Elucidation of Imamoto’s Organocerium Reagent

Abstract

In 1984 Tsueno Imamoto published a paper about the binary mixtures CeCl3/RLi (R = nBu, tBu) and their use in carbon-carbon-bond formation with various carbonyl compounds. The exact structure of these reagents, however, remained a mystery since then. These mixtures were studied via NMR experiments including 7Li NMR spectroscopy. Moreover, a procedure was developed to isolate organolithium/lanthanide complexes from these reactions. For example, Li3Ln(nBu)6(thf)4 (Ln = Sc, Y, La, Ce, Lu) and [Ce(tBu)4][Li(thf)4] could be accessed and fully characterized. The crucial part of these procedures is a seamless low-temperature syntheses and crystallization sequence, since the products are thermally very unstable. The working techniques were then applied to isolate the MAO species [(AlMe2)2O(thf)]2 from the simple reaction of AlMe3 and water in THF. MAO is the most important co-catalyst in commercial α-olefin polymerization. Although, its structure has remained elusive so far, many model complexes have been synthesized. These complexes, however, require laborious pathways or exotic precursors. Lastly, the field of organolanthanide complexes was revisited. Using the aza crown (1,4,7- Trimethyl-1,4,7-triazacyclononane = Me3TACN) the complexes (Me3TACN)Ln(nBu)3 (Ln = La, Sm) could be synthesized as the first terminal n-butyl lanthanide complexes, that contain no other metal. Further, the hitherto elusive lanthanum neosilyl complex [Li(thf)4] [La(CH2SiMe3)4(thf)] was isolated and fully characterized.