Pinacol as a New Green Reducing Agent: Molybdenum- Catalyzed Chemoselective Reduction of Sulfoxides and Nitroaromatics

Reduction of sulfoxides catalyzed by the commercially available manganese complex MnBr(CO)5

A new methodology for the reduction of a wide variety of aliphatic and aromatic sulfoxides catalyzed by the air-stable, cheap and commercially available manganese catalyst MnBr(CO)5 with excellent yields is reported in this work. The catalytic system MnBr(CO)5/PhSiH3 is highly chemoselective, allowing the effective reduction of the SO bond in the presence of different functional groups.

Triethyl amine as an effective reducing agent for sulfoxide deoxygenation

Enabled by triethyl amine (Et3N) and thionyl chloride (SOCl2), an efficient and practical protocol for deoxygenation of sulfoxide to sulfide was developed. This new method features a wide range of substrate scope, including diaryl, dialkyl and aryl alkyl substituted sulfoxides. Detailed mechanistic investigations reveal the crucial role played by Et3N as an electron-donating reductant rather than a hydrogen-atom donor.

Reactivity of metal dioxo complexes

Metal dioxo chemistry and its diverse reactivity are presented with an emphasis on the mechanisms of reactivity. Work from approximately the last decade is surveyed and organized by metal. In particular, the chemistry of cis-dioxo metal complexes is discussed at length. Reactions are grouped by generic type, including addition across a metal oxo bond, oxygen atom transfer, and radical atom transfer reactions. Attention is given to advances in deoxygenation chemistry, oxidation chemistry, and reductive transformations.

Direct synthesis of haloaromatics from nitroarenes via a sequential one-pot Mo-catalyzed reduction/Sandmeyer reaction

Herein, we report the direct synthesis of a wide variety of functionalized aromatic bromides, chlorides, iodides, and fluorides from nitroarenes in a sequential one-pot operation. This protocol is based on an air- and moisture-tolerant dioxomolybdenum-catalyzed reduction of nitroaromatics, employing pinacol as a reducing agent, which enables subsequent diazotization and halogenation steps. This methodology represents a step-economical, practical, and alternative procedure for synthesizing haloaromatics directly from nitroaromatics.

Synthesis of 1,4-ketoaldehydes and 1,4-diketones by Mo-catalyzed oxidative cleavage of cyclobutane-1,2-diols

A new two-step procedure for the synthesis of 1,4-dicarbonyls has been developed involving an efficient and clean Mo-catalyzed oxidative cleavage of cyclobutane-1,2-diols with DMSO, which is used as solvent and oxidant. The required starting glycols were prepared by nucleophilic additions of organolithiums and Grignard reagents to easily available 2-hydroxycyclobutanones.

Homocouplings of Sodium Arenesulfinates: Selective Access to Symmetric Diaryl Sulfides and Diaryl Disulfides

Symmetrical diaryl sulfides and diaryl disulfides have been efficiently and selectively constructed via the homocoupling of sodium arenesulfinates. The selectivity of products relied on the different reaction systems: symmetrical diaryl sulfides were predominately obtained under the Pd(OAc)2 catalysis, whereas symmetrical diaryl sulfides were exclusively yielded in the presence of the reductive Fe/HCl system.

Reduction of Sulfoxides in Multigram Scale, an Alternative to the Use of Chlorinated Solvents

In this manuscript, we describe the use of ethyl vinyl ether/oxalyl chloride as the reducing mixture for sulfoxides. The reaction is based on the high electrophilic character of chlorosulfonium salts, formed in situ by the reaction of oxalyl chloride and the sulfoxide. Thereafter, the nucleophilic vinyl ether acts as a chlorine scavenger, affording the corresponding sulfide. The method is applicable on a big scale and may be applied to highly functionalized sulfoxides. Chromatographic purification is only needed in exceptional cases of unstable substrates, and the final sulfide or the corresponding salt is usually obtained after simple evaporation of volatiles. The sole contaminants of this method are carbon dioxide, carbon monoxide and small (five-carbon maximum) aldol products, which are formed during the reaction process.

Deoxygenation reactions in organic synthesis catalyzed by dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes have been applied as efficient, inexpensive and benign catalysts to deoxygenation reactions of a diverse number of compounds in the last two decades. Dioxomolybdenum complexes have demonstrated wide applicability to the deoxygenation of sulfoxides into sulfides and reduction of N-O bonds. Even the challenging nitro functional group was efficiently deoxygenated, affording amines or diverse heterocycles after reductive cyclization reactions. More recently, carbon-based substrates like epoxides, alcohols and ketones have been successfully deoxygenated. Also, dioxomolybdenum complexes accomplished deoxydehydration (DODH) reactions of biomass-derived vicinal 1,2-diols, affording valuable alkenes. The choice of the catalytic systems and reductant is decisive to achieve the desired transformation. Commonly found reducing agents involved phosphorous-based compounds, silanes, molecular hydrogen, or even glycols and other alcohols.

