Bi2O3 catalyzed asymmetric oxidation of sulfides

Bismuth based catalytic system for the asymmetric oxidation of alkyl and aryl benzyl sulfides using t-BuOOH as the oxidant has been investigated. This method affords sulfoxides with high enantioselectivities (up to 98% ee) and good yields in reasonable time. The over oxidized product namely the sulfone was not observed. The described method has wide range of applications, exhibits chemoselectivity/enantioselectivity, and proceeds under mild and environmentally friendly reaction conditions.     

Titanium–salan‐catalyzed asymmetric sulfoxidations with H2O2: design of more versatile catalysts

Titanium–salan complexes with 3,3’‐diphenyl substituents in the salicylidene rings of the salan ligand are efficient sulfoxidation catalysts, capable of catalyzing the asymmetric oxidation of bulky aryl benzyl sulfides with H₂O₂ with good to high enantioselectivities. In this paper, substituent effects on titanium‐salan‐catalyzed enantioselective oxidation of sulfides to sulfoxides have been systematically investigated. Titanium–salan catalysts with halogen substituents at the 5,5’‐positions (3,3’‐H₂dihydrogen substituted) have been found to catalyze the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides with good to high enantioselectivities. Kinetic data witness a direct attack of the sulfide to the electrophilic active oxygen species; a consistent reaction mechanism is proposed. Copyright © 2013 John Wiley & Sons, Ltd.     

Copper-catalyzed asymmetric oxidation of sulfides

Copper-catalyzed asymmetric sulfoxidation of aryl benzyl and aryl alkyl sulfides, using aqueous hydrogen peroxide as the oxidant, has been investigated. A relationship between the steric effects of the sulfide substituents and the enantioselectivity of the oxidation has been observed, with up to 93% ee for 2-naphthylmethyl phenyl sulfoxide, in modest yield in this instance (up to 30%). The influence of variation of solvent and ligand structure was examined, and the optimized conditions were then used to oxidize a number of aryl alkyl and aryl benzyl sulfides, producing sulfoxides in excellent yields in most cases (up to 92%), and good enantiopurities in certain cases (up to 84% ee).     

Iron-catalyzed oxidation of thioethers by iodosylarenes: stereoselectivity and reaction mechanism

Catalytic properties of a series of iron(III)-salen (salen=N,N'-bis(salicylidene)ethylenediamine dianion) and related complexes in asymmetric sulfoxidation reactions, with iodosylarenes as terminal oxidants, have been explored. These catalysts have been found to efficiently catalyze oxidation of alkyl aryl sulfides to sulfoxides with high chemoselectivity (up to 100 %) and moderate-to-high enantioselectivity (up to 84 % with isopropylthiobenzene and iodosylmesitylene), the TON (TON=turnover number) approaching 500. The influence of the ligand (electronic and steric effects of the substituents), oxidant, and substrate structures on the oxidation stereoselectivity has been investigated systematically. The structure of the reactive intermediates (complexes of the type [Fe(III)(ArIO)(salen)] and the reaction mechanism have been revealed by both mechanistic studies with different iodosylarenes and direct in situ (1)H NMR observation of the formation of the reactive species and its reaction with the substrate.     

Asymmetric oxidation of sulfides catalyzed by (R)-6,6'-dibromo-BINOL derived titanium complex

Abstract

An efficient asymmetric oxidation of sulfides was achieved using (R)-6,6'-dibromo-BINOL as chiral ligand in combination with Ti(OⁱPr)₄ using 70% aqueous tertiary butyl hydroperoxide as oxidant. The resulting sulfoxides had high enantiopurities and good yields. A range of aryl alkyl and aryl benzyl sulfides were oxidized to the corresponding sulfoxides with 78–95% ee in 72–80% yields.     

Activation of oxaziridines by lewis acids: application in enantioselective sulfoxidation

The preparation of new and easily accessible chiral N-alkyloxaziridines for asymmetric sulfides oxidation was investigated. Most significantly, we report here that inert N-alkyloxaziridines can be activated by Lewis acids to achieve a fast, selective, and efficient oxygen atom transfer to sulfides. Asymmetric sulfoxidation by three new chiral oxaziridines (two of them were structurally characterized by X-ray analysis) afforded enantioselectivities ranging from 22% to 63% ee with the simplest aryl alkyl sulfides.     

Synthesis of sterically controlled chiral β-amino alcohols and their application to the catalytic asymmetric sulfoxidation of sulfides

Sterically hindered and enantiomerically pure β-amino alcohols 8a and 8b were prepared from the enantiomerically pure aziridine-2-carboxylic acid menthol ester 13. Vanadium complexes of the chiral Schiff-base ligands prepared from the β-amino alcohols catalyze an efficient enantioselective sulfoxidation of alkyl aryl sulfides, while enantioselectivities as high as 96% ee can be observed in the sulfoxidation of benzyl aryl sulfides.     

