Radiation-induced polymerization at high dose rate. VI. Butadiene in n-hexane solution

Polymerization of butadiene by electron-beam irradiation was studied in n-hexane solution. Kinetic features of the polymerization and microstructure of the product were similar to those in bulk polymerization, which indicated a cationic mechanism. Both the rate of polymerization and average molecular weight of the product decreased by about 20% in the solution system. Soluble polymer was obtained up to ca. 40% conversion at the monomer concentration of 50 mol%, whereas in the bulk system the gel formed in polymer conversion exceeded 10%. A kinetic equation was proposed to explain the change in rate of polymerization with the monomer concentration.     

Palladium-catalyzed intramolecular α-arylation of sulfoximines

The palladium-catalyzed cyclization of N-(2-bromobenzyl)- and N-(2-bromobenzoyl)-sulfoximines affords six-membered heterocycles in moderate to good yield. In both cases, the α-arylations of the sulfoximine methyl groups are catalyzed by combination of Pd(OAc)₂ and rac-BINAP in the presence of a base such as Cs₂CO₃ or K₂CO₃.     

NMR studies on the stereospecific polymerization of methyl vinyl ether. Part II. Polymerization by sulfuric acid–aluminum sulfate complex: Enantiomorphic catalyst sites model

Highly crystalline poly(methyl vinyl ether) (PMVE) was produced in toluene in a temperature range of 0 to ?20°C. with the use of sulfuric acid–aluminum sulfate complex (SA catalyst). It was found from the NMR spectra that these polymers contained more than 50% of the triad isotactic fraction and the melting point of the unfractionated polymer was about 130°C. However, PMVE containing a large amount of the isotactic fraction was insoluble in nitromethane, so the triad tacticity of highly crystalline PMVE could not be quantitatively determined. The molecular weight of PMVE increased with increasing conversion and increasing polymerization temperature. This behavior is different from that in metal halide catalysts. Also, the stereoregularity of PMVE decreased with increasing monomer concentration. However, addition of a polar solvent and increasing the polymerization temperature had little effect on the stereoregularity of the polymer. The increase in the isotactic fraction at high catalyst concentration and the difference in the monomer composition in the copolymerization of methyl vinyl ether with 2-chloroethyl vinyl ether by SA catalyst from that obtained by BF₃·O(C₂H₅)₂ suggest that the absorption of MVE on a catalyst surface is an important step in the propagation step by SA catalyst. The fraction of the triad tacticity calculated from the enantiomorphic catalyst sites model⁸ coincided with the experimental results. This fact shows that the steric structure of the adding monomer is determined only by the nature of the catalyst irrespective of the nature of a growing chain end. It is concluded, on considering also the results of the previous paper, that completely different factors can control the steric structure of a polymer even for the same monomer when different catalysts are used.     

Synthesis and Crystal Structure of a New 2D Honeycomb—like Cadmium （Ⅱ） Complex with Tripodal Ligand

A new cadmium (II) coordination polymer, [Cd(TTTMB)₂]‐(SO₄) · 21H₂O, where TTTMB =1,3, 5‐tris (imidazol‐1‐yl‐methyl)‐2,4,6‐trimethylbenzene, was obtained by self‐assembly of tripodal ligand TTTMB with CdSO₄·2.7H₂O in acetonitrile, and characterized by X‐ray crystallography. The crystal data belongs to monoclinic space group C_c with cell parameters a = 1.16891(4) nm, b=2.06671(6) nm, c = 2.48185(7) nm, β = 97.8560(10)°, R = 0.0487, wR = 0.1211. The results of structure analysis indicate that each TTTMB ligand coordinates three metal atoms and in turn each Cd(II) atom with octahedral coordination geometry connects six nitrogen atoms of imidazole group from six different TTTMB ligands to produce a 2D honeycomb network structure. There are a lot of water molecules linked by hydrogen bonds and occupied the channels formed intra‐ and inter‐sheets.     

