Microencapsulated VO(acac)2: preparation and use in allylic alcohol epoxidation

[reaction: see text]. Microencapsulated VO(acac)2 [MC-VO(acac)2] has been prepared and screened with success as a catalyst for the epoxidation of allylic alcohols using tert-butyl hydroperoxide as oxidant. The reactions are run in hexane at room temperature. The MC-VO(acac)2 is reusable without significant loss of activity.     

Aryl-bis(thioaryl)sulfonium salts and their use for the preparation of S-aryl-episulfonium salts

A new method for the preparation of S-arylepisulfonium salts has been developed and several compounds of this type have been isolated.     

Experimental investigation into the mechanism of the epoxidation of aldehydes with sulfur ylides

A mechanistic study of the epoxidation of aldehydes with sulfur ylides has been carried out. The DeltaG++ of the reaction was determined to be 22.2 kcal/mol at 298 K. A 13C kinetic isotope effect was determined to be 1.026 for the carbonyl carbon of benzaldehyde. A secondary deuterium isotope effect was determined to be 0.93 for the aldehydic hydrogen atom of benzaldehyde. Substituent effects on reaction rate were studied, and a Hammett rho of +2.50 was found.     

Design of sulfides with a locked conformation as promoters of catalytic and asymmetric sulfonium ylide epoxidation

[reaction: see text] A new generation of 2,5-dimethylthiolanes with a locked conformation was developed to promote the asymmetric addition of chiral sulfonium ylides to aldehydes. The novel chiral sulfur derivative 4 succeeded the synthesis of trans-stilbene oxide derivatives with enantiomeric ratios ranging from 95:5 to 98:2. This user-friendly organocatalytic process proved to be efficient with 20-10% of sulfide 4 in 1 or 2 days of reaction. An insight into the ylide intermediate conformation is given on the basis of a computational ab initio study.     

Direct organocatalytic asymmetric epoxidation of α,β-unsaturated aldehydes

The organocatalytic asymmetric epoxidation of α,β-unsaturated aldehydes with peroxides or sodium percarbonate is presented. Chiral pyrrolidine derivatives, proline and amino acid derived imidazolidinones mediate the asymmetric epoxidation of α,β-unsaturated aldehydes. For example, commercially available protected α,α-diphenyl-2-prolinol catalyzes the asymmetric formation of 2-epoxy-aldehydes in 81-95% conversion with up to 96:4 dr and 98% ee. The use of non-toxic catalysts, aqueous solvents and hydrogen peroxide or sodium percarbonate as the oxygen sources makes the reaction environmentally benign.     

6-Magnesiated purines: preparation and reaction with aldehydes

[reaction: see text] Halogen-metal exchange reaction of 9-benzyl-6-iodopurine with iPrMgCl in toluene at -80 degrees C proceeds almost quantitatively. Such a purine-derived Grignard reagent reacts selectively with aldehydes in toluene, giving the corresponding alcohols in 25-62% yield, while other functional groups such as ketones, esters, and nitriles do not react under these conditions. The reaction can be extended to protected 6-iodopurine ribonucleoside.     

Asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes with hydrogen peroxide

The first asymmetric organocatalytic epoxidation of alpha,beta-unsaturated aldehydes is presented. A chiral bisaryl-silyl-protected pyrrolidine acts as a very selective epoxidation organocatalyst using simple oxidation agents, such as hydrogen peroxide and tert-butyl hydroperoxide. The asymmetric epoxidation reactions proceed under environmental friendly reaction condition in, for example, water mixtures of alcohols, and the scope of the reaction is demonstrated by the formation of optically active alpha,beta-epoxy aldehydes in high yields and enantioselectivities >94% ee. Furthermore, the direct synthesis of the sex pheromone from an acaric mite by asymmetric epoxidation of citral is presented.     

烯烃及α,β-不饱和醛、酮在气相中的无催化环氧化反应

概述了碳碳双键在气相中无催化环氧化反应研究方法、反应机理、测定了2－甲基丙烯醛与过氧化乙酰基在393K和413K时环氧化反应的速率,实验结果表明，不同结构烯烃的环氧化反应速率主要取决于双键所连取代基的性质，取代基的给电子能力越强环氧化反应速率越快。     

Tris(trimethylsilyl)sulfonium and methylbis(trimethylsilyl)sulfonium ions: preparation, NMR spectroscopy, and theoretical studies(1)

Tris(trimethylsilyl)sulfonium 1 and methylbis(trimethylsilyl)sulfonium 2 ions were prepared as long-lived species by reacting trimethylsilane and trityl tetrakis(pentafluorophenyl)borate (Ph(3)C(+) TPFPB) in the presence of precursor sulfides and characterized by (1)H, (13)C, and (29)Si NMR spectroscopy at -78 degrees C. Attempted preparation of dimethyl(trimethylsilyl)sulfonium ion under similar conditions failed as a result of the formation of the more stabilized trimethylsulfonium ion. Structures, and (13)C and (29)Si NMR chemical shifts were calculated by density functional theory (DFT)/IGLO methods. The calculated results agree well with the experimental data.     

