The Synthesis and Reactivity of Group 4 Zwitterionic Complexes of the Type M_t⁺CH₂AlCl₃⁻: One‐Component Stereoselective Polymerization and Oligomerization Catalysts for Olefins and Acetylenes

Abstract

A reinvestigation of the interaction of TiCl4 with 2 equiv. of Me3Al in toluene between −78 °C and 25 °C over 24 h has now established that the ultimate black product obtained is an associated zwitterion of the type [Ti+−CH2−AlCl3−]n, supported by multinuclear NMR spectroscopy and mass spectrometric and gasometric analyses of the gases evolved (CH4, H2) upon its protolysis. Chemical reactions of the zwitterion have corroborated specific aspects of its proposed structure: 1) its methylene character, by its transformation of benzophenone into 1,1‐diphenylethylene; 2) its divalent titanium content, by the substantial reductive dimerization of benzophenone to tetraphenylethylene, and 3) its Lewis acidic Ti center, by its catalytic isomerization of trans‐stilbene oxide to 1,1‐diphenylacetaldehyde. Similar individual reactions of ZrCl4 or HfCl4 with Me3Al have led to the analogous zwitterions [Zr+−CH2−AlCl3−]n and [Hf+−CH2−AlCl3−]n, respectively. These zwitterions of Ti, Zr and Hf have been proven to be capable of the cyclotrimerization and/or polymerization of acetylenes with varying facility, as evidenced by their catalytic action on 1‐hexyne, phenylacetylene, di‐n‐butylacetylene, and diphenylacetylene. Furthermore, all three zwitterions were able to polymerize ethylene, without any added cocatalyst, with an activity following the order Zr > Ti > Hf. The Ti and Zr zwitterions effected the stereoselective polymerization of propylene to yield 50% of isotactic polymer, and all three catalysts induced the polymerization of 1‐hexene to yield 85% (Zr, Hf) or 100% (Ti) of isotactic polymer. These oligomerizations and stereoselective polymerizations of acetylenes and olefins can be rationalized through a model for the active site resembling a three‐membered metallacyclopropa(e)nium ion intermediate formed from the attack of the Group 4 metal zwitterion on the unsaturated hydrocarbon. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Synthesis and Characterization of Chiral Group 4 Metallocene Alkyne Complexes: (η5-menthyl-C5H4)2M(η2-Me3SiC2SiMe3), M = Ti, Zr

“…In solution this effect is found neither for the respective dihalides , nor for the zirconacyclopentane 3 . Therefore, it must be related to the presence of the metallacyclopropene core, for which an aromatic character was discussed. , This would create a magnetic anisotropy in its surrounding. Since the cyclopentadienyl and the cyclohexane ring take a roughly perpendicular orientation relative to each other [evident from an NOE between the cyclopentadienyl α-H (6.65 ppm) and 1-H ax and between the other α-H (5.35 ppm) and 6-H ax and 2-H ax on the opposite face of the cyclohexane], the methylene group 6 and in particular its equatorial hydrogen atom enter the sector above or below the metallacyclic plane and thus become shielded.…”

The Extraordinary Cocatalytic Action of Polymethylaluminoxane (MAO) in the Polymerization of Terminal Olefins by Metallocenes: Chemical Change in the Group 4 Metallocene Dimethyl Derivatives Induced by MAO

“…[8] Initial attempts to solve the MAO conundrum have run parallel to studies directed to studying the roles of the alkylaluminum halide cocatalyst in the activation of titanocene dichloride (1), the titanocene methyl(chloro) derivative (2) or titanocene dimethyl (3) for polymerization. [9][10][11][12][13][14][15][16] Starting with titanocene dichloride (1) the first role of cocatalyst Me x AlCl 3-x (4, x = 1, 2 or 3) is proposed to be the methylation of 1 with the formation of 2 and/or 3 [Equation (1)]. The second proposed role exerted by the by-product Lewis acid 5, either with 2 or 3, is the generation of titanocenium cation 6 as the active site for polymerization initiation [Equation (2)] by the Lewis-acidic extraction of anionic E by 5.…”

Illuminating the Unexpected Benzylic Carbon−Carbon Bond Cleavage of Arylated Ethanes with Di-n-Butylzirconium Diethoxide by Illumination: Transfer Epizirconation as Exclusively a Photochemical Process1

“…In our ongoing studies of early-transition-metal alkyls, − we have encountered with zirconium reagents three novel reactions of great potential in organic synthesis. First, di- n -butylzirconium diethoxide ( 2 ), generated from Zr(OEt) 4 ( 1 ) and 2 equiv of BuLi, can readily zirconate weaker carbon acids such as toluene and diphenylmethane ( 3 ) at the benzylic carbon in THF at 25 °C …”