Beta-hydride elimination

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Beta-hydride elimination is an organometallic reaction in which an alkyl group bonded to a metal centre is converted into a metal bonded hydride and an alkene.

The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty ( or vacant) site cis to the alkyl group for this reaction to occur.

The beta-hydride elimination can either be a vital step in a reaction or a tiresome side reaction which is unwanted.

For instance in the synthesis of RuHCl(CO)(PPh₃)₃ from ruthenium chloride, triphenylphosphine and methoxyethanol the alkoxide ligand needs to undergo a beta-hydride elimination to form the hydride ligand and the pi-bonded aldehyde which then is later converted into the carbonyl (carbon monoxide) ligand.

In the case of nickel and palladium catalyzed couplings of aryl halides with alkyl grignards, the beta-hydride elimination can cause the yield to be lowered. As the reaction starts to form alkenes rather than the required product. One method of stopping the unwanted beta-hydride elimination is to use a diphosphine where the two phosphorus atoms are fixed apart in space. One way of doing this is to use a ferrocene unit, the nickel and palladium complexes of 1,1'-diphosphino ferrocenes are arranged such that the metal has two phosphorus atoms in the trans sites. As these metals form square planar complexes, no vacant site cis to the alkyl group can be formed. Hence the beta-hydride elimination is prevented.