Silicon in Synthesis 2

Question 1

What is a Alkenyl Silane
What is the benefit of using them?

Answer 1

It is a silicon attached to an alkene
Adding the silicon group to the alkene massively increases its reactivity towards the electrophile, making it more electron rich

Question 2

The Shapiro reaction is the main example of a way to produce alkenyl silane
How does this reaction occur?

Answer 2

Addition of lithium
WHich is then exchanged with silicon

Question 3

What is the mechanism for the following reaction

Answer 3

• Sable salt is the driving force for the first reaction
• Stability of N₂ gas drives its elimination

Question 4

This reaction forming a different product can occur if the reaction is understaken at -78°C
What is the mechanism

Answer 4

• We can also alkylate the dilithiated intermediate
• We get a rotation of the R and H group (3rd intermediate) which allows the H to be picked up by the BuLi
• Results in the R group being trans to the nitrogen, and hence trans to the silicon when the alkenyl when its formed

Question 5

How can we synthesis alkenyl silanes from alkenyl halides?

Answer 5

Question 6

What happens when you react an Alkenyl Silane with an electrophile

Answer 6

• The silicon stabilises the β cation
• The SiMe3 essentially act like a big proton, and readily falls off
  The Si group is replaced with the electrophile with retention of the alkene geometry

Question 7

How does the following alkenyl silane react with iodine

Answer 7

• Iodine attacked from above the plane
• Silicon need to be anti-periplanar to the p-orbital to get maximum overlap - requires a 30°C rotation

Question 8

How does the following alcyl chloride react with alkenyl silane

Answer 8