Giorgio Modena

Asymmetric oxidation of 1,3-dithiolanes. A route to the optical resolution of carbonyl compounds

Abstract The asymmetric oxidation ( t -BuO 2 H, Ti(OPr- i ) 4 , DET) of a series of 1,3-dithiolanes was carried out to produce the corresponding S-oxides with high chemical and optical yields. By contrast, the oxidation of 1,3-dithianes and 1,3-oxathiolane prepared from the same carbonyl compounds gave much lower optical yields. The optical resolution of the model ketone, dl-menthone, via a) 1,3-dithiolane formation b) asymmetric S-oxidation c) chromatographic diasteromeric separation d) regeneration of the carbonyl group, (93% optical yield), is described.

Oxidations with peroxotungsten complexes: rates and mechanism of stoichiometric olefin epoxidations

Abstract The stoichiometric epoxidation reactions of a series of olefins with WO-(O2)2HMPT and MoO(O2)2HMPT have been studied in dichloroethane at 40 °C. The data obtained point out the superior oxidizing ability of W(VI) over Mo(VI). However, the two peroxo groups in WO(O2)2HMPT appear to react with the substrates at different rates, the transfer of the first peroxide oxygen being faster than of the second one, contrary to that observed in the behavior of MoO(O2)2HMPT. The crystal structure of WO(O2)2HMPT·H2O, determined by diffractometric techniques, does not show any significant difference compared to that reported for MoO(O2)2HMPT·-H2O, indicating that it might be difficult to correlate the solid state structures with the different behaviors experimentally observed in solution. In spite of the different features shown by the two peroxo complexes, the data collected in this investigation suggest that an identical mechanism of oxygen transfer from the two oxidants is operating. Moreover, this mechanism should not involve the occurrence of substrate-oxidant intermediates.

Metal catalysis in oxidation by peroxides part 8 [1] further insight on the mechanism of vanadium(V) catalyzed oxidation of sulphides and alkenes by hydrogen peroxide

Abstract The oxidation of p-chlorophenyl methyl sulphide, geraniol and cyclohexene with hydrogen peroxide in the presence of catalytic amounts of bis acetylacetonato oxovanadium(IV) [VaO(acac)2] in dioxane and dioxaneethanol has been studied, p-Chlorophenyl methyl sulphoxide and 2,3-epoxygeraniol are produced in quantitative yield; oxidation of cyclohexene is, on the other hand, much less selective, affording 2-cyclohexen-1-ol, 2-cyclohexen-1-one and cyclohexene oxide as major products. Kinetic studies indicate that the rate of sulphide oxidation, at relatively low hydrogen peroxide concentration, is first-order in sulphide, oxidant and catalyst whereas, at higher [H2O2]o, zero-order dependence on hydrogen peroxide is observed. Autodecomposition of H2O2, which takes place at [VaO(acac)2] much higher than that used in oxidation experiments shows zero-order dependence on [H2O2]o and second-order dependence on [VaO(acac)2]. The equilibrium formation of monoperoxovanadium(V) species as the oxidizing agent is suggested. The relevance of acid—base equilibria, involving vanadium peroxoacid, on its oxidizing efficiency is discussed, also on the ground of potentiometric experiments. The mechanism of geraniol epoxidation, which accounts for the high reactivity and regioselectivity observed, is also discussed.

Enantioselective oxidation of thioethers1: An easy route to enantiopure C2 symmetrical bis-methylsulfinylbenzenes

The direct oxidation of bis-methylthioethers 1 by t-butyl hydroperoxide, titanium tetra-iso-propilate and (+)-diethyltartrate, affords the almost enantiomerically pure dl bis-methylsulfinylbenzenes 2 (e.e.⩾99%) in a process which is also characterized by a very high diastereoselectivity.

