C2 Building blocks in the co-ordination sphere of electron-poor transition metals. Aspects of the chemistry of early-transition-metal carbenoide complexes

Abstract

Our understanding of the chemistry of the transition-metal–carbon σ bond is improved by investigations of the chemistry of alken-1-yl complexes of electron-poor transition metals. There is no other system known in which we can easily switch between the possible reaction pathways, depending on the nature of the metal, the ligands L and the alken-1-yl group. Only reductive elimination, α- and β-H elimination reactions give high selectivity. α-Hydrogen elimination from Cp* ₂ Ti(CHCH ₂ )R (Cp* = η-C ₅ Me ₅ ) derivatives leads to the versatile titana–allene intermediate [Cp* ₂ TiCCH ₂ ] 8. A wide range of cycloaddition products of high thermal stability can be prepared using 8. In reactions of 8 with copper and gold complexes, heterodinuclear µ-vinylidene compounds, Cp* ₂ Ti(µ-CCH ₂ )(µ- X)M′L, are formed. Additionally the first examples of intermolecular carbene–carbene coupling reaction of a Fischer- and a Schrock-carbene ligand are reported by using the strong nucleophilic vinylidene fragment 8.