Nucleophilic abstraction

Nucleophilic abstraction is a type of an organometallic reaction which can be defined as a nucleophilic attack on a ligand which causes part or all of the original ligand to be removed from the metal along with the nucleophile.

While nucleophilic abstraction of an alkyl group is relatively uncommon, there are examples of this type of reaction. In order for this reaction to be favorable, the metal must first be oxidized because reduced metals are often poor leaving groups. The oxidation of the metal causes the M-C bond to weaken, which allows for the nucleophilic abstraction to occur. G.M. Whitesides and D.J. Boschetto use the halogens Br₂ and I₂ as M-C cleaving agents in the following example of nucleophilic abstraction.

It is important to note that the product of this reaction is inverted with respect to the stereochemical center attached to the metal. There are several possibilities for the mechanism of this reaction which are shown in the following schematic.

In path a, the first step proceeds with the oxidative addition of the halogen to the metal complex. This step results in the oxidized metal center that is needed to weaken the M-C bond. The second step can proceed with either the nucleophilic attack of the halide ion on the α-carbon of the alkyl group or reductive elimination, both of which result in the inversion of stereochemistry. In path b, the metal is first oxidized without the addition of the halide. The second step occurs with a nucleophilic attack of the α-carbon which again results in the inversion of stereochemistry.

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