Organometallic compounds as intermediates

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Organometallic compoundsas intermediates
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Common metals

• Magnesium (Grigard reagents)
• Lithium
• Mercury
• Zinc
• Lithium cuprates (Gilman reagents)
• Less common, not dicussed here
• Cadmium
• Thallium
• Aluminum

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Other Organometallics

• Many C-metal bonds observed in catalytic
  processes, not isolated, often transient
• Other organometallics not usually used as
  intermediates (e.g. ferrocene)
• Organometallics discussed here are used as
  stochiometric quantity intermediates, are
  reactive, sigma bonded

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Electrostatic Potential Maps
Electrostatic potential maps show calculated
charge distributions Colors indicate
electron-rich (red) and electron-poor (blue)
regions
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Reactions of Alkyl Halides Grignard Reagents

• Reaction of RX with Mg in ether or THF
• Product is RMgX an organometallic compound
  (alkyl-metal bond)
• R is alkyl 1, 2, 3, aryl, alkenyl
• X Cl, Br, I
• Other common solvents other ethers

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Grignard Reagents

• Prepared by halogen metal exchange
• Easy to prepare
• Not very air sensitive
• Works for both aromatic and aliphatic halides
  (except F)
• Mg insertion not fully understood probably
  radical process

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Uses of Grignards
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Organolithium reagents

• Of type R-Li
• By metal-halogen exchange or metal-hydrogen
  exchange
• Very air sensitive, very reactive

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R-Li Preparation
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Gilman Reagents

• Alkyllithium (RLi) forms from RBr and Li metal
• RLi reacts with copper iodide to give lithium
  dialkylcopper (Gilman reagents)
• Lithium dialkylcopper reagents react with alkyl
  halides to give alkanes

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Use of Gilman Reagents
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Incompatibilities of Organometallics
(Mg, Li)

• Will react with
• Anything thats acidic (even low acidity!) e.g.
  alcohols, prim. amines, terminal alkynes
• Anything that has C-heteroatom multiple bonds
• e.g. NN, CO, NO, SO, nitriles

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Some Common Protecting Goups
No protecting groups for many other
functionalities!
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Mercury compounds

• Usually prepared by reacting mercury salts (e.g.
  mercury acetate) with alkenes or aromatics
• Much less reactive (usually not water sensitive)

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Hg CompoundsOxymercuration

• For laboratory-scale hydration of an alkene
• Use mercuric acetate in THF followed by sodium
  borohydride
• Markovnikov orientation
• via mercurinium ion

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Alkoxymercuration of Alkenes

• Demercuration with NaBH4 yields an ether
• Overall Markovnikov addition of alcohol to alkene
  
• React alkene with an alcohol and mercuric acetate
  or trifluoroacetate

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Aromatic mercuration
Application example