Renewable camphor-derived hydroperoxide: synthesis and use in the asymmetric epoxidation of allylic alcohols

Renewable enantiopure tertiary furyl hydroperoxide has been easily synthesized in two steps starting from low cost (+)-(1R)-camphor and it has been used in the asymmetric epoxidation and kinetic resolution of allylic alcohols (enantioselectivities up to 46%).     

Sulfur ylides via decarboxylation of carboxymethylsulfonium betaines: a novel and mild protocol for the preparation of oxiranes

A novel protocol for the generation of sulfur ylides is described. The overall process involves thermal decarboxylation of a carboxymethylsulfonium betaine to give a sulfur ylide that, in the presence of an aldehyde, affords the corresponding terminal oxirane. Yields were found to correlate with the electron deficiency of the aryl aldehyde. In situ generation of betaine in the presence of an aldehyde successfully afforded the desired oxirane in moderate yield, thus demonstrating the feasibility of a catalytic process. [reaction: see text]     

Synthesis and Structure of Some Aryl-substituted Thiiranes

A number of aryl-substituted 1,2-chlorohydrins was prepared by treating 1,2-epoxy-3-chlororopane with arylmagnesium bromide. The reaction of the aryl-substituted 1,2-chlorohydrins with sodium hydroxide solution furnished the corresponding oxiranes, the key compounds in the synthesis of a number of previously unknown thiiranes.     

Asymmetric epoxidation of olefins catalyzed by chiral iminium salts generated in situ from amines and aldehydes

[reaction: see text] A new approach for catalytic asymmetric epoxidation of olefins was developed that utilized chiral iminium salts, generated in situ from chiral amines and aldehydes, as catalysts. Epoxidation reactions can be conducted with 20 mol % of amines and aldehydes. The enantioselectivity of epoxides can be up to 65%. This modular approach obviates the difficulties inherent in the preparation and isolation of unstable exocyclic iminium salts.     

Polymeric DABCO-bromine complex: a mild oxidant for the preparation of ketones and aldehydes

A stable, shelf-ready polymeric oxidant was prepared from the addition of Br₂ to DABCO in CCl₄. This material was used to convert simple primary and secondary alcohols to the corresponding aldehydes and ketones in biphasic CH₂Cl₂/H₂O.     

Zn(ODf)2: preparation and application in asymmetric alkynylation of aldehydes

A new Lewis acid, Zn(ODf)2, was first prepared from commercially available 3,3,4,4-tetrafluoro[1,2]oxathietane 2,2-dioxide in four steps with 56% yields and also was applied to catalyze highly enantioselective alkynylation of aldehydes in the presence of ligand (1S,2S)-3-(tert-butyldimethylsilyloxyl)-2-N,N-dimethylamino-1- (p-nitrophenyl)-propane-1-ol or ligand (-)-N-methylephedrine to afford the corresponding propargylic alcohols in high yields with up to 99% ee.     

A versatile method for preparation of O-alkylperoxycarbonic acids: epoxidation with alkyloxycarbonylimidazoles and hydrogen peroxide

A variety of O-alkylperoxycarbonic acids ($\text{2}$) were conveniently prepared $\text{in}$$\text{situ}$ by utilizing alkyloxycarbonylimidazoles ($\text{1}$) as their precursors. Epoxidation of alkenes with such peroxy-acids was studied and their reactivities were compared with those of peroxycarboxylic acids.     

Aryl-substituted dithiafulvenes: synthesis,electronic properties,and redox reactivity

A series of aryl-substituted dithiafulvenes (DTFs) has been synthesized and characterized by single crystal X-ray diffraction analysis, UV–Vis absorption spectroscopy, and cyclic voltammetry. The studies indicate that the aryl-substituents not only affect the structures and electronic properties of the DTF derivatives, but also impose significant impact on their stability and reactivity when oxidized into radical cations.     

Electroorganic chemistry—XV : Electroreduction of sulfonium and ammonium salts. Formation of sulfonium ylids

In the electroreduction of sulfonium and ammonium salts, sulfonium ylid was generated from the sulfonium salt, whereas in the case of the ammonium salt, nitrogen ylid was not formed but the N+C bond was reductively cleaved. On the basis of the controlled potential electrolysis, it seems acceptable that the mechanism of formation of sulfonium ylid involves the direct electron transfer to the sulfonium salt as the initiation step.