Thiols were effectively oxidized into disulfides by reacting with hydrogen peroxide in the presence of a catalytic amount of iodide ion or iodine.The controlled oxidative coupling of thiols to disulfides is important in organic synthesis, and a wide range of methods have been developed for this transformation. 1 Thiols are among the functional groups which can be easily over oxidized, therefore, extensive studies have been carried out for their controlled oxidation with molecular oxygen, 2 peroxide, 3 metal oxidants, 4 halogens and derivatives, 5 sulfoxide, 6 and N-oxide. 7 In particular, the methods of oxidative coupling of disulfides with halogens such as I 2 , 5a I 2 /HI, 5b and Br 2 /KHCO 3 5c were carried out under mild reaction conditions, and gave the disulfides in good yields. However, these reactions require a stoichiometric amount of oxidants and long reaction times. Oae and co-workers developed the oxidative coupling of thiols to disulfides by dimethyl sulfoxide in the presence of a catalytic amount of molecular iodine. 5b Although this reaction affords the products in high yield under very mild reaction conditions, there are still some problems such as the requirement of a long reaction time and the production of foulsmelling dimethyl sulfide as a waste product.To develop an environmentally benign oxidative coupling of thiols, we planned to use hydrogen peroxide as a cooxidant for the oxidation of thiols with molecular iodine, because the iodide ion (iodoanion) is easily oxidized to molecular iodine by hydrogen peroxide.Hydrogen peroxide itself can oxidize thiols to the corresponding disulfides; however, alkaline conditions were required. 3a Recently, Bégué et al. reported that disulfides can be efficiently prepared from thiols using 30% hydrogen peroxide in fluoroalcohols at ambient temperature under neutral conditions. 3b Although this method provides the desired compounds in high yields in most cases, fluoroalcohols are expensive for industrial use. Furthermore, the thiols, which are less soluble in the fluoroalcohols, produced the desired disulfides in poor yields. In this paper, we describe a method for the oxidation of thiols 1 to disulfides 2 using 30% hydrogen peroxide catalyzed by iodide ion (Scheme 1). Scheme 1

The novel oxidant of sodium hypochlorite pentahydrate (NaOCl·5H2O) crystals is now available for industrial and laboratory use. It is superior to conventional aqueous sodium hypochlorite solutions (aq. NaOCl). The crystalline material is 44% NaOCl and contains minimal amounts of sodium hydroxide and sodium chloride, and the aqueous solution, which is prepared from NaOCl·5H2O and water, has a pH of 11–12. Examples of the selective organic synthesis using NaOCl·5H2O involve the oxidations of primary and secondary alcohols, selective oxidations to sulfoxide and sulfone, oxidative cleavage of disulfide to sulfonyl chloride and bromide, oxaziridine synthesis, and oxidative dearomatization of phenols.

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