Kinetic investigation on the highly efficient and selective oxidation of sulfides to sulfoxides and sulfones with t-BuOOH catalyzed by La₂O₃

Abstract

The selective oxidation of various sulfides to sulfoxides by a simple, efficient, and environmentally benign method is of prime focus. In this paper, we have explored a highly efficient protocol for the oxidation of alkyl aryl sulfides to sulfoxides with high selectivities catalyzed by La₂O₃ in the presence of 70% t-BuOOH solution (water). We obtained predominantly the monooxygenated product. The over oxidation of sulfides to sulfones was not observed under these conditions. The resulting products are obtained in good to excellent yields within a reasonable time without the use of ligands and other additives. The epoxidation of the double bond as well as allylic oxidation are not observed with allyl sulfides. Sulfones can be obtained quantitatively by altering the reaction conditions. The surface morphology and the catalyst reusability were verified by XRD, AFM and SEM techniques. The surface area of the La₂O₃ was measured using BET isotherms.

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1. Introduction

In recent years, the selective oxidation of sulfides to the corresponding sulfoxides and sulfones by environmentally benign and viable synthetic methodologies has been of growing interest owing to the increasing environmental, ecological and economic concerns.¹ In chemical sciences, pharmaceutical sciences and biology, sulfoxides have enormous applications in the preparation of significant molecules such as drugs, flavors, germicides, catabolism regulators,² ligands in asymmetric catalysis,³ and oxo-transfer reagents.⁴ These organosulfur compounds are also important building blocks as chiral auxiliaries in organic synthesis.⁵ These compounds are very useful in the extraction and separation of radioactive and less-common metals.⁶ Most importantly, chiral sulfoxides are widely used in asymmetric synthesis.^5b In addition, chiral sulfoxides are extensively used as therapeutic agents such as antiulcer (proton pump inhibitor), antibacterial, antifungal, antiatherosclerotic, anthelmintic, antihypertensive, and cardiotonic agents.^3a,5a,7 The most straightforward and convenient synthetic approach for the production of sulfoxides is the selective oxidation of sulfides. Until now various types of oxidants such as molecular oxygen,⁸ hydrogen peroxide,⁹ organic hydroperoxide,¹⁰ hypervalent iodine¹¹ and other halogen derivatives^1g have been well documented in the literature for the preparation of sulfoxides. With respect to the use of stoichiometric oxidants such as peracids, dioxiranes, NaIO₄, MnO₂, CrO₃, SeO₂, and PhIO,^1h,12 one major environmental concern is the generation of a large amount of noxious waste.¹³ The catalytic oxidation of sulfides with H₂O₂ using some metal catalysts is important because of the formation of water as the only by-product.^7a,14 Although H₂O₂, being easily available and environmentally benign the oxidation process requires an excess amount of H₂O₂ (1.1–8.0 equiv. with respect to sulfide) to attain high yields of the sulfoxides.^3b In that context, t-BuOOH appears to be a perfect alternative as it is inexpensive, high thermal stability and safe to handle even in large quantities for the efficient and selective production of sulfoxides.¹⁵ In addition, the probability of decomposition by trace metallic impurities is less than H₂O₂. The by-product of oxidation, t-BuOH can be easily removed by distillation or rotary evaporation, thus avoiding the need for aqueous work up.¹⁶ It is worth noting that metal catalysts are essential for the production of sulfoxides in high yields and selectivities. There are several reports available based on some metal-containing catalysts, such as V,¹⁷ W,^3c,d,f,18 Ti,¹⁹ Mn,²⁰ Co,^14a Cu,²¹ Ag,²² Re,²³ Ru,²⁴ Os,²⁵ Sc,²⁶ Cr,²⁷ Fe,²⁸ and Bi²⁹ for the oxidation of sulfides to sulfoxides. In recent times, the oxidation of sulfides employing vanadium based catalytic process along with H₂O₂ in the presence of ionic liquids as solvent was described.³⁰ However, this procedure was not suitable owing to the high cost and the viscosity associated with ionic liquids. Recently, Zhao et al. reported lanthanum-containing polyoxometalate for the oxidation of various substrates including sulfides for the production of sulfoxides and sulfones in high yields with only one equiv. H₂O₂ as oxidant at room temperature.³¹ Previous research has explored that peroxotungstates immobilized on mono-layered ionic liquid-modified silica as well as multilayer ionic liquid brushes-modified silica performs well for the oxidation of sulfides to sulfoxides and sulfones with high yields, short reaction time and high substrate conversion.³² Borax-catalyzed selective oxidation of organic sulfides to sulfoxides and sulfones is obtained by controlling the pH values in high yields and selectivities.³³ First time the development of imide-catalyzed NaOCl oxidation of sulfide to sulfone and sulfoxide displayed high selectivity for the formation of products within reasonable time at room temperature.³⁴ Recently, Fe–porphyrins supported on multi-walled carbon nanotubes in the heterogeneous oxidation of sulfides have widely investigated. The results showed high conversion and selectivity for the formation of sulfoxides and sulfones within very less time.³⁵ Previously, MoO₃ catalyzed chemoselective oxidation of sulfides to sulfoxides and sulfones in the presence of other oxidized functional groups in ethanol at 50 °C is well documented in the literature.³⁶ Later, from our research group we reported Bi₂O₃ catalyzed heterogeneous oxidation of sulfides to sulfoxides with high enantioselectivities (up to 98% ee) and good yields at room temperature.²⁹ Our primary goal for the current work was the quantitative production of both sulfoxides and sulfones with very high yields and selectivities. We are happy to inform that we were able to produce both sulfoxides and sulfones up to the 98% selectivity within reasonable time. To the best of our knowledge the synthesis of sulfoxides and sulfones by heterogeneous oxidation of sulfides using La₂O₃ is still unknown. In this aspect, the commercialization of an efficient protocol for the oxidation of different sulfides using La₂O₃ with an equimolar amount of oxidant with respect to the sulfide is still challenging.

