Oxidation Of Aryl Methyl Sulfides Research Articles

ChemInform Abstract: Synthesis of Novel β‐Amino Alcohols and Their Application in the Catalytic Asymmetric Sulfoxidation of Sulfides.

Abstract Novel chiral norephedrine-based β-amino alcohol ligands containing a thiophene ring were prepared from norephedrine and substituted furan carbaldehydes (methyl- or ethyl-substituted) and used in combination with VO(acac) 2 for the asymmetric oxidation of aryl methyl sulfides using H 2 O 2 as an oxidant. Amino alcohol derived Schiff bases 4 , 5a – b gave higher enantiomeric excesses than amino alcohol-derived reduced Schiff based ligands 6 , 7a – b . Of these chiral ligands, (1 S ,2 R )- 5b and (1 S ,2 R )- 7b gave high yields (90%) with moderate to high enantioselectivities (78%, 96% ee, respectively). The oxidation of other aryl methyl sulfides with (1 S ,2 R )- 5b and (1 S ,2 R )- 7b as ligands afforded the corresponding sulfoxides in 60–89% yields and with 92–99% ee.

Synthesis of novel β-amino alcohols and their application in the catalytic asymmetric sulfoxidation of sulfides

Novel chiral norephedrine-based β-amino alcohol ligands containing a thiophene ring were prepared from norephedrine and substituted furan carbaldehydes (methyl- or ethyl-substituted) and used in combination with VO(acac)2 for the asymmetric oxidation of aryl methyl sulfides using H2O2 as an oxidant. Amino alcohol derived Schiff bases 4,5a–b gave higher enantiomeric excesses than amino alcohol-derived reduced Schiff based ligands 6,7a–b. Of these chiral ligands, (1S,2R)-5b and (1S,2R)-7b gave high yields (90%) with moderate to high enantioselectivities (78%, 96% ee, respectively). The oxidation of other aryl methyl sulfides with (1S,2R)-5b and (1S,2R)-7b as ligands afforded the corresponding sulfoxides in 60–89% yields and with 92–99% ee.

ChemInform Abstract: Asymmetric Oxidation of Sulfides with H2O2 Catalyzed by Titanium Complexes of Schiff Bases Bearing a Dicumenyl Salicylidenyl Unit.

The sterically hindered Schiff bases (L3–L5), prepared from 3,5-dicumenyl salicylaldehyde and chiral amino alcohols, were used in combination with Ti(OiPr)4 for asymmetric oxidation of aryl methyl sulfides with H2O2 as terminal oxidant. Among the ligands L3–L5, L4 with a tert-butyl group in the chiral carbon of the amino alcohol moiety gave the best result with 89% yield and 73% ee for the sulfoxidation of thioanisole under optimal conditions [with 1 mol% of Ti(OiPr)4 in a molar ratio of 100:1:1.2:120 for sulfide:Ti(OiPr)4:ligand:H2O2 in CH2Cl2 at 0 °C for 3 h]. The reaction afforded good yield (84%) with a moderate enantioselectivity (62% ee) even with a lower catalyst loading from 1.0 to 0.5 mol%. The oxidations of methyl 4-bromophenyl sulfide and methyl 4-methoxyphenyl sulfide with H2O2 catalyzed by the Ti(OiPr)4–L4 system gave 79–84% yields and 54–59% ee of the corresponding sulfoxides in CH2Cl2 at 20 °C. The chiral induction capability of the cumenyl-modified sterically hindered Schiff bases for sulfoxidation was compared with the conventional Schiff bases bearing tert-butyl groups at the 3,5-positions of the salicylidenyl unit. Copyright © 2011 John Wiley & Sons, Ltd.

Asymmetric oxidation of sulfides with H2O2 catalyzed by titanium complexes of Schiff bases bearing a dicumenyl salicylidenyl unit

The sterically hindered Schiff bases (L3–L5), prepared from 3,5-dicumenyl salicylaldehyde and chiral amino alcohols, were used in combination with Ti(OiPr)4 for asymmetric oxidation of aryl methyl sulfides with H2O2 as terminal oxidant. Among the ligands L3–L5, L4 with a tert-butyl group in the chiral carbon of the amino alcohol moiety gave the best result with 89% yield and 73% ee for the sulfoxidation of thioanisole under optimal conditions [with 1 mol% of Ti(OiPr)4 in a molar ratio of 100:1:1.2:120 for sulfide:Ti(OiPr)4:ligand:H2O2 in CH2Cl2 at 0 °C for 3 h]. The reaction afforded good yield (84%) with a moderate enantioselectivity (62% ee) even with a lower catalyst loading from 1.0 to 0.5 mol%. The oxidations of methyl 4-bromophenyl sulfide and methyl 4-methoxyphenyl sulfide with H2O2 catalyzed by the Ti(OiPr)4–L4 system gave 79–84% yields and 54–59% ee of the corresponding sulfoxides in CH2Cl2 at 20 °C. The chiral induction capability of the cumenyl-modified sterically hindered Schiff bases for sulfoxidation was compared with the conventional Schiff bases bearing tert-butyl groups at the 3,5-positions of the salicylidenyl unit. Copyright © 2011 John Wiley & Sons, Ltd.

