Efficient asymmetric oxidation of sulfides and kinetic resolution of sulfoxides catalyzed by a vanadium-salan system

Chiral Thianthrenes

The absolute configuration and stability of two thianthrene chiral sulfoxides has been determined by means of X-ray single-crystal structure determinations. The analyses and configurations allow verification that the diastereomeric sulfoxides are stable in solution and are not interconverting, which has been suggested in some studies of sulfoxides. The two thianthrene sulfoxides have slightly different Rf values, which allowed their separation using flash chromatography on silica. The spots run back-to-back, which posed a challenge for their separation. The pure, separated compounds in solution remain as separate, single spots on a Thin Layer Chromatography (TLC) plate.

Catching an Oxo Vanadate Porous Acetylacetonate Covalent Adaptive Catalytic Network that Renders Mustard-Gas Simulant Harmless

In this work, we illustrated the design and development of a metal-coordinated porous organic polymer (POP) namely VO@TPA-POP via a post-synthetic metalation strategy to incorporate oxo-vanadium sites in a pristine polymer (TPA-POP) having acetylacetonate (acac) as anchoring moiety. The as-synthesized VO@TPA-POP exhibited highly robust and porous framework, which has been utilized for thioanisole (TA) oxidation to its corresponding sulfoxide. The catalyst demonstrated notable stability and recyclability by maintaining its catalytic activity over multiple reaction cycles without any significant loss in activity. The X-ray absorption spectroscopy (XAS) and density functional theory (DFT) analysis establish the existence of V(+4) oxidation state along with the VO(O)4 active sites into the porous network and the most energetically feasible mechanistic pathway involved in the TA oxidation, respectively, indicating the role of electron density associated with vanadium center during the catalytic transformation. Thus, this work aims at the demonstration of versatility and potential of VO@TPA-POP as a porous heterogeneous catalyst for the TA oxidation followed by decontamination of sulfur mustards (HD's) to their corresponding less toxic sulfoxides in a more efficient and greener way.

Peroxygenase-Catalysed Sulfoxidations in Non-Aqueous Media

Chiral sulfoxides are valuable building blocks in asymmetric synthesis. However, the biocatalytic synthesis of chiral sulfoxides is still challenged by low product titres. Herein, we report the use of peroxygenase as a catalyst for asymmetric sulfoxidation under non-aqueous conditions. Upon covalent immobilisation, the peroxygenase showed stability and activity under neat reaction conditions. A large variety of sulfides was converted into chiral sulfoxides in very high product concentration with moderate to satisfactory optical purity (e. g. 626 mM of (R)-methyl phenyl sulfoxide in approx. 89 % ee in 48 h). Further polishing of the ee value via cascading methionine reductase A (MsrA) gave>99 % ee of the sulfoxide. The robustness of the enzymes and high product titer is superior to the state-of-the-art methodologies. Gram-scale synthesis has been demonstrated. Overall, we demonstrated a practical and facile catalytic method to synthesize chiral sulfoxides.

Enzymatic Resolution and Decarboxylative Functionalization of α-Sulfinyl Esters

α-Sulfinyl esters can be readily prepared through thiol substitution of α-bromo esters followed by oxidation to the sulfoxide. Enzymatic resolution with lipoprotein lipase provides both the unreacted esters and corresponding α-sulfinyl carboxylic acids in high yields and enantiomeric ratios. Subsequent decarboxylative halogenation, dihalogenation, trihalogenation and cross-coupling gives rise to functionalized sulfoxides. The method has been applied to the asymmetric synthesis of a potent inhibitor of 15-prostaglandin dehydrogenase.

Recommended Tests for the Self-Disproportionation of Enantiomers (SDE) to Ensure Accurate Reporting of the Stereochemical Outcome of Enantioselective Reactions

The purpose of this review is to highlight the necessity of conducting tests to gauge the magnitude of the self-disproportionation of enantiomers (SDE) phenomenon to ensure the veracity of reported enantiomeric excess (ee) values for scalemic samples obtained from enantioselective reactions, natural products isolation, etc. The SDE always occurs to some degree whenever any scalemic sample is subjected to physicochemical processes concomitant with the fractionation of the sample, thus leading to erroneous reporting of the true ee of the sample if due care is not taken to either preclude the effects of the SDE by measurement of the ee prior to the application of physicochemical processes, suppressing the SDE, or evaluating all obtained fractions of the sample. Or even avoiding fractionation altogether if possible. There is a clear necessity to conduct tests to assess the magnitude of the SDE for the processes applied to samples and the updated and improved recommendations described herein cover chromatography and processes involving gas-phase transformations such as evaporation or sublimation.

Enantioselective Sulfoxidation of Thioanisole by Cascading a Choline Oxidase and a Peroxygenase in the Presence of Natural Deep Eutectic Solvents

A bienzymatic cascade for selective sulfoxidation is presented. The evolved recombinant peroxygenase from Agrocybe aegeritra catalyses the enantioselective sulfoxidation of thioanisole whereas the choline oxidase from Arthrobacter nicotianae provides the H₂ O₂ necessary via reductive activation of ambient oxygen. The reactions are performed in choline chloride-based deep eutectic solvents serving as co-solvent and stoichiometric reductant at the same time. Very promising product concentrations (up to 15 mM enantiopure sulfoxide) and catalyst performances (turnover numbers of 150,000 and 2100 for the peroxygenase and oxidase, respectively) have been achieved.

Five-Step Enantioselective Synthesis of Islatravir via Asymmetric Ketone Alkynylation and an Ozonolysis Cascade

A 5-step enantioselective synthesis of the potent anti-HIV nucleoside islatravir is reported. The highly efficient route was enabled by a novel enantioselective alkynylation of an α,β-unsaturated ketone, a unique ozonolysis-dealkylation cascade in water, and an enzymatic aldol-glycosylation cascade.

