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OrganoMagnesium Compounds

Discover the characteristics and uses of OrganoMagnesium compounds, such as Grignard reagents, highlighting reactivity, selectivity, and main applications in organic synthesis. Explore the Barbier reaction and the Schlenk equilibrium associated with these compounds.

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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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