Highly enantioselective sulfoxidation with vanadium catalysts of Schiff bases derived from bromo- and iodo-functionalized hydroxynaphthaldehydes

Two series of chiral Schiff bases, 4a− e and 5a− e, prepared from the condensation of the mono-, di-, tribromohydroxynaphthaldehyde or monoiodohydroxynaphthaldehyde with chiral amino alcohols, were used in combination with VO(acac) 2 for the asymmetric oxidation of aryl methyl sulfides using H 2O 2 as terminal oxidant. Among these Schiff bases, dibromo-functionalized 4d and iodo-functionalized 5e gave high yields (91–93%) with good enantioselectivities (80–82% ee) for the oxidation of thioanisole in dichloromethane. The asymmetric oxidation of thioanisole in toluene using these Schiff bases gave methyl phenyl sulfoxide in satisfactory isolated yields (48–62%) with high enantioselectivities (91–94% ee), which were further improved by a modified procedure with the ee value up to 98% in 62% yield. The oxidations of other aryl methyl sulfides in toluene with dibromo- and iodo-functionalized Schiff bases 5d and 5e as ligands using the modified procedure afforded the corresponding sulfoxides in 55–67% isolated yields with 95–99% ee.

Asymmetric oxidation of sulfides with hydrogen peroxide catalyzed by a vanadium complex of a new chiral NOO-ligand

A new chiral NOO-tridentate ligand (8 R )-2-(2-hydroxyphenyl)-4-methyl-5,6,7,8-tetrahydro-quinolin-8-ol ( 1 ) bearing a rigid tetrahydroquinoline framework was prepared and applied in the vanadium-catalyzed asymmetric oxidation of aryl methyl sulfides with H 2 O 2 as oxidant. Less toxic acetone was found to be the proper solvent for the enantioselective oxidation of sulfides. Under the optimal condition, the asymmetric oxidation of aryl methyl sulfides in acetone catalyzed by VO(acac) 2 / 1 at 0 °C gives good to high yields (80–95%) of sulfoxides with enantioselectivity up to 77% ee.

Mechanism of the Selective Sulfide Oxidation Promoted by HNO3/FeBr3

The oxidation of aryl methyl sulfides containing electron withdrawing and electron donating groups (p-NO(2), p-CHO, p-NCS, p-Br, H, p-CH(3), p-OCH(3)) was carried out in homogeneous solution in acetonitrile in the presence of catalytic amounts of HNO(3) and FeBr(3). The FeBr(3) is required for the reaction to proceed for compounds with strongly electron withdrawing groups (p-NO(2) and p-CHO) but is not necessary in the case of all the other compounds, although in the latter cases the yield decreases considerably. The rate of the reaction was measured as a function of substrate, FeBr(3) and HNO(3) concentration. From the experimental data a mechanism is suggested where there are two reaction pathways, one involving the formation of a ternary complex between the substrate, FeBr(3) and NO(3)(-) and one involving a complex formed between the sulfide and the HNO(3). From these complexes HNO(2) is generated, which then combines with HNO(3) to yield N(2)O(4), initiating a catalytic cycle where the sulfide is oxidized and O(2) from the air is stoichiometrically consumed.

Influence of substituents in the salicylaldehyde‐derived Schiff bases on vanadium‐catalyzed asymmetric oxidation of sulfides

AbstractA series of chiral Schiff bases (L1–L5) with different substituents in the salicylidenyl unit were prepared from condensation of 3‐aryl‐5‐tert‐butylsalicylaldehyde derivatives and optically active amino alcohols. Bromination of 3‐phenyl‐5‐tert‐butylsalicylaldehyde gave an unexpected product 3‐(4‐bromophenyl)‐5‐bromosalicylaldehyde, from which the corresponding Schiff base ligands L6 and L7, derived from (S)‐valinol and (S)‐tert‐leucinol, respectively, were prepared. Ligands L1–L7 were applied to the vanadium‐catalyzed asymmetric oxidation of aryl methyl sulfides. Under the optimal conditions, the oxidation of the thioanisole with H2O2 as oxidant in CH2Cl2 catalyzed by VO(acac)2‐L1–L7 gives good yields (74–83%) with moderate enantioselectivity (58–77% ee). Ligand L7, containing a 4‐bromophenyl group on the 3‐position and a Br atom on the 5‐position of the salicylidenyl moiety, displays an 80–90% ee for vanadium‐catalyzed oxidation of methyl 4‐bromophenyl sulfide and methyl 2‐naphthyl sulfide. Copyright © 2008 John Wiley & Sons, Ltd.

