Polymerization of phosphorus containing cyclic monomers: Synthesis of polymers related to biopolymers

The mechanism of polymerization is discussed, in which cyclic esters of phosphoric acid, and related compounds are converted into linear macromolecules, modelling nucleic and teichoic acid backbones. Structures like deoxyribose polyphosphate and glycerol polyphosphate were prepared from the corresponding cyclic compounds. These polymerizations involve heterolytic breaking of the P-O bond in the cyclic monomer and proceed by ionic mechanism. Both 5- and 6-membered monomers have been polymerized. The thermodynamic parameters of the ring-chain interconversion were determined; the 5-membered rings polymerization is driven by the exothermicity of the ring-opening, whereas polymerization of several 6-membered rings is endothermic and allowed because of the positive change of entropy. Anionic polymerization, and particularly the pseudo(coordinate)anionic polymerization provides, in contrast to the cationic process, high-molecular-weight polymers with more uniform structure. Anionic polymerization proceeds mostly (at the applied conditions) on the macroion – -pairs. The elementary reactions consist of the nucleophilic attack of the paired macroanions on the phosphorus atom in the cyclic monomer molecule. Rate constants of the elementary reactions for the model monomers will be presented. Stereochemistry of the propagation steps is shown to be governed by the statistical ring-opening, leading to the three kinds of polymer units (head-to-tail and two symmetrical units). Apart from the ring-opening, the polyaddition of diepoxides to phosphorous and phosphoric acids is described. Finally, a few examples of preparation of models of biopolymers are given, namely poly(deoxyribose phosphate) and poly(glycerol phosphate).     

Improvement of remainder term for the divisors problem

New bounds are obtained for the remainder term in the divisors problem.Translated from Matematicheskie Zametki, Vol. 6, No. 5, pp. 545–554, November, 1969.     

Force-field and quantum-mechanical binding study of selected SAMPL3 host-guest complexes

A Merck molecular force field classical potential combined with Poisson-Boltzmann electrostatics (MMFF/PB) has been used to estimate the binding free energy of seven guest molecules (six tertiary amines and one primary amine) into a synthetic receptor (acyclic cucurbit[4]uril congener) and two benzimidazoles into cyclic cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]) hosts. In addition, binding enthalpies for the benzimidazoles were calculated with density functional theory (DFT) using the B3LYP functional and a polarizable continuum model (PCM). Although in most cases the MMFF/PB approach returned reasonable agreements with the experiment (±2 kcal/mol), significant, much larger deviations were reported in the case of three host-guest pairs. All four binding enthalpy predictions with the DFT/PCM method suffered 70% or larger deviations from the calorimetry data. Results are discussed in terms of the molecular models used for guest-host complexation and the quality of the intermolecular potentials.     

Latent ruthenium carbene complexes with six-membered N- and S-chelate rings

New ruthenium carbene complexes with chelating N- and S-benzylidene ligands were synthesized by the reactions of second- and third-generation Grubbs catalysts with ortho-vinylbenzyl-substituted amines or sulfides. These complexes were shown to exhibit catalytic activity in ring-opening metathesis polymerization and ring-closing metathesis.     

The vibrational spectra and normal coordinate analysis of tetracyclopropyllead

All the fundamental frequencies observed in IR and Raman spectra have been assigned to the normal modes of the molecule (C₃H₅)₄Pb (C₃H₅ = cyclopropyl). The calculated force field is compared to that of (C₃H₅)₂Hg and the Cmetal stretching force constants are discussed along with those of Me₄Pb and Me₂Hg; The similarity of the spectra of cyclopropyllead and cyclopropyl-mercury proves that the vibrations of cyclopropyl structural units are isolated in both molecules.     

Stereodefined synthesis of O3'-labeled uracil nucleosides. 3'-[(17)O]-2'-Azido-2'-deoxyuridine 5'-diphosphate as a probe for the mechanism of inactivation of ribonucleotide reductases

Thermolysis of a 2'-[(16)O]-O-benzoyl-[(17)O]-5'-O-(tert-butyldimethylsilyl)-O(2),3'-cyclouridine derivative gave the more stable 3'-[(17)O]-O-benzoyl-[(16)O]- 5'-O-(tert-butyldimethylsilyl)-O(2),2'-cyclouridine isomer, which was converted into 3'-[(17)O]-2'-azido-2'-deoxyuridine by deprotection and nucleophilic ring opening at C2' with lithium azide. The 5'-diphosphate was prepared by nucleophilic displacement of the 5'-O-tosyl group with tris(tetrabutylammonium) hydrogen pyrophosphate. Model reactions gave (16)O and (18)O isotopomers, and base-promoted hydrolysis of an O(2),2'-cyclonucleoside gave stereodefined access to 3'-[(18)O]-1-(beta-D-arabinofuranosyl)uracil. Inactivation of ribonucleoside diphosphate reductase with 2'-azido-2'-deoxynucleotides results in appearance of EPR signals for a nitrogen-centered radical derived from azide, and 3'-[(17)O]-2'-azido-2'-deoxyuridine 5'-diphosphate provides an isotopomer to perturb EPR spectra in a predictable manner.     

Electrochemical and electrocatalytic studies of the N,N′-(1R,2R)-(–)-1,2-cyclohexylenebis(salicylideneiminato)cobalt(II) complex

The electrochemical properties and catalytic activity of a Co(II) complex with the optically active Schiff base derived from (1R,2R)-(–)-cyclohexanediamine and salicylaldehyde have been studied in non-aqueous solutions. When dissolved in deoxygenated non-aqueous solutions, the complex exhibits reversible redox properties for the Co(II)/Co(III) couple. Electrochemical reduction of oxygen and oxidation of cobalt(II) was observed on cyclic voltammograms of solutions containing both dioxygen and the Schiff base-cobalt(II) complex. An anodically formed film on a platinum electrode, studied by means of X-ray photoelectron spectroscopy, revealed the presence of the oxidized Co(III) species. Cyclic voltammetry of oxygenated solutions examined after a period of time indicates an electrochemical activity of coordinated superoxo/peroxo species in the 0.7–1.1 V potential range. In the presence of 4-methyl-1-cyclohexene the cyclic voltammetry curves reveal changes similar to those caused by the removal of oxygen. The GC-MS technique was used to identify some of the products formed by the catalytic oxidation of cyclohexene and 4-methyl-1-cyclohexene. Electronic Publication