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OrganoMagnesium Compounds

OrganoMagnesium Compounds. Peter H.M. Budzelaar. Organo-Mg and Be compounds. Like Organolithium compounds, but milder : Ionic character, electron-deficient oligomeric or polymeric structures Basic and nucleophilic Mg: applications in organic synthesis (Grignard reagents !)

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OrganoMagnesium Compounds

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  1. OrganoMagnesium Compounds Peter H.M. Budzelaar

  2. Organo-Mg and Be compounds Like Organolithium compounds, but milder: • Ionic character, electron-deficient • oligomeric or polymeric structures • Basic and nucleophilic • Mg: applications in organic synthesis (Grignard reagents !) • Be: extremely toxic, not used • Reducing • sometimes unwanted side reactions Organo Ca, Sr, ... compounds: ionic, insoluble, not used OrganoMagnesium Compounds

  3. Grignard reagents RMgX • Direct synthesis ("Grignard reaction"): • Mechanism is not simple! • almost certainly involves SET (Single Electron Transfer) • very sensitive to traces of impurities (Na, K, transition metals) in the Mg(vendors sell special "Grignard quality" Mg containing 1-2% Na/K) • nature of the Mg surface is important(for less reactive halides, surface activation is essential: I2, BrCH2CH2Br, MeI or HgCl2) OrganoMagnesium Compounds

  4. Grignard reagents RMgX • Reactivity: I > Br > Cl >> F • Selectivity: I gives largest amounts of side products • In ethers: • In Et2O cleaner than in THF, but also slower • Also in ether/hydrocarbon mixtures • Side reactions: • Wurtz coupling, especially for allylic halides • Radical rearrangements OrganoMagnesium Compounds

  5. Grignard reagents RMgX • Main application: addition to polar C=X bonds • C=O, C=N, C≡N • Sometimes radical mechanism(especially for aromatic ketones): • same product expected, but possibly different side products OrganoMagnesium Compounds

  6. Grignard reagents RMgX • Side reaction in C=X addition: reduction through b-H transfer • only for alkyls having a b-hydrogen... OrganoMagnesium Compounds

  7. Grignard reagents RMgX "Barbier reaction": • Selectivity differs from that of Grignard addition • Probably radical mechanism • Works even in water ! OrganoMagnesium Compounds

  8. The Schlenk equilibrium • Grignard reagents are usually mixturesof many compounds in dynamic equilibrium • This is often written as the Schlenk equilibrium but the situation is really more complicated • Crystal structures may not be representativeof species in solution • The equilibrium can be shifted through addition of ligands: OrganoMagnesium Compounds

  9. Structure of [MeMgCl(tmeda)]2 OrganoMagnesium Compounds

  10. Structure of MeMgBr(THF)(tmeda) OrganoMagnesium Compounds

  11. Structure of Me2Mg4Cl6(THF)6 OrganoMagnesium Compounds

  12. Structure of [tBu2Mg]2 OrganoMagnesium Compounds

  13. Structure of Me2Mg(pmdta) OrganoMagnesium Compounds

  14. Structure of (b-diiminate)MgMe OrganoMagnesium Compounds

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