An investigation by means of correlation analysis into the mechanisms of oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane in various solvents

Relative rate constants have been measured for the oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane in acetone, in mixtures of acetone with aprotic co-solvents of both higher and lower relative permittivity, and in aqueous acetone mixtures. Correlation analyses of the effects of substituents in the different solvents show that, with one exception, reactions take place via a single step mechanism in which the formation of the new SO bond and the elimination of acetone occur concertedly. The exception was oxidation of the sulfides in aqueous acetone containing the highest proportion of water of those studied (20% v/v). Here, the behaviour of the reaction is consistent with a two-step mechanism in which the oxidant reversibly attacks the sulfide to form an open-chain sulfonium betaine that subsequently fragments to sulfoxide and acetone. There is no evidence for the participation of an intermediate dioxathietane as has been found in the case of sulfide oxidations by (trifluoromethyl)methyldioxirane in CH(2)Cl(2) and similar aprotic solvents. It is not justified to generalise a mechanism involving a betaine, with or without a derived dioxathietane, to the reactions of dimethyldioxirane in acetone.

Kinetic studies on the oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane; absolute rate constants and activation parameters for 4-nitrophenyl methyl sulfide and sulfoxide

The oxidations of methyl 4-nitrophenyl sulfide and sulfoxide by dimethyldioxirane, in acetone and mixtures of acetone with water, methanol, acetonitrile and hexane, have been followed by UV-Vis spectroscopy to monitor the decay of the substrates. The data show that, under all the conditions studied, both oxidations obey second-order kinetics. Grunwald-Winstein and Kamlet-Taft analyses of the influence of solvents on the second-order rate constants have been used to obtain mechanistic information on the two reactions. Activation parameters for the two oxidations in acetone and aqueous acetone have been calculated from rate constants for reactions in the temperature range 283-313 K and compared with those from sulfide and sulfoxide oxidations with other oxidants. For sulfoxide oxidations in acetone and 1-20% v/v water in acetone, the results support a concerted nucleophilic displacement by sulfur of oxygen from dimethyldioxirane with the rate being dependent on the solvent's polarity. Sulfide oxidations in acetone and 1-5% v/v water in acetone also proceed by a concerted mechanism. However, in the most polar solvent system studied, 20% v/v water in acetone, the mechanism changes in favour of a two-step reaction involving a betaine intermediate. Importantly, the sulfide oxidation shows a different solvent dependence to that of the sulfoxide, with the rate of oxidation being determined by the hydrogen bond donor capacity and electron-pair donicity of the solvent.

Asymmetric oxidation of sulfides catalyzed by chiral (salen)Mn(III) complexes with a pyrrolidine backbone

AbstractCatalytic properties of a series of chiral (pyrrolidine salen)Mn(III) complexes for asymmetric oxidation of aryl methyl sulfides were evaluated. Moderate activity, good chemical selectivity and low enantioselectivity were attained with iodosylbenzene as a terminal oxidant. Enantioselectivity of sulfide oxidation was affected slightly by polar solvent and the sulfoxidation carried out in THF for thioanisole and in CH3CO2Et for electron‐deficient sulfides gave better enatioselctivities. The addition of the donor ligand PPNO (4‐phenylpyridine N‐oxide) or MNO (trimethylamine N‐oxide) only has a minor positive effect on the enantioselectivity. Also explored was the steric effect of the Naza‐substituent in the backbone of (pyrrolidine salen)Mn(III) complexes on the enantioselectivity of sulfide oxidation. The sulfides' access pathway is discussed based on the catalytic results. Copyright © 2006 John Wiley & Sons, Ltd.