In this paper we report for the first time a new methodology for the oxidation of sulfide to sulfoxides or sulfones in high yields and selectivities without the use of expensive ligands and other additives. The over-oxidized product (sulfones) can be produced quantitatively under controlled reaction conditions.

2. Experimental details

2.1. General information

All solvents were used as the commercial anhydrous grade. Column chromatography was carried out over silica gel (100–200 mesh). ¹H and ¹³C NMR spectra were recorded on a 400 MHz spectrometer. Chemical shifts for ¹H and ¹³C NMR spectra were referenced to residual solvent resonances and are reported as parts per million relative to SiMe₄. ESI-MS spectra of the samples were recorded using Waters Q-Tof micro mass spectrometer. GC-MS were recorded using Jeol JMS GC-Mate II instrument. HPLC analysis was done with Waters HPLC instrument fitted with Waters 515 pump and Waters 2487 dual λ absorbance detector.

2.2. Materials

All the substrates used in this study were purchased from Aldrich and used as received. The solvents, along with t-BuOOH, were purchased from Ranchem, India. Solvents were purified using standard methods. La₂O₃ salt used in this study was purchased from Aldrich.

2.3. General procedure for sulfoxidation

To a stirred suspension of sulfide (1 mmol) and La₂O₃ (10 mol%) in 2.5 mL of EtOAc, 70% t-BuOOH (water) (2 equiv.) was added. The mixture was heated to 90 °C and the progress of the reaction was monitored using TLC periodically until all sulfides were found to be consumed. After completion, the reaction mixture was washed with EtOAc and H₂O and the organic phase was separated and dried with Na₂SO₄. The EtOAc was evaporated and the crude residue was purified by chromatography on a silica gel column to obtain the pure products. For the preparation of sulfone, to a stirred suspension of sulfide (1 mmol) and La₂O₃ (10 mol%) in 2.5 mL of EtOAc, 70% t-BuOOH (water) (6 equiv.) was added. The mixture was heated to 150 °C and the progress of the reaction was monitored using TLC periodically until all sulfides were found to be consumed. The work-up procedure was same as above. The spectral data of the various sulfoxides were found to be satisfactory in accordance with the literature.

2.4. Procedure for the recovery of catalyst

The catalyst, La₂O₃ was recovered from the reaction mixture to examine the reusability of the catalyst. After the completion of the reaction the residue was filtered and washed with water followed by ethanol repeatedly. Next the residue was dried overnight and tested it's activity as a reusable catalyst. We tested the reusability up to 5 cycles.

2.5. X-ray diffraction (XRD)

The diffraction profile was recorded for each sample in order to examine the diffraction pattern of the material using Rigaku TTRAX 3XRD machine in the range of 10–50° (2θ) with CuKα (1.54 Å) as radiation source and 50 kV voltage and 100 mA intensity.

