Nonanebis(peroxoic acid) mediated efficient and selective oxidation of sulfide

Abstract

Two efficient and rapid protocols for the selective oxidation of sulfide in the presence of various oxidizable functionalities such as –CHO, –CH₂–OH, alkene and tert amine were investigated, using nonanebis(peroxoic acid) as an oxidant. The acetonitrile based system offers selective oxidation to sulfoxide and sulfone, while the water based system gives selective sulfone formation with a high conversion and 90–100% selectivity. Recovery and recycling of the parent acid of up to 75–80% was achieved in the water based protocol.

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Introduction

The growing interest in and applications of sulfoxide and sulfone have encouraged investigation into new methodologies of their synthesis. Sulfoxide and sulfone are two important sulphur compounds which are widely used in the synthesis of fine chemicals, ligands¹ in asymmetric catalysis, and oxo transfer reagents.² The recent applications of π-conjugated heterocyclic compounds containing vinyl sulfone and sulfoxide³ have exhibited a wide variety of optical, electrical and optoelectric properties⁴ and have drawn much attention due to their potential applications in organic optoelectronics.⁵

Ample oxidising systems have been reported for this oxidation purpose. Various transition metals catalysts such as Ti,⁶ Mo,⁷ Fe,⁸ V,⁹ W,¹⁰ Rh,¹¹ Ru,¹² Sc,¹³ Zr¹⁴ and Mn,¹⁵ Ce¹⁶ under homogeneous and heterogeneous conditions were used with hydrogen peroxide as an oxidant to carry out this transformation. Hydrogen peroxide shows a slow reactivity toward the oxidation of sulfide,¹⁷ for which reason it is either used in excess or in combination with a metal catalyst. Dioxirane is an efficient and mild oxygen transfer agent which works under both neutral and non nucleophilic conditions.^18–22 Dieva²³et al. have reported the use of chiral dioxirane for the synthesis of chiral sulfoxides. The oxidation of sulfide to sulfoxide and sulfone has been broadly studied using azohydroperoxides.^24–27 These peroxides are highly reactive and exhibit a better oxygen transfer ability. Their reactivity is on a par with that of peracids. Though dioxirane and azohydroperoxides are effective in oxidizing various sulfides to the corresponding sulfoxides and sulfones, they suffer from major drawbacks such as instability and a shock sensitive and explosive nature. Azohydroperoxides in particular are highly explosive under dry conditions, and therefore it is essential to store them in the dark under slightly moist conditions in benzene at −60 °C temperature. Though the above systems offer recycling of the catalyst and low waste generation, in most cases the reactions need to perform at a high temperature for a longer time,^28–31 and require halogenated solvents, promoters^13,32 or co-catalysts. In addition to this, over-oxidation occurs and in some cases metal residue remains in the product. Enzymes,³³ supported reagents,³⁴ cyanuril chloride,³⁵ silver nitrite with TBHP³⁶ and phosphorous oxychloride³⁷ are the other oxidizing agents used to carry out this transformation.

As a continuation of our research to explore the applications of aliphatic diperoxy acids in organic synthesis,^38–40 here we report two efficient protocols for the selective oxidation of sulfide in acetonitrile and water as solvents using nonanebis(peroxoic acid). The diperoxy acid used here is stable at room temperature, easy to handle and non-shock sensitive in nature which was conformed by DSC analysis. It is noticeable that we have recovered 75–80% of the parent acid and recycled it for up to three cycles.

Results and discussion

The reaction conditions were optimised using methyl phenyl sulfide 1a (MPS) (0.8 mmol, 1 equiv.) as a substrate and acetonitrile as a solvent at room temperature (Table 1). The effect of the oxidant on MPS was studied by varying its molar ratio from 0.5 to 1.6 equiv. (Table 1, entries 1–9). It was observed that 0.7 equiv. of the oxidant gives 94% of 2a in 7 min (Table 1, entry 3) while 1.5 equiv. of the oxidant gives 100% of 3a in 15 min (Table 1, entry 8). Along with acetonitrile, other solvents such as methanol, ethanol and ethyl acetate also show a higher conversion, but give a mixture of 2a and 3a (Table 1, entries 17–19). On the other hand, halogenated solvents such as dichloromethane, ethylene dichloride, chloroform (Table 1, entries 20–22) and acetic acid (Table 1, entry 23) demonstrate a good conversion as well as a good selectivity. Considering the hazards of using halogenated solvents and acetic acid we decided on acetonitrile as the best solvent for this reaction.

