Selected methods for the synthesis of sulfoxides and sulfones with emphasis on oxidative protocols

Microfluidic Photocatalytic Ring Expansion of Sulfonium Salts for the Synthesis of Cyclic Sulfides

Cyclic sulfides are relevant building blocks in medicinal and synthetic chemistry, with applications ranging from drug discovery to materials science. However, the synthesis of medium-sized cyclic sulfides (6-8-membered rings) remains largely underdeveloped. Herein, we report a photocatalytic ring-expansion strategy for sulfonium salts, granting access to six-, seven-, and eight-membered cyclic sulfides with very high regio- and diastereocontrol. The implementation of the method under continuous flow was key to increasing the efficiency and minimizing product decomposition. Mechanistic investigations revealed the formation of benzylic radicals and carbocation intermediates that control the high regio- and diastereoselectivity observed. Finally, the synthetic utility of this approach was demonstrated in the synthesis of cyclic sulfoxides and sulfones, which can be easily obtained from the corresponding sulfide products.

Indium Imidazo[4,5,-b]porphyrins as Photocatalysts for Oxidation of Sulfides

Over the past two decades, the application of photocatalytic reactions in organic synthesis has increased remarkably. Porphyrins, renowned for their exceptional photophysical properties, photostability, and prevalence in natural catalytic processes, are attracting significant attention as promising photocatalysts for reactions proceeding through energy transfer and one-electron transfer. In this work, we synthesized the indium(III) complex of 2-[4-(diethoxyphosphoryl)phenyl]-1H-imidazo[4,5-b]-5,10,15,20-tetramesitylporphyrin (InTMPIP) and explored its application as a photocatalyst for the oxidation of sulfides by dioxygen or air. Complex InTMPIP was found to generate singlet oxygen with quantum yield of 0.92 (toluene) and enables efficient photooxidation of sulfides to sulfoxides by dioxygen in "green" acetonitrile/water (4:1 v/v) or methanol/chloroform (2:1 v/v) solvent mixtures with almost quantitative yield. Furthermore, InTMPIP was grafted onto hydrated mesoporous titania and materials InTMPIP/TiO2-1 and InTMPIP/TiO2-2 with different In/Ti ratios were obtained and investigated. The composition and structure of the materials were studied using a combination of elemental analysis, various spectroscopic methods, gas adsorption measurements, and SEM imaging. Finally, the photocatalytic efficiency of InTMPIP/TiO2-2 was explored in aerobic photooxidation of sulfides. The heterogenized complex enables selective synthesis of sulfoxides under "green" conditions; however, it is prone to leaching into the solution when irradiated with both blue and red LEDs.

Water-soluble platinum and palladium porphyrins with peripheral ethyl phosphonic acid substituents: synthesis, aggregation in solution, and photocatalytic properties

Water-soluble porphyrins have garnered significant attention due to their broad range of applications in biomedicine, catalysis, and material chemistry. In this work, water-soluble platinum(II) and palladium(II) complexes with porphyrins bearing ethyl phosphonate substituents, namely, Pt/Pd 10-(ethoxyhydroxyphosphoryl)-5,15-di(p-carboxyphenyl)porphyrins (M3m, M = Pt(II), Pd(II)) and Pt/Pd 5,10-bis(ethoxyhydroxyphosphoryl)-10,20-diarylporphyrins (M1d-M3d; aryl = p-tolyl (1), mesityl (2), p-carboxyphenyl (3)), were synthesized by alkaline hydrolysis of the corresponding diethyl phosphonates M6m and M4d-M6d. NMR, UV-vis, and fluorescence spectroscopy revealed that the mono-phosphonates M3m tend to form aggregates in aqueous media, while the bis-phosphonates M3d exist predominantly as monomeric species across a wide range of concentrations (10-6-10-3 M), ionic strengths (0-0.81 M), and pH values (4-12). Single-crystal X-ray diffraction studies of the diethyl phosphonates Pt6d and Pd6d revealed that π-π stacking of the aromatic macrocycles is sterically hindered in the crystals, providing a rationale for the low degree of solution aggregation observed for ethyl phosphonate M3d. Photophysical studies of M3m and M1d-M3d demonstrated that these compounds are phosphorescent and generate singlet oxygen in aqueous solutions. Pd(II) complex Pd3d is an excellent photocatalyst for the oxidation of sulfides using di-oxygen in a solvent mixture (MeCN/H2O, 4 : 1 v/v). Under these conditions, various alkyl and aryl sulfides were quantitatively converted into the desired sulfoxides. For the oxygenation of mixed alkyl-aryl sulfides, Pd3d outperforms Pd(II) meso-tetrakis(p-carboxyphenyl)porphyrin (PdTCPP). This photocatalyst can be recycled and reused to afford sulfoxides with no loss of product yield.

