Reduction of esters to aldehydes using DIBAL
Description: Esters can be reduced to aldehydes with diisobutylaluminum hydride (DIBAL), a bulky source of hydride ion. [private_ReactionGuide]
Notes: DIBAL-H (sometimes called DIBAL or DIBAH) is a bulky reducing agent that will convert esters to aldehydes. Low temperatures help to prevent further reduction to the alcohol.
The second step is often just written “H2O”. Alternatively, acidic conditions can be used.
Notes: Note that example #4 is a reaction that many students have a hard time seeing.
Mechanism: Note that this mechanism is not generally considered to be important for the purposes of Org 1/ Org 2 but it shown here for completeness.
The reaction begins with attack of the carbonyl oxygen on aluminum (Step 1, arrows A). This leads to attack of the aluminum hydride on the carbonyl carbon in a 1,2-addition (Step 2, arrows B and C). This intermediate is stable at low temperatures until water is added in the workup. Water attacks aluminum (Step 3, arrow D) and after breakage of the O–Al bond (Step 4, arrow E) the resulting alkoxide then undergoes 1,2-elimination (arrows F and G) to give the aldehyde.
Notes: There are certainly other reasonable ways to show arrow pushing in steps 3-5 to give the aldehyde; this is just one possibility.