Highly active ethylene polymerization and copolymerization with norbornene using bis(imino‐indolide) titanium dichloride–MAO system

A catalytic system of new titanium complexes with methylaluminoxane (MAO) was found to effectively polymerize ethylene for high molecular weight polyethylene as well as highly active copolymerization of ethylene and norbornene. The bis (imino‐indolide)titanium dichlorides (L₂TiCl₂, 1 – 5 ), were prepared by the reaction of N‐((3‐chloro‐1H‐indol‐2‐yl)methylene)benzenamines with TiCl₄, and characterized by elemental analysis, ¹H and ¹³C NMR spectroscopy. The solid‐state structures of 1 and 4 were determined by X‐ray diffraction analysis to reveal the six‐coordinated distorted octahedral geometry around the titanium atom with a pair of chlorides and ligands in cis‐forms. Upon activation by MAO, the complexes showed high activity for homopolymerization of ethylene and copolymerization of ethylene and norbornene. A positive “comonomer effect” was observed for copolymerization of ethylene and norbornene. Both experimental observations and paired interaction orbital (PIO) calculations indicated that the titanium complexes with electron‐withdrawing groups in ligands performed higher catalytic activities than those possessing electron‐donating groups. Relying on different complexes and reaction conditions, the resultant polyethylenes had the molecular weights M_w in the range of 200–2800 kg/mol. The influences on both catalytic activity and polyethylene molecular weights have been carefully checked with the nature of complexes and reaction conditions. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3415–3430, 2007     

Quantum phase transition in one-dimensional arrays of resistively shunted small Josephson junctions

We have observed a superconductor-insulator transition in one-dimensional (1D) arrays of small Josephson junctions by changing both the resistance R(S) of normal metal resistors shunting each junction and the ratio of the Josephson coupling energy E(J) to the charging energy E(C). The phase boundary lies at R(S) approximately R(Q) (R(Q) identical with h/4e(2)=6.45 kOmega) when E(J)/E(C) is smaller than about unity. We discuss the obtained phase diagram in terms of theoretical models of the dissipation-driven quantum phase transition, with particular attention to differences from 2D arrays.     

Regio- and stereoselective cross-coupling of tert-propargyl alcohols with bis(trimethylsilyl)acetylene and its utilization in constructing a fluorescent donor-acceptor system

1,1-Disubstituted 3-aryl-2-propyn-1-ols undergo regio- and stereoselective cross-coupling on treatment with bis(trimethylsilyl)acetylene in the presence of a rhodium catalyst via cleavage of C(sp)-C(sp3) and C(sp)-Si bonds to produce the corresponding 2-hydroxymethyl-(E)-enynes. The subsequent desilylative Sonogashira coupling followed by base-promoted cyclization affords fluorescent dihydrofuran derivatives.     

Direct Aldol Strategy in Enantioselective Total Synthesis of Thuggacin B

An enantioselective total synthesis of thuggacin B, a natural product exhibiting antibiotic activity against Mycobacterium tuberculosis, is described. Asymmetric direct aldol reactions promoted by Cu and Zn catalysts play a pivotal role in constructing four stereogenic centers. The use of direct aldol reactions as the initial steps for the synthesis of two key fragments allowed the construction of the other stereogenic centers through chirality transfer.     

Radical CH Functionalization of Heteroarenes under Electrochemical Control

Electrochemical reactions are shown to be effective for the C? H functionalization of a number of heterocyclic substrates that are recalcitrant to conventional peroxide radical initiation conditions. Monitoring reaction progress under electrochemical conditions provides mechanistic insight into the C? H functionalization of a series of heterocycles of interest in medicinal chemistry.     

A facile and clean direct cyanation of heteroaromatic compounds using a recyclable hypervalent iodine(III) reagent

The facile and clean direct cyanating reaction of pyrroles and thiophenes has been achieved using a recyclable hypervalent iodine(III) reagent 1b by a simple solid-liquid separation of the products and the reagent.     

New crystalline polymers: poly(2,5‐dialkyl‐1,4‐phenylene oxide)s

New heat‐reversibly crystalline poly‐(alkylated phenylene oxide)s are described. the oxidative polymerization of 2,5‐dimethylphenol catalyzed by (1,4,7‐triisopropyl‐1,4,7‐triazacyclononane) copper dichloride produced poly(2,5‐dimethyl‐1,4‐phenylene oxide), which showed heat‐reversible crystallinity with a high melting point at ca. 300°C, although the isomeric polymer, poly(2,6‐dimethyl‐1,4‐phenylene oxide), never recrystallizes once melted. The polymerization of 2,5‐diethylphenol and 2,5‐dipropylphenol gave the polymers consisting of 1,4‐phenylene oxide units; the latter polymer possessed heat‐reversible crystallinity, however, the former one did not.