Silicon in Synthesis

Question 1

What is a silyl enol ether
also known as a “enolate in a bottle”

Answer 1

Question 2

What is the nucleophilic enhancement of alkenes using silicon?

Answer 2

(not really covered this module)
Called alkenyl silanes (2x left) or allyl silane (right)

Question 3

How is silicon used as a protecting group for alcohols and alkynes

Answer 3

Question 4

What are the two different Silyl enol ethers we can form from the following ketone starting product

Answer 4

depends on where we deprotonate

Question 5

Which product is considered the kinetic product or the thermodynamic product and what is the difference in the conditions required to produce them

Answer 5

• The LHS is considered the kinetic product due to the proton being less sterically hindered - this reaction is irreversible as such a strong base and low temp is used
• The RHS is considered the thermodynamic product due to being the lowest energy as the alkene with more substituents are more stable - hence only a weak base and elevated temp is needed to set up equilibrium

Question 6

How might we form a silyl enol ether from a enone

Answer 6

• The silicon will trap the regiospecific enolate which stops a mixture from forming

Question 7

What are the two ways you can regenerate an enolate from a silyl enol ether

Answer 7

• Can use methyl-lithium which acts as a nucleophile
• Or can use fluoride in the form of Bu₄N⁺F⁻ (TBAF)
• They both will displace the O⁻ as a leaving group

Question 8

What would we then react the following intermediate with to form a stable enolate species if we reacted it with an electrophile with bromide in it?

Answer 8

Question 9

What is the mechanism of the Lewis acid-promoted reaction?

Answer 9

TiCl₄ is the lewis acid
CH₂Cl₂ is the solvent

Question 10

These are the following reagents and conditions for the Mukaiyama Aldol reaction
What is the mechanism?

Answer 10

Question 11

These are the following reagents and conditions required for a Michael-Reaction
What is the mechanism?

Answer 11

Question 12

These are the following reagents and conditions required for a halogenation
What is the mechanism?

Answer 12

Forming a halo-ketone

Question 13

You can add a R group onto a enone using R₂CuLi and Me₃SiCl then reacting it with Cl-S-Ph and mCPBA
Describe the mechanism for this reaction

Answer 13

Question 14

What other group does a SYN elimination and what is the benefit of it?

Answer 14

Selenium
You can do the reaction at 0°C
(Less popular however due to the smell of the leaving compound)

Question 15

The following reaction is an a-hydroxylation of Silyl Enol Ethers (Rubottom oxidation)
Is it undertaken through the Butterfly mechanism
How does this occur?

Answer 15