1,721,002 research outputs found
The guanidinium unit in the catalysis of phosphoryl transfer reactions: From molecular spacers to nanostructured supports
No full textExamples of guanidinium-based artificial phosphodiesterases are illustrated in this review article. A wide set of collected catalytic systems are presented, from the early examples to the most recent developments of the use of this unit in the design of supramolecular catalysts. Special attention is dedicated to illustrate the operating catalytic mechanism and the role of guanidine/ium units in the catalysis. One or more of these units can act by themselves or in conjunction with other active units. The analogy with the mechanism of enzymatic systems is presented and discussed. In the last part of this overview, recent examples of guanidinophosphodiesterases based on nanostructured supports are reported, namely gold-monolayer-protected clusters and polymer brushes grafted to silica nanoparticles. The issue of the dependence of the catalytic performance on the preorganization of the spacer is tackled and discussed in terms of effective molarity, a parameter that can be taken as a quantitative measurement of this preorganization for both conventional molecular linker and nanosized supports
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
Guanidine–Guanidinium Cooperation in Bifunctional Artificial Phosphodiesterases Based on Diphenylmethane Spacers;gem-Dialkyl Effect on Catalytic Efficiency
No full textDiphenylmethane derivatives 1-3, decorated with two guanidine units, are effective catalysts of HPNP transesterification. Substitution of the methylene group of the parent diphenylmethane spacer with cyclohexylidene and adamantylidene moieties enhances catalytic efficency, with gem-dialkyl effect accelerations of 4.5 and 9.1, respectively. Activation parameters and DFT calculations of the rotational barriers around the C-Ar bonds indicate that a major contribution to the driving force for enhanced catalysis is entropic in nature. © 2013 American Chemical Society
Supramolecular Catalysis by Calixarenes
No full textCalixarenes 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
Organocatalyzed addition to activated C=C bonds
Get PDFIn this chapter, several examples of organocatalyzed additions to C=C bonds carried out through a domino approach are reviewed, from the early examples to recent applications of these strategies in industry
Organocatalyzed Addition to Activated Double Bonds: from proof-of-concept to the preparation of pharmaceutical and biologically relevant molecules
No full textOrganocatalytic reactions are especially attractive since they do not require anhydrous conditions and expensive catalysts.[1-9] Most of all, waste management is significant easier with respect to the processes employing transition metals, which present issues with the disposal of residual and the phenomena of “metal leaking” into the products. The most known organocatalytic process, the Hajos–Parrish–Eder–Sauer–Wiechert reaction[10-12] is actually a domino transformation which has found significant industrial application. Despite this key result, this field has surprisingly been forgotten for nearly thirty years before its renaissance at the beginning of the millennium. In particular, organocatalytic domino reactions are appealing transformations. The mild operational conditions required allow that in the same reaction vessel several processes can operate consecutively. This situation would be more complex when the several transformations composing a domino reaction had to be run in significantly different conditions.
The purpose of this chapter is to highlight and critically discuss some selected examples of organocatalytic domino reactions, not to write a comprehensive treaty on the subject. It will describe the first ground-breaking examples of organocatalytic domino reactions, (paragraph 1.1), then more specific applications from the chemistry of oxindoles (paragraph 2.1) and finally large-scale transformations, specifically the synthesis of (-)-Oseltamivir, and ABT-341 (paragraph 3.1) and the industrial scale process for the synthesis of chiral diene ligands (paragraph 4.1)
Guanidine-based polymer brushes grafted onto silica nanoparticles as efficient artificial phosphodiesterases
No full textPolymer brushes grafted to the surface of silica nanoparticles were fabricated by atom-transfer radical polymerization (ATRP) and investigated as catalysts in the cleavage of phosphodiesters. The surfaces of silica nanoparticles were functionalized with an ATRP initiator. Surface-initiated ATRP reactions, in varying proportions, of a methacrylate moiety functionalized with a phenylguanidine moiety and an inert hydrophilic methacrylate species afforded hybrid nanoparticles that were characterized with potentiometric titrations, thermogravimetric analysis, and SEM. The activity of the hybrid nanoparticles was tested in the transesterification of the RNA model compound 2-hydroxypropyl para-nitrophenylphosphate (HPNP) and diribonucleoside monophosphates. A high catalytic efficiency and a remarkable effective molarity, thus overcoming the effective molarities previously observed for comparable systems, indicate the existence of an effective cooperation of the guanidine/guanidinium units and a high level of preorganization in the nanostructure. The investigated system also exhibits a marked and unprecedented selectivity for the diribonucleoside sequence CpA. The results presented open up the way for a novel and straightforward strategy for the preparation of supramolecular catalysts
Guanidine based self-assembled monolayers on Au nanoparticles as artificial phosphodiesterases
No full textGold nanoparticles passivated with a long chain alkanethiol decorated with a phenoxyguanidine moiety were prepared and investigated as catalysts in the cleavage of the RNA model compound HPNP and diribonucleoside monophosphates. The catalytic efficiency and the high effective molarity value of the Au monolayer protected colloids points to a high level of cooperation between the catalytic groups. © 2014 the Partner Organisations
- …
