Molecular Catalysis of Rare-Earth Elements

The range of the series spans the entire Periodic Table and addresses structural and bonding issues associated with all the elements. Most of the applications deal with homogeneous catalysis but in some cases heterogeneous systems are also mentioned. Although the first organometallic compounds of the lanthanides, which are tris(cyclopentadienyl)lanthanide complexes, were already prepared in the 1950s, it was only in the late 1970s and early 1980s when a number of research groups began to focus on this class of compounds.

One of the first applications in this field was the use of lanthanide metallocenes for the catalytic polymerization of ethylene in the early 1980s. In the past two to three decades, a large number of inorganic and organometallic rare earth compounds have been synthesized and some of them have also been used as catalysts. This book contains four chapters covering some of the recent development of the use of molecular rare-earth metal compounds in catalysis.

Therefore, these two chapters are focused on homogeneous catalysis using lanthanidamidinates and guanidinates (Chapter by Edelmann) and the synthesis of rare earth post-metallocene catalysts with chelating amido ligands (Chapter by Li et al.). The organometallic lanthanide catalysts of the first generation, which are the metallocene catalysts with the general composition Œ.5-C5Me5/2LnR .RDCH.SiMe3/2, N.SiMe3/2, H), are mentioned in the first two chapters but are not covered of a separate synthetic contribution, because a number of excellent reviews on this topic have been published over the past years. Although σ-bond metathesis plays a central role in many rare earth metal-catalyzed polymerizations, the discussion of these processes is beyond the scope of this review and the interested reader can consult one of the associated reviews [21–24].

Fig. 2 Free energy (a) and relative electronic energies (b) for two possible reaction pathways for Cp 2 La-H C CH 4 (a) and rearrangement of Cp 2 La.H/.H-SiH 3 / (b) [15, 16]

Hydrosilylation of Alkenes

4 C1-Symmetrical Chiral Lanthanocene Catalysts for Asymmetric Hydrogenation, Hydrosilylation, and Hydroamination In general, rare earth metal-catalyzed hydrogenation has attracted considerably less attention in the last decade; therefore, the interested reader can consult previous reviews for more comprehensive coverage [19,20]. The “less constrained” geometry catalyst 5 shows a higher catalytic activity in the hydrosilylation of 1-decene than the “more constrained” complexes 4 (Table 1) [51]. However, complex 4b allows a better regiocontrol than 5 in the hydrosilylation of styrene, as the more open coordination sphere allows more facile 2,1-insertion of the styrene.

Note, however, that stoichiometric insertion reactions of styrene into the metal hydride bond give exclusively the 2,1-insertion product [48,64] due to the aryl-directing effect (see below), indicating that the linear product results from an unobserved 1, 2-entry product. Electron-donating substituents in the aromatic ring increase the turnover frequency as well as the regioselectivity, which is consistent with this hypothesis [29]. The selectivity of the hydrosilylation increases with increasing metal ionic radius and opening of the coordination sphere.

The cyclopentadienyl-free chiral yttrium diamidobiphenyl complex 8 was used for the asymmetric hydrosilylation of norbornene with high enantioselectivity. The yttrium silanolate complex 11 shows good catalytic activity with high anti-Markovnik regioselectivity in the hydrosilylation of 1-decene (Table 1) [59]. The sterically encumbered guanidinato complex 14b exhibits one of the highest catalytic activities among cyclopentadienyl-free hydrosilylation catalysts (Table 1) [61].

Fig. 5 Catalytic cycle for organo-rare-earth metal-mediated hydrosilylation [29, 34,35]

Hydrosilylation of Alkynes

Hydrosilylation/Carbocyclization

Postmetallocene catalysts can also facilitate this transformation, although they are generally less selective and a significant amount of acyclic hydrosilylation product can be formed if the rate of silylation of the initial alkene insertion product A (Fig. 9) is competitive with the rate of carbocyclization. Regioselective transformations of 1,6 and 1,7 enynes proceed via preferential insertion of the triple bond (24). 24) To date, the only example of an asymmetric hydrosilylation/carbocyclization sequence of α;ω-hexadienes and heptadienes uses the (R)-BINOL-derived yttrocene catalyst 16 to produce cyclopentanes and cyclohexanes in high yields, but only at low to moderate enantioselectivities. up to 50% ee (25) [75]. Enhanced reactivity of the tin-hydrogen bond results in a significant amount of distannane by-product formed via dehydrogenative σ-bond metathetic coupling.

The utility of the known hydroboration protocol has expanded considerably with the advent of metal-catalyzed processes [78,79]. 11 Proposed mechanism for intermolecular hydroboration of alkenes. The catalytic cycle is believed to be rate-determining, as the significantly higher reactivity of lanthanum compared to yttrium can be attributed to a metathetic transition state with lower σ bonds in the case of the larger La3Cion [81]. This fact can also be interpreted in terms of the fact that the uncatalyzed addition of the borane to the alkene is in some cases competitive with the catalyzed process [82,83].

