Allyl-lanthanide bond

The allyl group is able to form both a- and jr-bonded complexes with the actinides. The 71 complexes will be considered here because of the similarities of the homoal-lyls with the lanthanide and actinide homoalkyls. The limiting modes of bonding in metal allyl complexes and the ratio of PMR intensities from magnetically equi valent protons are illustrated in Fig. 14. 

The corresponding lanthanide allyl, Sm(C5H5)2(allyl), has recently been reported and preliminary indications, based upon the absence of infrared absorptions in 1610—1640 cm i region, are consistent with a jr-bonded structure 149). Since the jr-bonded structure would be formally eight coordinate and the a-bonded structure only six coordinate, this would be the predicted ground state. 

Lactones, via indium compounds, 9, 686 Lactonizations, via ruthenium catalysts, 10, 160 Ladder polysilanes, preparation and properties, 3, 639 Lanthanacarboranes, synthesis, 3, 249 Lanthanide complexes with alkenyls, 4, 17 with alkyls, 4, 7 with alkynyls, 4, 17 with allyls, 4, 19 with arenes, 4, 119, 4, 118 and aromatic C-F bond activation, 1, 738 bis(Cp ), 4, 73... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Reaction of tetranuclear lanthanide octahydrides with styrene provides lanthanide benzylic allyl heptahydride complexes through the insertion of a styrene molecule into one Ln-H bond. The lanthanide benzylic allyl complexes can be considered as the intermediates of styrene hydrogenation. Indeed, both the lanthanide octahydrides and the lanthanide benzylic allyl complexes can catalyze styrene hydrogenation efficiently in the presence of H2 [89]. Lanthanide hydrides react with 1,3-cyclohexadiene to form lanthanide allylic complexes via 1,4-addition [90]. However, these lanthanide hydride clusters can not catalyze the polymerization of styrene and 1,4-cyclohexadiene. 

Lanthanide (III) triflates (Ln(OTf)3) react with three equivalents of the l,3-bis(trimethylsilyl)allyl anion to produce neutral, solvated tris(allyl) species [l,3-(SiMe3)2C3H3]3Ln(thf) (Ln = Ce, Nd, Tb). The neodymium complex has three rf -coordinated allyl ligands, with an approximate threefold axis through the Nd-0 bond (11) Nd-C distances range from 2.64 to 2.78 A. 

In transition metal complexes, where the 18-electron rule is essentially obeyed, the bonding mode of the allyl ligand is mainly determined by the electronic requirements of the metal. However, this does not appear to be the case in the rare earth complexes. Instead, the bonding mode may be largely influenced by steric requirements. In the case of uranium, the coordination site necessary for w-bonding is highly constrained relative to that in the lanthanide complexes. 

It is generally accepted that, in the polymerisation of dienes on lanthanide catalysts, the growing chain is attached to the transition metal by an 7t-allyl bond and that the chain growth occurs by incorporation of the monomer via the metal-carbon o-bond. In the case of neodymium catalysts, the delocalised 7t-allyl type structure of the terminal unit has been observed by spectroscopic methods [8, 26, 28, 58-60]. The results reported in these papers show that the relative contents of cis-l,A- and tri2ns-1,4-units in polydienes depend on the type of solvent used, the polymerisation temperature, structure of diene monomer, and the composition of lanthanide-based catalysts. These data can be interpreted in terms of the concept of isomerisation equilibrium between anti- and syn-forms of n-allyl terminal unit. One of the arguments in favour of the existence of this isomerisation... 

The assumption about the bimetallic bridge structure of lanthanide catalytic systems was made in many works [66-69]. Nevertheless, the possibility must not be ruled out that active centres contain both types of bonds (tt-allyl and a-bridge). It is important that these bonds may differ dramatically in reactivities. In order to answer the question concerning the possible coexistence of two types of bonds, the polymerisation of butadiene on catalytic systems NdCl3 3L-AlR3, where R is /-C4H9 Ln is Nd or Tb L is TBP, prepared in the presence of a small amount of butadiene and piperylene [71, 72] was investigated. Table 3.3. 

The hydroamination of olefins has been shown to occur by the sequence of oxidative addition, migratory insertion, and reductive elimination in only one case. Because amines are nucleophilic, pathways are available for the additions of amines to olefins and alkynes that are unavailable for the additions of HCN, silanes, and boranes. For example, hydroaminations catalyzed by late transition metals are thought to occur in many cases by nucleophilic attack on coordinated alkenes and alkynes or by nucleophilic attack on ir-allyl, iT-benzyl, or TT-arene complexes. Hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of an olefin into a metal-amide bond. Finally, hydroamination catalyzed by dP group 4 metals have been shown to occur through imido complexes. In this case, a [2+2] cycloaddition forms the C-N bond, and protonolysis of the resulting metallacycle releases the organic product. 

The lanthanides and actinides also form a wide range of complexes with unsaturated organic ligands including allyls, cyclopentadienyls and cycloocta-tetraene derivatives. The bonding is strongly polar. Coordination numbers are determined largely by steric requirements around the metal centre. 

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