2.6. Atomic force microscopy (AFM)

The AFM measurements were performed on Agilent 5500 scanning probe microscope. AFM images were recorded in non-contact mode at room temperature. Commercial probes were used with a spring constant of 21–98 N m⁻¹ and a resonance frequency of about 146–236 kHz. For AFM analysis of the La₂O₃ before and after the reactions, a drop of sample suspension in EtOAc was allowed to dry on a mica surface.

2.7. Scanning electron microscopy (SEM)

The surface morphology of the samples were studied with the help of field emission scanning electron microscope (Hitachi, S-4800 FESEM) by applying accelerating voltage of 10 kV. Very little amount of the sample was put on a conducting carbon tape stuck on the stub and coated with platinum by using Hitachi E-1010 Ion Sputter system.

2.8. Surface area analyzer (BET)

BET surface area of La₂O₃ was measured by nitrogen physisorption isotherms at 77 K using the standard multipoint method using Smartsorb 92/93, India instrument. Prior to the analysis samples were degassed in a stream of nitrogen at 473 K for 24 h.

2.9. General procedure for the kinetic studies

To carry out the different kinetic experiments reversed-phase C18 HPLC (High Performance Liquid Chromatography) column and HPLC grade methanol as the eluting solvent were used. Before running the HPLC, the solvent was sonicated for 30 min. The reaction conditions were optimized by changing one parameter while keeping the others fixed (Fig. 1). The kinetic investigation for the determination of reaction order with respect to each parameter was carried out by varying the concentration of one component and keeping the rest of the components constant under standard reaction conditions. First we carried out four different sets of reaction varying the concentration of sulfide as 1 mmol, 0.5 mmol, 0.2 mmol and 0.1 mmol while keeping the other reagents constant. Then from each set of reactions we collected crude mixture at different time intervals. After working up the crude mixture, the HPLC was run and the kinetic plot was obtained from the percent conversion of reactants into the product (Fig. 7a). Similarly, the same result was obtained while varying the concentration of oxidant (t-BuOOH) and catalyst (Fig. 7b and c respectively). From the percentage conversion with time, a concentration plot was obtained which shows the disappearance of the starting material and the appearance of the corresponding products (sulfoxide and trace sulfone) with time (Fig. 5). From this data, log(rate) versus log C was plotted (Fig. 6). Again at different sulfide concentration ranging between 0.1 and 1 mmol, we plotted log(rate) vs. log C (Fig. 8).

Fig. 1 Influence of (a) amount of catalyst, (b) amount of oxidant, (c) reaction temperature, and (d) reaction time on the oxidation of thioanisole.

3. Results and discussion

Thioanisole was chosen as a model substrate to optimize the reaction conditions for the oxidation of various sulfides in the presence of different solvents, oxidants and 10 mol% of La₂O₃. The results are summarized in Table 1.

Table 1 Optimization of reaction conditions for the conversion of methyl(4-tolyl)sulfane to 1-methyl-4-(methylsulfinyl)benzene with different solvents, oxidants with La₂O₃ (10 mol%)^a

Entry	Catalyst	Solvent	Oxidant	Time^b (h)	Yield^c (%)	Selectivity^d (%)
a The methyl(4-tolyl)sulfane (1 mmol), 70% t-BuOOH (water) (2 equiv.) and La2O3 (10 mol%) refluxed in EtOAc.b Monitored using TLC until all the sulfide was found consumed.c Isolated yield after column chromatography of the crude product.d The selectivities of sulfide were analyzed by ^1H NMR.
1	La2O3	H2O	t-BuOOH	6	65	88
2	La2O3	CH2Cl2	t-BuOOH	5	78	90
3	La2O3	MeCN	t-BuOOH	4	85	94
4	La2O3	MeNO2	t-BuOOH	5	88	95
5	La2O3	Toluene	t-BuOOH	7	75	92
6	La2O3	EtOAc	t-BuOOH	4	90	98
7	La2O3	THF	t-BuOOH	7	81	90
8	La2O3	EtOAc	H2O2	10	79	89
9	—	EtOAc	t-BuOOH	4	10	65
10	LaCl3	EtOAc	t-BuOOH	10	54	69
11	La(OAc)3	EtOAc	t-BuOOH	8	68	83

