Catalysis by alkali and alkaline-earth metal ions in nucleophilic attack of methoxide ion on crown ethers bearing an intra-annular acetoxy group

Rates of reaction of methoxide ion with crown ethers bearing an intra-annular acetoxy group are markedly enhanced by alkali and alkaline-earth metal bromides as a result of much stronger interactions of the metal ions with transition states than with reactants.\ud \ud Rates of reactions of methoxide ion with crown ethers bearing an intra-annular acetoxy group markedly enhanced by alkali and alkaline-earth metal bromides as a result of much stronger interactions of the metal ion with transition state than with reactants

Covid-19 And Rheumatic Autoimmune Systemic Diseases: Role of Pre-Existing Lung Involvement and Ongoing Treatments

The Covid-19 pandemic may have a deleterious impact on patients with autoimmune systemic diseases (ASD) due to their deep immune-system alterations

The Ba(II) complex of a crown ether bearing a sulphydryl side-arm as turnover catalyst of ester cleavage

When activated by barium(II), 18-mercaptometyl-1,4,7,10,13,16-hexaoxacyclononadecane acts as turnover catalyst in the methanolysis of p-nitrophenyl acetate (pNPOAc) under slightly basic conditions in CH3CN-CH3OH 9:1 (v/v). The function of the complexed barium ion is to enhance the nucleophilicity of the pendant SH group towards the acetyl group of the ester. and to assist as a built-in electrophile the subsequent transfer of the acetyl group from the acetylated intermediate to the solvent. Besides the above double displacement mechanism of ester cleavage, a direct metal-ion assisted methanolysis involving reaction of pNPOAc with the ternary complex (5 BaOMe)(+) has been detected. (C) 1996 Elsevier Science Lt

Supramolecular Catalysis by Calixarenes

Calixarenes have been widely employed as building blocks in the design of supramolecular catalysts. As shown in the examples reported here, the molecular cavity itself can be involved as a recognition unit for inclusion of the substrate or of part structures of the substrate. Alternatively, the molecular framework of the calixarene can serve as a versatile molecular platform for the dynamic arrangement of one or more structural units that can serve as recognition and/or catalytic sites. In the catalytic mechanism of successful examples, a high level of cooperation between two or more ligated units is operative

The bis-barium complex of a butterfly crown ether as a phototunable supramolecular catalyst

Reversible phototuning of the catalytic efficiency of the bis-barium complex of azobis(benzo-18-crown-6) in the basic ethanolysis of anilide derivatives has been achieved by light-induced cis reversible arrow trans interconversion of the azobenzene spacer unit of the catalyst. The geometry of the productive catalyst-substrate complex is more favorable when the concave cis form of the catalyst is involved. Continuous photoregulation of the catalytic activity at any intermediate value between the "HIGH" and "LOW" levels was achieved by proper adjustment of the excitation wavelength or the irradiation time. The complete and relatively fast interconvertibility of photostationary states allowed the activity of the catalyst to be repeatedly photoswitched "HIGH" and "LOW" in the course of the same run