Table 1 Optimisation of reaction parameters^a

Entry	Solvent	Oxidant equiv.	Time (min)	Conv.^b (%)	Selectivity^b (%)
					2a	3a
a Reaction conditions: 1a (0.8 mmol, 1 equiv.), solvent (3 mL), oxidants: nonanebis(peroxoic acid); temperature: 30–32 °C (room temperature). b Conversion and selectivity determined by GC with area normalization; DCM: dichloromethane; DCE: dichloroethane. c Reaction temperature: 50–55 °C.
1	Acetonitrile	0.5	360	55	55	—
2		0.65	60	87	87	—
3		0.7	7	100	94	6
4		0.75	30	100	83	17
5		1	120	100	59	41
6		1.25	90	100	27	73
7		1.4	90	100	12	88
8		1.5	15	100	—	100
9		1.6	15	100	—	100
10	Water	0.7	12 h	80	24	56
11^c	Water	0.7	25	100	76	24
12^c	Water	1.5	25	100	—	100
13	Toluene	0.7	120	82	52	30
14	Hexane	0.7	120	15	2	13
15	Acetone	0.7	20	71	69	2
16	DMF	0.7	20	59	41	8
17	Methanol	0.7	20	100	77	23
18	Ethanol	0.7	20	100	81	19
19	Ethyl acetate	0.7	20	97	85	12
20	DCM	0.7	20	100	98	2
21	DCE	0.7	20	100	94	6
22	Chloroform	0.7	20	100	97	3
23	Acetic acid	0.7	20	100	98	2

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Acetonitrile plays a vital role in this reaction. The reaction proceeds in the homogeneous phase which allows the proper mixing of the substrate with peracid. We also believe that the nitrogen atom of the –CN stabilizes the partial positive charge on the sulfur atom in the transition state (as shown in the mechanism).^41–44

The rate of the addition of peracid plays a vital role in higher selectivity for sulfoxide. When the oxidation of 1a to 2a was carried out by adding nonanebis(peroxoic acid) (0.7 equivalents) in one portion, exotherm was observed which further oxidizes the 2a formed in the reaction to 3a. The reaction resulted in the formation of 85% of 2a and 15% of 3a, whereas a slow addition of peracid over 2 to 3 min gives 94% of 2a and only 6% of 3a. Thus from these results, it was clear that the slow addition of peracid favours a higher selectivity for 2a.

During solvent free reaction conditions the strong exotherm observed was due to the vigorous reaction between the peracid and sulfide (which results in the rapid decomposition of the peracid). Also the improper mixing of the reaction mass leads to a mixture of products. The oxidation of 1a to 2a under solvent free conditions gives a mixture of 2a (29%) and 3a (32%), along with unreacted 1a (32%). Similarly, the oxidation of 1a to 3a under similar conditions results in a mixture of 2a (19%) and 3a (81%).

During the solvent study, we observed that when water was used as a solvent, 56% of 3a was obtained which was much higher than in all other oxidants under the sulfoxide formation conditions (Table 1, entry 10). Out of curiosity, we performed the same reaction at 50–55 °C and varied the oxidant equiv. During this, it was found that at 0.7 equiv. of oxidant, 76% of 2a was obtained in 25 min while at 1.5 equiv. of oxidant, 100% of 3a was obtained in 25 min (Table 1, entries 11 & 12). This indicates that at a higher temperature, selective sulfone formation occurs in water. With these studies in hand, we optimized two protocols; one with acetonitrile for 2a and 3a using 0.7 and 1.5 equiv. of oxidant at room temperature, and the second with water for 3a using 1.5 equiv. of oxidant at 50–55 °C.

