Beta-hydride elimination

Beta-hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. [Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6] 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 an unproductive side reaction. The Shell higher olefin process relies on beta-hydride elimination to produce alpha-olefins which are used to produce detergents. Illustrative of a sometimes undesirable beta-hydride elimination, beta-hydride elimination in Ziegler-Natta polymerization results in polymers of decreased molecular weight. 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.

In some cases, beta-hydride elimination is the first in a series of steps. For instance in the synthesis of RuHCl(CO)(PPh₃)₃ from ruthenium trichloride, triphenylphosphine and methoxyethanol, an intermediate alkoxide complex undergoes 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.

Avoiding β-hydride elimination

There are several strategies for avoiding β-hydride elimination. The first is for the alkyl ligand that lacks a beta-hydrogen in the first place such as methyl or neopentyl. Beta-hydride elimination is inhibited when the reaction would produce a strained alkene. This situation is illustrated by the stability of metal complexes containing four norbornyl ligands. [Barton K. Bower and Howard G. Tennent "Transition metal bicyclo [2.2.1] hept-1-yls"Journal of American Chemical Society 1972, Volume 94, pp 2512 - 2514; DOI: 10.1021/ja00762a056] Alternatively, the beta position may be blocked by non-hydrogen atoms. Fluorine is not suitable because metal-fluorine bonds are often strong, thus the abstraction of fluoride is thermodynamically favorable.

Bulky alkyl ligands, such as t-butyl or trimethylsilyl, the hydrogen atom may not be able to approach a coplanar configuration with respect to the metal, and the α and β atoms.

If the metal center does not have empty coordination sites, for example, by the complex already have 18 electron configuration, β-hydride elimination is not possible as well.

In some cases, the coligands can impose geometries that inhibit beta-hydride elimination. For the above example, the unwanted beta-hydride elimination is prevented by using 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'-diphosphinoferrocenes 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.

References