Organic carbonates as alternative solvents for palladium-catalyzed substitution reactions

Two novel Ln8 clusters bridged by CO32- effectively convert CO2 into oxazolidinones and cyclic carbonates

It is difficult and challenging to design and construct high-nuclearity Ln(III)-based clusters due to the high coordination numbers and versatile coordination geometries of Ln(III) ions. Herein, two novel octanuclear Ln(III)-based clusters [Ln₈(H₂L^-)₄(HL^2-)₄(NO₃)₆ (CO₃)₂](NO₃)₂·2CH₃CN (Ln = Nd (1) and Sm (2)) have been synthesized under solvothermal conditions. The X-ray single analysis reveals that both 1 and 2 are octanuclear structures and the eight central Ln(III) ions are bridged by two CO₃^2- anions. Catalytic study revealed that 1 and 2 can effectively catalyze the cycloaddition reaction of CO₂ and aziridines or epoxides simultaneously under mild conditions. What is more, cluster 1, as a heterogeneous catalyst, can be reused at least three times without obvious loss in catalytic activity for coupling of CO₂ and epoxides. To our knowledge, cluster 1 is the first Ln(III)-based cluster catalyst used for the conversion of CO₂ with aziridines or epoxides simultaneously. This work provides a successful strategy to integrate high-nuclear Ln(III)-based clusters for CO₂ conversion, which may open a new space for the construction of multifunctional high-nuclear Ln(III)-based clusters as efficient catalysts for CO₂ conversion.

P-Stereogenic Ir-MaxPHOX: A Step toward Privileged Catalysts for Asymmetric Hydrogenation of Nonchelating Olefins

The Ir-MaxPHOX-type catalysts demonstrated high catalytic performance in the hydrogenation of a wide range of nonchelating olefins with different geometries, substitution patterns, and degrees of functionalization. These air-stable and readily available catalysts have been successfully applied in the asymmetric hydrogenation of di-, tri-, and tetrasubstituted olefins (ee's up to 99%). The combination of theoretical calculations and deuterium labeling experiments led to the uncovering of the factors responsible for the enantioselectivity observed in the reaction, allowing the rationalization of the most suitable substrates for these Ir-catalysts.

Green Chemistry in Organic Synthesis: Recent Update on Green Catalytic Approaches in Synthesis of 1,2,4-Thiadiazoles

Green (sustainable) chemistry provides a framework for chemists, pharmacists, medicinal chemists and chemical engineers to design processes, protocols and synthetic methodologies to make their contribution to the broad spectrum of global sustainability. Green synthetic conditions, especially catalysis, are the pillar of green chemistry. Green chemistry principles help synthetic chemists overcome the problems of conventional synthesis, such as slow reaction rates, unhealthy solvents and catalysts and the long duration of reaction completion time, and envision solutions by developing environmentally benign catalysts, green solvents, use of microwave and ultrasonic radiations, solvent-free, grinding and chemo-mechanical approaches. 1,2,4-thiadiazole is a privileged structural motif that belongs to the class of nitrogen–sulfur-containing heterocycles with diverse medicinal and pharmaceutical applications. This comprehensive review systemizes types of green solvents, green catalysts, ideal green organic synthesis characteristics and the green synthetic approaches, such as microwave irradiation, ultrasound, ionic liquids, solvent-free, metal-free conditions, green solvents and heterogeneous catalysis to construct different 1,2,4-thiadiazoles scaffolds.

Oxygen vacancies generated by Sn-doped ZrO2 promoting the synthesis of dimethyl carbonate from methanol and CO2

Oxygen vacancy sites on a catalyst surface have been extensively studied and been proved to promote the adsorption and activation of carbon dioxide. We use Sn-doped ZrO₂ to prepare a Zr/Sn catalyst rich in oxygen vacancies (OVs) by co-precipitation. The yield of dimethyl carbonate is 5 times that of ZrO₂. Compared with the original ZrO₂, Zr/Sn exhibits a higher specific surface area, number of acid–base sites and a lower band gap, which improves the conductivity of electrons and creates more surface. The number of reaction sites greatly enhances the adsorption and activation capacity of CO₂ molecules on the catalyst surface. In situ infrared spectroscopy shows that CO₂ adsorbs on oxygen vacancies to form monomethyl carbonate, and participates in the reaction as an intermediate species. This work provides new clues for the preparation of ZrO₂-based catalysts rich in oxygen vacancies to directly catalyze the synthesis of dimethyl carbonate from methanol and CO₂.

