Rare‐Earth‐Metal Alkyl and Aryl Compounds in Organic Synthesis

Rare‐earth‐metal (Ln) alkyl and aryl compounds feature highly reactive, thermally labile [Ln−C] σ‐bonds which result in rapid and violent decomposition in the presence of air and moisture. Nevertheless, such [Ln−C] moieties display important intermediate species in numerous organic transformations. Reagents containing [Ln−C] bonds are deliberately generated in situ like Imamoto‐type reagents Ln(III)Cl 3 /RLi (routinely at low temperatures) or “heavy” lanthanide‐Grignard compounds RLnX. Samarium(III)‐alkyl species are supposed intermediates of SmI 2 ‐promoted Barbier‐type carbon–carbon coupling reactions. Alkyl/aryl ligand exchange at Ln(III) centers has been identified as the crucial step of lanthanide–halogen‐exchange reactions. In the meantime, several such mixed alkyl/halogenido complexes, devoid of an ancillary ligand, could be isolated and scrutinized with regard to structure and reactivity. More recently, Ln(III)‐alkylidene complexes could be accessed, structurally authenticated and successfully employed in Tebbe‐like olefination reactions of ketones.