6.3.3: Nucleophilic and Electrophilic Addition and Abstraction
-
- Last updated
- Save as PDF
Learning Objectives
In this lecture you will learn the following
- Ligand activation by metal that leads to a direct external attack at the ligand.
- Nucleophilic addition and nucleophilic abstraction reactions.
- Electrophilic addition and electrophilic abstraction reactions.
The nucleophilic and electrophilic substitution and abstraction reactions can be viewed as ways of activation of substrates to allow an external reagent to directly attack the metal activated ligand without requiring prior binding of the external reagent to the metal. The attacking reagent may be a nucleophile or an electrophile. The nucleophilic attack of the external reagent is favored if the L n M fragment is a poor π−base and a good σ−acid i . e ., when the complex is cationic and/or when the other metal bound ligands are electron withdrawing such that the ligand getting activated gets depleted of electron density and can undergo an external attack by a nucleophile Nu − , like LiMe or OH − . The attack of the nucleophiles may result in the formation of a bond between the nucleophiles and the activated unsaturated substrate, in which case it is called nucleophilic addition, or may result in an abstraction of a part or the whole of the activated ligand, in which case it is called the nucleophilic abstraction. The nucleophilic addition and the abstraction reactions are discussed below.
Nucleophilic addition
An example of a nucleophilic addition reaction is shown below.
Carbon monoxide (CO) as a ligand can undergo nucleophilic attack when bound to a metal center of poor π−basicity, as the carbon center of the CO ligand is electron deficient owing to the ligand to metal σ−donation not being fully compensated by the metal to ligand π−back donation. Thus, activated CO ligand undergoes nucleophilic attack by the lithium reagent to give an anionic acyl ligand, which upon alkylation generates the famous Fischer carbene complex.
Nucleophilic abstraction
An example of a nucleophilic abstraction reaction is shown below.
Electrophilic addition
Similar to the nucleophilic addition and abstraction reactions, the electrophilic counterparts of these reactions also exist. An electrophilic attack is favored if the L n M fragment is a good π−base and a poor σ−acid i.e., when the complex is anionic with the metal center at low−oxidation state and/or when the other metal bound ligands are electron donating such that the ligand getting activated becomes electron rich from the π−back donation of the metal center and thus can undergo an external attack by an electrophile E + like H + and CH 3 I. The attack of the electrophiles may result in the formation of a bond between the electrophile and the activated unsaturated substrate, in which case it is called electrophilic addition, or may result in an abstraction of a part or the whole of the activated ligand, in which case it is called the electrophilic abstraction.
Electrophilic abstraction
An example of an electrophilic abstraction reaction is shown below.
Alkyl abstractions are often achieved by Hg 2 + that can proceed in two ways, ( i ) by an attack at the α−carbon of a metal alkyl bond leading to an inversion of configuration at the alkyl carbon and ( ii ) by an attack at the metal center leading to retention of configuration at the alkyl carbon. The inversion of configuration proceeds by the following pathway.
The retention of configuration proceeds by the following pathway.