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The conversion of methyl phenyl sulfide to methylsulfinylbenzene is extremely facile in the presence of 10 mol% La₂O₃ and 2 equiv. 70% t-BuOOH (water) as the oxidant in EtOAc under reflux condition for 4 h without the use of expensive ligands and other additives. In EtOAc, the reaction was very fast and high product conversion was observed. In a protic solvent such as water the reaction was slow and resulted in very low conversion of reactant into product (Table 1, entry 1). We observed less yield and prolonged reaction time upon using H₂O₂ instead of t-BuOOH (Table 1, entry 8). The conversion was almost negligible (∼10%) in the absence of La₂O₃. After optimizing the reaction conditions, in order to test the efficiency of the developed catalytic system, various aryl alkyl and aryl benzyl sulfides were studied for the oxidation reaction. The results are summarized in (Table 2). Various sulfides were oxidized to the corresponding sulfoxides in high yields (∼91%) and selectivities (∼98%) without much affected by the steric and electronic effects of the substituents on the aromatic rings (Table 2). The solvation effect of the sulfoxide by water deters the rate of further oxidation to the sulfone by decreasing the nucleophilicity of the sulfur atom.³⁷ Thus; the reactivity of the sulfoxide towards peroxide oxidation is lowered. The probability of sulfide oxidation is high over the possibility of allylic oxidation as well as epoxidation of a double bond (Table 2, entry 15).³⁸

Table 2 La₂O₃ catalyzed selective oxidation of sulfides to sulfoxides^a

Entry	Substrate	Time^b (h)	Yield^c (%)	Conversion^d (%)	Selectivity^d (%)
a Reactions performed in EtOAc with sulfide (1 mmol), La2O3 (10 mol%) and 70% t-BuOOH (2 equiv.) under reflux condition.b Monitored using TLC until all sulfide was found consumed.c Isolated yield after column chromatography of the crude reaction mixture.d The conversion of sulfide and selectivities were analyzed by ^1H NMR.
1		4	90	96	98
2		5	88	94	97
3		4	91	95	98
4		5.5	86	93	96
5		6	88	94	96
6		5.5	87	95	97
7		4.5	90	96	97
8		6	86	93	95
9		4	90	96	97
10		5	88	94	95
11		8	83	93	95
12		5	85	94	96
13		6	89	95	95
14		7	90	96	97
15		6	86	93	96

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Later we optimized the reaction conditions kinetically by changing one specific parameter while keeping the others unaltered. The influences of the reaction parameters such as the amount of catalyst, amount of t-BuOOH, reaction temperature, and time on the catalytic performances are shown in Fig. 1. For the efficient and selective production of sulfoxide, the optimal reaction conditions were found to be 10 mol% La₂O₃, 2 equiv. 70% t-BuOOH (water), 90 °C reaction temperatures, and 4 h reaction time.

With increases in the amount of the oxidant t-BuOOH, reaction temperature and time, the selectivity for the formation of monooxygenated product (sulfoxide) decreases while the selectivity for the formation of dioxygenated product (sulfone) increases quickly. After this interesting result, we were curious to see whether the sulfone could be produced quantitatively using more specified conditions. It can be seen from the data in Fig. 2 that sulfone could be quantitatively obtained at a higher reaction temperature of 150 °C and reaction time of 8 h (Table 3). This conversion needed higher amount (6 equiv.) of t-BuOOH. These results are well matched with the previous literature report.³⁹

Fig. 2 Influence of the reaction temperature on the yield for methyl phenyl sulfone in the oxidation of thioanisole with t-BuOOH at different reaction times.

Table 3 La₂O₃ catalyzed selective oxidation of sulfides to sulfones^a

Entry	Substrate	Time^b (h)	Yield^c (%)	Conversion^d (%)	Selectivity^d (%)
a Reactions performed in EtOAc with sulfide (1 mmol), La2O3 (10 mol%) and 70% t-BuOOH (6 equiv.) under reflux condition.b Monitored using TLC until all sulfide was found consumed.c Isolated yield after column chromatography of the crude reaction mixture.d The conversion of sulfide and selectivities were analyzed by ^1H NMR.
1		8	88	94	96
2		9.5	85	91	94
3		8	89	93	96
4		10	83	90	93
5		10.5	86	92	94
6		10	84	92	94
7		9	88	94	95
8		10.5	83	90	92
9		8.5	88	94	95
10		9.5	85	91	92
11		13	81	91	93
12		9.5	83	92	93
13		11	86	92	92
14		12	88	94	95
15		11	83	90	94

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We are delighted to inform that the catalyst can be recovered easily after washing the residue with water and ethanol repeatedly. The residue was dried overnight and recycled conveniently. We have tested up to five cycles choosing thioanisole as the model substrate. The results are depicted in Fig. 3 and it can be concluded that no significant loss of conversion and selectivity was observed after five runs, indicating reusability of catalyst (Table 4).

