Andrzej M. Piotrowski

The titanocene methylene-zinc halide complex: a convenient synthesis and its methylenating action on unsaturated carbon centers☆

Abstract The titanocene methylene-zinc halide complex, which can be conveniently prepared by treating titanocene dichloride with methylenezinc iodide in THF solution, readily methylenates ketones, nitriles, and alkynes.

Die oxidative Addition von NickeI(0)-KompIexen an Kohlenstoff-Kohlenstoff-Bindungen im Cyclobutadien. Zur Frage der Überführbarkeit von Cyclobutadien-Nickel(O)-Komplexen in Nickelaringe / Oxidative Addition of Nickel(O) Complexes to Carbon-Carbon Bonds in Cyclobutadiene. The Question of the Interconvertibility of Cyclobutadiene-nickel(O) Complexes and the Nickelaring Systems

Abstract Bis(triethylphosphine)(η4-tetraphenylcyclobutadiene)nickel (4) was synthesized by the reduction of (η4-tetraphenylcyclobutadiene)nickel(II)bromide (3) with t-butyllithium in the presence of Et3P, and its structure was determined by X-ray crystallography. Furthermore, its reactivity towards CO, CH3CO2H, PhC≡CPh, LiAlH4 and O2 were investigated. 1,1-Bis(triethylphos-phine)-2,3,4,5-tetraphenylnickelole (14) was synthesized from (E,E)-1,4-dilithio-1,2,3,4-tetraphenyl-1,3-butadiene (15) and bis(triethylphosphine)nickel(II)bromide. Since the resulting crystals of the nickelole were not suitable for X-ray structure determination, the compound was characterized by elemental analyses, spectral data and carbonylation to yield tetraphenylcyclo-pentadienone (6). Analogous reductions of (η4 -tetraphenylcyclobutadiene)nickel(II)bromide (3) in the presence of Ph3P or Ph2PCH2CH2PPh2 , followed by carbonylation, led to 6 in 40% yield, demonstrating that about half of the cyclobutadiene rings in 3 undergo cleavage upon reduction to give the nickelole. Reactions of the dilithium reagent 15 with NiBr2 complexed with Me2PCH2CH2PMe2 ,Ph3P or Et2PCH2CH2PEt2 , led to the formation of thermolabile nickeloles, as demonstrated by carbonylàtion which yielded 6. Warming of the nickeloles and subsequent treatment with CH3CO2H led to the formation of 1,2,3,4,5,6,7,8-octaphenyl-1,3,5,7-octatetraene (8) and, in one case, octaphenyl-cyclooctatetraene (5). The relevance of these findings to the mechanism of the Reppe nickel-catalyzed oligomerization of alkynes is discussed.

Nature of the Active Sites in Soluble Ziegler-Polymerization Catalysts Generated from Titanocene Halides and Organoaluminum Lewis Acids

That the active site in Ziegler-Natta polymerization catalysts is the transition metal-carbon bond receives strong support from the finding that pure transition metal alkyls, such as tetrabenzyltitanium [1], can themselves polymerize ethylene and propylene, even in the complete absence of any main-group alkyl. However, it is also evident that combinations of transition-metal salts or organometallics with main-group alkyls often display greatly enhanced catalytic activity and stereoregular polymerization in such processes [2]. The isotactic polymerization of propylene by TiCl3 and Et2AlCl [3] and the greatly accelerated polymerization of ethylene by Cp2ZrMe2 and (MeAlO)n [4] are cases in point.