Organometallic Chemistry between organic and inorganic_12_ by malj



          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 !)
        – Be: extremely toxic, not used

    • Reducing
        – sometimes unwanted side reactions

    Organo Ca, Sr, ... compounds: ionic, insoluble, not used

2                                                                 OrganoMagnesium Compounds
               Grignard reagents RMgX

    • Direct synthesis ("Grignard reaction"):
           Mg + RX                R M gX

    • Mechanism is not simple!
        – almost certainly involves SET (Single Electron Transfer)

           R X + M gs            R X + M gs            R + X M gs              R M g X s o lv

        – 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)

3                                                                        OrganoMagnesium Compounds
                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
              RM gX + RX              R    R + M gX2

        – Radical rearrangements

4                                                          OrganoMagnesium Compounds
                  Grignard reagents RMgX

    •   Main application: addition to polar C=X bonds
         –   C=O, C=N, C≡N

                      
             R        Mg         R    Mg               Mg

                                              R        O

    • Sometimes radical mechanism
      (especially for aromatic ketones):
         – same product expected, but possibly different side products
                      
             R        Mg         R    Mg           R       Mg            Mg

                                                                   R     O
                                      O                    O

5                                                                 OrganoMagnesium Compounds
                Grignard reagents RMgX

    • Side reaction in C=X addition: reduction through b-H transfer
        – only for alkyls having a b-hydrogen...


            H                          H             O
                       M gX                        M gX

                                       O              H
                                   M gX                   O   M gX

6                                                              OrganoMagnesium Compounds
              Grignard reagents RMgX

    "Barbier reaction":

                    1)       O

         Mg + RX                 R   OH
                     2) H+

    • Selectivity differs from that of Grignard addition
    • Probably radical mechanism
    • Works even in water !

7                                                          OrganoMagnesium Compounds
              The Schlenk equilibrium
    • Grignard reagents are usually mixtures
      of many compounds in dynamic equilibrium

    • This is often written as the Schlenk equilibrium

           R 2M g + M gX 2            2 R M gX

       but the situation is really more complicated

    • Crystal structures may not be representative
      of species in solution

    • The equilibrium can be shifted through addition of ligands:
           2 R M g X + d io x a n e     R 2 M g + M g X 2 ( d io x a n e )

8                                                                  OrganoMagnesium Compounds
    Structure of [MeMgCl(tmeda)]2

9                          OrganoMagnesium Compounds
     Structure of MeMgBr(THF)(tmeda)

10                          OrganoMagnesium Compounds
     Structure of Me2Mg4Cl6(THF)6

11                         OrganoMagnesium Compounds
     Structure of [tBu2Mg]2

12                       OrganoMagnesium Compounds
     Structure of Me2Mg(pmdta)

13                       OrganoMagnesium Compounds
     Structure of (b-diiminate)MgMe
                                t                     t
                                    Bu                    Bu

                                         N        N
                                    Ar                    Ar


                                    Ar =

14                          OrganoMagnesium Compounds

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