyde (0.21 g, 2 mmol) and 2-aminothiophenol (0.25 g,
2 mmol) and the mixture was stirred magnetically at rt. After
completion of reaction (TLC, 1 h), the reaction mixture was diluted
18 Z. B. Alfassi, R. E. Hui, B. L. Milman and P. Neta, Anal. Bioanal.
Chem., 2003, 377, 159.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1797–1799 1799