Lithium Halogen Exchanges

Lithium Halogen Exchanges - Myers RLi + R'X RX + R'Li...

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I I Li I Li t -BuLi I CH 3 Br I Br H H OEt 1.1 eq n -BuLi Li H H OEt OH CH 3 CH 3 Myers RLi + R'X RX + R'Li • Lithium-halogen exchange reactions are kinetically controlled. The position of the equilibrium varies with the stabilities of the carbanion intermediates involved (sp >> sp 2 >> sp 3 ). n- PrLi + PhI PrI + PhLi 1:10,000 + + K eq <<1 + LiI • In the above example, internal trapping of the newly formed alkyllithium reagent by alkylation drives an otherwise unfavorable exchange reaction. • Alkyliodides are more reactive than the corresponding bromides. Alkylchlorides are essentially inert. Lithium-Halogen Exchange • Lithium-halogen exchange of vinyl halides is stereospecific, proceeding with retention of configuration. Chem 215 • Lithium-halogen exchange reactions using -BuLi typically employ two or more equivalents of -BuLi. The first equivalent is used for the exchange and the second equivalent reacts with the -BuI produced, to form isobutene, isobutane, and lithium iodide.
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Lithium Halogen Exchanges - Myers RLi + R'X RX + R'Li...

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