1,720,995 research outputs found
Size-selective catalysis of ester and anilide cleavage by the dinuclear barium(II) complexes of cis- and trans-stilbenobis(18-crown-6)
No full textThe basic ethanolysis of esters and anilides endowed with a distal carboxylate anchoring group is catalyzed by the dinuclear Ba(II) complexes of cis- and trans-stilbenobis(18-crown-6). Comparison with the catalytic efficiency of a mononuclear model catalyst demonstrates that the two metal ions in the dinuclear complexes act synergically. The cis form of the catalyst is more efficient than the trans form and much more sensitive to the size of the substrate, showing that the concave shape imposed by the cis double bond is better suited for catalysis. An experiment in which the less active trans form is photoactivated in situ is also reported
General base-guanidinium cooperation in bifunctional artificial phosphodiesterases
No full textArtificial phosphodiesterases that combine a guanidinium unit with a general base connected by a m-xylylene linker catalyze the transesterification of the RNA model compound 2-hydroxypropyl p-nitrophenyl phosphate (HPNP). The bifunctional catalysts presented in this work show varying extents of cooperation between catalytic units and a rate enhancement of 4 x 10(4) in the most favorable case
On the ' livingness ' of a dynamic library of cyclophane formaldehyde acetals incorporating calix[4]arene subunits
No full textThe acid catalyzed transacetalation of cyclophane formaldehyde acetals incorporating calix[4]arene subunits generates a short-lived dynamic library of macrocycles. The side reaction responsible for the loss of 'livingness' is the unexpected decomposition of monomeric units into a bridged ether and formaldehyde. A plausible mechanism is suggested, in which the crucial step is the formation of benzyl carbocations strongly stabilized by the alkoxy substituents at the lower rim of the calix[4]arene moieties
Effective molarities in supramolecular catalysis of two-substrate reactions
No full textReactivity data related to processes in which molecular receptors promote the reaction of two simultaneously complexed reactants have been surveyed and analyzed in terms of effective molarity (EM). Methods and criteria for the calculation of reliable EM’s have been highlighted. Extension of a previous extrathermodynamic treatment of intramolecular reactions of bifunctional chains to the intracomplex reactions of the ternary complexes involved in twosubstrate catalyzed reactions has provided a sound framework for a comparative analysis of reactivity and catalytic efficiency in structurally diverse and apparently unrelated systems
Alkaline-earth metal complexes of thiol-pendant crown ethers as turnover catalysts of ester cleavage
No full textMetathesis reaction of formaldehyde acetals: An easy entry into the dynamic covalent chemistry of cyclophane formation
No full textThe acid-catalyzed transacetalation of formaldehyde acetals (formal metathesis) is a suitable reaction for the generation of well-behaved Dynamic Libraries of cyclophane formals. The composition of the equilibrated mixtures solely depends on concentration, and is totally independent of whether the feedstock is any of the pure cyclic oligomers, or a mixture of oligomers/polymers. Effective Molarities related to the formation of the lower cyclic oligomers were directly measured as their equilibrium molar concentrations above the critical monomer concentration. The finding that silver cation acts as a selective binder toward the cyclic dimer C2, coupled with the “proof reading and editing” capability of our quickly equilibrating system, translated into significant amplifications of C2 when the equilibrated mixtures were exposed to the action of the silver template. These results highlight the potential of Dynamic Combinatorial Chemistry as a powerful tool for the preparation in synthetically useful amounts of an otherwise elusive macrocyclic compound. The possibility of using a mixture of high molecular weight byproducts as feedstock adds considerably to the practical value of the procedure
Calixarenes as Supramolecular Catalysts Endowed with Esterase and Phosphodiesterase Activity
No full textThe molecular framework of calixarenes has been widely used as a versatile scaffold for the arrangement of active units working as catalytic and/or
recognition sites, attaining, in many cases, high levels of cooperation. The calixarene cavity itself can also play a role in the catalytic process, acting as a recognition unit by inclusion of the substrate or of part of the substrate. This chapter focuses on the high performance of calixarene Catalysts active in acyl and phosphoryl transfer processes
A calix[4]arene with acylguanidine units as an efficient catalyst for phosphodiester bond cleavage in RNA and DNA model compounds
No full textA calix[4]arene scaffold, blocked in the cone conformation and decorated at the upper rim with two acylguanidine units, effectively catalyzes the cleavage of phosphodiester bonds of HPNP and BNPP under neutral pH conditions. The catalyst performance is discussed in terms of acceleration over background hydrolysis and effective molarity (EM). The combination of potentiometric acid-base titrations with pH-rate profiles for HPNP and BNPP cleavage in the presence of 2 center dot 2HCl additives points to a marked synergic action of an acylguanidine/acylguanidinium catalytic dyad in 2H(+), via general base-electrophilic bifunctional catalysis. Acceleration factors over background larger than 3 orders of magnitude are obtained. The connection of the guanidine/guanidinium dyad to the calixarene scaffold by means of carbonyl joints has a double advantage: (i) the acidity of the guanidinium moiety is enhanced by the electron-withdrawing carbonyl group and maximum conversion into the catalytically active form 2H(+) occurs at almost neutral pH, lower than the pH needed for the monoprotonated form 1H(+) devoid of carbonyl groups; (ii) the EM value for HPNP cleavage with 2H(+) is definitely higher than that with 1H(+), suggesting a highly preorganized catalyst that perfectly fits in a strainless ring-shaped transition state in the catalyzed process. DFT calculations also provide useful insights into the reaction mechanisms and transition states
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