1,721,045 research outputs found
Calixarenes: From chemical curiosity to a rich source for molecular receptors
No full textCalixarenes, the macrocyclic compounds derived from the condensation of p-substituted phenols and formaldehyde under basic conditions, have been widely used in the last ten years in Supramolecular Chemistry. Their huge success is due to several factors. They are readily accessible in large quantities, can be easily functionalized both at the phenolic OH groups and at the aromatic nuclei, and, depending on the number of monomeric units (n) in the macrocyclic structure, they present different size, shape and conformational properties. The regio- and stereoselective control in the fuctionalization of calix[4]- and calix[6]arenes here reported, provides the fundamental synthetic tools to model the shape of these macrocycles and allows one to obtain receptors able to recognize cations, anions, and neutral molecules. The high preorganization and the high complementarity with the guests that can be achieved with calixarene-based hosts make these macrocycles a rich source for molecular receptors
An equal terms comparison of the proficiency of artificial phosphodiesterases by using simple models of RNA or DNA as benchmarks–the takeaway to design next generation supramolecular catalysts
Full text linkThis comprehensive review aims at identifying the structural features and general rules governing the design of enzyme mimics and supramolecular catalysts having the ability to hydrolytically cleave the phosphodiester bonds. Rate and binding constants of the artificial phosphodiesterases so far proposed and tested by using the model compounds, bis (p-nitrophenyl) phosphate (BNPP) and 2-hydroxypropylp-nitrophenyl phosphate (HPNP) as widely recognized model substrates have been collected, elaborated and compared. These substrates have been extensively used over time to evaluate the performance of artificial phosphodiesterases, providing consistent and unique bases for comparing different catalysts. Notably, no other substrates have been tested as extensively and over such a prolonged period. A wide variety of supramolecular phosphodiesterases have been considered, comprising metal-free- and metallocatalysts, acyclic, macrocyclic or even nanostructured ones. The scope and limits of the use of Effective Molarity to evaluate the enhanced reactivity of some of these supra- molecular catalysts are also discussed. The information collected allows to give the reader a take-home message for the design of next generation artificial phosphodiesterases
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
Halogen-bonded architectures of multivalent calix[4]arenes
Full text linkA small family of novel halogen-bonded crystalline supramolecular architectures of calixarenes was obtained by the co-crystallization of cone (1) and 1,3-alternate tetrakis(3-iodopropargyloxy)calix[4]arene (4) as tetradentate halogen donors with different multidentate acceptors. Particularly interesting is the interpenetrated diamondoid network of 4 with DABCO, which represents the first example of a 2D network of calixarene macrocycles where halogen bonding is the key interaction for self-organization
A Combined Solution and Solid-State Study on the Tautomerism of an Azocalix[4]arene Chromoionophore
No full textAzocalixarenes functionalized with cation binding sites are popular chromoionophores due to the ease of synthesis and the large complexation-induced shifts of their absorption band that originate from an azo-phenol-quinone-hydrazone tautomerism. Despite their extensive use, however, a thorough investigation of the structure of their metal complexes has not been reported. We describe herein the synthesis of a new azocalixarene ligand (2) and the study of its complexation properties with the Ca2+ cation. Through a combination of solution (H-1 NMR and UV-vis spectroscopies) and solid-state (X-ray diffractometry) techniques, we demonstrate that metal complexation induces a shift of the tautomeric equilibration towards the quinone-hydrazone form, while deprotonation of the complex results in the reversion to the azo-phenol tautomer
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Synthesis of fluorenyl alcohols: Via cooperative palladium/norbornene catalysis
No full textHerein, we report a novel catalytic synthesis of substituted 9H-fluoren-9-ols starting from aryl iodides and secondary ortho-bromobenzyl alcohols in the presence of palladium/norbornene as a catalytic system. The present protocol exhibits high functional group tolerance, mild reaction conditions and moderate to good yields. This transformation is based on two sequential pathways: (i) Pd(ii)-mediated oxidation of the secondary alcohol to the corresponding ketone and (ii) Pd(0)/norbornene-catalyzed reaction of the in situ generated ortho-bromoacetophenone with the aryl iodide
Diguanidinocalix[4]arenes as effective and selective catalysts of the cleavage of diribonucleoside monophosphates
No full textCalix[4]arenes derivatives 1 and 2, featuring two guanidine units at the upper rim, catalyze the transesterification of diribonucleoside monophosphates much more effectively than that of HPNP. Rate accelerations relative to the background range from 10(3) to 10(4)-fold, and approach 10(5)-fold with the most favorable substrate-catalyst combinations
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