Carbocyclization of α; of ω-dienes proceeds smoothly in the presence of Cp2Sm(THF) of divalent samaracene, and the metal-alkyl intermediate is blocked by a 1,3-diaza-2-boracyclopentane to afford the cyclic hydroboration product (29). Among the various systems of transition metal and main group hydroamination catalysts, rare earth metal complexes remain one of the most active and selective [88,97].

Fig. 10 Diastereospecific decaline formation

Intramolecular Hydroamination

12 Thermodynamics of the elementary steps in the rare-earth metal-catalyzed hydroamination/cyclization of aminoalkenes (a), aminoalkynes (b), aminoallenes (c) and aminodienes (d) (nD. The subsequent protonolysis of the primary rare-earth alkyl species is quite exothermic , to a lesser extent also for the secondary rare earth alkyl species Protonolysis of the resulting vinyl (in the case of alkynes and allenes) or η3-allyl (in the case of conjugated dienes) rare earth metal species are approximately thermoneutral (for the vinylic species) to slightly endothermic (for the allylic species) due to the significant stabilization of these species.

However, recent DFT analyzes of the catalytic cycle of the rare earth metal-catalyzed hydroamination of dienes and allenes suggest that the protonolysis of the rare earth metal3-allyl species (in the hydroamination of dienes), namely, the vinylic species (in the hydroamination of allenes ), is the rate-determining step [111-113]. As discussed in the previous section, the first step of the catalytic cycle involves insertion of the alkene into the rare earth metal amido bond with a seven-membered transition state (for nD1). Amines, coordinating solvents, and other external bases can adversely affect the reactivity of the rare metal center, especially if the metal center is easily accessible.

C–C triple bond insertion proceeds much faster than double bond insertion due to the exothermic nature of the step (Figure 12). DFT calculations suggest that protonolysis is the rate-determining step of the process [113], although this idea contradicts some experimental observations [143,144].

Fig. 12 Thermodynamics of the elementary steps in rare-earth metal-catalyzed hydroamina- hydroamina-tion/cyclization of aminoalkenes (a), aminoalkynes (b), aminoallenes (c), and aminodienes (d) (n D 1–3) [103–108]

Hydroamination/Carbocyclization

Intermolecular Hydroamination

Molecular weight determinations indicated a low polydispersity (Mw=MnD1–1:5), the formation of only one polymer chain per neodymium and the linear increase of the degree of polymerization with the conversion, as observed for living polymerization. They explained that the steric hindrance and C2 symmetry of the (NCNdipp/Y unit made isoprene coordinate to the Y center preferentially in a 3;4-η2 manner, leading to the formation of isotactic 3,4-polyisoprene (Scheme 15); when 5 equiv.. Detailed study revealed that the Mn of the polymer increased linearly with the conversion and that the polydispersity remained narrow.

Thus, until about 30 years ago, organometallic compounds of rare earth metals remained a curiosity. At this point, it should be emphasized that the organometallic chemistry of the lanthanide (and actinide) elements differs significantly from the chemistry of the ted-transition metals [4,5]. After the early discovery of tris(cyclopentadienyl) complexes of lanthanide elements by Wilkinson and Birmingham [1], most of the organolanthanide compounds studied were sandwich-type complexes containing unsubstituted or ring-substituted cyclopentadienyl ligands [3].

The first lanthanide amidinate complexes ever reported in the literature were homoleptic tris(amidinates) of the type ŒRC6H4C.NSiMe3/23Ln (Scheme 10), which can be regarded as the amidinate analogs of the homoleptic lanthanide tris(cyclopentadienyl) complexes Cp3Ln [6,7, 22]. A large number of such complexes were synthesized and fully characterized in the course of the early studies. These represent a new family of Lewis base-free hydrido complexes of the rare earth elements.

In this compound only one of the two NMe2 functionalities is coordinated to the metal center [6,7]. Further reaction of the resulting monomeric chloro complex with NaCp in DME afforded LYbCp(DME) in high yield (Scheme47, CpD5-cyclopentadienyl) [6,7,43]. Based on the initial region of the polymerization kinetic curve measured by the dilatometric method (up to the conversion rate of 20%), the Sm complex exhibited higher activity compared to the Nd analogue [35,36].

It was proposed that a cooperation of the lanthanide and lithium metals in this process would contribute to the high activity of these catalysts [66,67]. All three possible oxidation states (C2; C3 and C4) of the lanthanide ions can be stabilized using amidinate ligands. Summary This overview deals with the synthesis and catalytic application of non-cyclopentadienyl complexes of rare earth metals.

In recent years, the number of contributions to the chemistry of lanthanide metal amides has increased dramatically. The focus of this review is on the synthesis of catalytically active post-metallocene lanthanide complexes. The catalytic activity of the complexes in the polymerization of lactides or isoprene was investigated [48,49].

Studies on the catalytic activity of anilido-phosphinimine rare-earth complexes were reported exclusively by Cui and colleagues.

Fig. 21 Anti-Markovnikov hydroamination of vinyl arenes [100]