It is well reported that sulfoxide is less nucleophilic in nature, due to which the oxidation of sulfoxide to sulfone require a longer time.^42,45 Also, the rate of oxidation of sulfoxide to sulfone was lower when peracids were used as an oxidant. The reaction rate was enhanced either by adding an excess of oxidant or incorporating a metal catalyst along with an oxidant as an activator for the oxidant.⁴⁵

The reaction conditions optimized using acetonitrile as a solvent were further used to investigate the effect of other oxidizing agents on the oxidation of 1a in to 2a (Table 2, entries 1–11). Amongst these oxidants, a higher conversion was obtained for 50% H₂O₂ and m-CPBA, but both failed to give a high selectivity for 2a (Table 2, entries 1 and 3). Other oxidants were found to give a poor conversion even after keeping the reaction at room temperature for 2 h (Table 2, entries 2 and 4–9). The study with the other diperoxy acids under the present conditions failed to give a high conversion, but offered a high selectivity for 2a in 2 h (Table 2, entries 10 and 11). On the other hand, when we performed the oxidation of 1a in to 3a in water using these diperoxy acids, it was observed that hexanebis(peroxoic acid) gives 100% conversion while dodecanebis(peroxoic acid) gives only 20% conversion to 3a (Table 2, entries 12 and 13).

Table 2 Comparative study with other oxidants^a

Entry	Oxidant	Conv.^b (%)	Selectivity^b (%)
			2a	3a
a Reaction conditions: 1a (0.8 mmol, 1 equiv.), other oxidants (1.4 equiv.), bisperoxy acids (0.7 equiv.), temperature: 30–32 °C (room temperature), acetonitrile was used as solvent for entries 1–11 (3 mL), time: 20 min. b Conversion and yield determined by GC with the area normalization method. c Time: 2 h. d Solvent used: acetonitrile : methanol (3 : 2). e Water (3 mL) was used as solvent, temperature 50–55 °C.
Method-A
1^c	50% H2O2	85	66	19
2^c	Oxone	40	37	3
3^c	m-CPBA	99	79	20
4^c^,^d	Sodium perborate	2	2	—
5^c	tert Butyl hydrogen peroxide	33	33	—
6^c	Urea hydrogen peroxide	Traces	—	—
7^c	Ammonium persulphate	Traces	—	—
8^c	Performic acid	08	08	—
9^c	Peracetic acid	19	19	—
10^c	Hexanebis(peroxoic acid)	32	32	—
11^c	Dodecanebis(peroxoic acid)	56	56	—
Method-B
12^e	Hexanebis(peroxoic acid)	100	—	100
13^e	Dodecanebis(peroxoic acid)	20	—	20

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It was observed that the higher conversion and selectivity for the oxidation of sulfide to sulfoxide and sulfone is solubility dependent. A higher conversion and selectivity is observed when the reaction proceeds under homogeneous conditions. Hexanebis(peroxoic acid) is completely insoluble while dodecane(bisperoxoic acid) is partially soluble in acetonitrile (Method-A). Due to this, both peracids show a lower conversion to 2a even after keeping the reaction for 2 h, but the more nucleophilic nature of sulfide over sulfoxide favours a high selectivity for sulfoxide (Table 2, entries 10 and 11).

In order to investigate other peracids for the oxidation of sulfide to sulfone in water at a temperature of 50–55 °C (Method-B), we carried out reactions using hexanebis(peroxoic acid) and dodecanebis(peroxoic acid) under similar conditions. It was observed that the water soluble nature of hexanebis(peroxoic acid) favours 100% conversion to sulfone, whereas a lower conversion was observed for dodecanebis(peroxoic acid) which is insoluble in water (Table 2, entries 12 and 13).

After optimizing the reaction conditions for both methods, we investigated the substrate scope using various aryl–alkyl and diaryl sulfides bearing oxidizable functionalities (Table 3, entries 1–9). The sulfoxide formation occurred in 5 to 15 min and sulfone formation occurred in 15 to 60 min using Method-A. When using Method-B, sulfone formation occurred in 25 min. Bigi et al. have reported that the oxidation of 1b to 2b and 3b occurred in 24 h.⁴⁶ In our case, when using Method-A, the formation of 2b occurred in 12 min and 3b in 15 min, whereas by using Method-B, the formation of 3b was observed in 25 min (Table 3, entry 2). For the substrate bearing acid group 1c, no oxidation product of the acid was observed in either method (Table 3, entry 3). Similar results were obtained for the substrate bearing alcohol, tert amine (Table 3, entries 5 and 6). It was noticeable that for the oxidation of substrate 1g to 3g by using Method-A 60 min was required, whereas the same transformation occurred in 25 min by using Method-B (Table 3, entry 7). Also in both methods, traces of acid formation was observed. The influence of electron donating and withdrawing substrates on the rate of reaction was not observed for either protocol. The substrate bearing electron donating as well as the withdrawing groups were smoothly oxidized to the corresponding sulfoxide and sulfone in 15 and 25 min respectively by Method-A. On the other hand it took 25 min to produce the corresponding sulfone using Method-B (Table 3, entries 8 and 9).

