Polymerization of β-cyanopropionaldehyde. V. Anionic polymerization initiated by benzophenone–alkali metal complexes

Anionic polymerization of β-cyanopropionaldehyde was studied with use of benzophenone-monosodium, -disodium, and -dilithium complexes as initiators. The resulting poly(cyanoethyl)oxymethylene was compared with that obtained by cationic and coordinated initiators previously reported. Polymer of higher stereoregularity but lower molecular weight was formed in the present system. The marked influence of the initiator concentration on the polymer yield and stereoregularity is explained on the basis of the difference in the degree of association of the alcoholate ion pair, i.e., the associated ion pair may form stereoregular polymer and the nonassociated or less-associated ion pair may form a large amount of amorphous atactic polymer. The initiation with benzophenone-dialkali metal complex was found to be bond-formation type. Chain transfer with active hydrogen of β-cyanopropionaldehyde frequently occurs.     

Regioselective reductive coupling of alkynes and aldehydes leading to allylic alcohols

Treatment of a mixture of a terminal alkyne and an aldehyde with CrCl(2) and a catalytic amount of NiCl(2) and triphenylphosphine in the presence of water in DMF at 25 degrees C gives a 1,2-disubstituted allylic alcohol regioselectively.     

Magnetically separable phase-transfer catalysts

Magnetic nanoparticles-supported quaternary ammonium and phosphonium salts were prepared and evaluated as phase-transfer catalysts. Some of them exhibited good activities that were comparable to that of tetra-n-butylammonium iodide. The catalysts were readily separated using an external magnet and reusable without significant loss of their catalytic efficiency.     

Sequence-Controlled Catalytic One-Pot Synthesis of Siloxane Oligomers

Silicones are highly valuable poly- and oligomeric materials with a broad range of applications due to their outstanding physicochemical properties. The core framework of silicone materials consists of siloxane (Si−O−Si) bonds, and thus, the development of efficient siloxane-bond-forming reactions has attracted much attention. However, these reactions, especially “catalytic” siloxane-bond-forming reactions that enable the selective formation of unsymmetrical siloxane bonds, remain relatively underdeveloped. On the other hand, controlled iteration has become a powerful tool for the sequence-controlled synthesis of poly- and oligomeric compounds. Recently, control over the siloxane sequence has been achieved by the one-pot iteration of a B(C₆F₅)₃-catalyzed dehydrocarbonative cross-coupling of alkoxysilanes with hydrosilanes and a B(C₆F₅)₃-catalyzed hydrosilylation of carbonyl compounds. Thus, it is now possible to generate linear, branched, and cyclic sequence-specific oligosiloxanes in a highly selective manner under chloride-free conditions.     

Mechanistic Insights into Rhenium-Catalyzed Regioselective C-Alkenylation of Phenols with Internal Alkynes

A (μ-aryloxo)rhenium complex was isolated and confirmed as a key precatalyst for rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes. The reaction exclusively provided ortho-alkenylphenols; the formation of para or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products was not observed. Several mechanistic experiments excluded a classical Friedel–Crafts-type mechanism, leading to the proposed phenolic hydroxyl group assisted electrophilic alkenylation as the most plausible reaction mechanism. For this purpose, the use of rhenium, a metal between the early and late transition metals in the periodic table, was key for the activation of both the soft carbon–carbon triple bond of the alkyne and the hard oxygen atom of the phenol, at the same time. ortho-Selective alkenylation with allenes also provided the corresponding adducts with a substitution pattern different from that obtained by the addition reaction with alkynes.     

Negative muon capture ratios for nitrogen oxide molecules

Journal of Radioanalytical and Nuclear Chemistry - Per-atom muon capture ratios are precisely determined from characteristic muonic X-ray measurements of three nitrogen oxide samples (NO, N2O, and...     

Solid-state 17O NMR study of the electric-field-gradient and chemical shielding tensors in polycrystalline gamma-glycine

We report the first experimental determination of the carboxylate oxygen electric-field-gradient (EFG) and chemical shielding (CS) tensors in polycrystalline γ-glycine. Analysis of magic-angle spinning (MAS) and stationary ¹⁷O NMR spectra of [¹⁷O]-γ-glycine obtained at 9.4, 14.1, 16.4, and 18.8 T yields the magnitudes of the ¹⁷O EFG and CS tensors and the relative orientations between the two tensors. Extensive quantum chemical calculations at both the restricted Hartree–Fock and density functional levels have been performed to present the absolute tensor orientations in term of the molecular frame. We have demonstrated that ¹⁷O NMR tensor information could be unambiguously derived by the multiple field analyses of stationary ¹⁷O NMR spectra.     

Baryon distributions in ultrarelativistic nucleus-nucleus collisions

We have measured distributions in transverse momentum and rapidity of protons from interactions of 14.6 GeV/nucleon²⁸Si projectiles with targets of Al and Pb. The transverse momentum spectra exhibit a thermal shape with a rapidity dependent temperature parameter. For very central or violent collisions the proton rapidity distributions exhibit the large rapidity shifts and (for Si+Al) a peak at midrapidity as required for full stopping.