Chapter 15: Alcohols, Diols and Thiols

Hydroboration / Oxidation of Alkenes
(review of Chapter 6)

Summary.

• Overall transformation :  C=C to H-C-C-OH
• Reagents (two steps) 1. BH₃ or B₂H₆ then 2) NaOH/ H₂O₂
• Regioselectivity : Anti-Markovnikov, since the B is the electrophile.
• Stereoselectivity : Syn since the C-B and C-H bonds form simultaneously from the BH₃.
• The alcohol is formed over a series of steps involving the B center (see below), with retention of configuration at the C.
• Compliments simple hydration with opposite regiochemistry and stereospecificity

• Hydration of Alkenes
• Oxymercuration-Demercuration

MECHANISM FOR REACTION OF ALKENES WITH BH3
Step 1: A concerted reaction. The p electrons act as the nucleophile with the electrophilic B and the H is transferred to the C with syn stereochemistry.
Step 2: First step repeats twice more so that all of the B-H bonds react with C=C
Step 3: Peroxide ion reacts as the nucleophile with the electrophilic B atom.
Step 4: Migration of C-B bond to form a C-O bond and displace hydroxide. Stereochemistry at the C center is unchanged i.e. it is retained.
Step 5: Attack of hydroxide as a nucleophile with the electrophilic B displacing the alkoxide.
Step 6: An acid / base reaction to form the alcohol.

© Dr. Ian Hunt, Department of Chemistry