Enantioselective oxidation of sulfides with hydrogen peroxide catalyzed by vanadium complex of sterically hindered chiral Schiff bases

Sterically hindered chiral Schiff base ligands 4a-d were prepared from an aldehyde derived from BINOL. The vanadium complexes of the ligands catalyze an efficient, enantioselective H₂O₂-promoted sulfoxidation of alkyl aryl sulfides, and enantioselectivities as high as 98-99% ee are observed in the sulfoxidation of benzyl aryl sulfides.     

Readily Synthesized Chiral Sulfides as Reagents for Asymmetric Epoxidation

Chiral oxathianes were designed, synthesized, and successfully used for asymmetric sulfur ylide mediated epoxidation. A considerable emphasis has been placed upon the design of sulfides with suitable architecture in a small number of steps (three or four). The use of (4aR,6S,8aR)‐6‐isopropenyl‐8a‐methyloctahydro‐1,4‐benzoxathiane in asymmetric epoxidation resulted in good diastereo‐ and enantioselectivity in the formation of stilbene oxide, and (2S,6S)‐2‐allyl‐2,3,3,6‐tetramethyl‐1,4‐oxathiane produced even better results. Moderate to good diastereoselectivities with essentially complete enantioselectivities were observed in the formation of alkyl–aryl‐, vinyl–aryl‐, and propargyl–aryl‐substituted epoxides. The selectivities were rationalized and supported by density functional theory calculations.     

Application of a novel 1,3-diol with a benzyl backbone as chiral ligand for asymmetric oxidation of sulfides to sulfoxides

A chiral 1,3-diol with a benzyl backbone has been used for the asymmetric oxidation of sulfides to sulfoxides. Moderate to good yields and enantioselectivity (upto 87% ee) have been observed.     

Synthesis of C3- and C2-symmetric tris- and bis-sulfoxide ligands by asymmetric oxidation

A series of tris- and bis-sulfoxides were synthesized by multiple asymmetric oxidation of the corresponding sulfides. High enantioselectivities were obtained based on Horeau-type amplification of selectivity. Naphthyl-substituted sulfoxides allowed for higher selectivity and greater ease of purification. These sulfoxides were tested as ligands in rhodium-catalyzed olefin hydroacylation and 1,4-addition of phenyl boronic acid to 2-cyclohexen-1-one. While no enantioinduction was observed in hydroacylation, up to 80% ee was obtained for a tris-sulfoxide and a bis-sulfoxide ligand in the 1,4-addition. Bis-sulfoxides with flexible backbones gave lower and inversed enantioselectivity, suggesting backbone rigidity plays a key role in enantioinduction.     

Catalytic enantioselective oxidation of sulfides and disulfides by a chiral complex of bis-hydroxamic acid and molybdenum

A chiral bis-hydroxamic acid (BHA)-molybdenum complex was used for the catalytic asymmetric oxidation of sulfides and disulfides utilizing one equivalent of alkyl peroxide with yields up to 83% and ee up to 86%. An extension of our methodology combines the asymmetric oxidation with kinetic resolution providing excellent enantioselectivity (ee 92–99%).     

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

Catalytic 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 N_aza‐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.     

Biocatalytic Synthesis of Allylic and Allenyl Sulfides through a Myoglobin‐Catalyzed Doyle–Kirmse Reaction

The first example of a biocatalytic [2,3]‐sigmatropic rearrangement reaction involving allylic sulfides and diazo reagents (Doyle–Kirmse reaction) is reported. Engineered variants of sperm whale myoglobin catalyze this synthetically valuable C?C bond‐forming transformation with high efficiency and product conversions across a variety of sulfide substrates (e.g., aryl‐, benzyl‐, and alkyl‐substituted allylic sulfides) and α‐diazo esters. Moreover, the scope of this myoglobin‐mediated transformation could be extended to the conversion of propargylic sulfides to give substituted allenes. Active‐site mutations proved effective in enhancing the catalytic efficiency of the hemoprotein in these reactions as well as modulating the enantioselectivity, resulting in the identification of the myoglobin variant Mb(L29S,H64V,V68F), which is capable of mediating asymmetric Doyle–Kirmse reactions with an enantiomeric excess up to 71 %. This work extends the toolbox of currently available biocatalytic strategies for the asymmetric formation of carbon–carbon bonds.     

Asymmetric Hydrogenation of Pyridinium Salts with an Iridium Phosphole Catalyst

Iridium‐catalyzed asymmetric hydrogenation of N‐alkyl‐2‐alkylpyridinium salts provided 2‐aryl‐substituted piperidines with high levels of enantioselectivity. Simple benzyl and other alkyl groups successfully activated the challenging pyridine substrates toward hydrogenation. The use of the unusual chiral‐phosphole‐based MP²‐SEGPHOS was the key to the success of this approach which provides a versatile and practical procedure for the synthesis of chiral piperidines.     

