Review of Organometallic Insertions

Insertion Reactions in Organometallic Chemistry

Overview of Insertion Reactions

• Two main types of insertion reactions exist in organometallic chemistry.

  • Commonly referred to as migratory insertion for historical reasons.

• The reaction involves the addition of a nucleophile (often bound to a metal) to a carbon monoxide (CO) ligand or an analogous ligand.

• Common nucleophiles include metal alkyls (like methyl) and hydrides, especially from metals on the left-hand side of the periodic table.

1.1: Insertion of Alkyl Ligand into Carbon Monoxide

• Complex Formation: When carbon monoxide binds to the metal:

  • The metal can be shown with a formal negative charge.

  • Carbon monoxide has a positively charged oxygen and negatively charged carbon.

  • This structure emphasizes the electron density transfer to the metal.

• Nucleophilic Alkyl Movement: The alkyl group (nucleophile) from the metal can transfer its electrons to the carbon of the CO ligand:

  • This movement allows the oxygen to regain its lone pair, forming a new complex.

  • This represents a 1:1 insertion where the nucleophile attaches to the carbon atom of the carbon monoxide ligand.

1.2: 1,2 Insertion Reaction with Aldehydes and Ketones

• In the case of aldehydes or ketones, the mechanism differs:

  • Oxygen lone pairs coordinate with the metal, facilitating electron transfer.

  • The metal complex may again be drawn with a negative charge for emphasis.

• Activation of Electrophile: The positive charge on the coordinated oxygen makes it an activated electrophile, primed to react with nucleophiles, such as a metal hydride:

  • In this reaction, a nucleophilic hydride is commonly involved.

  • The outcome is the formation of a methoxy group, indicative of a 1,2 addition.

1.3 Insertion Reaction with Alkenes

• When reacting with alkenes:

  • Instead of a lone pair donation from oxygen, a pi bond from the alkene donates electrons to the metal.

  • This process does involve some charge transfer from the alkenes to the metal (not formally charged).

• Electrophilic Behavior: Although alkenes are typically viewed as nucleophiles, this can be seen as acting similarly to an electrophile in this context:

  • As charge is transferred, and a hydride is moved to the coordinated alkyne (in this case), it creates a new alkyl ligand.

  • The result of this reaction is the conversion of an ethylene into an ethyl group, emphasizing that the hydride insertion yields an alkyl ligand from an alkyne.