C-2 building blocks in the co-ordination sphere of electron-poor transition metals. Aspects of the chemistry of early-transition-metal carbenoide complexes

Our understanding of the chemistry of the transition-metal-carbon sigma 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, alpha- and beta-H elimination reactions give high selectivity. alpha-Hydrogen elimination from Cp*Ti-2(CH=CH2)R (Cp*=eta-C5Me5) derivatives leads to the versatile titana-allene intermediate [Cp*Ti-2=C=CH2]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 mu-vinylidene compounds, Cp*Ti-2(mu-C=CH2)(mu-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.