N-Heterocycle-Substituted Hexa-peri-Hexabenzocoronenes with Windmill Architectures

We describe the synthesis and computational investigation of N-heterocycle-substituted hexa-peri-hexabenzocoronenes (HBCs). Following our method for the preparation of thioether-substituted HBCs, we prepared pyrrole-, indole-, carbazole-, and 1H-benz[g]indole-substituted HBCs from the corresponding fluorinated precursors under microwave irradiation. A series of polysubstituted benzoindole-HBCs with windmill architectures was also synthesized using the polyfluorinated HBC analogs, and the substituent effects on the electronic properties of the HBC core were investigated. Similar to the thioether substituted HBCs, the nature of the heterocycle does not influence the optoelectronic properties of the HBC core. The attachment of multiple benzoindole substituents leads to a bathochromic shift of the absorption and emission spectra, comparable to our previous studies. Due to the circular arrangement of the benzoindole moiety, the attachment of multiple substituents results in the presence of multiple conformers at room temperature. The rotation barrier can be overcome by heating the compounds to 323-333 K. Additionally, the investigation of the relaxed geometries shows two π-stacking motifs within the conformers.

Phosphonate-substituted porphyrins as efficient, cost-effective and reusable photocatalysts

Recent advances in visible light photocatalysis represent a significant stride towards sustainable catalytic chemistry. However, its successful implementation in fine chemical production remains challenging and requires careful optimization of available photocatalysts. Our work aims to structurally modify bioinspired porphyrin catalysts, addressing issues related to their laborious synthesis and low solubility, with the goal of increasing their efficiency and developing reusable catalytic systems. We have demonstrated the catalytic potential of readily available meso-tetrakis[4-(diethoxyphosphoryl)phenyl]porphyrins (M(TPPP)). Novel metal (Pd(II), Co(II) and In(III)) complexes with this ligand were prepared in good yields. These chromophores were characterized in solution using spectroscopic (NMR, UV-vis, fluorescence) and electrochemical methods. The introduction of phosphonate groups on the phenyl substituents of meso-tetraphenylporphyrins (M(TPP)) improves solubility in polar organic solvents without significantly altering the photophysical properties and photostability of complexes. This structural modification also leads to easier reductions and harder oxidations of the macrocycle for all investigated complexes compared to the corresponding TPP derivatives. The free base porphyrin, zinc(II), palladium(II), and indium(III) complexes were studied as photocatalysts for oxidation of sulfides to sulfoxides using molecular oxygen as a terminal oxidant. Both dialkyl and alkyl aryl sulfides were quantitatively transformed into sulfoxides under blue LED irradiation in the acetonitrile-water mixture (10 : 1 v/v) with a low loading (0.005-0.05 mol%) of porphyrin photocatalysts, where H2(TPPP) and Pd(TPPP) were found to be the most efficient. The reaction mechanism was studied using photoluminescence and EPR spectroscopies. Then, to access reusable catalysts, water-soluble derivatives bearing phosphonic acid groups, H2(TPPP-A) and Pd(TPPP-A), were prepared in high yields. These compounds were characterized using spectroscopic methods. Single-crystal X-ray diffraction analysis of Pd(TPPP-A) reveals that the complex forms a 3D hydrogen-bonded organic framework (HOF) in the solid state. Both H2(TPPP-A) and Pd(TPPP-A) were found to catalyze the photooxidation of sulfides by molecular oxygen in the acetonitrile-water mixture (1 : 1 v/v), while only Pd(TPPP-A) resulted in selective production of sulfoxides. The complex Pd(TPPP-A) was easily recovered through extraction in the aqueous phase and successfully reused in five consecutive cycles of the sulfoxidation reaction.