Table 3 Oxidation of sulfide to sulfoxide and sulfone^a

Method-A
Entry	Substrate	Time (min)		Yield^b (%)
		Sulfoxide	Sulfone	Sulfoxide	Sulfone
a Reaction conditions: Method-A sulfide (1 equiv.), acetonitrile (3 mL), nonanebis(peroxoic acid): 0.7 equiv. for sulfoxide, 1.5 equiv. for sulfone, temperature: 30–32 °C (room temperature). b Yield – isolated yield, Method-B sulfide (1 equiv.), water (3 mL), nonanebis(peroxoic acid): 1.5 equiv., temperature 50–55 °C.
1		7	15	93	87
2		12	15	87	83
3		12	15	89	85
4		7	20	88	98
5		13	20	89	86
6		8	15	85	75
7		5	60	88	80
8		15	25	92	94
9		15	25	90	95

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Method-B
Entry	Substrate	Time (min)	Yield^b (%)
		Sulfone	Sulfone
1		25	95
2		25	90
3		25	88
4		25	95
5		25	87
6		25	80
7		25	80
8		25	95
9		25	96

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Various oxidizing systems were reported for the chemoselective oxidation of sulfide, such as urea hydrogen peroxide with a Mn-complex,⁴⁷ sodium perborate and sodium percarbonate with an Amberlyst support under solvent free conditions,⁴⁸ ammonium nitrate and citric acid with MBr,⁴⁹ IBX with TEAB as a catalyst,⁵⁰and oxone under solvent free conditions.⁵¹ It was observed that in almost all cases (except oxone) a catalyst was used for the chemoselective oxidation of sulfide. To investigate the chemoselectivity, we have also carried out some competitive reactions (Scheme 1) for the oxidation of 1a to 2a in the presence of the substrate bearing oxidizable groups such as benzyl alcohol, styrene and aliphatic and aromatic tert amine using Method-A. When the GC analysis was performed for these reactions, selective 2a formation was observed with a 91 to 98% selectivity. No oxidation product of alcohol, double bond or tert amine was observed. Thus the present approach offers the chemoselective oxidation of sulfide under catalyst free conditions.

Scheme 1 Chemoselective reaction.

The recyclability study was carried out on 5 g of 1a (Table 4). The main problem we were facing while using these diperoxy acid was the recovery and recycling of the parent acid. Here we succeeded in recovering 75 to 80% of the parent acid which was further used up to a third cycle using Method-B. The detailed results are depicted in Table 4. The diperoxy acid prepared using the recovered parent acid was analysed using iodometric titration for % active oxygen content. The results obtained were consistent and in good agreement with the reported active oxygen content value. In all cycles, 100% selectivity for 3a was observed with a 92 to 95% isolated yield. Similarly, we performed the same study using Method-A, where only 66% recovery of the parent acid was obtained (Table 4, entry 1).

Table 4 Scale up and recyclability study in water^a

Entry	No. of cycles	% AOC of oxidant^d	Isolated yield	Isolated yield	Selectivity^e (%)
			Azealic acid	Sulfone	Sulfone
a Reaction conditions: 1a (1 equiv.), nonane(bis peroxoic)acid (1.5 equiv.), temperature: 50–55 °C, solvent: water. b Peracid used was synthesised from recovered azealic acid. c Solvent: acetonitrile. d AOC – active oxygen content determined using iodometric titration. e Selectivity determined by GC.
1^c	Fresh	14.3	66	89	100
2	Fresh	14.3	75	95	100
3^b	First	14.1	77	94	100
4^b	Second	14.2	80	92	100
5^b	Third	14.1	79	93	100

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Plausible mechanism

A plausible mechanism^{41–43,52,53} for the oxidation of sulfide to sulfoxide and sulfone is shown in Fig. 1. We believe that the presence of intra-molecular hydrogen bonding between the carbonyl oxygen and hydrogen increases the electrophilic nature of the peroxy oxygen which is then attacked by the nucleophilic sulfide atom. This leads to the formation of positive charge on the sulfur atom from sulfide.⁵² This partial positive charge on sulfur may be stabilized by the electron donating nature of the nitrogen from acetonitrile. The parent acid, nonanedioic acid, formed during the reaction was recovered and recycled further by converting it to nonanebis(peroxoic acid).

