β-Hydride Elimination

β-hydride eliminations are used to form alkenes from metal-alkyll complexes. This reaction involves the formation of a metal hydrogen bond and the formation of a π bond. The hydride will attack the empty orbital on the metal center only if the M-C_α and C_β-H bonds are syn coplanar, or on the same side of the C_α-C_β bond.

The metal donates electrons while also recieving electrons from the leaving group, giving the metal a Π ligand. The Π bonf is formed as well as a metal-hydrogen bond.

Requirements

• Open coordination site
• Empty orbital on metal center
• M-C_α and C_β-H  must be able to align syn coplanar
• Complex must have 16 or less electrons since the total electron count will increase by 2

Importance

• β-hydride eliminations allow for the formation of a metal-hydride and alkene.
• This reaction is used in the synthesis of hydrocarbons in many reactions such as;
  • olefin polymerization
  • hydroformylation
  • Heck and Suzuki reactions
  • alkene metathesis
• They are often reversible.
• β-hydride elimination is an important step in the catalytic cycle as it forms alkenes and regenerates the metal catalyst that allows reactions to continue occuring. 

Mechanism

Stereospecificity

β-hydride elimination is stereospecific. Diastereomers will lead to an E and Z olefin.

Examples

Alkoxide can lose a beta hydrogen to become a ketone or aldehyde