Brook rearrangement

The Brook rearrangement in organic chemistry is a rearrangement reaction in which an organosilyl group switches position with a hydroxyl proton over a carbon to oxygen covalent bond under the influence of a base. It is named for the Canadian chemist Adrian Gibbs Brook (1924–2013). The reaction product is a silyl ether.

The silyl substituents can be aliphatic (methyl) or aromatic (phenyl) and the alcohol is secondary or tertiary with aliphatic or aryl groups. The base is an amine, sodium hydroxide, an organolithium reagent or an alkali metal alloy such as sodium/potassium. When the reactant is a silylmethanol the reaction is a 1,2-brook rearrangement but rearrangements over larger carbon skeletons are also possible.

The reaction mechanism for this rearrangement starts with proton abstraction of the hydroxyl group by base to the alkoxide anion. This nucleophile attacks the silicon atom in a nucleophilic displacement with the methylene group as the leaving group. The proposed transition state for this reaction step is a three-membered ring with the degree of Si-O bond making in step with the Si-C bond breaking process. The additional electron pair is now transferred from oxygen to a carbanion which quickly abstracts a proton from a proton source such as solvent to form the final silyl ether.

When the reactant is (triphenylsilyl)methylphenylmethanol the activation energy is found to be relatively low but that the entropy of activation has a large negative value which supports the cyclic transition state structure. The Hammett results for a group of para-substituted phenyl methanols conform that electron withdrawing groups help to stabilize the negative charge built up in the carbanionic intermediate.

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