Fig. 3 Recycling of the catalyst in the oxidation of thioanisole with t-BuOOH.

Table 4 Results for the La₂O₃ catalyzed selective oxidation of sulfides to sulfoxides in different cycles^a

Cycle no.	Time^b (h)	Yield^c (%)
a Reactions performed in EtOAc with sulfide (1 mmol), La2O3 (10 mol%) and 70% t-BuOOH (2 equiv.) under reflux condition.b Monitored using TLC until all sulfide was found consumed.c Isolated yield after column chromatography of the crude reaction mixture.
1	4	90
2	4	90
3	4.1	89
4	4.1	89
5	4.2	88.5

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Next, the catalyst reusability was verified by the XRD, AFM and FESEM techniques. The powder X-ray diffraction (XRD) patterns of the La₂O₃ before and after the reaction are presented in Fig. 4a–c in the angular range of 10–50° 2θ. By careful observation of all the peaks and the 2d-spacing of different peaks in the XRD diffraction pattern shows that the catalyst is unaltered and can be conveniently recovered and recycled without notable changes in the yield and the catalytic activity (Fig. 4a–c). The 2d-spacing of different peaks of fresh La₂O₃ (11.32 Å, 6.37 Å, 4.56 Å and 3.74 Å) and La₂O₃ after 3 (11.30 Å, 6.38 Å, 4.56 Å and 3.74 Å) and 5 (11.35 Å, 6.38 Å, 4.56 Å and 3.74 Å) run and the representative XRD patterns are identical, thus proving the reusability of the catalyst.³¹ To further confirm the reusability of the catalyst, AFM was done in non-contact mode. AFM images of the catalyst, La₂O₃ before and after the reactions, are exhibited in Fig. S1a–c (see ESI†). The same triangle like morphological representation of the fresh and recovered La₂O₃ proves the reusability of the catalyst employed (Fig. S1a–c) (see ESI†). The SEM images of the pure La₂O₃ and the recovered La₂O₃ after 2, 4 and 5 run represent the same cotton-like porous nature of the fresh and recovered La₂O₃. The images also show the same irregularly shaped agglomerations of fresh and recovered La₂O₃ (Fig. 4d–g). The surface property and the micro structural analysis confirm that the catalytic property of the La₂O₃ after 5 run is retained thus can be used as a recyclable catalyst.

Fig. 4 X-ray diffraction patterns of the catalyst: (a) before reaction, (b) after 3 run, and (c) after 5 run; FESEM images of the catalyst: (d) before reaction, (e) after 2 run, (f) after 4 run, and (g) after 5 run.

Next, in order to gain some insight about the surface area of the catalyst used in this study, we carried out Brunauer–Emmett–Teller (BET) surface area analysis of the catalyst employed. BET⁴⁰ surface area results implied a surface area of ∼11.92 m² g⁻¹. The adsorption–desorption isotherms and BET surface area results are depicted in Fig. S2 (see ESI†).

After that, we explored kinetic studies for the oxidation of sulfide choosing thioanisole as a testing substrate. High Performance Liquid Chromatography (HPLC) was used for carrying out the experiments as it can determine the concentration of various starting materials and product present as a function of time. The reaction kinetics was investigated by monitoring the conversion of sulfide to corresponding sulfoxides at different time intervals. A concentration plot shows the disappearance of the starting material and the appearance of the corresponding product with time (Fig. 5). We observed that initially the conversion increased rapidly, reached a maximum (∼98%) at 4 h reaction time and displayed no change with further reaction time (Fig. 5). In addition the selectivity for the formation of sulfone, which is caused by further oxidation of the sulfoxides, remained almost same over the reaction period of 4 h (Fig. 5).

Fig. 5 Concentration of sulfide, sulfoxide and sulfone as a function of reaction time with 10 mol% La₂O₃ and 2 equiv. 70% t-BuOOH in EtOAc reflux at 90 °C.