Evidence concerning peroxovanadate structures in solution and their role in catalytic oxidation process

Abstract Evidence concerning the nature of peroxometal intermediates in the oxidation of organic substrates by hydrogen peroxide or t-butyl hydroperoxide by vanadium(V) compounds is critically reviewed; it suggests formation of either metal μ-peroxoester (or peroxoacid) 2 or side-bonded peroxometal intermediates 3. Kinetic and spectroscopic data, as well as comparison with analogous systems, indicate that an entirely different situation arises in the interaction of alkyl hydroperoxides or hydrogen peroxide with the metal center, in that while alkyl hydroperoxides generate metal μ-peroxoester intermediates, H 2 O 2 produces vanadium(V)—side-bonded peroxocomplexes. It is suggested that formation constant ( K t ) values of the peroxometal species and kinetic behaviour in catalytic oxidation permit distinction between these two significantly different modes of binding of the peroxide to the metal.

Asymmetric reduction of carbon–nitrogen, carbon–oxygen, and carbon–carbon double bonds by homogeneous catalytic hydrogenation

Asymmetric homogeneous hydrogenations of carbon-nitrogen, carbon-oxygen, and carbon-carbon double bonds have been achieved with the same RhI-chiral diphosphine [(–)-diop] catalyst; all the products have the R configuration and a small increase in optical yields is obtained by changing the solvent from methanol to propan-2-ol.

Asymmetric oxidation of thioethers. Optical resolution of [1,1′-binaphthalene]-2,2′-dithiol☆

Almost optically pure (e.e. > 98%) [1,1′-binaphthalene]-2,2′-dithiol (2) is obtained by resolution of racemic 2 via the transformation of the sulfidryl functions into the corresponding thioethers 3 which are asymmetrically oxidized to diastereomeric chiral monosulfoxides 4 and then reconverted into 2. The diastereo and enantioselectivity is dependent on the structure of the thioether; i.e. the dimethylthioether 3a gives two diastereomeric sulfoxides 4a in ca. 1:1 ratio, each of them in > 98% e.e., while cyclic dithioethers 3b–d afford a single diastereomeric sulfoxide 4b–d in 46–78% e.e..

Asymmetric oxidation of thioethers: Enantioselective synthesis of β-hydroxysulfoxides by direct oxidation☆

Abstract S-methyl β-hydroxysulfoxides of fairly high optical purity (up to 80%) may be prepared by direct asymmetric oxidation [Bu t O 2 H, Ti(OPr i ) 4 , (+)-DET] of acetylated or silylated S-methyl β-hydroxysulfides and subsequent deprotection. The upgrading of the optical purity from 78% up to >98% by simple crystallization has been obtained.

Kinetics and mechanism of the tungsten-catalyzed oxidation of organic sulphides and alkenes by hydrogen peroxide

Abstract The oxidation of a series of phenyl-substituted arylmethyl sulphides and of cyclohexene and 1-methylcyclohexene by hydrogen peroxide in the presence of catalytic amounts of WO 5 · HMPT · H 2 O and W(CO) 6 has been studied in ethanol (HMPT = hexamethyl phosphoric acid triamide). The reactions afford the corresponding sulphoxides and epoxides in quantitative yield. A subsequent ethanolysis of the oxirane ring occurs in solution. Kinetic studies indicate that the reaction is first-order in the substrate and in the catalyst, whereas a zero-order in hydrogen peroxide is found. The identity of the catalytic efficiency of the two W(VI) species employed suggests that HMPT ligand is displaced in solution. The substituent effect on the oxidation rates of arylmethyl sulphides follows the Hammett free-energy relationship (ϱ ≈ −1). Sulphides are much more reactive than cyclohexene which, in turn, is 2.5-fold less reactive than 1-methylcyclohexene. Reaction rates are remarkably enhanced by addition of CH 3 SO 3 H, whereas added HMPT at relatively high concentration inhibits the oxidation. The data are discussed in the light of the results of previous studies on metal-hydrogen peroxide systems, pointing out similarities and differences. The conclusion is reached that W(VI)—H 2 O 2 is a particularly efficient oxidant.

Hydroxylation of aromatics with hydrogen peroxide catalyzed by vanadium(V) peroxocomplexes

Abstract Benzene and substituted benzenes XC6H5 (X = CH3, F, Cl, Br, NO2) are hydroxylated to the corresponding monophenols by H2O2 in CH3CN in the presence of catalytic amounts of VO (O2) (PIC) (H2O)2 where PIC is the anion of picolinic acid. Fair yields of hydroxylated materials are obtained by adding the oxidant in portions, or continuously, and keeping the conversion of the substrate rather low (