Chem 335 - Winter 2002
Organic Chemistry II
Dr. Carl C. Wamser

Chapter 14 Notes - Organometallics

Nomenclature

• organometallic implies a bond between a carbon and a metal
• named as alkylmetal, e.g., methyllithium or phenylmagnesium bromide

Structure and Bonding

• polar bond with negative carbon
• strongly basic and nucleophilic

Preparations

• from alkyl halides + active metal
  • R-X + 2 Li --> R-Li + LiX
  • R-X + Mg --> R-Mg-X

Reactions - Basicity

• pKa of corresponding conjugate acid (C-H bonds) very high (see table in text)
• highly reactive with water and most other proton sources
• can be used to incorporate D by reaction with D2O

Grignard Syntheses

• Grignard reagents add a new C-C bond and make alcohols
  • alkyllithium reagents react similarly, somewhat more reactive
  • acetylide anions react similarly, making alkynols

• Grignard + formaldehyde --> 1° alcohol
• Grignard + aldehyde --> 2° alcohol
• Grignard + ketone --> 3° alcohol
• Grignard (2 equiv) + ester --> 3° alcohol
• useful synthetic reaction
  a new C-C bond is formed
  builds up a larger carbon skeleton
  
• retrosynthetic analysis
  • visualize carbon framework buildup by disconnections
    
• limitations: Grignard reagents are incompatible with most other polar functional groups
  (OH, COOH, NH2, etc.) and with atmospheric water and oxygen

Coupling Reactions (Dialkylcuprates)

• preparation of Gilman reagents: 2 RLi + CuX --> R2CuLi + LiX
• coupling with alkyl halides: R2CuLi + R'X --> R-R' + RCu + LiX
• useful method for making larger alkanes from smaller pieces
• alkyl halide ( R' ) should be CH3 or 1° (SN2 reaction)
• alkyl cuprate ( R ) should be unhindered ( CH3 or 1° or aryl or vinyl )

Carbenes and Carbenoids

• carbene: divalent C with only 6 electrons (highly electron-deficient)
• carbenoid: carbene-like compound, usually with another molecule complexed
• alpha-elimination: removal of HX from the same carbon
  • CHCl3 + KOtBu --> KCl + tBuOH + [ :CCl2 ] ( dichlorocarbene )
  • dihalocarbene adds to a double bond to make a cyclopropane
• Simmons-Smith reaction
  • CH2I2 + Zn --> I-CH2-Zn-I (carbenoid)
  • carbenoid adds CH2 to a double bond to make a cyclopropane

Transition Metal Organometallics

• 18-electron rule
  • equivalent of the octet rule for the transition metal series (full shell, including d orbitals)
  • ligand electrons surrounding the central metal ion total 18 for best stability
  • ligand exchange reactions can lead to catalysis of many reactions
• metallocenes
  • "sandwich" compounds" with aromatic anion ligands (e.g., cyclopentadienyl anion)
• Ziegler-Natta polymerization catalysts
  • addition of substituted alkenes to make polyalkenes
  • stereoregular polymers can be produced, e.g., isotactic polypropylene