Question

What is the mechanism to convert an enol to a ketone under the basic conditions of the oxidation reaction that follows hydroboration?

Answer 1

The earlier steps in the hydroboration of a disubstituted alkyne are:

3RC≡CR + BH₃ → (RCH=CR-)₃B

(RCH=CR-)₃B + 3H₂O₂ → (RCH=CR-O)₃B + 3H₂O

(RCH=CR-O-)₃B + 3OH⁻ → 3RCH=CR-O⁻ + B(OH)₃

The mechanism for the conversion to a ketone starts here.

Step 1. The enolate ion is a resonance hybrid:

RCH=CR-O⁻ ⟷ RCH⁻-CR=O

Step 2. The carbanion end of the hybrid attacks a water molecule:

RCH⁻-CR=O + H-OH → RCH₂-CR=O + ⁻OH

The product is a ketone.