Synthesis of sulfoxides by oxidation of thioethers

α,β-Dehydrogenation Adjacent to Sulfur- and Phosphorus- Containing Compounds

Here, we report a robust nickel-catalyzed α,β-dehydrogenation process designed for substrates that contain electron-withdrawing sulfur and phosphorus groups. Leveraging the formation of organozinc intermediates and the utilization of a mild oxidant, allyl methyl carbonate, this methodology exhibits remarkable efficiency and outstanding diastereoselectivities across a diverse array of substrates, achieving E : Z ratios exceeding 20 : 1. Investigation through deuterium incorporation studies and an analysis of the reaction sequence leading to the formation of the dehydrogenative allylation side product, provide useful insights into reaction optimization.

Manganese-Mediated Electrochemical Oxidation of Thioethers to Sulfoxides Using Water as the Source of Oxygen Atoms

Oxygen-atom transfer reactions are a prominent class of synthetic redox reactions that often use high-energy oxygen-atom donor reagents. Electrochemical methods can bypass these reagents by using water as the source of oxygen atoms through pathways involving direct or indirect (mediated) electrolysis. Here, manganese porphyrins and related mediators are shown to be effective molecular electrocatalysts for selective oxidation of thioethers to sulfoxides, without overoxidation to the sulfone. The reactions proceed by proton-coupled oxidation of a MnIII-OH2 species to generate a MnIV-OH and MnV═O species. This methodology is compared to direct electrolysis methods initiated by single-electron oxidation of the thioether, and chloride-mediated electrochemical oxidation of thioethers. The Mn-mediated reactions operate at lower applied potential and exhibit improved substrate scope and functional group compatibility relative to direct electrolysis, and the tunability of the Mn-based mediators allows for improved performance relative to chloride-mediated electrolysis. An electrochemical parallel screening platform is developed and applied to a library of pharmaceutically relevant thioethers.

In Situ Plasma Polymerization of Self-Stabilized Polythiophene Enables Dendrite-Free Lithium Metal Anodes with Ultra-Long Cycle Life

Constructing a flexible and chemically stable multifunctional layer for the lithium (Li) metal anodes is a highly effective approach to improve the uneven deposition of Li+ and suppress the dendrite growth. Herein, an organic protecting layer of polythiophene is in situ polymerized on the Li metal via plasma polymerization. Compared with the chemically polymerized thiophene (C-PTh), the plasma polymerized thiophene layer (P-PTh), with a higher Young's modulus of 8.1 GPa, shows strong structural stability due to the chemical binding of the polythiophene and Li. Moreover, the nucleophilic C─S bond of polythiophene facilitates the decomposition of Li salts in the electrolytes, promoting the formation of LiF-rich solid electrolyte interface (SEI) layers. The synergetic effect of the rigid LiF as well as the flexible PTh-Li can effectively regulate the uniform Li deposition and suppress the growth of Li dendrites during the repeated stripping-plating, enabling the Li anodes with long-cycling lifespan over 8000 h (1 mA cm-2, 1 mAh cm-2) and 2500 h (10 mA cm-2, 10 mAh cm-2). Since the plasma polymerization is facile (5-20 min) and environmentally friendly (solvent-free), this work offers a novel and promising strategy for the construction of the forthcoming generation of high-energy-density batteries.

Aerobic Photocatalysis: Oxidation of Sulfides to Sulfoxides

Sulfoxides constitute one of the most important functional groups in organic chemistry found in numerous pharmaceuticals and natural products. Sulfoxides are usually obtained from the oxidation of the corresponding sulfides. Among various oxidants, oxygen or air are considered the greenest and most sustainable reagent. Photochemistry and photocatalysis is increasingly applied in new, as well as traditional, yet demanding, reaction, like the aerobic oxidation of sulfides to sulfoxides, since photocatalysis has provided the means to access them in mild and effective ways. In this review, we will summarize the photochemical protocols that have been developed for the oxidation of sulfides to sulfoxides, employing air or oxygen as the oxidant. The aim of this review is to present: i) a historical overview, ii) the key mechanistic studies and proposed mechanisms and iii) categorize the different catalytic systems in literature.

