Lanthanocene complexes

Scheme 21 Formation of five- and six-membered ring heterocycles by lanthanocene complexes...

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

The synthesis, structural characterization, and catalytic behavior of one-atom-bridged fluorenyl cyclopentadienyl lanthanocene complexes with Cs- or Cj-symmetry have been reviewed by Qian et al. Two Yb(ll) ansa-indenyl complexes were synthesized by the reaction between Li2[(CH2)2(l-Ind )2l and YbClj in diethyl ether and subsequent reduction of the product with sodium metal in THF yielding [/V7f-(CH2)2( l-Ind )2 Yb(TI IF)2 where Ind =C9H6 and 4,7-(CH3)2C9H4. Solid-state structural data of both complexes are basically identical to each other with values in the expected areas.659... 

Scheme 11.7 Amine induced epimerization of chiral lanthanocene complexes.

Early transition metal and lanthanide complexes also catalyze the hydroboration of alkenes. As shown in Equation 16.44, titanocene complexes catalyze the hydroboration of vinylarenes, and as shown in Equation 16.45, lanthanocene complexes catalyze the hydroboration of alkenes. Ligand-less lanthanides and zirconocene complexes have also been reported to catalyze these reactions, but the apparent catalysis of the hydroboration in these cases more likely occurs by catalytic generation of BH3 from catecholborane and uncatalyzed addition of BHj to the olefin. 

Reactions catalyzed by titanocene and lanthanocene complexes occur by different mechanisms (Schemes 16.13 and 16.14). The mechanism of the hydroboration of vinylarenes catalyzed by titanium complexes is shown in Scheme 16.13. In this mechanism, a titanocene bis-borane complex dissociates borane to generate a 16-electron complex that coordinates the alkene or alkyne. Coupling of one carbon of the resulting metallacycle with the boron of the coordinated borane forms the final product. The mechanism of the lanthanocene-catalyzed reactions, shown in Scheme 16.14, relates to the mechanism of... 

The bulk of the studies on intramolecular hydroamination of alkenes catalyzed by lanthanide complexes have been conducted using lanthanocene complexes or half-sandwich lanthanide complexes. The prototypical cyclizations of aminoalkenes to form five- and six-membered rings are shown in Equation 16.61. These reactions occur with exclusive Markovnikov selectivity. These reactions have also been conducted using arylamines, as shown in Equation 16.62. The intramolecular reactions of amines catalyzed by certain lanthanide complexes occur with 1,1- and 1,2-disubstituted olefins (Equation 16.63), although such reactions require high temperatures. 

Perrin L, Eisenstein O, Maron L (2007) Chemoselectivity in o bond activations by lanthanocene complexes from a DFT perspective reactions of Cp2LnR (R = CH3, H, SiH3> with Sia, and CHj-SiHj. New J Chem 31 549-555... 

Trivalent lanthanocene complexes that are isoelectronic (14e, (P) to cationic group 4 metallocenes are also active for olefin polymerization, although their activity is much less than that of Ti and Zr [28,29]. Mashima et al. have reported monocyclopentadienyl rf-diene complexes of tantalum and niobium that are isoelectronic to group 4 metallocenes. These complexes catalyze ethylene polymerization, albeit with low activity [30]. 

Compared to homogeneous catalysts, heterogeneous catalysts have the significant advantage that the catalyst may be easily removed from the reaction mixture and can potentially be recycled. Lanthanocene complexes may be attached to amine-functionalized cross-linked polystyrene supports (2) [104]. The supported catalysts, e.g., 18, displayed activities similar to their homogeneous analogs and could be recycled at least two times with moderate loss of activity (Table 3). The immobilized form of the catalyst is released from the support via transamination by the aminoalkene substrate and the catalytic cycle proceeds then homogeneously in solution. After all the substrate has been consumed, the catalyst can remrn to the polymer support in order to allow catalyst separation and recycle. 

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