Radical approaches to C-S bonds

Organosulfur functionalities are ubiquitous in nature, pharmaceuticals, agrochemicals, materials and flavourants. Historically, these moieties were introduced almost exclusively using ionic chemistry; however, radical-based methods for the installation of sulfur-based functional groups have recently come to the fore. These radical methods have enabled their late-stage introduction into complex molecules, avoiding the need to preserve labile organosulfur moieties through multistep synthetic sequences. Here, we discuss homolytic C-S bond-forming processes, with a particular emphasis on radical substitution approaches to sulfide, disulfide and sulfinyl products, and the use of sulfur dioxide and its surrogates to build sulfonyl products. We also highlight the mechanistic considerations that we hope will guide further development of radical-based strategies compatible with the various organosulfur moieties that feature in modern chemistry.

Photoinduced Photocatalyst-Free Cascade Cyclization of Alkynes with Sodium Sulfinates for the Synthesis of Benzothiophenes and Thioflavones

The subject of this investigation is a new method for the construction of sulfonylated heterocycles which overcomes the limitations of classical approaches using a cheap feedstock sulfonylating agent, especially under photocatalyst- and metal-free conditions.

Hybrid Materials Based on Imidazo[4,5-b]porphyrins for Catalytic Oxidation of Sulfides

Heterogenized metalloporphyrin catalysts for oxidation reactions are extensively explored to improve chemical production. In this work, manganese meso-tetraarylporphyrins were immobilized on hydrated mesoporous titanium dioxide (SBET = 705 m2 g−1) through carboxylate or phosphonate anchoring groups separated from the macrocycle by the 2-arylimidazole linker fused across one of the pyrrolic rings of the macrocycle. The element composition of two mesoporous hybrid materials thus obtained were investigated and the integrity of the immobilized complexes was shown by different physicochemical methods. Finally, the catalytic efficiency of the more stable material Mn(TMPIP)/TiO2 with the phosphonate anchor was evaluated in the selective oxidation of sulfides to sulfoxides by molecular oxygen in the presence of isobutyraldehyde (IBA). The heterogenized complex has shown excellent catalytic activity exhibiting a turnover (TON) of ~1100 in a single catalytic run of the sulfoxidation of thioanisole. The catalyst was successfully reused in seven consecutive catalytic cycles.

Synthesis of Adagrasib (MRTX849), a Covalent KRASG12C Inhibitor Drug for the Treatment of Cancer

A concise, transition-metal and protection-free synthesis of adagrasib (MRTX849), a novel KRAS^G12C inhibitor drug recently approved by the FDA, is reported. Introduction of two chiral building blocks to the tetrahydropyridopyrimidine core was accomplished via two sequential S_NAr reactions. Extensive reaction optimization led to a robust, transition-metal-free oxidation of the sulfide intermediate. A judicious choice of the leaving group with favorable steric and electronic characteristics at the 4-OH position of the tetrahydropyridopyrimidine core enabled a facile S_NAr displacement to introduce the chiral piperazine. This new, five-step, chromatography-free synthesis of adagrasib from readily available starting materials obviated the palladium catalysis and protecting group manipulations in the current commercial route and significantly improved the efficiency of the process in 45% overall yield.

Decarboxylative Sulfinylation Enables a Direct, Metal-Free Access to Sulfoxides from Carboxylic Acids

The intermediate oxidation state of sulfoxides is central to the plethora of their applications in chemistry and medicine, yet it presents challenges for an efficient synthetic access, limiting the structural diversity of currently available sulfoxides. Here, we report a data-guided development of direct decarboxylative sulfinylation that enables the previously inaccessible functional group interconversion of carboxylic acids to sulfoxides in a reaction with sulfinates. Given the broad availability of carboxylic acids and the growing synthetic potential of sulfinates, the direct decarboxylative sulfinylation is poised to improve the structural diversity of synthetically accessible sulfoxides. The reaction is facilitated by a kinetically favored sulfoxide formation from the intermediate sulfinyl sulfones, despite the strong thermodynamic preference for the sulfone formation, unveiling the previously unknown and chemoselective radicalophilic sulfinyl sulfone reactivity.

