Sulfoxide complexes, catalytic hydrogenation

This chapter reports principally on studies with ruthenium chiral phosphine and chiral sulfoxide complexes and their use for catalytic hydrogenation. We have used the familiar diop ligand, [2R,3R-(—)-2,3-Oisopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino) butane] (7) a related chiral chelating sulfoxide ligand dios, the bis(methyl sulfinyl)butane analog (21) (S,R S,S)-(+)-2-meth-ylbutyl methyl sulfoxide(MBMSO), chiral in the alkyl group and R-(+)-methyl para-tolyl sulfoxide(MPTSO), chiral at sulfur. Preliminary data on some corresponding Rh(I) complexes are presented also. 

A Cr(VI) sulfoxide complex has been postulated after interaction of [CrOjtClj] with MejSO (385), but the complex was uncharacterized as it was excessively unstable. It was observed that hydrolysis of the product led to the formation of dimethyl sulfone. The action of hydrogen peroxide on mesityl ferrocencyl sulfide in basic media yields both mesityl ferrocenyl sulfoxide (21%) and the corresponding sulfone (62%) via a reaction similar to the Smiles rearrangement (165). Catalytic air oxidation of sulfoxides by rhodium and iridium complexes has been observed. Rhodium(III) and iridium(III) chlorides are catalyst percursors for this reaction, but ruthenium(III), osmium(III), and palladium(II) chlorides are not (273). The metal complex and sulfoxide are dissolved in hot propan-2-ol/water (9 1) and the solution purged with air to achieve oxidation. The metal is recovered as a noncrystalline, but still catalytically active, material after reaction (272). The most active precursor was [IrHClj(S-Me2SO)3], and it was observed that alkyl sulfoxides oxidize more readily than aryl sulfoxides, while thioethers are not oxidized as complex formation occurs. 

Recently we presented a new methodological approach for catalytic sulfoxidation which makes use of water as solvent in the presence of anionic surfactant, and hydrogen peroxide as terminal oxidant activated by an easily prepared chiral Pt(ll) complex, all under mild conditions (Figure 9.7). Moreover, the enantioenriched sulfoxides are isolated from the catalyst by means of simple diethyl ether extraction (Figure 9.8) which does not dissolve the catalyst. 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

Copper(II) complex (5) catalyses the oxidation of sulfides to sulfoxides with hydrogen peroxide in high yields. Addition of a catalytic amount of TEMPO to the reaction mixture enhances the conversion and selectivity.181... 

In 1995, Bolm and Bienewald introduced a new, very practical method for the asymmetric catalytic oxidation of sulfides [44]. In the presence of vanadium complex prepared in situ from VO(acac)2 and 23 reactions of various sulfides or dithianes fike 24 with aqueous hydrogen peroxide afforded the corresponding sulfoxides with enantiomeric excesses of up to 85% (Eq. 2). Only traces of the corresponding sulfones were observed. The transformation can easily be carried out in open vessels at room temperature using inexpensive H2O2 as oxidant. 

Activation of carbon—hydrogen bonds in benzyl phenyl sulfoxides catalysed by palladium may lead to dibenzothiophene derivatives (39). lodoarenes and silver salts are required as additives, and product formation requires several consecutive catalytic cycles. The copper-catalysed conversion of bisaryloxime ethers to 2-arylbenzoxazoles is likely to involve complexation of the catalyst at... 

This particular thiourea used by Russo for sulfoxidation, gives a catalytic turnover that competes well with transition metal complexes generally used for sulfoxidation reactions. The effectiveness of the TBHP activation could be rationalised according to a donble hydrogen-bonding interaction of the thiourea with the proximal oxygen of TBHP, which should enhance the elctrophilic character of the distal oxygen attacked by a snlfide. Formation of the TBHP and thiourea complex (Fig. 1.11) was confirmed by H NMR spectroscopic analysis, where the chemical shift of H moves downfield from 7.88 to 7.91 ppm, and the proton in the TBHP also shifts downfield from 7.14 to 7.42 ppm. 

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