Montmorillonite K10 Catalyzed Selective Oxidation of Sulfides to Sulfoxides Using Hydrogen Peroxide

Abstract

Abstract: A clean and highly selective oxidation of sulfides to sulfoxides with 35% hydrogen peroxidecatalyzed by Montmorillonite K10 in methanol is described. A series of structurally different sulfides wereoxidized selectively at room temperature in excellent yields. This reaction is very simple and proceeds withoutover-oxidation under mild condition, and shows many advantages over the existing methodologies. Usingthis procedure, sulfides bearing functional groups such as hydroxyl, methoxy, amino, nitrile, aldehyde, andolefinic double bond remained intact. Keywords: Montmorillonite K10, hydrogen peroxide, sulfide, sulfoxide, catalyzed sulfoxidation.Convenient synthesis of organic sulfoxides is achallenging research in synthetic organic chemistry becauseof the importance of the sulfoxides as intermediates inchemically and biologically active compounds [1].Oxidation of sulfides is a very useful route for preparation ofsulfoxides and several oxygenating reagents as well ashalogenating ones are available. This type of transformationby various oxidation methods has been extensively reviewed[2]. However, in conventional oxidation methods undesir-able overoxidation to sulfone [3], and in halogenatingreactions the C-S bond cleavage, formation of halo-derivatives or halogenation of aromatic ring are quitefrequent competing reactions [4]. Besides the extendedreaction period, most of the oxidation methods useexpensive, hazardous peracids and toxic heavy metal or rareoxidant that are difficult to prepare and detract their practicalapplication in many cases. The selective oxidation ofsulfides to sulfoxides has also been developed usingtransition metal catalyzed sulfoxidation with hydrogenperoxide and derivatives [5], and metal Schiff basecomplexes in combination with a variety of oxidant havebeen used as catalysts for this purpose [6]. Furthermore, anenzymatic catalysis has also been reported [7]. However themajor shortcoming of all heme-dependent peroxidases andSchiff base-metal complexes is their low operation stabilitydue to facile oxidative degradation of heterocyclic orporphyrine ring. Heterogeneous catalyzed oxidativetransformations by solid supports have gained popularityduring the past decades thanks to distinct advantages ofselectivity, mildness and operational simplicity. Anincreasingly popular category of supported catalysts andreagents have been developed in an effort to achieve facile,efficient, cheap and selective methods [8]. In the course ofour studies on selective oxidative organic transformations [9,10], and using solid support mediated reaction [11], and toelaborate safer and simpler oxidation process, we haveinvestigated the role of natural clays in catalytic oxidation ofsulfides to corresponding sulfoxides using hydrogenperoxide. So far, the capability of hydrogen peroxide as anefficient oxidant has been studied considerably both fromenvironmental and economical viewpoints. It is ideal wasteavoiding oxidant and its aqueous solution shows safety inusage, storage and handling [12, 13]. But, as mentioned inthe literature, without activation with a catalyst, it has to beused in controlled manner to avoid over-oxidation to sulfone[14]. Drabowicz [15] reported that for selective oxygenationof sulfides to sulfoxides by hydrogen peroxide in theabsence of catalyst, the reaction time was extremely long andsome other functional groups were easily destroyed. This isthe first report of the clay catalyzed selective oxidation ofsulfides to sulfoxides using 35% H