π Bonds: Electrophilic Attack & Hydrogenation

Question 1

reason for regioselectivity in first part of hydroboration process

Answer 1

Borane = Lewis acid, v. strong attraction for double bond e-
C-B bond formation goes slightly ahead of C-H bond formation
so B and C are partially charged in four-centred transition state

reasons for regioselectivity:

• electronics: π e- are being donated into empty p orbital of B (sp2 hybridised and lewis acidic when not dimerised), +ve charge that this generates needs to go on the carbon best able to stabilised +ve charge
• sterics: BH2 larger than H so ends up where there is less steric hinderance

Question 2

second step of hydroboration

Answer 2

breakdown of trialkylborane w/ hydroperoxide anion

Question 3

hydroboration whole point

Answer 3

to get to product which gives opposite regioselectivity (least stable carbocation intermediate) - i.e. primary carbocation rather than tertiary