Facile preparation of dihydro-1,4-benzothiazine derivatives via oxidative ring-expansion of 2-aminobenzothiazoles with olefins

A concise and efficient approach to prepare dihydro-1,4-benzothiazine derivatives is described via oxidative ring-expansion of 2-aminobenzothiazoles with olefins under metal-free conditions. This protocol is applicable for a wide range of readily accessible 2-aminobenzothiazoles and olefins with moderate-to-good yields. The [4+2] heteroannulation between the intermediacy of oxidative ring-opening of 2-aminobenzothiazoles and olefins is suggested to rationalize the formation of the product.

A Base-Catalyzed Approach for the anti-Markovnikov Hydration of Styrene Derivatives

The base-catalyzed addition of 1-cyclopropylethanol to styrene derivatives with an acidic reaction workup enables anti-Markovnikov hydration. The use of either catalytic organic superbase or crown ether-ligated inorganic base permits hydration of a wide variety of styrene derivatives, including electron-deficient, ortho-substituted and heteroaryl variants. This protocol complements alternative routes to terminal alcohols that rely on stoichiometric reduction and oxidation processes. The utility of this method is demonstrated through multigram scale reactions and its use in a two-step hydration/cyclization process of ortho-halogenated styrenes to prepare 2,3-dihydrobenzofuran derivatives.

The base-catalyzed addition of 1-cyclopropylethanol to vinyl (hetero)arenes sequenced with an acidic reaction workup enables anti-Markovnikov hydration in a complementary fashion to traditional hydroboration/oxidation protocols.

Iron-catalyzed regioselective synthesis of (E)-vinyl sulfones mediated by unprotected hydroxylamines

An Fe-catalyzed unprotected hydroxylamine mediated Heck-type coupling between sulfinic acids and alkenes furnished structurally important (E)-vinyl sulfones with moderate to good yields, high atom-economy and regioselectivity.

Mild Oxidation of Organosulfur Compounds with H2O2 over Metal-Containing Microporous and Mesoporous Catalysts

Mild catalytic oxidation of thioethers and thiophenes is an important reaction for the synthesis of molecules with pharmaceutical interest, as well as for the development of efficient processes able to remove sulfur-containing pollutants from fuels and wastewater. With respect to the green chemistry principles, hydrogen peroxide (H2O2) is the ideal oxidant and the Me-containing porous materials (Me = Ti, V, Mo, W, Zr) are among the best heterogeneous catalysts for these applications. The main classes of catalysts, including Me-microporous and mesoporous silicates, Me-layered double hydroxides, Me-metal–organic frameworks, are described in this review. The catalytic active species generated in the presence of H2O2, as well as the probable oxidation mechanisms, are also addressed. The reactivity of molecules in the sulfoxidation process and the role played by the solvents are explored.

Silver-promoted synthesis of vinyl sulfones from vinyl bromides and sulfonyl hydrazides in water

The synthesis of vinyl sulfones via silver-promoted cross-coupling of vinyl bromides with sulfonyl hydrazides was realized. Water was used as the sole solvent. Multisubstituted vinyl sulfones were easily prepared with excellent alkyl group tolerance. A mechanism involving nucleophilic attack of a sulfinate anion was proposed.

Methanesulfinylation of Benzyl Halides with Dimethyl Sulfoxide

A phenyltrimethylammonium tribromide-mediated nucleophilic substitution/oxygen transformation reaction of benzyl halides with DMSO has been developed. In this transition-metal-free reaction, DMSO acts as not only a solvent but also a "S(O)Me" source, thus providing a convenient method for the efficient and direct synthesis of various benzyl methyl sulfoxides.

Aerobic oxidation catalyzed by polyoxometalates associated to an artificial reductase at room temperature and in water

Four polyoxometalates (POMs) were combined with an artificial reductase based on polyethyleneimine (PEI) and flavin mononucleotide (FMN) which is capable of delivering single electrons upon addition of nicotinamide adenine dinucleotide (NADH).