Fig. 1 Plausible mechanism for sulfoxide and sulfone formation.

Conclusion

In conclusion, nonanebis(peroxoic acid) was found to be effective for oxidizing various sulfides to the corresponding sulfoxides and sulfones. The developed protocol was found to be compatible with various oxidizable groups. In acetonitrile, the reaction smoothly proceeded at room temperature in 5–25 min, whereas in water the reaction needed to be carried out at 50–55 °C to give full selectivity for sulfone in 25 min. Nonanedioic acid generated at the end of the reaction in Method-B was effectively recovered and reused for up to three cycles by converting it to nonanebis(peroxoic acid).

Experimental

General

Common reagent grade chemicals were purchased from Alfa Aesar, Spectrochem, Sigma Aldrich and Sd fine chemicals. FT-IR spectra were recorded on a Bomen Hartmann and Braun MB-Series FT-IR spectrometer. GC-MS was carried out with a GC-MS-QP 2010 instrument. The ¹H NMR spectroscopic data were recorded with an Agilent 500 MHz spectrometer with CDCl₃ and DMSO as the solvent. GC analysis was carried out with a Thermo scientific, column-TR-1, 30 m × 0.25 mm, ID × 0.25 μm film, FID detector and with a sample size of 0.11 μL.

Procedure for the preparation of nonanebis(peroxoic acid)

Nonanebis(peroxoic acid) was synthesized and its purity was determined from its active oxygen content using an iodometric titration method.^38,54

General procedure for oxidation of sulfide to sulfoxide and sulfone

Method-A. In 2 N RBF, 1 equiv. of sulfide was mixed with 3 mL acetonitrile at room temperature. To this, 0.7/1.5 equiv. of nonanebis(peroxoic acid) was added slowly over 2 to 3 min to avoid the over-oxidation of the sulfide. The progress of the reaction was monitored by TLC. The reaction was quenched by adding 5 mL of 5% sodium bicarbonate solution. The isolated solid was filtered and washed with water. The liquid products were extracted using ethyl acetate after the sodium bicarbonate treatment. In the case of the acid group containing substrate, the reaction mass was quenched using 0.1 N sodium thiosulphate solution and the crude product obtained was directly filtered without a base treatment. The crude product was further purified by column chromatography using ethyl acetate : hexane as an eluting system.

Method-B. In 2 N RBF, 1 equiv. of sulfide was dissolved into 3 mL of water as a solvent, and then 1.5 equiv. of nonanebis(peroxoic acid) was added slowly under stirring. The reaction was maintained for 25 min at 55 °C temperature, and the progress of the reaction was monitored using TLC. The reaction was quenched by sodium thiosulphate, followed by adding the sodium bicarbonate solution to get sulfone.

Iodometric titration. 0.15 to 4 g of peroxy compound was added to a 250 mL iodine flask, and dissolved in 20 mL of acetic acid and chloroform (3 : 2). 2 mL of 50% potassium iodide solution was added, and it was immediately stoppered and placed in an ice bath for 5 min. 50 mL of distilled water was added, and it was titrated against 0.1 M sodium thiosulfate solution. Starch indicator was added towards the endpoint. The endpoint is dark brown to colourless.³⁸

Spectroscopic data of compounds

(1) (Methylsulfinyl)benzene (2a):⁵⁵ pale yellow oil; yield: 93%; ¹H NMR (500 MHz, CDCl₃): δ = 7.56–7.53 (m, 2H), 7.44–7.37 (m, 3H), 2.62 (s, 3H); IR: ν/cm⁻¹ = 1080; GC-MS (EI, 70 eV): m/z, [M]⁺ = 140.

