Reagents for Organic Synthesis

Question 1

What are the properties of Boron chemistry (ie. borane)?

Answer 1

Borane-like molecules tend to have an empty p orbital which makes them lewis acids and results in the compounds being pyrophoric. This is due to borons trivalent nature. Note that borane-like compounds will also tend to dimerize.

Question 2

2. The most common form of reactions using borane is hyrdoboration. Draw a hydroboration reaction

Answer 2

Note that this reaction may continue with excess alkenes

Question 3

Why does Boron addition tend to take place at the terminal carbon

Answer 3

Marknownikows Rule, H is added to the most substituted C in double bond

Question 4

When conducting hydroboration on alkynes. What is the stereochemistry of the addition?

Answer 4

The addition will be syn

Question 5

Write down a simple protonolysis using a hydroborane using a carboxylic acid

Answer 5

Question 6

Write down a simple oxidation reaction using a hydroborane.

Why is NaOH and H₂O₂ required for the reaction?

Answer 6

NaOH and H₂O₂ react together to form HO₂ required for the mechanism

Note that Boric Acid is extremely stable and drives reaction equilibrium toward the products

Question 7

Explain how Homologation takes place

Answer 7

Primary alcohol has prochiral protons removed using stereospecific catalyst (-)-spartine which removes pro-S proton

Borane then reacts with electronegative center and undergoes 1,2-shift before being oxidised with NaOH/H₂O₂

Yeilds 99% stereoselective product

Question 8

Explain how allyl boranes react with carbonyl compounds.

How is the E/Z stereochemistry important?

Answer 8

Stereo specific outcome results in E-allyl compounds creating an anti product while the Z-allyl creates a syn product

Can be decided using the transition state