Metal-free catalytic oxidation of sulfides to sulfoxides with ammonium nitrate, ammonium hydrogen sulfate and ammonium bromide as catalyst

ARTICLE

Metal-free catalytic oxidation of sulfides to sulfoxides with ammonium nitrate, ammonium hydrogen sulfate and ammonium bromide as catalyst

Arash Ghorbani-ChoghamaraniI, ^*; Mohammad Ali ZolfigolII, ^*; Roia Ayazi-Nasrabadi^II

^IDepartment of Chemistry, Faculty of Science, Ilam University, P.O. Box 69315516, Ilam, Iran

^IIFaculty of Chemistry, Bu-Ali Sina University, P.O. Box 6517838683, Hamadan, Iran

ABSTRACT

A general and metal-free catalytic oxidation of aliphatic and aromatic sulfides to their corresponding sulfoxides via combination of ammonium nitrate (NH₄NO₃), supported ammonium hydrogen sulfate on silica gel (NH₄HSO₄-SiO₂) and a catalytic amount of ammonium bromide (NH₄Br) in the presence of wet SiO₂ (50%, w/w) has been investigated. The reactions were carried out heterogeneously and selectively in short reaction times in CH₂Cl₂ at room temperature. This protocol is mild and efficient compared to other reported methods.

Keywords: sulfides, sulfoxide, ammonium hydrogen sulfate (NH₄HSO₄), ammonium nitrate (NH₄NO₃), ammonium bromide (NH₄Br), homoselectivity

RESUMO

A oxidação catalítica geral e sem o uso de metais, de sulfetos alifáticos e aromáticos aos seus sulfóxidos correspondentes via combinação de nitrato de amônio (NH₄NO₃), hidrogenossulfato de amônio suportado em sílica gel (NH₄HSO₄-SiO₂) e uma quantidade catalítica de brometo de amônio (NH₄Br) na presença de SiO₂ aquoso (50%, m/m) foi investigada. As reações ocorreram em meio heterogêneo e seletivamente em curtos tempos de reação, em CH₂Cl₂ a temperatura ambiente. Este procedimento é simples e eficiente comparado a outros métodos relatados.

Introduction

The chemoselective oxidation of sulfides to their corresponding sulfoxides is of interest for organic chemists, both for fundamental research and for a variety of applications. Sulfoxides are valuable synthetic intermediates for the synthesis of chemically and biologically significant molecules.^1-3 Sulfoxides are also valuable materials in C-C bond-forming^4,5 and molecular rearrangements.^6-8 Additionally some of biologically active sulfoxides play an important role as therapeutic agents such as anti-ulcer,^9-11 antibacterial,¹² anti-atherosclerotic,^13,14 among others. Although a wide variety of oxidizing systems has been applied for the oxidation of sulfides to the sulfoxides such as H₂O₂/silica sulfuric acid,¹⁵ urea hydrogen peroxide (UHP)/Mn(III),¹⁶ Me-IBX,¹⁷ H₂O₂/VO(acac)₂,¹⁸N-bromosuccinimide,¹⁹ H₂O₂/NBS,²⁰ H₂O₂/Cr(III),²¹ 1-benzyl-4-aza-1-azonia-bicyclo[2.2.2]octane tribromide,²² BF₄-based ionic liquid/UHP,²³ H₂O₂/1,1,1-trifluoroacetone,²⁴ some of these methods still suffer from some drawbacks like overoxidation to sulfones, low selectivity, low yields of products, tedious work-up, toxicity, and expensive reagents or catalysts.

In the last few years, heterogeneous reagents and catalysts are used increasingly in organic functional group transformation.^25-28 The development of efficient and new catalytic systems for various organic transformations is an active research area with the aim to develop milder reaction conditions.²⁹ Finding molecules which are able to catalyze the reaction between others is an important contribution of molecular chemists to increase the efficiency of chemical reactions whereby our daily life based on consumption of chemicals is shifted closer to an ecologically and economically tolerable equilibrium with our environment.³⁰

Results and Discussion

Recently we have introduced different approaches for the in situ generation of bromonium ion (Br⁺), which effectively applied for the oxidation of different types of organic compounds.^31-36 In continuation of this investigation we decided to explore catalytic and metal-free media for the in situ generation of Br⁺. Meanwhile we were interested to use ammonium nitrate (NH₄NO₃) as very cheap, nontoxic and commercially available oxidizing material. Ammonium nitrate is used in fertilizer industry and as an explosive in mining operations. It has been also used widely in different organic transformations such as: cleavage of THP ethers, acetals and acetonides,³⁷ nitration of arenes,³⁸ dethioacetalization,³⁹ coordination of ammonium nitrate to crown ethers.⁴⁰

However, the low oxidizing power of ammonium nitrate is the main drawback to its application. Therefore we decide to activate this reagent with supported ammonium hydrogen sulfate on silica gel (NH₄HSO₄-SiO₂). Therefore a mixture of NH₄NO₃, NH₄HSO₄-SiO₂ (50%, w/w) and catalytic amounts of NH₄Br in the presence of wet SiO₂ (50%, m/m), as metal free catalytic media, was combined for the in situ generation of Br⁺. Consequently, we disclosed a new heterogeneous catalytic protocol for the chemo and homoselective oxidation of sulfides to the sulfoxides using combination of ammonium nitrate, ammonium hydrogen sulfate and wet SiO₂ (50%, w/w) in the presence of catalytic amounts of NH₄Br.