Carbon Nanotubes Modified by Venturello Complex as Highly Efficient Catalysts for Alkene and Thioethers Oxidation With Hydrogen Peroxide

In this work, we elaborated heterogeneous catalysts on the basis of the Venturello complex [PO4{WO(O2)2}4]3- (PW4) and nitrogen-free or nitrogen-doped carbon nanotubes (CNTs or N-CNTs) for epoxidation of alkenes and sulfoxidation of thioethers with aqueous hydrogen peroxide. Catalysts PW4/CNTs and PW4/N-CNTs (1.8 at. % N) containing 5-15 wt. % of PW4 and differing in acidity have been prepared and characterized by elemental analysis, N2 adsorption, IR spectroscopy, HR-TEM, and HAADF-STEM. Studies by STEM in HAADF mode revealed a quasi-molecular dispersion of PW4 on the surface of CNTs. The addition of acid during the immobilization is not obligatory to ensure site isolation and strong binding of PW4 on the surface of CNTs, but it allows one to increase the PW4 loading and affects both catalytic activity and product selectivity. Catalytic performance of the supported PW4 catalysts was evaluated in H2O2-based oxidation of two model substrates, cyclooctene and methyl phenyl sulfide, under mild conditions (25-50°C). The best results in terms of activity and selectivity were obtained using PW4 immobilized on N-free CNTs in acetonitrile or dimethyl carbonate as solvents. Catalysts PW4/CNTs can be applied for selective oxidation of a wide range of alkenes and thioethers provided a balance between activity and selectivity of the catalyst is tuned by a careful control of the amount of acid added during the immobilization of PW4. Selectivity, conversion, and turnover frequencies achieved in epoxidations over PW4/CNTs catalysts are close to those reported in the literature for homogeneous systems based on PW4. IR spectroscopy confirmed the retention of the Venturello structure after use in the catalytic reactions. The elaborated catalysts are stable to metal leaching, show a truly heterogeneous nature of the catalysis, can be easily recovered by filtration, regenerated by washing and evacuation, and then reused several times without loss of the catalytic performance.

Tailoring chemoenzymatic oxidation via in situ peracids

Epoxidation chemistry often suffers from the challenging handling of peracids and thus requires in situ preparation. Here, we describe a two-phase enzymatic system that allows the effective generation of peracids and directly translate their activity to the epoxidation of olefins. We demonstrate the approach by application to lipid and olefin epoxidation as well as sulfide oxidation. These methods offer useful applications to synthetic modifications and scalable green processes.

Mechanistic reaction model for oxidation of sulfur mustard simulant by a catalytic system of nitrate and tribromide

A new metal-free catalytic reaction system is developed to selectively oxidize 2-chloroethyl ethyl sulfide (CEES), a surrogate of sulfur mustard. The combination of two catalytic precursors, tribromide and nitrate, allows a rapid sulfoxidation of CEES even at ambient conditions. The kinetic behaviours at various reaction conditions are investigated to identify the most probable reaction pathways of the development of catalytic loop and the overall reaction steps of CEES sulfoxidation. The mechanistic study demonstrates that the catalytic loop does not require an addition of mineral acid or water, which is common in most other reaction systems. Incomplete catalytic systems with one precursor are also examined to uncover the complex network of sulfoxidation in the catalytic reaction system. The results reveal that the complex between CEES and bromine is a reactive intermediate, bromosulfonium, which can be further catalysed and converted into sulfoxide by nitrate. Based on the proposed reaction mechanism, a predictive kinetic model fully describing most reaction behaviours is developed.

Benzyltriphenylphosphonium Dichromate as a Mild Reagent for Oxidation of Thiols and Sulfides

Oxidation of thiols and sulfides under non-aqueous and aprotic conditions uses benzyltriphenylphosphonium dichromate (1, PhCH2PPh3)2 Cr2O7) which is very easily prepared by mixing an aqueous solution of benzyltriphenylphosphonium chloride with CrO3 in 3N HCl at room temperature. This reagent, a stable orange powder which may be stored for a month without loss of activity, is soluble in acetonitrile, chloroform and dichloromethane and slightly soluble in carbon tetrachloride, ether and hexane.