Syntheses of (5E)-PGE2 and New 6-Functionalized Derivatives by the Use of Palladium-Catalyzed Decarboxylative Allylic Alkylation

(5 )-Prostaglandin E₂ (7) was synthesized fron ( )-4- -butyldimethylsilyloxy-2-cyclopentenone (1) by 2-alkenyloxycarbonylatlon of the organocopper conjugate-addition adduct (3) followed by intramolecular palladium-catalyzed decarboxylative allylic alkylation. The (5 )-prostaglandin E₂ skeleton was also obtained from the β-keto allylic ester (11) by a similar decarboxylative allylic alkylation. The decarboxylative allylic alkylation of another type of the three-component coupling product (12) gave new 6-methyleneprostaglandin E1 skeleton (15a), which was converted into new 6-methylprosta-glandin I methyl ester (20) 6-methyleneprostaglandin F₁ derivative (16) by two different ways. The stereochemistry of this intramolecular decarboxylative allylic alkylation was discussed in the reaction of 2-[( )- or ( )-2-butenyloxy-carbonyl] cyclopentanone systems.     

一个新型碗状双核配合物的合成及结构表征

采用挥发法，在无水条件下合成了一个新型具有碗状结构的双核稀土金属配合物：[Y₂(L)(HL)(NO₃)₆(HCOO)]·3CH₃OH[L=1，3，5-三(1-咪唑基-亚甲基)-2，4，6-三甲基苯] (1)。并通过元素分析、FT-IR、电喷雾质谱及X-射线单晶衍射对其进行了表征。结构分析表明该化合物属于正交晶系，空间群为Pnnm，晶胞参数为a=1.952 42(15) nm，b=1.875 28(19) nm，c=1.682 25(15) nm，晶胞体积V=6.159 3(10) nm³，Z=4。在化合物1中，两个独立的Y(Ⅲ)均为九配位，并通过甲酸根桥联，而两个配体采用不同的构象作为二齿配体参与与Y(Ⅲ)的配位，一个为cis，cis，cis-构象，另一个为cis，trans，trans-构象。     

Noncanonical RNA Nucleosides as Molecular Fossils of an Early Earth—Generation by Prebiotic Methylations and Carbamoylations

The RNA‐world hypothesis assumes that life on Earth started with small RNA molecules that catalyzed their own formation. Vital to this hypothesis is the need for prebiotic routes towards RNA. Contemporary RNA, however, is not only constructed from the four canonical nucleobases (A, C, G, and U), it also contains many chemically modified (noncanonical) bases. A still open question is whether these noncanonical bases were formed in parallel to the canonical bases (chemical origin) or later, when life demanded higher functional diversity (biological origin). Here we show that isocyanates in combination with sodium nitrite establish methylating and carbamoylating reactivity compatible with early Earth conditions. These reactions lead to the formation of methylated and amino acid modified nucleosides that are still extant. Our data provide a plausible scenario for the chemical origin of certain noncanonical bases, which suggests that they are fossils of an early Earth.     

Analyzing powers in the 12C( \vec d ,p)13C reaction at the energy T d = 270 MeV

The preliminary results on the tensor T₂₀ analyzing power for the ¹²C(,p)¹³C* reaction with the excitation of ¹³C levels and the d(,p)X reaction at the energies T _d = 140, 200 and 270 MeV at the emission angle Θ _cm = 0° are presented. The data on the tensor A _yy and vector A _y analyzing powers for the ¹²C(,p)¹³C* reaction at the energy T _d = 270 MeV in the angular range from 4° to 18° in the laboratory are also obtained.     

The effects of Majorana phases in three-generation neutrinos

Neutrino-oscillation solutions for the atmospheric neutrino anomaly and the solar neutrino deficit can determine the texture of the neutrino mass matrix according to three types of neutrino mass hierarchy: Type A: , Type B: , and Type C: , where is the absolute mass of the ith generation neutrino. The relative sign assignments of the neutrino masses in each type of mass hierarchy play crucial roles in the stability against quantum corrections. Actually, two physical Majorana phases in the lepton flavor mixing matrix connect the relative sign assignments of the neutrino masses. Therefore, in this paper we analyze the stability of the mixing angles against quantum corrections according to three types of neutrino mass hierarchy (Type A, B, C) and two Majorana phases. The two phases play crucial roles in the stability of the mixing angles against quantum corrections. Received: 9 May 2000 / Revised version: 23 May 2000 / Published online: 8 September 2000