Obviously, solvent plays an important role in organic transformations, and hence it was decided to investigate the solvent effect and also find an appropriate solvent for the oxidation of sulfides. However, we screened different solvents for the oxidation of dibenzyl sulfide, as a typical example. The oxidation of dibenzyl sulfide was carried out using a mixture containing 1mmol of dibenzyl sulfide, NH₄NO₃ (1.2 mmol), NH₄HSO₄-SiO₂ (50%, w/w) (1.2 mmol), NH₄Br (0.02 mmol) and 0.2 g of wet SiO₂ (50%, w/w) in 5 mL of solvent at room temperature. As is evident from Table 1 oxidation reaction proceeds more rapidly and more selectively in dichloromethane compared to other solvents.

Eventually, we wish to report here the selective oxidation of wide range of aliphatic and aromatic sulfides 1 to the corresponding sulfoxides 2 using NH₄NO₃ I, supported ammonium hydrogen sulfate on silica [NH₄HSO₄-SiO₂ (50%, w/w)] II and catalytic amounts of NH₄Br III in the presence of wet SiO₂ (50%, w/w) in dichloromethane at room temperature with good to excellent yields (Scheme 1 and Table 2).

NH₄Br is the catalytic part of this oxidizing system. Therefore to investigate the catalytic role of NH₄Br dibenzyl sulfide was subjected to the oxidation reaction in the absence of catalyst. However, no sulfoxide was observed after 3 h (Table 1, entry 4). Also the necessity of wet SiO₂ (50%, w/w), as source of water, and

NH₄HSO₄-SiO₂ (50%, w/w) were considered. Therefore to show the role of supported ammonium hydrogen sulfate on silica in this system, oxidation of dibenzyl sulfide was carried out in the absence of this reagent. Interestingly it was observed that the reaction did not proceed in the absence of NH₄HSO₄-SiO₂ (Table 1, entry 5). Also is evident from entry 6 of Table 1, that wet SiO₂ is necessary for the described system.

It is of interest to note that the mentioned oxidizing system allowed the homoselective^31,33 oxidation of thianthrene and bis-(methyl thio)-methane to their monosulfoxide derivatives (Scheme 2, entries 11 and 18).

Finally to show the chemoselectivity of the described system, two sulfides containing a hydroxy group were subjected to the sulfoxidation reaction, but alcohol oxidation did not occur under mentioned conditions, and primary hydroxyl group remained intact in the course of the reaction (Table 2, entries 8 and 12, Scheme 3).

A possible mechanism of this oxidizing system is shown in Scheme 4 based on our previously reported works.^31-36

In the first place, NH₄NO₃ reacts with NH₄HSO₄ to produce in situ HNO₃. Subsequently, auto ionization of HNO₃ generates nitronium ion (NO₂⁺), which is able to convert bromide ion (Br^-) to bromonium (Br⁺). In the last step reaction of bromonium ion with sulfide in the presence of water might generate the corresponding sulfoxide.

In summary herein we report a novel catalytic protocol for the chemo and homoselective oxidation of sulfides to the sulfoxides under metal-free, mild and heterogeneous conditions. This method offers the advantage of shorter reaction times, high selectivity, non toxic conditions, cost effective reagents and catalyst and easy workup. We believe that the present methodology could be an important addition to the existing methodologies.

Experimental

Chemicals were purchased from Fluka, Merck and Aldrich chemical companies. The oxidation products were characterized by comparison of their spectral (IR, ¹H NMR, and ¹³C NMR) and physical data with authentic samples.

Oxidation of dibenzyl sulfides 1c to dibenzyl sulfoxide 2c using NH₄NO₃, NH₄HSO₄-SiO₂ and NH₄Br as a typical procedure

NH₄NO₃ (0.096 g, 1.2 mmol), NH₄Br (0.0019 g, 0.02 mmol), NH₄HSO₄-SiO₂ (50%, w/w), (0.276 g, 1.2 mmol) and 0.2 g of wet SiO₂ (50%, w/w) were added to a solution of dibenzyl sulfide 1c (0.214 g, 1 mmol) in CH₂Cl₂ (5 mL). The resulting mixture was stirred at room temperature for 20 min (the reaction progress was monitored by TLC) and then filtered. The residue was washed with CH₂Cl₂ (4×5 mL). Anhydrous Na₂SO₄ (1.5 g) was added to the filtrate and filtered off after 20 min. Finally CH₂Cl₂ was removed and the yield was 0.221 g (98%).

Acknowledgments

Financial support for this work by the Center of Excellence of Development of Chemical Methods (CEDCM) of Bu-Ali Sina University, Hamadan, Iran is gratefully acknowledged.

Received: February 19, 2009

Web Release Date: October 9, 2009

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Publication Dates

• Publication in this collection
  19 Feb 2010
• Date of issue
  2010

History

• Received
  19 Feb 2009
• Accepted
  09 Oct 2009