Selective oxidations of sulfides to sulfoxides using immobilised cerium alkyl phosphonate Mohammed Al-Hashimi, Gopa Roy, Alice C. Sullivan * and John R. H. Wilson Department of Chemistry, Queen Mary, University of London, Mile End Road, London E1 4NS, UK Received 2 March 2005; revised 14 April 2005; accepted 20 April 2005 Available online 10 May 2005 Abstract—A range of sulfides can be selectively oxidised to the corresponding sulfoxides in good yields using catalytic quantities of immobilised cerium alkyl phosphonate and either sodium bromate or tert-butyl hydroperoxide as oxidants. Ó 2005 Elsevier Ltd. All rights reserved. 1. Introduction The selective oxidation of sulfides to sulfoxides remains a challenge and is of interest because of the importance of sulfoxides as synthetic intermediates in organic syn- thesis. 1 Stoichiometric oxidation can be accomplished with reagents like HNO 3 , KMnO 4 , MnO 2 and m-CPBA 1 but cleaner methods based on catalytic promotion of oxidation using aqueous hydrogen peroxide, or tert- butyl hydroperoxide (TBHP) are preferred. Noyori and co-workers reported the use of various W catalysts (based on sodium tungstate, phenylphosphonic acid and methyltri-n-octylammonium hydrogensulfate, which acts as a phase-transfer catalyst) for the oxidation of aromatic and aliphatic sulfides using 30% H 2 O 2 . 1 Espen- son reported the use of oxorhenium dithiolates and TBHP for the selective oxidation of a number of sul- fides. 2 A number of studies have been reported on hetero- geneous oxidation of sulfides. Titanium silicates TS-1 promote sulfide oxidation, however bulky sulfides were not oxidised due to limited access to the Ti active sites. 3 Ti-beta 3 and Ti-MCM-41 4 allow the oxidation of bulky sulfides to mixtures of sulfoxide and sulfone. Better selectivity was observed for the oxidant UHP (crystal- line urea adduct of H 2 O 2 ). 3 Mayoral and co-workers re- ported that TiðOPr i Þ 4 supported on silica is an efficient and selective catalyst for the oxidation of sulfides to sulf- oxides, 5 but the Ti was gradually leached from the sup- port. Product selectivity (i.e., sulfoxide/sulfone) was better for H 2 O 2 than for TBHP but the rate of reaction was faster with the latter. Choudary et al. reported the catalytic oxidation of sulfides selectively to sulfoxides with heterogeneous tungstate exchanged Mg-Al-LDH (layered double hydroxide) catalysts, 30% H 2 O 2 as the oxidant and water as the solvent. 6 These heterogeneous catalysts displayed superior activity over their homoge- neous counterparts but were not always selective for sulfoxide formation. Kropp and co-workers reported that silica gel and alu- mina mediate the oxidation of sulfides and sulfoxides by TBHP and a 2:1:1 mixture KOSO 2 OOH, KHSO 4 , K 2 SO 4 (OXONE) with minimal over-oxidation to the sulfone. Mechanisms for the surface mediated oxygen atom transfer were proposed. 7,8 We recently reported 9,10 the synthesis and characterisa- tion of the first examples of phosphonate and phosphonic acid modified silicas, [O 2 Si] x [O 3/2 Si(CH 2 ) 2 - PO(OH) 2 ] y ; x:y = 1:1–10:1, that is, ethylphosphonate substituted silica 1:1 to 1:10, EPS1 to EPS10, obtained from diethyl phosphonatoethyltriethoxysilane (EtO) 3 - Si(CH 2 ) 2 PO(OEt) 2 and tetraethoxysilane, TEOS, in appropriate ratios. In addition we have reported 11 on related phosphonate materials [O 2 Si] x [O 3/2 Si(CH 2 ) 2 PO(OH) 2 ] y [O 3/2 SiCH- (CH 2 PO(OH) 2 )CH 2 CH 2 SiO 3/2 ] z ,Q x Ta y Tb z , x:y:z = 0.0: 0.5:0.5–10:0.5:0.5, obtained from TEOS (Q source) di- methyl phosphonatoethyltrimethoxysilane (Ta source) and 1-dimethylphosphonatobutyl-2,4-bis(trimethoxysi- lane), (MeO) 3 SiCH(CH 2 PO(OMe) 2 )CH 2 CH 2 Si(OMe) 3 , (Tb source). Since these materials have both ethylphos- phonate and butylphosphonate functionalities in a 0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2005.04.079 Keywords: Sulfide; Sulfoxide; Oxidation; Cerium; Phosphonate; Silica; Immobilised reagents. * Corresponding author. Tel.: +44 20 7882 3274; fax: +44 20 7882 7794; e-mail: a.c.sullivan@qmul.ac.uk Tetrahedron Letters 46 (2005) 4365–4368 Tetrahedron Letters