Hydroboration and manipulation of C-B bonds

Question 1

What is hydroboration?

Answer 1

Addition of B and H across a double (or triple) bond

Question 2

What influences the rate of hydroboration and how?

Answer 2

In the case of BH3, all 3 H substituents can be transferred. Rate is dependent on a number of factors but sterics most important. Less hindered alkenes react most readily. Substituents on borane decrease rate of hydroboration and hence the relative rate of transfer of the 3 H substituents in BH3. With particularly hindered alkenes it is possible to stop the reaction after transferring 2 Hs.

Question 3

What is the regiochemistry of the hydroboration reaction?

Answer 3

B substituent usually ends up on less substituted end of alkene. Dialkylboranes generally give higher regiocontrol than BH3.

Question 4

What are the origins of regioselectivity?

Answer 4

Sterics; polariation of B-H bond (also favours B on less substituted end of alkene).

Question 5

What is the stereochemistry of the hydroboration reaction?

Answer 5

2 new bonds formed on same face of alkene (syn stereospecific). Can also be stereoselective - borane reacts on sterically less hindered face of alkene when the two faces are sterically differentiated

Question 6

Describe the oxidation reaction of organoboranes

Answer 6

Mechanism of oxidation of C-B bond proceeds with complete retention of configuration at migrating centre. Standard conditions for oxidation involve H2O2/NaOH. Phosphate buffer to maintain pH 8. Hydroboration followed by oxidation overall represents the formal anti-Markovnikov hydration of an alkene

Question 7

What is carbonylation?

Answer 7

Introduction/addition of CO. Convergent route to ketones

Question 8

How can sp3 C-B bonds be manipulated?

Answer 8

Oxidation (to make alcohols) or carbonylation (to make ketones)