Organometallic Compounds: Chapter 11

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Organometallic CompoundsChapter 11
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Organometallic Compounds

• Most metals are less electronegative than carbon
• In general a carbon bonded to a metal is
  nucleophilic and carbanion-like (C)
• Four major classes of organometallic compounds
  are
• Organolithium compounds
• Grignard reagents
• Cuprates
• Organopalladium compounds

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Grignard Lithium Reagents

• Grignard and lithium Organometallic reagents made
  by reacting a halide with elemental metal
• Mg essentially slides into C-halogen bond
• Li simply replaces halogen

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Grignard Lithium Reagents

• Although the carbonmetal bonds are not
  completely ionic, organolithium compounds and
  Grignard reagents react as if the carbon portion
  were a carbanion
• Consider the reaction of a Grignard reagent with
  an ethylene oxide
• Ethylene oxide is a good way to extend a carbon
  chain by two carbons via use of halide to make
  organometallic cpd with negative charge on C

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Gilman Reagents

• Gilman reagents, also called organocuprates, are
  prepared from the reaction of an organolithium
  reagent with copper(I) iodide in diethyl ether or
  THF

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Gilman Reagents

• When a Gilman reagent reacts with an alkyl halide
  (except F-) one of the alkyl groups replaces the
  halide
• Alkyl groups can substitute halogens attached to
  alkene or aromatic C with Gilman reagent
  impossible with SN1 or SN2 reaction
• Mechanism unknown, probably radical

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Suzuki and Heck Reactions

• Various palladium catalysts substitute ordinary
  unfunctionalized alkenes for Br, I, or triflate
  (OTf) leaving group attached to benzene or alkene
  (Heck).
• Reagent can even be made to substitute alkyl
  groups if a dialkoxyalkylborane reagent is used
  with it (Suzuki).

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Grubbs Schrock Metathesis

• Terminal alkenes couple eliminating ethylene with
  rhodium catalyst (Grubbs).
• Terminal alkynes couple eliminating acetylene
  (ethyne) using molybdenum or tungsten catalyst
  (Schrock).

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Retrosynthetic Analysis Using Ethylene Oxide (EO)

• Product has 2 extra Cs plus CN
• 2 extra Cs mean use EO
• Always work back to an ROH for EO analysis
• Retro EO addn removes 2 Cs and an OH
• Here CN has subd for an ROH-derived LG (leav
  grp)
• Make OTs the LG derive the ROTs from ROH
• Cyclohexyl C- attacks EO to make this ROH