[cswarth]

Can someone tell me why the alkoxide anion released during reduction of an ester by AlH(t-BU)2 (DIBAL) does not turn around and attack the carbonyl carbon of the aldehyde that is formed?  (See step 3 of the diagram.)

Isn't the alkoxide anion a great nucleophile?  Or is the alkoxide anion neutralized by all the water that is around?

[jeffrey.struss]

Neutralization should run high (methoxide anion should be very nicely reactive). Also, if it attacks the carbonyl you get back the starting product. That is just quite simply the reverse reaction. So in summary it likely does happen a bit, but is dominated by the forward reaction.

[g-bones]

actually, once the tetrahedral intermediate is formed, it is a stable complex at -78 C in a fairly non-polar solvent like toluene (not something with lone pairs like THF).   This is maintained until work up at which time you quench the reaction and methanol (or methoxide if warmed without protic source) and destroy the reducing agent so therefore no further reduction occurs.  the methanol that remains can attack the aldehyde (especially in a protic workup) but forms a hemi-acetal which will rapidly hydrolyze to the thermodynamically more stable carbonyl.