Practical synthesis of active O-2-(2-propylsulfinyl)benzyl (OPSB) glycosides via a catalytic and metal free oxidation of latent O-2-(2-propylthiol)benzyl (OPTB) glycosides

A catalytic and metal free sulfoxidation of O-2-(2-propylthiol)benzyl (OPTB) glycosides to O-2-(2-propylsulfinyl)benzyl (OPSB) glycosides has been developed by introducing NOBF₄ as catalyst, oxygen as terminal oxidant and TBAB as additive. Wide variety of OPTB glycosides were efficiently oxidized without observation of over oxidation. The allowance of large scale synthesis, easy operation and purification highlighted its practical application in construction of complex oligosaccharides and glycoconjugates employing interrupted Pummerer reaction mediated glycosylation strategy.

Thiol-ene/oxidation tandem reaction under visible light photocatalysis: synthesis of alkyl sulfoxides

The photocatalyzed synthesis of sulfoxides from alkenes and thiols has been carried out using Eosin Y.

The photocatalyzed synthesis of sulfoxides from alkenes and thiols has been carried out using Eosin Y. This is a metal-free method which uses a low catalyst loading, atmospheric oxygen as the oxidant, and visible light conditions (green light). A mechanism has been proposed that is consistent with the experimental results.

Polyoxometalate LUMO engineering: a strategy for visible-light-responsive aerobic oxygenation photocatalysts

We report efficient visible-light-responsive oxygenation photocatalysis via the strategy of LUMO engineering of polyoxometalates.

Tuning of Catalytic Property Controlled by the Molecular Dimension of Palladium-Schiff Base Complexes Encapsulated in Zeolite Y

Planar palladium-Schiff base complexes are synthesized, maintaining the order of their molecular dimensions as PdL1 < PdL2 < PdL3 < PdL4 < PdL5 in free state, as well as encapsulated in zeolite Y, where L1: N,N'-bis(salicylidene)ethylenediamine and L2, L3, L4, and L5 are derivatives of L1. All encapsulated complexes have shown better catalytic activity for the sulfoxidation of methyl phenyl sulfide in comparison to their homogeneous counter parts. These hybrid systems are characterized with the help of different characterization techniques such as X-ray diffraction analysis, scanning electron microscopy-energy-dispersive X-ray spectrometry, X-ray photoelectron spectroscopy, Fourier transform infrared, and UV-visible spectroscopy; all of these studies have suggested that the largest complex deviates by the maximum from its free-state properties, and a radical change in the reactivity of the complex is observed.

Metal- and additive-free oxygen-atom transfer reaction: an efficient and chemoselective oxidation of sulfides to sulfoxides with cyclic diacyl peroxides

Metal- and additive-free sulfoxidation was developed using cyclic diacyl peroxides as oxygen sources and a single two-electron transfer mechanism was suggested.

Molybdenum-doped α-MnO2 as an efficient reusable heterogeneous catalyst for aerobic sulfide oxygenation

In the presence of Mo⁶⁺-doped α-MnO₂ (Mo–MnO₂), various sulfides could efficiently be oxidized to the corresponding sulfoxides as the major products. In addition, Mo–MnO₂ could repeatedly be reused.

B(C6F5)3-Catalyzed Hydrodesulfurization Using Hydrosilanes--Metal-Free Reduction of Sulfides

B(C6F5)3-catalyzed hydrodesulfurization of carbon-sulfur bonds was achieved using triethylsilane as the reducing agent. The corresponding products were obtained in good yields under mild reaction conditions. This protocol could be applied to the reduction of sulfides, including benzyl and alkyl sulfides and dithianes, with high chemoselectivities.

Electrochemically mediated enantioselective reduction of chiral sulfoxides

The respiratory DMSO reductase from Rhodobacter capsulatus catalyzes the reduction of dimethyl sulfoxide to dimethyl sulfide. Herein, we have utilized this Mo enzyme as an enantioselective catalyst to generate optically pure sulfoxides (methyl p-tolyl sulfoxide, methyl phenyl sulfoxide and phenyl vinyl sulfoxide) from racemic starting materials. A hexaaminecobalt coordination compound in its divalent oxidation state was employed as the mediator of electron transfer between the working electrode and DMSO reductase to continually reactivate the enzyme after turnover. In all cases, chiral HPLC analysis of the reaction mixture revealed that the S-sulfoxide was reduced more rapidly leading to enrichment or isolation of the R isomer.