New Type of Catalyst for Efficient Aerobic Oxidative Desulfurization Based On Tungsten Carbide Synthesized by the Microwave Method

Herein, we present a new type of high-performance catalyst for aerobic oxidation of organosulfur compounds based on tungsten carbide. The synthesis of tungsten carbide was performed via microwave irradiation of the precursors, which makes it possible to obtain a catalyst in just 15 min. The synthesized catalyst was investigated by a variety of physicochemical methods: X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, electron microscopy, and N₂ adsorption/desorption. It was shown that active centers containing tungsten in the transition oxidation state (+4) play a key role in the activation of oxygen. The main factors influencing the conversion of dibenzothiophene (DBT) were investigated. It should be noted that 100% conversion of DBT can be achieved under relatively mild conditions: 120 °C, 3 h, 6 bar, and 0.5% wt catalyst. The catalyst retained its activity for at least six oxidation/regeneration cycles. The simplicity and speed of synthesis of the proposed catalyst in combination with its high activity and stability open broad prospects for its further use both for oxidative desulfurization and for other reactions of aerobic oxidation of organic substrates.

Recent Progress and Emerging Technologies towards a Sustainable Synthesis of Sulfones

Sulfones play a pivotal role in modern organic chemistry. They are highly versatile building blocks and find various applications as drugs, agrochemicals, or functional materials. Therefore, sustainable access to this class of molecules is of great interest. Herein, the goal was to provide a summary on recent developments in the field of sustainable sulfone synthesis. Advances and existing limitations in traditional approaches towards sulfones were reviewed on selected examples. Furthermore, novel emerging technologies for a more sustainable sulfone synthesis and future directions were discussed.

Hydroxylamine-Derived Reagent as a Dual Oxidant and Amino Group Donor for the Iron-Catalyzed Preparation of Unprotected Sulfinamides from Thiols

An iron catalyzed reaction for the selective transformation of thiols (-SH) to sulfinamides (-SONH2 ) by a direct transfer of -O and free -NH2 groups has been developed. The reaction operates under mild conditions using a bench stable hydroxylamine derived reagent, exhibits broad functional group tolerance, is scalable and proceeds without the use of any precious metal catalyst or additional oxidant. This novel, practical reaction leads to the formation of two distinct new bonds (S=O and S-N) in a single step to chemoselectively form valuable, unprotected sulfinamide products. Preliminary mechanistic studies implicate the role of the alcoholic solvent as an oxygen atom donor.

A mild and chemoselective CALB biocatalysed synthesis of sulfoxides exploiting the dual role of AcOEt as solvent and reagent

A mild, chemoselective and sustainable biocatalysed synthesis of sulfoxides has been developed exploiting CALB and using AcOEt with a dual role of more environmentally friendly reaction solvent and enzyme substrate. A series of sulfoxides, including the drug omeprazole, have been synthesised in high yields and with excellent E-factors.

Progress and perspectives of electrochemical insights for C–H and N–H sulfonylation

A comprehensive electrosulfonylation study has been carried out via cathodic and anodic approaches for the production of organosulfone and sulfonamide derivatives.

6-Nitro-7-tosylquinazolin-4(3H)-one

Sulfones are important building blocks in the construction of biologically active molecules or functional materials. The sulfonyl functional group in sulfones is so versatile that it can act as either a nucleophile, an electrophile, or a radical in different organic reactions. Recently, quinazoline sulfones have been used to build asymmetrical ether derivatives as inhibitors of signaling pathways governed by tyrosine kinases and the epidermal growth factor-receptor. In this paper, we report a facile synthesis of a novel quinazoline sulfone, 6-nitro-7-tosylquinazolin-4(3H)-one (III), using the modified protocol from 7-chloro-6-nitroquinazolin-4(3H)-one (I) and sodium p-toluenesulfinate (II). The structure of the title compound III was determined using mass-spectrometry, FT-IR, 1H-NMR, 13C-NMR, DEPT, HSQC (Heteronuclear single quantum coherence), HMBC (Heteronuclear Multiple Bond Correlation Spectroscopy) spectroscopies, and PXRD analysis.

One pot synthesis of trifluoromethyl aryl sulfoxides by trifluoromethylthiolation of arenes and subsequent oxidation with hydrogen peroxide

Hydrogen peroxide was used for oxidation of various aryl trifluoromethyl sulfides. Trifluoroacetic acid was used as an activating solvent that enables non-catalyzed oxidation and increases selectivity for sulfoxide formation. As shown by oxidation of thianthrene TFA enhances electrophilic character of the oxidant and further oxidation of sulfoxide group is blocked. We have joined trifluoromethylthiolation of arenes using a modified Billard reagent (p-ClPhNHSCF₃) with oxidation of aryl trifluoromethyl sulfides using 1.2 equiv. of 30% aqueous hydrogen peroxide and this one-pot process has superior yields than would have been obtained in a two step process.