Rare Earth Metal Based Catalysts

Chiral rare earth metal based catalysts for the asymmetric hydroamination/ 

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 

Internal 1,1 or 1,2 disubstituted olefins 26 and 28 are much less reactive for hydroamination and require significantly harsher reaction conditions [39, 41 44]. The formation of pyrrolidines and piperidines often proceeds at comparable rates (Eq. 11.7), contrasting the general trend of significantly faster five membered ring formation observed with terminal aminoalkenes [39]. Despite these harsh reaction conditions, moderate enantioselectivities of up to 58% ee at 100 C (up to 68% at 60 °C) were observed. 

Equation 11.7. Enantioselective hydroamination/cyclization of internal aminoalk enes [39]. 

Significant progress in this area has been made since 2003 utilizing noncyclo 

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Figure 5.12 Schematic presentation of the backbiting coordination involving penultimate and pen-penultimate cis- 1,4-butadiene units in butadiene/styrene copolymerisation with transition metal- and rare-earth metal-based catalysts...

The multidentated-type coordination in the case of the rare-earth metal-based catalysts may arise from 4f and 5d orbital features [225],... 

Explain why polybutadienes obtained with rare-earth metal-based catalysts exhibit a higher degree of stereoregularity (higher contents of cis-1,4 monomeric units) than those derived from polymerisations in the presence of transition metal-based catalysts. 

Fig. 7 Cyclopentadienyl-based postmetallocene rare-earth metal-based catalysts for hydrosilyla-tion [48-52]...

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]. 

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. 

Group 4 metal based catalysts have been studied intensively in hydroamination reactions involving alkynes and allenes [77 81], but (achiral) hydroamination reac tions involving aminoalkenes were only recently reported [82 84]. The reactivity of these catalysts is significantly lower than that of rare earth, alkali, and alkaline earth metal based catalysts. In most instances, gem dialkyl activation [37] of the aminoalk ene substrate is required for catalytic turnover. 

The first chiral rare earth metal-based hydroam-ination catalysts were reported in 1992 using chiral lanthanocene. Organolanthanide complexes catalyze regios-elective intermolecular hydroamination of alkenes, alkynes. 

Diastereoselective cycUzations of chiral aminoalkenes were also achieved for zirconium catalysts (Table 6). Interestingly, the cyclization of primary aminoalkenes gave predominately tran -disubstituted pyrrolidines in accordance to observations for rare earth metal-based hydroamination catalysts [17, 67, 74, 80-82,99,121,122], while the c -diastereomer was favored in case of the secondary aminoalkene. Plausible transition states are shown in Fig. 9. The chair-like transition state leading to the traws-isomer of the primary aminoalkene is less encumbered due to reduced 1,3-diaxial interactions, whereas gauche interactions of the (V-substituent make the c -pyrrolidine the preferred product in case of secondary aminoalkenes. 

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. 

The Oxidative Transformation of Methane over the Nickel-based Catalysts Modified by Alkali Metal Oxide and Rare Earth Metal Oxide... 

We expected to control the direction of OTM reaction over NiO by sur ce modification, namely making use of the interaction between NiO and other conq>onents to beget a synergistic effect. In this paper, two completely different behaviors of the oxidative transformation of methane were performed over the nickel-based catalysts because of the different modifications by alkali metal oxide and rare earth metal oxide and the different interactions between nickel and supports. Furthermore, the two completely different reactions were related with the acid-base properties of catalysts and the states of nickel present. 

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-... 

As for heterogeneous olefin polymerization catalysis, the activity of rare-earth metal catalysts may be also enhanced in organic transformations by the use of silica supports or other carriers [7]. Indeed, several catalytic C-C and C-X (with X = H/D, Si, O) bond formation reactions as weU as functional group transformations witness to the potential of SOLn/AnC-based heterogeneous catalysts for fine chemical synthesis. 

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