Meerwein-Pondorf-Verley Reduction

Reduction of aldehydes and ketones to alcohols is most commonly carried out by metal hydride reagents, dissolving metal reagents, and sometimes by catalytic hydrogenation. A table listing these reagents and the conditions under which they are best used  as show in below.

Reducing Reagents

Prior to the development of these new and powerful reduction methods, the conversion of carbonyl compounds to alcohols was often effected by hydrogen transfer from an alkoxide salt. This procedure, known as the Meerwein-Pondorf-Verley reaction, is illustrated by the following equation and mechanism ( the hydride-like hydrogen is colored red). Aluminum isopropoxide has been the most common hydrogen source in most cases, but lanthanide salts, such as ROSmI₂ have been used with good results. This reduction is specific for aldehyde and ketone carbonyl functions, so other easily reduced functions such as nitro groups and halogen are unaffected.

Not only are two hydrogens delivered independently from the least hindered (convex) side of the cis-decalin substrate in example 1, but the easily reduced double bond of the enedione remains unchanged. The initially formed cis-diol undergoes lactonization with the neighboring methyl ester. It should be noted that a similar reductive hydride transfer takes place when large alkyl Grignard reagents react with hindered ketones, as shown in equation 2.

The MVP reduction is also an oxidation, as evidenced by the conversion of isopropoxide to acetone. Consequently, the reaction can be converted into an oxidation of alcohols to ketones or aldehydes. This procedure is called the Oppenauer oxidation. The reaction displayed below is an example of the Oppenauer oxidation in which benzophenone is the oxidant. Two significant features may be noted. First, the oxidation is specific for alcohols, and does not oxidize other sensitive functions such as amines and sulfides. Second, although aluminum or other coordinating metals are often used as cationic partners, alkali metals alone will suffice.