Meerwein-Ponndorf-Verley Reduktion: Mechanism
The mechanism could also be considered a Cannizaro reaction induced by aluminum alkoxide (or magnesium alkoxide). Aluminum, being a very strong Lewis base, coordinates in the first step with the carbonyl oxygen, a Lewis acid .
Coordination between aluminum and oxygen allows for the transfer of a hydride ion to the carbonyl carbon atom. The use of mainly aluminum isopropoxide places the hydride in a favorable position for the transfer. The reaction proceeds via a six-membered transition state. By using aluminum isopropoxide, the hydride transfer generates acetone which is continously removed from the reaction mixture by distillation, moving the equilibrium to the right. The alcohol, produced from the ketone, is obtained by hydrolysis.
The solvent for the reaction is isopropanol allowing for the hydrolysis of the newly generated alkoxide and regeneration of aluminum isopropoxide.