Question

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

Short answer

Cyclohexanol and trimethylsilyl fluoride are produced when LiF reacts with cyclohexyl TMS ether.

Step-by-step solution

Identify Reactive Species

In this reaction, the cyclohexyl TMS ether is the substrate, and the fluoride ion ( F^− ) from lithium fluoride ( LiF ) is the nucleophile. The fluoride ion is known for its strong affinity to silicon, which plays a crucial role in the mechanism.

Attack of Fluoride Ion

The fluoride ion ( F^− ) attacks the silicon atom in the TMS group of cyclohexyl TMS ether because silicon is electron-deficient and can form a strong bond with the highly electronegative fluoride ion.

Breaking the Si-O Bond

As the fluoride ion forms a bond with the silicon atom, the Si-O bond in the TMS ether weakens and undergoes cleavage. This results in the formation of an alkoxide ion where the cyclohexyl group is attached to oxygen.

Formation of Fluorotrimethylsilane

The trimethylsilyl fluoride ( ( ext{CH}_3)_3 ext{SiF} ) is formed as the TMS group is now attached to the fluorine atom. The Li^+ ion remains as a spectator ion throughout the reaction.

Resulting Products

The final products of the reaction are cyclohexanol (formed from protonation of the alkoxide ion) and trimethylsilyl fluoride.

Key concepts

Nucleophilic Attack

In the realm of organic chemistry, nucleophilic attack is a fundamental mechanism that involves the donation of a pair of electrons from a nucleophile to an electrophile. It serves as a starting point for many reactions, including the one involving TMS ether cleavage. Here, the role of the nucleophile is played by the fluoride ion (\(F^-\)), which seeks out electron-deficient centers to bond with. In the case of cyclohexyl TMS ether, silicon in the TMS group is the electrophile.

• Fluoride ions, due to their small size and high electronegativity, are very effective nucleophiles, especially towards silicon.
• Electron-deficient silicon atoms in the TMS group are very attractive to the negatively charged fluoride ions.

This nucleophilic attack initiates the reaction and paves the way for subsequent bond-breakage and formation steps, making it a crucial concept to understand.

Fluoride Ion

The fluoride ion plays a significant role in the cleavage of TMS ethers due to its unique properties. As an ion, fluoride (\(F^-\)) brings strong nucleophilic characteristics to reactions, particularly those involving silicon atoms.

• Fluoride's small size allows it to approach electrophilic centers very closely, enhancing its reactivity.
• Its high electronegativity makes it prioritize bonding with less electronegative atoms such as silicon.

In the cleavage of TMS ethers, the fluoride ion not only initiates nucleophilic attack but also selectively targets silicon atoms. This targeting effectively sets the stage for the breaking of the silicon-oxygen bond, demonstrating why fluoride is often chosen for such reactions.

Silicon-Oxygen Bond Cleavage

The cleavage of the silicon-oxygen bond in TMS ethers is a critical part of the reaction mechanism when employing fluoride ions. This process ultimately leads to the formation of smaller, more stable molecules as by-products.

• After the nucleophilic attack by the fluoride ion, the bond between silicon and oxygen experiences increased tension.
• The affinity of silicon for the incoming fluoride is greater than its attachment to oxygen, prompting the bond cleavage.
• This bond-breaking forms an alkoxide ion, allowing the rest of the molecule to transform accordingly, forming products like cyclohexanol.

Understanding the dynamics of this bond cleavage is key to mastering ether cleavage reactions and anticipating product formation, making it essential knowledge for students studying organic chemistry.