Titanium alkoxides can catalyze efficient and selective synthesis of cyclic carbonates through CO2 insertion into epoxides when tetrabutylammonium bromide or iodide is used as a cocatalyst. Of the series of synthesized titanium alkoxides, the most active catalyst was one bearing an aliphatic bidentate alkoxide ligand, and it produced propylene carbonate under benign conditions (100 °C, 15 bar CO2, 3 h) with turnover frequencies of up to 116 h–1. The effect of reaction parameters such as pressure, temperature, and catalyst/cocatalyst molar ratio were also studied in order to optimize the catalytic system.
A new procedure was developed for the synthesis of a broad range of ansa zirconocenes containing bis(2 methyl 4 arylindenyl)dimethylsilane ligands. The method is based on the palladium catalyzed reaction of halogen substituted bis(indenyl)dimethylsilanes with various organozinc compounds. The aryl substituted bridging ligands thus prepared serve as the starting compounds for the synthesis of ansa zirconocenes, which can be used as components of promising catalysts for propylene polymerization. (4 bromo 2 methylinden 1 yl)dimethylsilane.Recent extensive research into the relationship be tween the structures of ansa metallocenes and the activity and stereoselectivity of the corresponding catalysts for propylene polymerization showed that the best results were achieved for catalysts based on the so called improved ansa zirconocenes A containing bis(2 methyl 4 aryl indenyl)dimethylsilane ligands, as well as on related com plexes B containing 3 aryl 2,5 dimethylcyclopenta[b] thienyl moieties.
First members of a novel family of constrained geometry complexes of titanium and zirco nium containing a short bridge between the amide and inden 4 yl fragments were obtained. When activated with methylaluminoxane, these complexes can be used as highly active cata lysts for ethene polymerization producing high molecular weight polymers.Constrained geometry complexes (CGC) are analogs of ansa metallocenes in which one cyclopentadienyl moi ety is replaced by an anionic monoatomic (e.g., alkyl amide) fragment. The first complex of a transition metal (scandium) with a ligand of this type (complex 1, see below) was synthesized in 1990. 1,2 Shortly afterwards, ExxonMobil Chemical Co. 3 and Dow Chemical Co. 4 were nearly simultaneously granted patents for the use of CGC of titanium subgroup metals (e.g., complex 2) as precata lysts for olefin polymerization.The term "constrained geometry complex" reflects the fact that the angle Cp(centroid)-M-N (M is the metal atom) in these silylene bridged complexes is smaller than the angle Cp(centroid)-M-Cp´(centroid) in ansa met allocenes containing a similar bridge. Because of this, Ti CGC catalysts are highly active in olefin polymeriza tion and allow copolymerization of ethene with higher olefins, the resulting copolymers showing the rich content of the latter. 5In most of the documented CGC, an amide (or like) fragment is connected through a bridge directly to the cyclopentadienyl (or like) anionic polyhapto coordinated fragment. 5 Here we are the first to report on the synthe sis of CGC 3 (M = Ti or Zr) containing no direct bond between the silylene bridge and the cyclopentadie nyl fragment (the bridging group is attached to indene at position 4).Obviously, the angle Cp(centroid)-M-N in struc tures 3 will be smaller than that in complexes 2, so there will be more room in the nearest vicinity of the metal for its coordination with an olefin in cationic complexes, which are actual catalytic species. That is why activation of complexes 3 with methylaluminoxane (MAO) can pro duce catalysts for olefin (notably, ethene) polymerization that exhibit unique properties and allow the synthesis of so called ultra high molecular weight polyethylene (UHMWPE, M w > 1.5 MDa) of great practical value. 6
Results and DiscussionThe first step in the synthesis of bridging ligands and CGC involves a reaction of a Grignard reagent (pre pared from 7 bromo 2 methyl 1H indene and magnesium in THF) with an excess of (dichloro)(dimethyl)silane (Scheme 1).
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