Next, we wanted to calculate the order (n) and rate constant (k) using van't Hoff differential method. At different concentration, the rate of the reaction was calculated by estimating the slope of the tangent at each point on the concentration plot which shows the disappearance of the starting material and the appearance of the corresponding product with time. From this data, log(rate) vs. log C was plotted (Fig. 6) and the order (n) and rate constant (k) were obtained from the slope of the line and its intercept. From the plot we observe it follows second order kinetics (n = 2.003 ≈ 2) and the rate constant is 5.62 × 10⁻⁵ L mol⁻¹ s⁻¹.

Fig. 6 van't Hoff differential plot for the oxidation of thioanisole with 10 mol% La₂O₃ and 2 equiv. 70% t-BuOOH in EtOAc reflux at 90 °C.

In the next segment of our study, we wanted to investigate the reaction order with respect to each parameter by varying the concentration of one component while keeping the rest of the components constant under standard reaction conditions. In the case of sulfide, the reaction order was found to be nearly one at the range of 0.1–1 mmol (Fig. 7a). This result was expected and only shows that sulfide is involved in the rate-limiting step, in good agreement with literature reports.⁴¹ Again, at low sulfide concentration product conversion is less and at higher sulfide concentration product concentration is more. Performing the reaction by varying the concentration of sulfide and keeping the rest constant, the concentration of the monooxygenated product was maximum with 1.0 mmol of sulfide (Fig. 7a). When the amount of oxidant (t-BuOOH) was varied from 0.5–2 equiv., the concentration of the resulting product was maximum at 2 equiv. of t-BuOOH. The reaction order was approximately one over the entire range of 0.5–2 equiv. of t-BuOOH (Fig. 7b).

Fig. 7 Reaction profile with various concentration of (a) sulfide, (b) oxidant, and (c) catalyst (La₂O₃) (5% error margin).

Next, we were interested to see the effect of catalyst loadings on the reaction environment. In the case of catalyst (La₂O₃), the reaction order was found to be approximately one at the range of 0.5–10 mol% catalytic loadings (Fig. 7c). Here too, the maximum product formation was noted with 10 mol% of La₂O₃ when the amount of catalyst was varied from 0.5–10 mol%. At low catalytic loadings of La₂O₃ we observe very less conversion of reactants into the product but at higher catalyst loadings product concentration is more.

Next, we studied the reaction order for sulfide. We observed sulfide displayed variable reaction order in the concentration range of 0.1–1 mmol. The reaction orders appeared to be weakly positive at lower sulfide concentration but strongly negative at relatively higher concentrations (Fig. 8).⁴²

Fig. 8 Reaction order for sulfides from 0.1–1.0 mmol concentration.

The plausible mechanism for the oxidation of sulfide (Scheme 1) can be explained based on the reports available in the literature.^28b The metal first binds with t-BuOOH and forms La(IV) active oxidant. These species reacts with the sulfide and forms oxidant–substrate complex and the transfer of oxygen takes place. The reaction rate is dependent on the concentrations of the La(IV) active oxidant and sulfide substrate. This is supported by the kinetic studies providing second order reaction. The active oxidant and oxidant–substrate complex intermediate species are transient in nature thereby allowing the unusual oxidation state on the La metal.

Scheme 1 Proposed reaction pathway for the oxidation of sulfide.

4. Conclusions

In conclusion, we have explored a new method for the selective oxidation of various aromatic and aliphatic sulfides to the corresponding sulfoxides with 70% t-BuOOH (water) as the oxidant catalyzed by La₂O₃ without the use of ligands and other additives. The described method in this paper clearly indicate that the methodology involves mild, environmentally friendly and selective synthesis of sulfoxides from both aromatic and aliphatic sulfides. The resulting products were obtained in good yields. By changing the reaction conditions sulfones can be prepared quantitatively. Epoxidation and allylic oxidation products are hardly observed. Kinetic studies gave information about the optimal reaction conditions for the oxidation reaction to occur and rate constant of this reaction. The order of the reaction with respect to the different reaction parameters is also studied. The catalyst reusability was verified by careful observation of the surface morphology using XRD, AFM and SEM techniques. The surface area of the La₂O₃ was measured using BET isotherm. The results of our method are comparable with the previous metal oxides reported protocol.

Acknowledgements

The authors thank Indian Institute of Technology Patna for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: AFM images and BET isotherm. See DOI: 10.1039/c4ra14391d

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