Nucleophilic abstraction is a type of anorganometallic reaction which can be defined as anucleophilic attack on aligand which causes part or all of the original ligand to be removed from the metal along with thenucleophile.[1][2]
While nucleophilic abstraction of analkyl group is relatively uncommon, there are examples of this type of reaction. In order for this reaction to be favorable, the metal must first beoxidized because reduced metals are often poorleaving 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.[3]
It is important to note that the product of this reaction is inverted with respect to thestereochemical center attached to the metal. There are several possibilities for themechanism of this reaction which are shown in the following schematic.[1]
In path a, the first step proceeds with theoxidative addition of thehalogen 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 theα-carbon of the alkyl group orreductive 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.
Trimethylamine N-oxide (Me3NO) can be used in the nucleophilic abstraction ofcarbonyl. There is an nucleophilic attack of Me3NO on the carbon of the carbonyl group which pushes electrons on the metal. The reaction then proceeds to kick out CO2 and NMe3.[4][5]
An article from theBulletin of Korean Chemical Society journal showed interesting results where one iridium complex undergoes carbonyl abstraction while a very similar iridium complex undergoeshydride extraction.[6]
Nucleophilic abstraction can occur on a ligand of a metal if the conditions are right. For instance the following example shows the nucleophilic abstraction of H+ from anarene ligand attached to chromium. Theelectron withdrawing nature of the chromium allows for the reaction to occur as afacile reaction.[1]
AFischer carbene can undergo nucleophilic abstraction where amethyl group is removed. With the addition of a small abstracting agent, the abstracting agent would normally add to the carbene carbon. In this case however, thesteric bulk of the abstracting agent that is added causes the abstraction of the methyl group. If the methyl group is replaced with ethyl, the reaction proceeds 70 times slower which is to be expected with aSN2 displacement mechanism.[7]
Asilylium ion is a silicon cation with only three bonds and a positive charge. The abstraction of the silylium ion is seen from theruthenium complex shown below.[8]
In the first step of this mechanism one of theacetonitrile groups is replaced by a silicon molecule where the bond between the silicon and the hydrogen is coordinating to the ruthenium. In the second step aketone is added for the nucleophilic abstraction of the silylium ion and the hydrogen is left on the metal.
One example of nucleophilic abstraction of anα-acyl group is seen when MeOH is added to the following palladium complex. The mechanism follows atetrahedral intermediate which results in the methylester and the reduced palladium complex shown.[9]
The following year a similar mechanism was proposed where oxidative addition of anaryl halide followed bymigratory CO insertion and is followed by nucleophilic abstraction of the α-acyl by MeOH. One of the advantages of this intermolecular nucleophilic abstraction is the production of linear acyl derivatives. The intramolecular attack of these linear acyl derivatives gives rise tocyclic compounds such aslactones orlactams.[10]
