Q16P - Organic Chemistry (Mcmurry)

Question

TMS ethers can be removed by treatment with fluoride ion as well as by acid-catalyzed hydrolysis. Propose a mechanism for the reaction of cyclohexyl TMS ether with LiF. Fluorotrimethylsilane is a product.

Step-by-Step Solution

Answer

The mechanism for the reaction of cyclohexyl TMS ether with LiF is shown as,

Step-by-Step Solution Step 1: Protection of Alcohol

TMS ethers are unreactive in protecting alcohols and don’t have any acidic hydrogens. That’s why they don’t undergo a reaction with Grignard reagents and oxidizing agents. Instead, they react with fluoride ions or an aqueous acid to regenerate alcohol as a product.

Step 2: The mechanism for the reaction of cyclohexyl TMS ether with LiF

The reaction of cyclohexyl TMS ether with LiF is an ${\text{S}}_{\text{N}}\text{2}$  reaction in which the nucleophile is a fluoride ( ${\text{F}}^{\text{-}}$) ion that attacks the silicon of TMS ether and displaces the alkoxide ion as a leaving group. Through this reaction, the alcohol compound is regenerated. The mechanism shows the attack of fluoride ion on the silicon of TMS ether, and fluorotrimethylsilane product is formed.

Formation of fluorotrimethylsilane