(2) (Methylsulfonyl)benzene (3a):⁵⁶ white solid; yield: Method-A: 87%, Method-B: 95%; mp 88–90 °C; ¹H NMR (500 MHz, CDCl₃): δ = 7.95–7.93 (m, 2H), 7.67–7.63 (m, 1H), 7.58–7.55 (m, 2H), 3.04 (s, 3H); IR: ν/cm⁻¹ = 1142, 1282; GC-MS (EI, 70 eV): m/z, [M]⁺ = 156.

(3) Sulfinyldibenzene (2b):⁵⁷ white solid; yield: 87%; mp 68 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.65 (m, 4H), 7.50–7.41 (m, 6H); IR: ν/cm⁻¹ = 1035, 1085; GC-MS (EI, 70 eV): m/z, [M]⁺ = 202.

(4) 2-(Benzylsulfinyl)acetic acid (2c):⁵⁸ white solid; 89% yield; mp 124–126 °C; ¹H NMR (500 MHz, DMSO): δ 13.12 (s, 1H, –COOH), 7.30–7.39 (m, 5H), 4.24–4.21 (d, 2H), 4.23–4.12 (d, J = 10 Hz, 1H, SOCH₂COOH), 4.03–4.05 (d, J = 15 Hz, 1H, ArCH₂SO), 3.84–3.81 (d, J = 15 Hz, 1H, ArCH₂SO), 3.54–3.52 (d, J = 10 Hz, 1H, SOCH₂COOH); IR: ν/cm⁻¹ = 1069, 1698; MS calculated m/z = 198.2, observed m/z = 196.85 [M − 1].

(5) 2-(Benzylsulfonyl)acetic acid (3c):⁵⁸ white solid; yield: Method-A: 85%, Method-B: 88%; mp 138 °C; ¹H NMR (500 MHz, DMSO) δ 7.40 (m, 5H), 4.62 (s, 2H), 4.15 (s, 2H); IR: ν/cm⁻¹ = 1137, 1299, 1697; GC-MS (EI, 70 eV): m/z, [M]⁺ = 214.

(6) Ethyl-2-(benzylsulfonyl)acetate (3d):⁵⁹ white solid; yield: Method-A: 98%, Method-B: 95%; mp 42 °C; ¹H NMR (500 MHz, CDCl₃): δ 7.39–7.41 (m, 3H), 7.48–7.50 (m, 2H), 4.51 (s, 2H,); 4.28–4.32 (q, 2H), 3.77 (s, 2H), 1.33–1.36 (t, 3H); IR: ν/cm⁻¹ = 1138, 1290, 1716; GC-MS (EI, 70 eV): m/z, [M]⁺ = 242.

(7) 2-(Benzylsulfinyl) ethanol (2e):⁶⁰ yellowish liquid; yield: 89%; ¹H NMR (500 MHz, CDCl₃) δ 7.31 (m, 5H), 4.07 (s, 2H), 4.05–3.98 (m, 2H), 3.28 (s, 1H), 2.87–2.70 (m, 2H); IR: ν/cm⁻¹ = 1088, 3448; GC-MS (EI, 70 eV): m/z, [M]⁺ = 184.

(8) 2-(Benzylsulfonyl) ethanol (3e):⁶¹ white solid; yield: Method-A: 86%, Method-B: 87%; mp 66 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.48–7.37 (m, 5H), 4.35 (s, 1H), 4.09 (dd, 2H), 3.11–3.07 (t, 2H), 2.51 (s, br, 1H); IR: ν/cm⁻¹ = 1280, 1249, 3425; GC-MS (EI, 70 eV): m/z, [M]⁺ = 200.

(9) 10-Butyl-10H-phenothiazine 5-oxide (2f):⁶² white solid; yield: 85%, mp 130 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.94 (dd, 2H), 7.65–7.59 (m, 2H), 7.42 (d, 2H), 7.25 (dd, 2H), 4.26–4.19 (t, 2H), 1.95 (m, 2H), 1.62–1.52 (m, 2H), 1.07 (t, 3H); IR: ν/cm⁻¹ = 1023; GC-MS (EI, 70 eV): m/z, [M]⁺ = 271.

(10) 10-Butyl-10H-phenothiazine 5,5-dioxide (3f):⁶³ white solid; yield: Method-A: 75%, Method-B: 80%; mp 148 °C; ¹H NMR (500 MHz, CDCl₃) δ 8.13 (dd, 2H), 7.65–7.59 (m, 2H), 7.35 (d, 2H), 7.27 (dd,2H), 4.20–4.13 (t, 2H), 1.92 (m, 2H), 1.56–1.46 (m, 2H), 1.03 (t, 3H); IR: ν/cm⁻¹ = 1376, 1279; GC-MS (EI, 70 eV): m/z, [M]⁺ = 287.

