Organolanthanide-catalyzed Polymerization Reactions

The most thoroughly investigated group of organolanthanide-catalyzed reactions are monoolefin transformations. These include the following processes a) hydrogenation, b) oligomerization, c) polymerization, d) hydroamination, e) hydrosilylation, and f) hydroboration, which will be discussed in the above order. 

Early mechanistic studies concerning organolanthanide-catalyzed olefin polymerization reactions showed that insertion of the unsaturated hydrocarbon into the lanthanide-carbon cr-bond is a key step. It was first demonstrated for the... 

In spite of countless applications of rare earth activation in industrial heterogeneous catalysis, most soluble complexes have long been limited to more or less stoichiometric reactions. An early example is the Kagan C-C coupling mediated by samarium(II) iodide [126]. Meanwhile, true catalytic reactions have become available. Highlights are considered the organolanthanide-catalyzed hydroamina-tion of olefins [127], the living polymerization of polar and nonpolar monomers [128], and particularly the polymerization of methyl methacrylate [129]. In the first case, lanthanocene catalysts of type 27 are employed [127]. 

Recently, diphenylphosphine has been shown to be an efficient chain-transfer agent in organolanthanide-catalyzed ethylene polymerization, yielding phosphine-terminated polyethylenes. This reaction is a versatile, efficient way of incorporating an electron-rich functional group into an otherwise inert polymer.929... 

Phosphine-terminated polyethylenes have been prepared in related organolanthanide-catalyzed reactions [31, 32]. After initiation by insertion of ethylene into the Ln-P bond of a phosphide complex, polymerization occurs by repeated ethylene insertion. Termination results from protonolysis of the growing polymer chain via a four-centered a-bond metathesis transition state, as seen in Scheme 19, to give a polymeryl-phosphine and regenerate the lanthanum phosphide complex (Scheme 20). 

W.J.Evans (33,34) prepared some divalent organolanthanides by co-condensation at low temperature of lanthanide metal vapours with unsaturated hydrocarbons (cyclopentadienes, alkynes) containing acidic hydrogen. Some organolanthanides showed catalytic activity. Thus, Sm(C Me ) (THF) catalyzes hydrogenation of 3-hexyne into cis-hexene (cis trans > 99 1 under mild conditions 25°C, 1 atm of hydrogen. The reaction is believed to involve the addition of a hydride Ln-H to the triple bond followed by hydrogenolysis with H (35). The same complex polymerizes ethylene (35). /... 

Chiral Cyclopentadienyl Complexes. Since the discovery of the polymerization activity of cyclopentadienyl complexes, they also play a key role in asymmetric catalysis (Fig. 13). Titanocene complexes of chiral tricyclic monocy-clopentadienyl ligand catalyze the enantioselective hydrogenation of unfunctionalized oleflns (105). A similar reaction has been performed with related catalysts such as chiral Ziegler-Natta systems (106) and organolanthanide systems (107). 

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