Mo-Catalyzed One-Pot Synthesis of N-Polyheterocycles from Nitroarenes and Glycols with Recycling of the Waste Reduction Byproduct. Substituent-Tuned Photophysical Properties

A catalytic domino reduction-imine formation-intramolecular cyclization-oxidation for the general synthesis of a wide variety of biologically relevant N-polyheterocycles, such as quinoxaline- and quinoline-fused derivatives, and phenanthridines, is reported. A simple, easily available, and environmentally friendly dioxomolybdenum(VI) complex has proven to be a highly efficient and versatile catalyst for transforming a broad range of starting nitroarenes involving several redox processes. Not only is this a sustainable, step-economical as well as air- and moisture-tolerant method, but also it is worth highlighting that the waste byproduct generated in the first step of the sequence is recycled and incorporated in the final target molecule, improving the overall synthetic efficiency. Moreover, selected indoloquinoxalines have been photophysically characterized in cyclohexane and toluene with exceptional fluorescence quantum yields above 0.7 for the alkyl derivatives.

Nickel phosphide nanoalloy catalyst for the selective deoxygenation of sulfoxides to sulfides under ambient H2 pressure

Exploring novel catalysis by less common, metal-non-metal nanoalloys is of great interest in organic synthesis. We herein report a titanium-dioxide-supported nickel phosphide nanoalloy (nano-Ni2P/TiO2) that exhibits high catalytic activity for the deoxygenation of sulfoxides. nano-Ni2P/TiO2 deoxygenated various sulfoxides to sulfides under 1 bar of H2, representing the first non-noble metal catalyst for sulfoxide deoxygenation under ambient H2 pressure. Spectroscopic analyses revealed that this high activity is due to cooperative catalysis by nano-Ni2P and TiO2.

Electrophilic Chlorine from Chlorosulfonium Salts: A Highly Chemoselective Reduction of Sulfoxides

Herein, we describe a selective late-stage deoxygenation of sulfoxides based on a novel application of chlorosulfonium salts and demonstrate a new process using these species generated in situ from sulfoxides as the source of electrophilic chlorine. The use of highly nucleophilic 1,3,5-trimethoxybenzene (TMB) as the reducing agent is described for the first time and applied in the deoxygenation of simple and functionalized sulfoxides. The method is easy to handle, economic, suitable for gram-scale operations, and readily applied for poly-functionalized molecules, as demonstrated with more than 45 examples, including commercial medicines and analogues. We also report the results of competition experiments that define the more reactive sulfoxide and we present a mechanistic proposal based on substrate and product observations.

Photocatalytic Deoxygenation of Sulfoxides Using Visible Light: Mechanistic Investigations and Synthetic Applications

The photocatalytic deoxygenation of sulfoxides to generate sulfides facilitated by either Ir[(dF(CF₃)ppy)₂(dtbbpy)]PF₆ or fac-Ir(ppy)₃ is reported. Mechanistic studies indicate that a radical chain mechanism operates, which proceeds via a phosphoranyl radical generated from a radical/polar crossover process. Initiation of the radical chain was found to proceed via two opposing photocatalytic quenching mechanisms, offering complementary reactivity. The mild nature of the radical deoxygenation process enables the reduction of a wide range of functionalized sulfoxides, including those containing acid-sensitive groups, in typically high isolated yields.

Phosphonate Decomposition-Induced Polyoxomolybdate Dumbbell-Type Cluster Formation: Structural Analysis, Proton Conduction, and Catalytic Sulfoxide Reduction

The reaction of MoO₄^2- with a number of phosphonic acids [bis(phosphonomethyl)glycine, R,S-hydroxyphosphonoacetic acid, 1-hydroxyethane-1,1-diphosphonic acid, phenylphosphonic acid, aminotris(methylene phosphonic acid), and 1,2-ethylenediphosphonic acid] under oxidizing (H₂O₂) hydrothermal conditions at low pH leads to rupture of the P-C bond, release of orthophosphate ions, and generation of the octanuclear, phosphate-bridged, polyoxometalate molybdenum cluster (NH₄)₅[Mo₈(OH)₂O₂₄(μ₈-PO₄)](H₂O)₂ (POMPhos). This cluster has been fully characterized and its structure determined. It was studied as a proton conductor, giving moderate values of σ = 2.13 × 10^-5 S·cm^-1 (25 °C) and 1.17 × 10^-4 S·cm^-1 (80 °C) at 95% relative humidity, with E_a = 0.27 eV. The POMPhos cluster was then thermally treated at 310 °C, yielding (NH₄)_2.6(H₃O)_0.4(PO₄Mo₁₂O₃₆) together with an amorphous impurity containing phosphate and molybdenum oxide. This product was also studied for its proton conductivity properties, giving rise to an impressively high value of σ = 2.43 × 10^-3 S·cm^-1 (25 °C) and 6.67 × 10^-3 S·cm^-1 (80 °C) at 95% relative humidity, 2 orders of magnitude higher than those corresponding to the "as-synthesized" solid. The utilization of POMPhos in catalytic reduction of different sulfoxides was also evaluated. POMPhos acts as an efficient homogeneous catalyst for the reduction of diphenyl sulfoxide to diphenyl sulfide, as a model reaction. Pinacol was used as a low-cost, environmentally friendly, and highly efficient reducing agent. The effects of different reaction parameters were investigated, namely the type of solvent and reducing agent, presence of acid promoter, reaction time and temperature, loading of catalyst and pinacol, allowing to achieve up to 84-99% yields of sulfide products under optimized conditions. Substrate scope was tested on the examples of diaryl, alkylaryl, dibenzyl, and dialkyl sulfoxides and excellent product yields were obtained.

