Β-Hydride elimination

The Shell higher olefin process relies on β-hydride elimination to produce α-olefins which are used to produce detergents. β-Hydride elimination interferes with the Ziegler–Natta polymerization, leading to decreased molecular weight. The production of branched polymers from ethylene relies on chain walking, a key step of which is β-hydride elimination. Nickel- and palladium-catalyzed couplings mainly focus on aryl-aryl couplings. Aryl-alkyl and especially alkyl-alkyl couplings are less successful because of β-hydride elimination can lower the yield. In Hydroformylation, β-hydride elimination can act as a side reaction that influences product regioselectivity. For example, in the hydroformylation of open chain unsaturated ethers, it reverses the formation of branched metal-alkyl intermediates at high temperatures, leading to a greater yield of linear products. β-Hydride elimination is one step in the synthesis of some metal hydrides. For instance in the synthesis of RuHCl(CO)(PPh3)3 from ruthenium trichloride, triphenylphosphine and 2-methoxyethanol, an intermediate alkoxide complex undergoes a β-hydride elimination to form the hydride ligand and the pi-bonded aldehyde which then is later converted into the carbonyl (carbon monoxide) ligand. == Mechanism ==

β-Hydride elimination transforms a metal-alkyl complex into a metal-hydrido-alkene complex. Starting with an unsaturated complex, the transformation proceeds in stages: 1) Dissociation of a ligand from a metal alkyl complex, yielding a coordinatively unsaturated derivative. 2) Alignment of the beta hydrogen. In this step, a vacant site on the metal forms an agostic complex by binding a C-H bond of the alkyl (or alkoxide). 3) Hydride Transfer/Alkene Formation. In this step, the M-H bond forms concomitant with cleavage of a C-H bond and the development of a double bond in what was once an alkyl (or alkoxide) ligand. This was suggested primarily based on the observed order of the L-type ligand in the rate law derived from kinetic studies. This mechanism appears to be operative for the minority of reactions studied. == Structure-reactivity relationships ==

Relative to an arbitrary reference complex, β-hydride elimination is faster in a complex with the following characteristics: • More electron-deficient metal center. This can be as a result of less donating ancillary ligands. • More labile ancillary ligands, such as weakly coordinating (e.g. solvent) or sterically demanding ligands. == Avoiding β-hydride elimination ==

Several strategies exist for avoiding β-hydride elimination. The most common strategy is to employ alkyl ligands that lack hydrogen atoms at the β position. Common substituents include methyl and neopentyl. β-Hydride elimination is also inhibited when the reaction would produce a strained alkene. This situation is illustrated by the stability of metal complexes containing norbornyl ligands, where the β-hydride elimination product would violate Bredt's rule. ==Further reading==