Synthesis and nano-Pd catalyzed chemoselective oxidation of symmetrical and unsymmetrical sulfides

A highly chemoselective, efficient and nano-Pd catalyzed protocol for the rapid construction of sulfoxides and sulfones via the oxidation of symmetrical and unsymmetrical sulfides using H2O2 as an oxidant has been developed, respectively. The ready availability of starting materials, easy recovery and reutilization of the catalyst, wide substrate scope, and high yields make this protocol an attractive alternative. The process also involves the metal-free and microwave-promoted synthesis of symmetrical diarylsulfides, and FeCl3-mediated preparation of symmetrical diaryldisulfides through the reaction of arenediazonium tetrafluoroborates with Na2S·9H2O as a sulfur source. In addition, unsymmetrical sulfides were generated via the K2CO3-mediated reaction of arenediazonium tetrafluoroborates with symmetrical disulfides.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Access towards enantiopure α,α-difluoromethyl alcohols by means of sulfoxides as traceless chiral auxiliaries

A new methodology to access enantiopure α,α-difluoromethyl alcohols is hereby being described. The strategy relies on the use of an enantiopure aryl α,α-difluoromethyl sulfoxide employed as chiral and removable auxiliary for the stereoselective difluoromethylation of carbonyl derivatives. The obtained α,α-difluoro-β-hydroxysulfoxides displayed unprecedented diastereomeric ratios.

Chemoenzymatic Deracemization of Chiral Sulfoxides

The highly enantioselective enzyme methionine sulfoxide reductase A was combined with an oxaziridine-type oxidant in a biphasic setup for the deracemization of chiral sulfoxides. Remarkably, high ee values were observed with a wide range of substrates, thus providing a practical route for the synthesis of enantiomerically pure sulfoxides.

Bovine serum albumin-cobalt(ii) Schiff base complex hybrid: an efficient artificial metalloenzyme for enantioselective sulfoxidation using hydrogen peroxide

An artificial metalloenzyme (BSA-CoL) based on the incorporation of a cobalt(ii) Schiff base complex {CoL, H2L = 2,2'-[(1,2-ethanediyl)bis(nitrilopropylidyne)]bisphenol} with bovine serum albumin (BSA) has been synthesized and characterized. Attention is focused on the catalytic activity of this artificial metalloenzyme for enantioselective oxidation of a variety of sulfides with H2O2. The influences of parameters such as pH, temperature, and the concentration of catalyst and oxidant on thioanisole as a model are investigated. Under optimum conditions, BSA-CoL as a hybrid biocatalyst is efficient for the enantioselective oxidation of a series of sulfides, producing the corresponding sulfoxides with excellent conversion (up to 100%), chemoselectivity (up to 100%) and good enantiomeric purity (up to 87% ee) in certain cases.

Chiral sulfoxides: advances in asymmetric synthesis and problems with the accurate determination of the stereochemical outcome

Chiral sulfoxides are in extremely high demand in nearly every sector of the chemical industry concerned with the design and development of new synthetic reagents, drugs, and functional materials. The primary objective of this review is to update readers on the latest developments from the past five years (2011-2016) in the preparation of optically active sulfoxides. Methodologies covered include catalytic asymmetric sulfoxidation using either chemical, enzymatic, or hybrid biocatalytic means; kinetic resolution involving oxidation to sulfones, reduction to sulfides, modification of side chains, and imidation to sulfoximines; as well as various other methods including nucleophilic displacement at the sulfur atom for the desymmetrization of achiral sulfoxides, enantioselective recognition and separation based on either metal-organic frameworks (MOF's) or host-guest chemistry, and the Horner-Wadsworth-Emmons reaction. A second goal of this work concerns a critical discussion of the problem of the accurate determination of the stereochemical outcome of a reaction due to the self-disproportionation of enantiomers (SDE) phenomenon, particularly as it relates to chiral sulfoxides. The SDE is a little-appreciated phenomenon that can readily and spontaneously occur for scalemic samples when subjected to practically any physicochemical process. It has now been unequivocally demonstrated that ignorance in the SDE phenomenon inevitably leads to erroneous interpretation of the stereochemical outcome of catalytic enantioselective reactions, in particular, for the synthesis of chiral sulfoxides. It is hoped that this two-pronged approach to covering the chemistry of chiral sulfoxides will be appealing, engaging, and motivating for current research-active authors to respond to in their future publications in this exciting area of current research.

Asymmetric catalytic sulfoxidation by a novel VIV8 cluster catalyst in the presence of serum albumin: a simple and green oxidation system

Enantioselective oxidation of a series of alkyl aryl sulfides catalyzed by a novel VIV8 cluster is tested in an aqueous medium in the presence of serum albumin. The procedure is simple, environmentally friendly, selective, and highly reactive.

Kinetic investigation on the highly efficient and selective oxidation of sulfides to sulfoxides and sulfones with t-BuOOH catalyzed by La2O3

An efficient and highly selective methodology for the oxidation of various sulfides with 70% t-BuOOH as oxidant in the presence of catalytic amounts of La₂O₃ is described.

In situ-generated chiral iron complex as efficient catalyst for enantioselective sulfoxidation using aqueous H2O2 as oxidant

This study represents the rare combination of non-toxic Fe based catalyst/H₂O₂ as an efficient catalytic protocol for asymmetric sulfoxidation reaction.

Enantioselective oxidation of sulfides with H2O2 catalyzed by a pre-formed manganese complex

A facile and environmentally friendly method is presented for the asymmetric oxidation of sulfides with H₂O₂ by an in situ-formed manganese complex, affording the corresponding chiral sulfoxides in high yields and excellent enantioselectivities.