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Lecture 36 Organometallic reactions and catalysis 1) Reactions involving modification of ligands

Lecture 36 Organometallic reactions and catalysis 1) Reactions involving modification of ligands. We will consider: (a) migratory insertion; (b) b -hydride elimination; (c) a -abstraction.

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Lecture 36 Organometallic reactions and catalysis 1) Reactions involving modification of ligands

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  1. Lecture 36 Organometallic reactions and catalysis1) Reactions involving modification of ligands • We will consider: (a) migratory insertion; (b) b-hydride elimination; (c) a-abstraction. • Insertion. An insertion reaction involves coupling of two ligands in cis-position,neutral unsaturated (X), and anionic (Y),to form another anionic ligand (XY) at the same metal: • New X-Y bond forms and the coordination vacancy appears. The vacancy is necessary for the reversal reaction which is called deinsertion or elimination. • To prevent the reversal reaction from happening one can add a ligand which will coordinate to the open coordination site at the metal.

  2. 2) Mechanisms of ligand migratory insertion • Two mechanisms are known for insertion reactions, which involve either neutral ligand X migration to M-Y or anionic group Y migration to M-X. The reaction below proceeds via Me migration (most typical). 2:1 Mixture of cis- and trans- isomers is observed experimentally

  3. 3) Olefin migratory insertion / b-hydride elimination • Another important type of insertion is olefin insertion (1,2 insertion). Olefin insertion into M-H bond is reversible. The microscopic reverse is called b-hydride elimination. • b-Hydride elimination is responsible for the instability of great variety of transition metal alkyl complexes where alkyl possesses b-hydrogen. • Olefin insertion into M-C bond is usually irreversible. It forms a base for numerous systems for catalytic olefin polymerization:

  4. 4) Carbene migratory insertion / a-hydride abstraction • Carbene can also insert into a number of M-X bonds • The reversal reaction is called a-hydride abstraction. • a-Hydride abstraction can also result from intramolecular a-CH bond oxidative addition to a metal. • This is the rare case when an elimination reaction causes metal formal oxidation state to change. CO or olefin insertion / elimination reactions do not involve change in metal oxidation state.

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