Organic reactions in chiral micelles: 7. The structural effects on the asymmetric oxidation of prochiral sulfides in chiral micelles

A variety of phenyl alkyl sulfides were oxidized enantioselectively with NaIO₄ in chiral micellar systems formed from eight chiral surfactants, to give optical active sulfoxides. The enantiomer excesses ranged from 1.6 to 15.0%. To understand the mechanistic detail of this asymmetric oxidation in chiral micelle, the effects of structure both in substrates and surfactants on the optical yield of the oxidation were studied and discussed. Generally, increasing the alkyl chain length both in surfactants and in substrates enhances the optical yield, also the surfactant with hydroxy group at its appropriate position gives better enantioselectivity, suggesting the enzymic characteristics of the chiral micelle.     

Effect of temperature on the enantioselectivity in the oxazaborolidine-catalyzed asymmetric reduction of ketones. Noncatalytic borane reduction, a nonneglectable factor in the reduction system

The effect of temperature on the enantioselectivity of the oxazaborolidine-catalyzed asymmetric borane reduction of ketones has been investigated carefully using alkyl aryl ketones with a variety of functional groups and a B-methoxyoxazaborolidine derived from trimethyl borate and (S)-alpha,alpha-diphenylprolinol as a catalyst. The reductions were carried out over a range of temperatures in THF and toluene with or without the catalyst. The reductive rates increase along with increasing reaction temperature with or without the catalyst by determining the conversion of the ketones to alcohols by GC analysis. However, the rates of the catalytic reductions increase faster than those without the catalyst. The results indicate that the noncatalytic borane reduction is an important factor to the enantioselectivity in the reduction. The highest enantioselectivities were usually obtained between 20 and 30 degrees C in the asymmetric reduction.     

Stereoselective sulfoxidation catalyzed by achiral Schiff base complexes in the presence of serum albumin in aqueous media

Four coordination complexes ML derived from an achiral Schiff base ligand (H₂L?=?2,2′-[(1,2-ethanediyl)bis(nitrilopropylidyne)]bisphenol) have been synthesized and characterized. A method is described for the enantioselective oxidation of a series of aryl alkyl sulfides using the coordination complexes in the presence of serum albumins (SAs) in an aqueous medium at ambient temperature. The mixture of metal complexes with serum albumins is useful for inducing asymmetric catalysis. The complex, albumin source and substrate influence stereoselective sulfoxidation. At optimal pH with the appropriate oxidant, some of ML/SA systems are identified as very efficient catalysts, giving the corresponding sulfoxides in excellent chemical yield (up to 100%) and good enantioselectivity (up to 94% ee) in certain cases. UV–visible spectroscopic data provide evidence that stronger binding between the complex and serum albumin lead to higher enantioselectivity.     

Highly enantioselective (OC)Ru(salen)-catalyzed sulfimidation using N-alkoxycarbonyl azide as nitrene precursor

Enantioselective imidation of alkyl aryl sulfides with N-alkoxycarbonyl azide as a nitrene precursor was effected by using (OC)Ru(salen) complex 1 as catalyst. The steric and electronic nature of the N-alkoxycarbonyl group was found to strongly affect the enantioselectivity and the reaction rate, and high enantioselectivity (up to 99% ee) and good chemical yields were achieved by using 2,2,2-trichloro-1,1-dimethylethoxycarbonyl azide as the nitrene precursor at room temperature.     

Rapid Synthesis of Chiral 1,2-Bisphosphine Derivatives through Copper(I)-Catalyzed Asymmetric Conjugate Hydrophosphination

1,2-Bisphosphines have been identified as one class of important and powerful chiral ligands in asymmetric catalysis with transition metals. Herein, a copper(I)-catalyzed asymmetric hydrophosphination of α,β-unsaturated phosphine sulfides was developed with the assistance of “soft–soft” interaction between copper(I)-catalyst and the phosphine sulfide moiety, which afforded 1,2-bisphosphine derivatives with diversified electronic nature and steric hindrance in high to excellent yields with high to excellent enantioselectivity. Moreover, the challenging catalytic asymmetric hydrophosphination/protonation reaction was achieved with excellent enantioselectivity. Strikingly, the dynamic kinetic resolution of racemic diarylphosphines was also successfully carried out with high to excellent diastereo- and enantioselectivities. Interestingly, the nucleophilic copper(I)-diphenylphosphide species was characterized by ³¹P NMR spectrum and mass spectrum. At last, three products were transformed to chiral 1,2-bisphosphines, which were employed as ligands in Rh-catalyzed asymmetric hydrogenation of α-amino-α,β-unsaturated ester. The α-amino acid derivative was produced in high enantioselectivity, which demonstrated the utility of the present methodology.