The heterocyclization of some reactive polymers by a 3 + 2 dipolar cyclo-addition reaction—II. Polyamides and polyhydrazides

Based on quantitative transformation of unsaturated polyesters into Δ₂-pyrazoline polymers when reacted with 1,3-dipoles, attempts were made to apply the same reaction to unsaturated polyamides and polyhydrazides having double bonds conjugated with a carbonyl group. One could not obtain heterocyclic polymers because of factors discussed in the paper. The presence of an active hydrogen atom in the molecule of polyamide or polyhydrazide causes a decrease of the dipolarophile capacity of the double bond. Substitution of these hydrogen atoms, both in model compounds and polymers, leads to the expected results. Thus N-alkylpolyamide was quantitatively transformed into the Δ₂-pyrazoline polyamide.     

Free subgroups of free products and combinatorial hypermaps

We derive a generating series for the number of free subgroups of finite index in ${\Delta }^{+}={\mathbb{Z}}_{p}1{\mathbb{Z}}_q$ by using a connection between free subgroups of ${\Delta }^{+}$ and certain hypermaps (also known as ribbon graphs or “fat” graphs), and show that this generating series is transcendental. We provide non-linear recurrence relations for the above numbers based on differential equations that are part of the Riccati hierarchy.We also study the generating series for conjugacy classes of free subgroups of finite index in ${\Delta }^{+}$, which correspond to isomorphism classes of hypermaps. Asymptotic formulas are provided for the numbers of free subgroups of given finite index, conjugacy classes of such subgroups, or, equivalently, various types of hypermaps and their isomorphism classes.     

Site-dependent activity of atomic Ti catalysts in Al-based hydrogen storage materials

Doping catalytically inactive materials with dispersed atoms of an active species is a promising route toward realizing ultradilute binary catalyst systems. Beyond catalysis, strategically placed metal atoms can accelerate a wide range of solid-state reactions, particularly in hydrogen storage processes. Here we analyze the role of atomic Ti catalysts in the hydrogenation of Al-based hydrogen storage materials. We show that Ti atoms near the Al surface activate gas-phase H(2), a key step toward hydrogenation. By controlling the placement of Ti, we have found that the overall reaction, comprising H(2) dissociation and H spillover onto the Al surface, is governed by a pronounced trade-off between lowering of the H(2) dissociation barrier and trapping of the products near the active site, with a sharp maximum in the overall activity for Ti in the subsurface layer. Our findings demonstrate the importance of controlling the placement of the active species in optimizing the activity of dilute binary systems.     

Discovery of an entropically-driven small molecule streptavidin binder from nucleic acid-encoded libraries

Dehydrocholic acid was identified as a selective streptavidin binder from a PNA-tagged library. Isothermal calorimetry titration measurements showed this interaction to be entropically driven. Peptides tagged with dehydrocholic acid can be captured on a streptavidin resin and released under thermal conditions.     

Experimental and DFT studies of gold nanoparticles supported on MgO(111) nano-sheets and their catalytic activity

A wet chemical preparation of MgO with the (111) facet as the primary surface has recently been reported and with alternating layers of oxygen anions and magnesium cations, this material shows unique chemical and physical properties. The potential to utilize the MgO(111) surface for the immobilization of metal particles is intriguing because the surface itself offers a very different environment for the metal particle with an all oxygen interface, as opposed to the typical (100) facet that possesses alternating oxygen anion and magnesium cation sites on the surface. Gold nanoparticles have demonstrated a broad range of interesting catalytic properties, but are often susceptible to aggregation at high temperatures and are very sensitive to substrate effects. Here, we investigate gold-supported on MgO(111) nanosheets as a catalyst system for the aerobic oxidation of benzyl alcohol. Gold nanoparticles deposited on MgO(111) show an increased level of activity in the solvent-free benzyl alcohol aerobic oxidation as compared to gold nanoparticles deposited on a typical MgO aerogel. TEM studies reveal that the gold nanoparticles have a hemispherical shape while sitting on the main surface of MgO(111) nanosheets, with a large Au-MgO interface. Given that the gold nanoparticles deposited on the two types of MgO have similar size, and that the two types of unmodified MgO show almost the same activities in the blank reaction, we infer that the high activity of Au/MgO(111) is due to the properties of the (111) support and/or those of the gold-support interface. To understand the binding of Au on low-index MgO surfaces and the charge distribution at the surface of the support, we have performed density functional theory (DFT) calculations on all low-index MgO substrates (with and without gold), using a model Au(10) cluster. Due to similar lattice constants of Au(111) and MgO(111) planes, the Au cluster retains its structural integrity and binds strongly on MgO(111) with either oxygen or magnesium termination. Furthermore, we have found that for the (001) and (110) substrates the charges of the ions in the top surface layer have similar values as in bulk MgO, but that on (111) surfaces these charges are significantly different. This difference in surface charge determines the direction of the electronic transfer upon adsorption of gold, such transfer occurring so as to restore the bulk MgO charge values. Using the results from theoretical calculations, we provide an explanation of our observations of increased catalytic activity in the case of the Au/MgO(111) system.     

Thermal behavior,decomposition mechanism by TG/MS/FTIR technique and theoretical study of some symmetric and asymmetric bent-core liquid crystals based on 2,7-dihydroxynaphthalene

Journal of Thermal Analysis and Calorimetry - The thermal stability of some symmetric and asymmetric bent-core liquid crystals derived from 2,7-dihydroxynaphthalene as well as the degradation...     

Solid-phase parallel synthesis of 17alpha-substituted estradiol sulfamate and phenol libraries using the multidetachable sulfamate linker

We report an application of the multidetachable sulfamate linker in the synthesis of two model libraries of N-derivatized 17alpha-piperazinomethyl estradiols (phenols and sulfamates) by solid-phase parallel chemistry. The solid-phase precursor, a 3-sulfamoyl-17alpha-(N-trifluoroacetyl-piperazinomethyl) estradiol, was synthesized in solution from estrone and loaded efficiently onto trityl chloride resin as polymeric support. After cleavage of the trifluoroacetyl protecting group, sequential acylation reactions with five Fmoc-protected amino acids and five carboxylic acids were performed to introduce two levels of molecular diversity. Finally, the resins were split into two parts, and acidic (5% trifluoroacetic acid in dichloromethane) and nucleophilic (piperazine in tetrahydrofuran) cleavages were used to generate libraries A (5 x 5 sulfamates) and B (5 x 5 phenols) members in overall yields of 18-66% and high HPLC purities (87-96%) without purification steps. A preliminary screening test for inhibition of steroid sulfatase showed that the phenols were clearly weaker inhibitors, as compared to their sulfamate analogues. The most potent inhibitors were those with suitable hydrophobic amino acid and carboxylic acid substituents. Thus, compounds with a phenylalanine residue as the first element of diversity inhibited over 90% of steroid sulfatase activity at a concentration of 1 nM in homogenates of HEK-293 transfected cells, being as potent as the leading inhibitor 17alpha-tert-butylbenzyl estradiol 3-O-sulfamate previously reported. These results suggest that the steroid sulfatase inhibitory potency of estradiol derivatives, sulfamoylated or not, can be increased by the hydrophobic effect of a suitable substituent introduced in the proximity of the D ring of the steroid. The present work also demonstrated the efficiency and the cleavage versatility of the sulfamate linker to generate libraries of compounds with relevant biological importance, phenols and sulfamates.