General Trends for a- and -Eliminations

By far the most common elimination is p-hydride elimination. One example is given in Eq. 12.35. These reactions are very exothermic because a weak M-C bond and a C-H bond are converted to two stronger bonds overall an M-H bond and a C-C n bond, along with coordination of the IT bond to the metal. This reaction can occur whenever a hydrogen is P to the metal and the metal has an open coordination site. 

P-Hydride elimination is preceded by a bridging complex where the hydrogen to be eliminated is weakly coordinated to the metal. This coordination is referred to as an agostic interaction. Sometimes these bridging species are actually isolated, and one example is shown in the margin. In this complex the Ti—H distance is only 2.29 A, and the Ti-C-C bond angle in the ethyl group is only 86°. 

P-Alkyl elimination is much less common than hydride elimination. This is because it is difficult to have an alkyl group form an intramolecular interaction with the metal, as is required prior to the elimination, and the reaction is now sometimes endothermic because an M-C and a C-H bond are traded for another M-C bond and a C-C -it bond. However, P-alkyl elimination can occur. For example, Eq. 12.36 shows an example involving Lu, where 

CHAPTER 12 ORGANOTRANSITION METAL REACTION MECHANISMS AND CATALYSIS 

With highly electrophilic early transition metal complexes, a-hydride elimination is a common reaction. If other alkyl groups are present on the metal center besides the group undergoing elimination, the facile nature of reductive elimination will lead to formation of an alkane. An example of this is given in Eq. 12.38. Whether a- or p-hydride elimination occurs, neopentane is the product that arises from reductive elimination of a neopentyl ligand with a hydride ligand. 

---