Oxygen vacancy sites on a catalyst surface have been extensively studied and been proved to promote the adsorption and activation of carbon dioxide.

Conversion of dilute CO2 to cyclic carbonates at sub-atmospheric pressures by a simple indium catalyst

A simple indium halide with an ammonium salt catalyst can catalyze effectively the cycloaddition of epoxide and dilute CO₂. A detailed mechanistic investigation is conducted using kinetics, isotope labeling, and in situ NMR and IR experiments.

Deposition of Palladium Nanoparticles by the Coating of the Carbonaceous Layer from Wastepaper-Derived Bio-Oil

A highly active and recyclable Pd-deposited catalyst has been successfully prepared for the Heck reaction. Bio-oil liquid, a byproduct from the microwave pyrolysis of wastepaper, is employed to immobilize palladium nanoparticles on a solid support. FTIR, GC, and NMR results indicate the self-polymerization feature of bio-oil, thus giving rise to a uniform carbonaceous layer coated around the surface of the catalyst. Characteristic analysis of the catalyst indicates that palladium nanoparticles are well-dispersed on the parent SBA-15 solid substrate, which is attributed to the carbonaceous layer that is derived from bio-oil carbonization, allowing a high catalytic performance as a heterogeneous catalyst for the Heck reaction. The as-synthesized catalyst demonstrates remarkable recyclability with firm deposition of palladium nanoparticles on the solid support and could be reused without a dramatic decrease in catalytic activity.

Lewis acid sites in MOFs supports promoting the catalytic activity and selectivity for CO esterification to dimethyl carbonate

We studied the effect of Lewis acidity in metal–organic frameworks (MOFs) on their activity as catalyst supports for the esterification of CO to dimethyl carbonate.

Self-assembly of tetranuclear 3d–4f helicates as highly efficient catalysts for CO2 cycloaddition reactions under mild conditions

A series of 4-nuclear lanthanide clusters supported by organic ligands Zn3LnL4 (Ln = Dy(1), Gd(2), Er(3)) were synthesized. These helicates could be used to convert CO₂ into cyclic carbonates with TOF up to 38 000 h⁻¹, without being influenced by moisture or air.

Catalytic Annulation of Epoxides with Heterocumulenes by the Indium-Tin System

In the synthesis of five-membered heterocycles by the annulation of epoxides with heterocumulenes such as carbon dioxide and isocyanates, we developed the indium-tin catalytic system and synthesized various cyclic adducts including novel types products under mild reaction conditions.

Coupling Reactions of Carbon Dioxide with Epoxides Catalyzed by Vanadium Aminophenolate Complexes

A series of vanadium compounds supported by tetradentate aminobis(phenolate) ligands were screened for catalytic reactivity in the reaction of propylene oxide (PO) with CO₂ : [VO(OMe)(O₂ NO^BuMeMeth )], [VO(OMe)(ON₂ O^BuMe )], [VO(OMe)(O₂ NN^BuBuPy )], and [VO(OMe)(O₂ NO^BuBuFurf )]. They showed similar reactivities, but reaction rates were higher for [VO(OMe)(ON₂ O^BuMe )], which was studied in more detail. Turnover frequencies for conversion of PO over 500 h^-1 were observed. Activation energies were determined experimentally through in situ IR spectroscopy for propylene carbonate (48.2 kJ mol^-1 ), styrene carbonate (45.6 kJ mol^-1 ), and cyclohexene carbonate (54.7 kJ mol^-1 ) formation.