Gold-Catalyzed Oxidation of Thioalkynes To Form Phenylthio Ketene Derivatives via a Noncarbene Route

Gold-catalyzed oxidations of thioalkynes with 8-methylquinoline oxides afford 2-phenylthioketenes that can be trapped efficiently with alcohols. The synthetic utility is manifested by terminal and internal thioalkynes over a wide scope, bearing esters, ketones, alkyl, and oxime substituents. Our density functional theory calculations suggest that gold-catalyzed oxidations of terminal and internal thioalkynes with 8-methylquinoline oxides generate gold-bound ketene intermediates without the intermediacy of α-oxo gold carbene.

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.

Synthesis of Mesoporous γ-Alumina-Supported Co-Based Catalysts and Their Catalytic Performance for Chemoselective Reduction of Nitroarenes

Mesoporous γ-alumina (γ-MA)-supported cobalt oxides (Co₃O₄) with large surface areas and narrow pore size distributions were first prepared through one-pot hydrolysis of metal nitrates. The obtained Co₃O₄/γ-MA materials were impregnated with a water-ethanol solution of 1,10-phenanthroline, followed by treatment at 700 °C in N₂ atmosphere, generating Co-NC/γ-MA catalysts containing N-doped graphitic carbon (NC). The Co-NC/γ-MA catalysts maintained the mesoporous structure of γ-MA, and Co₃O₄ was reduced to metallic Co nanoparticles highly dispersed in the γ-MA frameworks. Metallic Co species had a strong interaction with NC in the matrices, avoiding the surface oxidation of Co particles. The Co-NC/γ-MA catalysts exhibited superior catalytic activity and quantitatively reduced a variety of functionalized nitroarenes to the corresponding arylamines with hydrazine hydrate in ethanol at near room temperature, affording yields of >99%. The recycling test of 2-chloronitrobenzene as a model reaction showed no detectable change in catalyst performance after 10 cycle reactions.

Rhodium-calix[4]pyrrole and rhodium-tetraphenyl porphyrin: preparation, surface grafting and their catalytic application in nitro-benzene reduction

Rhodium containing macromolecules calix[4]pyrrole (RhCP) and tetraphenyl porphyrin (RhTPP) were prepared, grafted on functionalized SBA-15 and demonstrated as promising catalysts for nitro-arene reduction.

Nitrogen-doped graphene-activated metallic nanoparticle-incorporated ordered mesoporous carbon nanocomposites for the hydrogenation of nitroarenes

Herein, nanoscale metallic nanoparticle-incorporated ordered mesoporous carbon catalysts activated by nitrogen-doped graphene (NGr) were fabricated via an efficient multi-component co-assembly of a phenolic resin, nitrate, acetylacetone, the nitrogen-containing compound 1,10-phenanthroline, and Pluronic F127, followed by carbonization. The obtained well-dispersed nitrogen-doped graphene-activated transition metal nanocatalysts possess a 2-D hexagonally arranged pore structure with a high surface area (∼500 m² g⁻¹) and uniform pore size (∼4.0 nm) and show excellent activity for the selective hydrogenation–reduction of substituted nitroarenes to anilines in an environmentally friendly aqueous solution. The high catalytic performance and durability is attributed to the synergistic effects among the components, the unique structure of the nitrogen-doped graphene layer-coated metallic nanoparticles, and electronic activation of the doped nitrogen.

The non-noble M/NGr@OMC-T catalyst exhibits excellent catalytic activity and chemoselectivity for the hydrogenation of nitroarenes to anilines in water with hydrogen.

Deoxygenation of sulphoxides to sulphides with trichlorophosphane

An efficient route to deoxygenation of sulphoxides to sulphides with PCl₃ under mild reaction condition was developed.

Platinum-supporting hollandite-type vanadium–chromium mixed oxides as efficient heterogeneous catalysts for deoxygenation of sulfoxides under atmospheric H2 pressure

In the presence of hollandite-based catalysts, various sulfoxides and pyridine N-oxides could be converted into deoxygenated products under atmospheric H₂ pressure.

Photoredox catalysis for oxygenation/deoxygenation between sulfides and sulfoxides by visible-light-responsive polyoxometalates

We report the visible-light-responsive photoredox catalysis of a lacunary silicotungstate for selective aerobic oxygenation of sulfides and deoxygenation of sulfoxides.

A practical and chemoselective Mo-catalysed sulfoxide reduction protocol using a 3-mercaptopropyl-functionalized silica gel (MPS)

3-Mercaptopropyl-functionalized silica gel (MPS) has been described as a new reducing agent for the chemoselective deoxygenation of sulfoxides under dioxomolybdenum(vi)-catalysis.