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