Lanthanocene catalyst

Due to its marked atom economy, the intramolecular hydroamination of alkenes represents an attractive process for the catalytic synthesis of nitrogen-containing organic compounds. Moreover, the nitrogen heterocycles obtained by hydroamination/cyclisation processes are frequently found in numerous pharmacologically active products. The pioneering work in this area was reported by Marks et al. who have used lanthanocenes to perform hydroamination/cyclisation reactions in 1992. These reactions can be performed in an intermolecular fashion and transition metals are by far the more efficient catalysts for promotion of these transformations via activation of the... 

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. 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

Without the third component no activity at all was observed. They proposed a mechanism similar to the one given by Yasuda et al. [216, 217] for polymerization of methylmethacrylate by lanthanocenes which are isoeleetronie with alkylzirconocenium ions. The role of the third component in this mechanism is not very clear. Nevertheless polymerization of polar monomers by metallocene catalysts is an open field of research and investigations are just beginning. 

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]. 

A comprehensive review on lanthanocene catalysts in selective organic synthesis has been published by Molander and Romero.37 Petrov et al,39 have reviewed the synthesis, structure, and reactivity organolanthanides RLnX (R = alkyl, aryl X = halogen) and lanthanide compounds with aromatic hydrocarbon dianions. 

Reactions of intramolecular hydroamination of alkenes, alkynes, and allenes with the formation of A-heterocycles in the presence of lanthanocene catalysts 02CRV2161. 

Fig. 4 Cl-symmetric chiral lanthanocene catalysts for asymmetric hydrogenation, hydrosUylation, and hydroamination [27-29]...

Pioneering preparative reactions have been performed with lanthanocene catalysts [40 3], with yttrium complexes exhibiting the highest reactivities and selec-tivities in most cases that allow reactions to be performed at ambient temperatures (7) [44],... 

The resting state of the catalyst is believed to be an amine adduct of the catalytic active Ln-amide. For lanthanocene catalysts such an amido amine species of the type Cp 2Ln(NFiR)(NH2R) has been spectroscopically and crystallographically characterized [103]. Amines, coordinating solvents and other external bases may adversely affect the reactivity of the rare-earth metal center, in particular if the metal center is readily accessible. Sterically open anya-lanthanocenes and constrained-geometry catalysts (CGC) [27,104,108] and more recently also sterically readily accessible nonmetallocene catalysts [101,115,116] have displayed product inhibition (leading to apparent first-order kinetics) or substrate inhibition (resulting in self-acceleration). 

Initial studies focused on lanthanocene-based catalyst systems that proofed to be efficient in the exo-specific cyclization of terminal aminoalkenes to form five-, six-, and seven-membered azacycles (Scheme 2). The reactions are predictably faster for the formation of smaller five-membered rings and in the presence of em-dialkyl substituents [117]. An increasing metal ionic radius and a more open coordination sphere, for example, in onra-lanthanocenes, are also beneficial for higher cyclization rates [103]. A further increase in catalytic activity is observed when sterically more open and more electrophilic CGC 17 (Fig. 15) are applied [118]. 

Although lanthanocene catalysts initially developed for aminoalkene hydroami-nation are highly sensitive and not readily available, their catalytic activity remains unsurpassed as of now and only a few postmetallocene rare-earth metal complexes can reach comparable levels of catalytic efficiency. Besides constrained-geometry (Fig. 15) [118, 119] and other half-sandwich [102, 120, 121] rare-earth metal complexes, a large number of cyclopentadienyl-free catalyst systems have been... 

Enantioselective cyclizations of amino-octadiene with chiral lanthanocene catalysts provide a facile access to (-l-)-coniine with 63% ee after hydrogenolysis of the... 

Postmetallocene catalysts such as the bis(benzamidinato)yttrium alkyl 38 (Fig. 25) [201] have been studied with respect to their activity in these alkyne dimerizations as well (55). However, the catalytic activity is significantly lower in comparison to the lanthanocenes, and the catalyst is not applicable to nonhindered alkynes such as propyne. Larger substituents (e.g., Ph, Bu) as well as elevated temperatures are required [201]. 

The lanthanocene catalysts display high catalytic activity, which is proportional to the ionic radius of the rare earth metal (Table 11.1). The rate of cyclization depends on the ring size of the azacyclic product (5 > 6 7) and the presence of rate... 

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