(11) 10-Hexyl-10H-phenothiazine-3-carbaldehyde-5-oxide (2g): white solid; yield: 88%; mp 138 °C; ¹H NMR (500 MHz, CDCl₃) δ 10.01 (s, 1H), 8.44 (d, J = 2.0 Hz, 1H) 8.14 (dd, J = 8.9, 2.0 Hz, 1H), 7.99 (dd, J = 7.7, 1.6 Hz, 1H), 7.69 (ddd, J = 8.8, 7.3, 1.7 Hz, 1H), 7.50 (t, J = 8.8 Hz, 2H), 7.38–7.34 (m, 1H), 4.30–4.25 (m, 2H), 2.04–1.95 (m, 2H), 1.60–1.52 (m, 2H), 1.47–1.35 (m, 4H), 0.94 (t, J = 7.1 Hz, 3H); IR: ν/cm⁻¹ = 1026, 1679; MS calculated m/z = 327.4, observed m/z = 328.02 (M + 1).

(12) 10-Hexyl-10H-phenothiazine-3-carbaldehyde-5,5-dioxide (3g): white solid; yield: Method-A: 80%, Method-B: 80%; mp 178 °C; ¹H NMR (500 MHz, CDCl₃) δ 10.01 (s, 1H), 8.602–8.598 (d, 1H, J = 2), 8.16–8.13 (m, 1H, J = 8, 1.5 Hz), 7.702–7.667 (m, 1H, J = 9, 2 Hz), 7.453–7.359 (m, 3H), 4.233–4.201 (t, 2H), 1.977–1.914 (m, 2H), 1.530–1.470 (m, 2H), 1.400–1.339 (m, 4H), 0.930–0.901 (t, 3H); IR: ν/cm⁻¹ = 1135, 1284; MS calculated m/z = 343.4, observed m/z = 344.13 (M + 1).

(13) 1-Methoxy-4-(methylsulfinyl)benzene (2h):³⁴ colourless oil; yield: 92%, ¹H NMR (500 MHz, CDCl₃) δ 7.58–7.55 (m, 2H), 7.02–6.98 (m, 2H), 3.82 (s, 3H), 2.68 (s, 3H); IR: ν/cm⁻¹ = 1088, GC-MS (EI, 70 eV): m/z, [M]⁺ = 170.1.

(14) Methoxy-4-(methylsulfonyl)benzene (3h):³⁴ white crystals; yield: Method-A: 94%, Method-B: 95%; mp 120 °C; ¹H NMR (500 MHz, CDCl₃) δ 7.90–7.84 (d, 2H), 7.05–7.00 (d, 2H), 3.89 (s, 3H), 3.03 (s, 3H). IR: ν/cm⁻¹ = 1139, 1291; GC-MS (EI, 70 eV): m/z, [M]⁺ = 186.

(15) 1-(Methylsulfinyl)-4-nitrobenzene (2i):³⁴ pale yellow solid; yield: 90%; mp 150 °C; ¹H NMR (500 MHz, CDCl₃) δ 8.40 (d, J = 8.5 Hz, 2H), 7.84 (d, J = 8.5 Hz, 2H), 2.80 (s, 3H); IR: ν/cm⁻¹ = 1085; GC-MS (EI, 70 eV): m/z, [M]⁺ = 185.

(16) 1-(Methylsulfonyl)-4-nitrobenzene (3i):³⁴ pale yellow solid; yield: Method-A: 95%, Method-B: 96%; mp 140 °C; ¹H NMR (500 MHz, CDCl3) δ 8.45–8.41 (d, 2H), 8.19–8.14 (d, 2H), 3.13 (s, 3H); IR: ν/cm⁻¹ = 1149, 1287; GC-MS (EI, 70 eV): m/z, [M]⁺ = 201.

Acknowledgements

The authors are thankful to the UGC-GREEN TECH and UGC-CAS for providing financial assistance, and MS Indoco Remedies Ltd, Rabale, Navi Mumbai for recording the EI-MS spectra.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02616d

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