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-Cα and Cβ-H bonds are syn coplanar, or on the same side of the Cα-Cβ 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-Cα and Cβ-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