Active Pd(ii) complexes: enhancing catalytic activity by ligand effect for carbonylation of methyl nitrite to dimethyl carbonate

The catalytic activities of Pd(ii) complexes for carbonylation of methyl nitrite to dimethyl carbonate could be enhanced by ligand effect.

Development of pyridine based o-aminophenolate zinc complexes as structurally tunable catalysts for CO2 fixation into cyclic carbonates

Structure–reactivity correlation in o-aminophenolate zinc catalysts provides an atom-efficient protocol for the conversion of CO₂ into valuable cyclic carbonates.

Propylene oxide as a dehydrating agent: potassium carbonate-catalyzed carboxylative cyclization of propylene glycol with CO2 in a polyethylene glycol/CO2 biphasic system

Propylene carbonate was directly prepared in 78% yield from 1,2-propylene glycol and CO₂ in polyethylene glycol/CO₂ biphasic system promoted by K₂CO₃ with propylene oxide as the dehydrating agent.

Highly recyclable and magnetic catalyst of a metalloporphyrin-based polymeric composite for cycloaddition of CO2to epoxide

A highly recyclable and magnetic catalyst made of a metalloporphyrin-based polymeric composite efficiently promotes chemical fixation of CO₂to cyclic carbonate.

Bifunctional catalyst of a metallophthalocyanine-carbon nitride hybrid for chemical fixation of CO2 to cyclic carbonate

Hybrid materials of metallophthalocyanine and carbon nitride are recyclable catalysts for CO₂ fixation to cyclic carbonate.

Titanate nanotube-promoted chemical fixation of carbon dioxide to cyclic carbonate: a combined experimental and computational study

Titanate nanotubes are efficiently used as air- and water-tolerant, and recyclable Lewis acid catalysts for CO₂ fixation to cyclic carbonate.

Eco-friendly solvents for palladium-catalyzed desulfitative C-H bond arylation of heteroarenes

Herein, we report the Pd-catalyzed regioselective direct arylation of heteroarenes in which benzenesulfonyl chlorides are used as coupling partners through a desulfitative cross-coupling that can be performed in diethyl carbonate (DEC) or cyclopentyl methyl ether (CPME) as green and renewable solvents or even in neat conditions instead of dioxane or dimethylacetamide (DMA). Under these solvent conditions, the reaction proceeds with a wide range of heteroarenes. C2- or C5-arylated products were obtained with furan and pyrrole derivatives. Benzofuran was also arylated regioselectively at the C2-position, whereas the reaction proceeds selectively at the C3- or C4-positions if thiophenes and benzothiophenes are used. Moreover, in some cases, especially with 1-methylindole, solvent-free conditions afforded better regioselectivities and/or yields than the reaction performed in the presence of solvents.

Cycling of waste fusel alcohols from sugar cane industries using supercritical carbon dioxide

The present work describes the clean synthesis of non-phosgene organic carbonates (NPOCs) with two important by-products from sugar and alcohol industries, namely, fusel alcohols and carbon dioxide, in the presence of DBU, DBN or TBD and an alkylating agent.

Biferrocene-Based Diphosphine Ligands: Synthesis and Application of Walphos Analogues in Asymmetric Hydrogenations

A total of four biferrocene-based Walphos-type ligands have been synthesized, structurally characterized, and tested in the rhodium-, ruthenium- and iridium-catalyzed hydrogenation of alkenes and ketones. Negishi coupling conditions allowed the biferrocene backbone of these diphosphine ligands to be built up diastereoselectively from the two nonidentical and nonracemic ferrocene fragments (R)-1-(N,N-dimethylamino)ethylferrocene and (S_Fc)-2-bromoiodoferrocene. The molecular structures of (S_Fc)-2-bromoiodoferrocene, the coupling product, two ligands, and the two complexes ([PdCl₂(L)] and [RuCl(p-cymene)(L)]PF₆) were determined by X-ray diffraction. The structural features of complexes and the catalysis results obtained with the newly synthesized biferrocene-based ligands were compared with those of the corresponding Walphos ligands.

