Rare earth metal based catalyst systems

This limitation of chiral cyclopentadienyl-based hydroamination catalysts has stimulated the development of a large number of cyclopentadienyl-free rare earth metal-based catalyst systems [67, 68, 73,121, 122, 215-239]. A detailed discussion of the large number of catalytic systems is beyond the scope of this review and the interested reader should refer to one of the comprehensive reviews on this topic [9-14]. Some prominent catalyst systems are shown in Fig. 15 and a brief survey of catalytic results is listed in Table 14. 

The cationic aminophenolate complex (5)-75 readily cyclizes secondary amino-alkenes with enantioselectivities of up to 82% ee (Table 19, entries 1-3) [62], For catalyst solubility reasons, reactions are conunonly performed in bromobenzene and require reaction temperatures of 100°C and catalyst loadings of 10 mol%. The mechanism of this cationic system is thought to proceed similar to the a-bond metathesis mechanism of rare earth metal-based catalyst systems (Scheme 2) [61,62]. 

The development of group-4-metal-based catalysts for intramolecular hydro-amination of aUcenes has also led to several advanced systems for asymmetric hydroamination (Fig. 19). Most group 4 metal catalyst systems exhibit inferior reactivity and substrate scope (Table 19) in comparison to most rare earth metaland alkaline earth metal-based catalyst systems. They typically require high catalyst loadings and elevated reaction temperatures. However, the recent development of zwitterionic zirconium catalysts with significantly improved reactivities and selectivities [60, 118] promises to close this gap. 

Even though Ziegler catalysts have been known for almost half a century, rare earth metals (Ln), particularly neodymium (Nd)-based Ziegler catalyst systems, only came into the focus of industrial and academic research well after the large scale application of titanium, cobalt and nickel catalyst systems. As a direct consequence of the late recognition of the technological potential of rare earth metal Ziegler catalysts, these systems have attracted much attention. 

Related to these catalysts are the systems based on lanthanide metal systems or rare earth metal complexes [46, 47]. The main problem with these catalyst systems is their instability. When the catalyst solution is prepared by reachng a metallocene with an organolithium compound in a polar solvent, the prepared catalyst soluhon is unstable and decomposes quickly, even under a nitrogen atmosphere. The activity of these catalysts can be high only if the catalyst is added to the polymer soluhon immediately after preparation. Attempts have been made to overcome the stability problem by using an additive in the system to improve the stability and the activity of the catalyst [33-35, 41, 57, 58, 61]. Re-... 

Beyond this exclusive lanthanide Ziegler-Natta model, Ziegler-type multicomponent systems ( Mischkatalysatoren ) represent the only class of homogeneous rare-earth metal catalysts of considerable commercial relevance [40-43]. High-czs-1,4-polydienes are industrially produced from 1,3-dienes (butadiene and isoprene) in aliphatic or aromatic hydrocarbons by a number of Mischkatalysatoren based on the transition metals titanium, cobalt, and nickel, and the lanthanide element neodymium [40-47]. The... 

Numerous binary and ternary diene polymerization initiator systems with neodymium as the rare-earth metal component have been designed empirically and investigated since the early discoveries in the 1960s. Commercially used neodymium-based catalysts mostly comprise Nd(III) carboxylates, aluminum alkyl halides, and aluminum alkyls or aluminum alkyl hydrides [43, 48,50-52]. Typically, the carboxylic acids, which are provided as mixtures of isomers from petrochemical plants carry solubilizing aliphatic substituents R. They are treated with the alkylaluminum reagents to generate the active catalysts in situ (Scheme 11). 

Heterogeneous diene polymerization catalysts based on modified and unmodified silica-supported lanthanide complexes are known as efficient gas-phase polymerization catalysts for a variety of support materials and activation procedures (see Sect. 9). Metal siloxide complexes M(()SiR3 )x are routinely employed as molecular model systems of such silica-immobilized/ grafted metal centers [196-199]. Structurally authenticated alkylated rare-earth metal siloxide derivatives are scarce, which is surprising given that structural data on a considerable number of alkylated lanthanide alkoxide and aryloxide complexes with a variety of substitution patterns is meanwhile available. 

The f-transition metal catalysts were first described by von Dohlen [98] in 1963, Tse-chuan [99] in 1964 and later by Throckmorton [100]. In the 1980s Bayer [14] and Enichem [101] developed manufacturing processes based on neodymium catalysts. The catalyst system consists of three components [102] a carboxylate of a rare earth metal, an alkylaluminum and a Lewis acid containing a halide. A typical catalyst system is of the form neodymium(III) neodecanoate/diisobutylaluminum hydride/butyl chloride [103]. Neodymium(III) neodecanoate has the advantage of very high solubility in the nonpolar solvents used for polymerization. The molar ratio Al/Nd/Cl = 20 1 3. Per 100 g of butadiene, 0.13 mmol neodymium(III) neodecanoate is used. With respect to the monomer concentration, the kinetics are those of a first-order reaction. 

As already mentioned, there has been significant progress in the development of chiral catalysts for asymmetric hydroamination reactions over the last decade. However, significant challenges remain, such as asymmetric intermolecular hydro aminations of simple nonactivated alkenes and the development of a chiral catalyst, which is applicable to a wide variety of substrates with consistent high stereochemical induction and tolerance of a multitude of functional groups as well as air and moisture. Certainly, late transition metal based catalysts show promising leads that could fill this void, but to date, early transition metal based catalysts (in particular, rare earth metals) remain the most active and most versatile catalyst systems. 

Some catalyst systems based on rare earth metal complexes, such as Nd(P507)3/H2O/TIBA (P507 = RP(0R)(0)0-, R = CH3(CH2)3CH(Et)CH2-), are also active for styrene polymerization and give mixtures of atactic and isotactic polystyrenes (Tn, = 220 C). ... 

In contrast to the large number of publications on group 4-based catalysts for syndiospecific styrene polymerization, reports on analogous rare-earth metal catalysts are stiU hmited [5], Early investigations on neodymium-based sandwich complexes (1-3) activated by methylaluminoxane (MAO) (Fig. 7.1) have shown that these systems are promising candidates for styrene polymerization catalysis with the resulting polymers syndiotactically enriched. The selectivity as well as the activity of these systems are moderate [6]. 

In Sect. 2.1, heterobimetallic transition metal (Cu)/rare earth metal (Sm) system with Lewis acid/Brpnsted basic properties was introduced. By suitably selecting metal combinations depending on the targeted reactions, variety of chiral bimetallic Lewis acid/Br0nsted base bifunctional catalysts could be created, such as aPd/La/la... 

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