Asymmetric Oxidation of Sulfides Catalyzed by Vanadium(IV) Complexes of Dibromo- and Diiodo-Functionalized Chiral Schiff Bases

Abstract The catalyst system of VO(acac) 2 and Schiff base ligands derived from 3,5-dibromo- or 3,5-diiodosalicylaldehyde and inexpensive chiral amino alcohols was prepared. This catalyst has good yields and moderate-to-high enantioselectivity for the asymmetric oxidation of aryl methyl sulfides at room temperature when 1% catalyst (VO(acac) 2 /ligand molar ratio of 1:2) and H 2 O 2 oxidant were used. The ligands derived from ( S )-valinol exhibit considerably higher enantioselectivity than those derived from ( S )-phenylalaninol and ( R )-leucinol. The enantiomeric excess values are improved up to 88% for methyl phenyl sulfoxide and 92% for methyl p -bromophenyl sulfoxide by slow dropwise addition of H 2 O 2 with the ligand prepared from 3,5-diiodosalicylaldehyde and ( S )-valinol. The catalytic efficiency of VO(acac) 2 /Schiff base systems cannot be improved by the addition of carboxylic acids or carboxylate salts.

The reactivity–selectivity principle in the oxidation of aryl methyl sulfides with sodium hypochlorite catalysed by (salen)MnIII complexes

The kinetics of oxygen atom transfer from four oxo(salen)manganese(V) complexes to various para-substituted phenyl methyl sulfides have been studied spectrophotometrically in 90% acetonitrile-10% water(v/v) at 20oC. Electron-releasing substituents in sulfides and electron-withdrawing substituents in oxo(salen)manganese(V) complexes enhance the rate of oxidation. Correlation analyses establish that there is an inverse relationship between reactivity and selectivity in both the sulfide and the complex series. Mathematical treatment of the results shows the operation of a valid reactivity-selectivity principle in this redox system.

Kinetics and mechanism of oxidation of aromatic sulfides and arylmercaptoacetic acids by N-chlorosuccinimide

Kinetic measurements of the oxidation of divalent organic sulfur compounds by N-chlorosuccinimide in acetonitrile–water mixture at constant [H+] show that the reaction is first order in both the oxidant and the organic sulfur compound. While the rate of oxidation of methyl phenyl sulfide or diphenyl sulfide increases with [H+], that of phenylmercaptoacetic acid decreases. Therefore it has been concluded that protonated N-chlorosuccinimide and N-chlorosuccinimide are the active oxidising species in the oxidation of aromatic sulfides and N-chlorosuccinimide is the active species in the case of phenylmercaptoacetic acids. Structure–reactivity correlations for the oxidation of aryl methyl sulfides, diaryl sulfides and arylmercaptoacetic acids result in a high negative reaction constant, providing evidence for the formation of a chlorosulfonium ion intermediate.

Oxidation of aryl methyl sulfides and sulfoxides on irradiated TiO 2

TiO 2 photocatalyzed oxidation of aryl methyl sulfides results in the formation of sulfoxides as the major product with small amounts of sulfones and C-S bond cleaved products. Increase in the irradiation time leads to disproportionation reaction (irradiation time determines the composition of the product). However, TiO 2 acts as an effective photocatalyst for the oxidation of aryl methyl sulfoxides to give exclusively sulfones. Photoelectrochemical studies provide support for electron transfer to occur from sulfide to the hole.

Catalytic asymmetric oxidation of aryl methyl sulfides mediated by a ( S, S)-1,2-diphenylethan-1,2-diol/titanium/water complex

The asymmetric oxidation of aryl methyl sulfides to sulfoxides with hydroperoxides has been achieved using catalytic amounts of the complex formed in situ between Ti( i-PrO) 4, ( S, S)-diphenylethan-1,2-diol 1 and water. The sulfoxides are obtained in 60% yield and 67–80% enantiomeric excess.

ChemInform Abstract: Reactivity and Selectivity in the Oxidation of Aryl Methyl Sulfides and Sulfoxides by Hydrogen Peroxide Mediated by Acetonitrile.

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Reactivity and selectivity in the oxidation of aryl methyl sulfides and sulfoxides by hydrogen peroxide mediated by acetonitrile

In homogeneous methanol solution containing potassium carbonate, hydrogen peroxide and acetonitrile, the active oxidant in the conversion of p-substituted-phenyl methyl sulfides into the sulfoxides is selective (ρ=–1.0 at 0 °C) but the oxidation of the related sulfoxides at 0 and 24 °C shows low selectivity, which, taken in conjunction with direct kinetic data, throws new light on the active oxidizing species in these nitrile-mediated reactions.

Horseradish peroxidase catalysed sulfoxidation is enantioselective

Horseradish peroxidase catalysed oxidation of aryl methyl sulfides by H2O2 preferentially gives (–)-(S)-sulfoxides, enantiomeric excess 30–68%.