Organic carbonates as stabilizing solvents for transition-metal nanoparticles

Biodegradable, non-toxic, "green" and inexpensive propylene carbonate (PC) solvent is shown to function as a stabilizing medium for the synthesis of weakly-coordinated transition-metal nanoparticles. Kinetically stable nanoparticles (M-NPs) with a small and uniform particle size (typically <5 ± 1 nm) have been reproducibly obtained by easy, rapid (3 min) and energy-saving 50 W microwave irradiation under an argon atmosphere from their metal-carbonyl precursors in PC. The M-NP/PC dispersions are stable for up to three weeks according to repeated TEM studies over this time period. The rhodium nanoparticle/PC dispersion is a highly active catalyst for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to and 1875 (mol product) (mol Rh)(-1) h(-1) and near quantitative conversion at 4 to 10 bar H(2) and 90 °C. From the PC dispersion the M-NPs can be coated with organic capping ligands such as 3-mercaptopropionic acid or trioctylphosphine oxide for further stabilization.

Kinetics and mechanism of vanadium catalysed asymmetric cyanohydrin synthesis in propylene carbonate

Propylene carbonate can be used as a green solvent for the asymmetric synthesis of cyanohydrin trimethylsilyl ethers from aldehydes and trimethylsilyl cyanide catalysed by VO(salen)NCS, though reactions are slower in this solvent than the corresponding reactions carried out in dichloromethane. A mechanistic study has been undertaken, comparing the catalytic activity of VO(salen)NCS in propylene carbonate and dichloromethane. Reactions in both solvents obey overall second-order kinetics, the rate of reaction being dependent on the concentration of both the aldehyde and trimethylsilyl cyanide. The order with respect to VO(salen)NCS was determined and found to decrease from 1.2 in dichloromethane to 1.0 in propylene carbonate, indicating that in propylene carbonate, VO(salen)NCS is present only as a mononuclear species, whereas in dichloromethane dinuclear species are present which have previously been shown to be responsible for most of the catalytic activity. Evidence from ⁵¹V NMR spectroscopy suggested that propylene carbonate coordinates to VO(salen)NCS, blocking the free coordination site, thus inhibiting its Lewis acidity and accounting for the reduction in catalytic activity. This explanation was further supported by a Hammett analysis study, which indicated that Lewis base catalysis made a much greater contribution to the overall catalytic activity of VO(salen)NCS in propylene carbonate than in dichloromethane.

Cyclic carbonate synthesis catalysed by bimetallic aluminium-salen complexes

The development of bimetallic aluminium-salen complexes [{Al(salen)}(2)O] as catalysts for the synthesis of cyclic carbonates (including the commercially important ethylene and propylene carbonates) from a wide range of terminal epoxides in the presence of tetrabutylammonium bromide as a cocatalyst is reported. The bimetallic structure of one complex was confirmed by X-ray crystallography. The bimetallic complexes displayed exceptionally high catalytic activity and in the presence of tetrabutylammonium bromide could catalyse cyclic carbonate synthesis at atmospheric pressure and room temperature. Catalyst-reuse experiments demonstrated that one bimetallic complex was stable for over 60 reactions, though the tetrabutylammonium bromide decomposed in situ by a retro-Menschutkin reaction to form tributylamine and had to be regularly replaced. The mild reaction conditions allowed a full analysis of the reaction kinetics to be carried out and this showed that the reaction was first order in aluminium complex concentration, first order in epoxide concentration, first order in carbon dioxide concentration (except when used in excess) and unexpectedly second order in tetrabutylammonium bromide concentration. Further kinetic experiments demonstrated that the tributylamine formed in situ was involved in the catalysis and that addition of butyl bromide to reconvert the tributylamine into tetrabutylammonium bromide resulted in inhibition of the reaction. The reaction kinetics also indicated that no kinetic resolution of racemic epoxides was possible with this class of catalysts, even when the catalyst was derived from a chiral salen ligand. However, it was shown that if enantiomerically pure styrene oxide was used as substrate, then enantiomerically pure styrene carbonate was formed. On the basis of the kinetic and other experimental data, a catalytic cycle that explains why the bimetallic complexes display such high catalytic activity has been developed.