Hydrogen Eliminations and Abstractions

In most cases, the immediate product of a-hydrogen elimination is not observed. Instead, reductive elimination of a hydride and an alkyl group to form an alkane frequently occius (Equation 10.30). In other cases, a carbene forms from a dialkyl complex of a d metal that cannot accommodate the additional valency required to form an alkylidene and a hydride ligand. In these cases, an alternative four-center pathway involving the transition state shown in Equation 10.31 that does not involve formation of a metal-hydride complex is followed. When the valency of the metal allows either pathway to occur, it is difficult to distinguish between the a-elimination and a-hydrogen abstraction pathways.  

Carbene complexes have also been shown to undergo a-hydrogen elimination, in this case to form alkylidyne complexes. This reaction is much less common than the reaction of alkyl complexes to form alkylidenes, but a few examples are well documented. Two examples of this transformation are shown in Equations 10.39 and 10.40. Schrock reported the first direct observation of this transformation (Equation 10.39). ° In this first example, an isolated carbene complex converted to an alkylidyne hydride complex upon abstraction of a chloride ligand with trimethylaluminum. In a second example, a double C-H activation process by two sequential a-hydrogen elimination reactions converts the starting tungsten-methyl complex in Equation 10.40 into a methylidyne complex.  

Ta(=CHCMe3)(dmpe)2CI + AIMea == [Ta(=CCMe3)(H)(dnipe)2](CIAIMe3) 

This section closes with examples of a-hydrogen elimination from amido complexes to form imido complexes. This reaction is described in detail in Chapter 13 (metal-ligand multiple bonds). To illustrate two forms of this reaction. Equations 10.41 and 10.42 are shown here. In these reactions, elimination of arene and elimination of amine occur to generate imido complexes that have been trapped by dative ligands or allowed to imdergo [2-i2] cycloadditions with alkynes. -  

Elimination reactions occur from complexes containing all of the transition metals. The class and rate of elimination reaction depends on the type of reactive ligand, the electronic properties of the ancillary ligands, and the conformational flexibility of the complex. 

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The mechanism or mechanisms are believed to involve formation of an alkyl followed by selective a hydrogen abstraction to give an alkylidene, as shown in Eq. 25, although it is not yet known what factor or factors tip the energetic balance in favor of the alkylidene in these examples. Selective a hydrogen elimination and abstraction have been observed in other circumstances, especially those in which (3 processes are sterically blocked. [135-138] These... 

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