Operationally Simple, Efficient, and Diastereoselective Synthesis of <i>cis</i>-2,6-Disubstituted-4-Methylene Tetrahydropyrans Catalyzed by Triflic Acid

A highly efficient (0.01 mol % of TfOH), operationally simple (room temperature, inexpensive, and commercially available catalyst), and diastereoselective (up to >98% de) method for Brønsted acid-catalyzed reaction of enol ethers to form cis-2,6-disubstituted tetrahydropyrans is disclosed

Continuous-Flow Stereoselective Organocatalyzed Diels–Alder Reactions in a Chiral Catalytic “Homemade” HPLC Column

Continuous-flow organocatalyzed Diels–Alder reactions have been performed with excellent enantioselectivity for the first time in a chiral “homemade” HPLC column, packed with silica on which a MacMillan catalyst has been supported by a straightforward immobilization procedure. The versatility of the system was also proven by running with the same column continuous-flow stereoselective reactions with three different substrates, showing that the catalytic reactor may efficiently work <i>in continuo</i> for more than 150 h; the regeneration of the HPLC column was also demonstrated, allowing to further extend the activity of the reactor to more than 300 operating hours

Enantioselective Synthesis of Cyclic Enol Ethers and All-Carbon Quaternary Stereogenic Centers Through Catalytic Asymmetric Ring-Closing Metathesis

The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts were screened. Although chiral Ru catalysts (those that do not bear a phosphine ligand) promote ARCM in some cases, such transformations proceed in <10% ee. In contrast, Mo-based alkylidenes give rise to efficient ARCM and deliver the desired products in the optically enriched form. Thus, Mo-catalyzed enantioselective transformations allow access to various five- and six-membered cyclic enol ethers in up to 94% ee from readily available achiral starting materials. The first examples of catalytic ARCM that lead to the formation of all-carbon quaternary stereogenic centers are also disclosed. Mechanistic models that offer a plausible rationale for the identity of major enantiomers as well as the observed levels of enantioselectivity are provided. Representative examples demonstrate that the enol ether moiety and the unreacted alkene of the ARCM products can be discriminated with excellent site selectivity (>98%)

Enantioselective Synthesis of Cyclic Enol Ethers and All-Carbon Quaternary Stereogenic Centers Through Catalytic Asymmetric Ring-Closing Metathesis

The first examples of catalytic asymmetric ring-closing metathesis (ARCM) reactions of enol ethers are reported. To identify the most effective catalysts, various chiral Mo- and Ru-based catalysts were screened. Although chiral Ru catalysts (those that do not bear a phosphine ligand) promote ARCM in some cases, such transformations proceed in <10% ee. In contrast, Mo-based alkylidenes give rise to efficient ARCM and deliver the desired products in the optically enriched form. Thus, Mo-catalyzed enantioselective transformations allow access to various five- and six-membered cyclic enol ethers in up to 94% ee from readily available achiral starting materials. The first examples of catalytic ARCM that lead to the formation of all-carbon quaternary stereogenic centers are also disclosed. Mechanistic models that offer a plausible rationale for the identity of major enantiomers as well as the observed levels of enantioselectivity are provided. Representative examples demonstrate that the enol ether moiety and the unreacted alkene of the ARCM products can be discriminated with excellent site selectivity (>98%)

Monodisperse Octahedral α-MnS and MnO Nanoparticles by the Decomposition of Manganese Oleate in the Presence of Sulfur

Octahedral monodisperse α-MnS and MnO nanoparticles have been synthesized by decomposing manganese oleate and elemental sulfur in octadecene at high (250−320 °C) temperature. The chemical composition of the obtained NPs depends on the Mn:S ratio in an unexpected way. Pure α-MnS NP samples are obtained when S:Mn ≥ 2:1, whereas pure MnO NPs require S:Mn ≤ 0.6. Variation of several parameters (concentration of sulfur, heating rate and aging temperature and time) resulted in a α-MnS NP size interval of 11−14 (from Mn monooleate) and 18−30 nm (from Mn dioleate). For MnO NPs only, size control is also possible by addition of free oleic acid (14−24 nm). Analysis of TEM tilting experiments and electron diffraction shows that both α-MnS and MnO nanoparticles have octahedral shape and spontaneously form ordered arrays with strong texture in the {111} direction. Measurement of the magnetic properties showed that α-MnS nanoparticles consist of an antiferromagnetic core and a ferromagnetic-like shell that are exchange coupled below the blocking temperature of the shell (23 K for 29 nm α-MnS NP)

Chiral Hybrid Inorganic–Organic Materials: Synthesis, Characterization, and Application in Stereoselective Organocatalytic Cycloadditions

The synthesis of chiral imidazolidinones on mesoporous silica nanoparticles, exploiting two different anchoring sites and two different linkers, is reported. Catalysts <b>1</b>–<b>4</b> were prepared starting from l-phenylalanine or l-tyrosine methyl esters and supporting the imidazolidinone onto silica by grafting protocols or azide–alkyne copper­(I)-catalyzed cycloaddition. The four catalysts were fully characterized by solid-state NMR, N₂ physisorption, SEM, and TGA in order to provide structural assessments, including an evaluation of surface areas, pore dimensions, and catalyst loading. They were used in organocatalyzed Diels–Alder cycloadditions between cyclopentadiene and different aldehydes, affording results comparable to those obtained with the nonsupported catalyst (up to 91% yield and 92% ee in the model reaction between cyclopentadiene and cinnamic aldehyde). The catalysts were recovered from the reaction mixture by simple filtration or centrifugation. The most active catalyst was recycled two times with some loss of catalytic efficiency and a small erosion of ee