Organolanthanide Catalysis

A review article entitled "Bulky amido ligands in rare-earth chemistry Syntheses, structures, and catalysis" has been published by Roesky. Benzamidinate ligands are briefly mentioned in this contexD The use of bulky benzamidinate ligands in organolanthanide chemistry was also briefly mentioned in a review article by Okuda et al. devoted to "Cationic alkyl complexes of the rare-earth metals S mthesis, structure, and reactivity." Particularly mentioned in this article are reactions of neutral bis(alkyl) lanthanide benzamidinates with [NMe2HPh][BPh4] which result in the formation of thermally robust ion pairs (Scheme 55). ... 

The text of this chapter on Features of Organolanthanide Chemistry has been geared to the precursor chemistry of potential applications in the fields of catalysis and material sciences, which are certainly two of the most-discussed topics in current publications. However, organolanthanide chemistry has also penetrated other important areas for example, in a text on macrocyclic derivatives the topic probes in life is to be found [92]. 

The chiral organolanthanides have been especially designed for asymmetric catalysis. Thus far several enantioselective olefin transformations (hydrogenation, hydroamination/cyclization, hydrosilylation) as well as the polymerization of methyl methacrylate mediated by these chiral organolanthanide metallocenes have been investigated. 

Anwander, R. and Herrmann, W.A. (1996) Features of organolanthanide complexes. Topics in Current Chemistry, 179 (Organolanthoid Chemistry Synthesis Structure Catalysis) 1-32. 

Yasuda, H., Euro, M., Yamamoto, H. etal. (1992) New approach to block copolymerizations of ethylene with alkyl methacrylates and lactones by unique catalysis with organolanthanide complexes. Macromolecules, 25, 5115. 

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]. 

Keywords Amidinates Guanidinates Lanthanides Organolanthanide chemistry Polymerization catalysis... 

Several routes are currently applied to synthesize cationic organolanthanide species, including the halide abstraction from heteroleptic Ln(III) compounds [Eq. (25)] [152], the oxidation of divalent metallocenes [Eqs. (26) and (27)] [153], the protolysis of lanthanide alkyl and amide moieties [Eqs. (28) and (29)] [154,155], and anion exchange [Eqs. (30) and (31)] [84,156]. In the absence of a coordinating solvent such extremely electrophilic species attain stabilization via arene interactions with the BPh4 anion (Sect.5.1) [153b]. Cationic rare earth species have been considered as promising candidates for Lewis acid catalysis [157]. 

Hence, synthetic organolanthanide chemistry puts the main emphasis on the adaption of prevailing precatalyst types to the requirements of highly enantiose-lective catalysis. This is impressively demonstrated by tied-back cyclopentadienyl complexes [207], even water-stable BINOL systems [43], and fluorinated (3-diketonate complexes (Fig. 11) [208]. 

During the last two decades, lanthanide catalysis has been extensively explored [3], considering the unique properties and the absence of toxicity of these "heavy" metals which make them environmentally friendly. Olefin transformations catalysed by organolanthanides such as oligomerisation, hydrogenation, hydrosilylation, hydroamination, polymerisation, have attracted much attention. The two latter reactions can be initiated by hydrides (which act as precatalysts, such as for MMA polymerisation [4]), but do not involve hydrides as intermediates in the catalytic cycle and therefore will not be considered in the present review. 

Organolanthanide complexes differ from late d-block transition metal complexes in several aspects. They are electrophilic, kinetically labile and lack conventional oxidative addition/reductive elimination pathways in their reactions. They have alternative mechanisms to perform catalytic transformations and are being increasingly used in homogeneous catalysis. The hydrophosphination reaction was proposed to proceed through the cycle depicted in... 

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). 

New reports on the application of organoactinides in catalysis also mention catalytic hydrosilylation of alkynes (106) and alkenes (for a recent review, see (107)). For alkenes, the hydrosilylation reaction catalyzed by organoactinides is much slower than by the respective organolanthanides. 

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