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.
`
`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
`dioxygen directly. However, this needs dioxygen activation1 by
`various chemical models2 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
`sustainable development3 with major focus on the use of green
`reaction media.4 Water is most preferred on the basis of solvent
`selection guide of the pharmaceutical industry5 and its use in
`organic reactions has gained momentum.6 Yet its use is
`currently limited due to poor aqueous solubility of organic
`compounds and efforts are made to overcome the problem by
`using surfactants.7 However, the general assumption that ions
`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.
`We followed the cyclocondensation of benzaldehyde 1
`(2.5 mmol) with 2-aminothiophenol 2 (2.5 mmol) in water in
`the presence and absence of surfactant by GCMS (Scheme 1).8z
`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
`2-phenylbenzothiazoline 3b with the 3a : 3b selectivity varying
`from 41 : 59 to 98 : 2 (3a was obtained in 32–91% yield after
`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
`
`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
`amount (4–15%) of 3b.8 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
`the oxidative conversion of benzothiazoline to benzothiazole.8
`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
`uptake and the observed 3a : 3b selectivities (Fig. 1).8z
`The oxygen content/uptake was found to be in higher
`amounts in the presence of SDOSS, SDS and SDC than that
`of the others8 and followed the order SDOSS > SDS > SDC
`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.
`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).
`The oxygen activation/fixation9 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
`water10 and the propensity of the anion of alkali metal salts to
`form hydrogen bonds with water molecules,11 (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
`
`Scheme 1 Surfactant-induced reaction of 1 with 2 in water.
`
`This journal is c The Royal Society of Chemistry 2011
`
`Chem. Commun., 2011, 47, 1797–1799
`
`1797
`
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`(ESI MS) has the ability to efficiently generate ions of non-
`covalent species in the gas phase15 and is the frontline
`biochemical
`research
`tool
`to
`study
`non-covalent
`interactions.16 We recently demonstrated ‘ion fishing’ by
`MALDI and ESI MS to detect/identify non-covalently
`formed supramolecular assemblies of small molecules.17
`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
`as [A + H]+, [A]+ and [A H]+, respectively.8 Further
`structural proof of the ion of m/z 494.57 was obtained by the
`MS–MS (MS2) that resulted in daughter ions [A H]+,
`[A C8H17]+ and [A 2C8H17 + H]+ at m/z 493.52, 381.36
`and 269.24, respectively.8 The characteristic ion peaks observed
`in the TIC of similar MS experiments on SDS indicated the
`involvement of B.8 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.
`The MS2 (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
`[A C8H17]+ and 355.0814 [SDOSS + Na C8H17 + 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
`(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
`area)18 of ions at m/z 492.5–496.5 using a fixed amount (10 mL)
`
`Fig. 4 (a) (+ve) ESI HRMS TIC of sample of 5 mol% SDOSS in 1 : 1
`MeCN–water. (b) MS2 of ion at m/z 494.3898 (TIC Fig. 4a).
`
`Fig. 1 Oxygen uptake by different surfactants.
`
`Fig. 2 Oxygen uptake with varying amounts of SDOSS.
`
`Fig. 3 The oxygen adducts of SDOSS (A) and SDS (B).
`
`cytochrome P450,12 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
`groups forms a bridgehead through the bifurcated13 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
`for heme metabolism14 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
`spectrometry
`adduct A). Electrospray ionisation mass
`
`1798
`
`Chem. Commun., 2011, 47, 1797–1799
`
`This journal is c The Royal Society of Chemistry 2011
`
`Published on 03 December 2010. Downloaded by Reprints Desk on 3/12/2020 6:01:32 PM.
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`

`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
`(27 1C, 18.2 O) was recorded. The actual oxygen content was
`determined by subtracting the blank reading from the corresponding
`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.
`
`issue on dioxygen activation by metalloenzymes and
`1 Special
`models, Acc. Chem. Res., 2007, 40, 465; J. M. Bollinger and
`C. Krebs, Curr. Opin. Chem. Biol., 2007, 11, 151.
`2 Fe (II) complexes: S. Hong, Y.-M. Lee, W. Shin, S. Fukuzumi and
`W. Nam, J. Am. Chem. Soc., 2009, 131, 13910; Au–TiO2/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.,
`2009,
`1416; Multi-Ru-substituted
`polyoxymetalates:
`131,
`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.
`3 M. Poliakoff and P. Licence, Nature, 2007, 450, 810.
`4 C. Capello, U. Fischer and K. Hungerbu¨ hler, Green Chem., 2007, 9,
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`5 K. Alfonsi, J. Colberg, P. J. Dunn, T. Fevig, S. Jennings,
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`D. A. Perry and M. Stefaniak, Green Chem., 2008, 10, 31.
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`Reactions in Aqueous Media, Wiley, New York, 1997; N. Azizi and
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`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.,
`2008, 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,
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`12 A. R. Groenhof, A. W. Ehlers and K. Lammertsma, J. Am. Chem.
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`
`Fig. 5 Ion current of the species at m/z 492.5–496.5 measured using
`different amounts of SDOSS.
`
`of stock solutions containing various amounts of SDOSS.8z A
`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
`5 mol%. An optimum value of the ion current was obtained
`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.
`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!
`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.
`
`Notes and references
`z Typical procedures. Determination of benzothiazoline : benzothiazole
`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
`
`This journal is c The Royal Society of Chemistry 2011
`
`Chem. Commun., 2011, 47, 1797–1799
`
`1799
`
`Published on 03 December 2010. Downloaded by Reprints Desk on 3/12/2020 6:01:32 PM.
`
`View Article Online
`
`Opiant Exhibit 2317
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`IPR2019-00685, IPR2019-00688, IPR2019-00694
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`