Procedure:  1000 mL 1-neck flask, stirbar, condenser, septum, N₂ inlet

                  Prepared a suspension of 15.191 g of LiAlH₄ in 250 mL of dry THF.  Stirred at rt.  Added a solution of 38.964 g of diester I in 150 mL of THF using a cannula.  After addition the reaction mixture was refluxed for 4 h.  TLC showed product spot at Rf 0.44 (Et₂O, PMA).  The reaction mixture was then cooled to rt and quenched by the sequential addition of 15.2 mL of H₂O, 15.2 mL of 15w/v% aq. NaOH and 45.6 mL of H₂O.  The mixture was then poured into 200 mL of Et₂O and stirred for 30 min.  The insoluble material was removed by filtration through Celite and the solvent was removed from the filtrate by rotary evaporation.  The product was isolated by flash chromatography on silica gel using 50:50 Et₂O-hexanes as eluant.  The product was a clear, colorless oil. 

 

notes

 

The reduction of esters to alcohols with LiAlH₄ is a concerted process whereby an O-Al bond is formed as addition of hydride to the carbonyl proceeds.  All four hydrides from AlH₄ are available for addition and this complicates the reaction mechanism which is depicted below for a single hydride transfer.  After hydride transfer an elimination of ethoxide must necessarily occur to afford the aldehyde.  A second hydride transfer followed by hydrolysis liberates the free hydroxyl and completes the reaction. 

 

The quench employed for LiAlH₄ reductions calls for the successive slow addition of n mL of water followed by n ml of 15% aq. NaOH followed by 3n mL of water for every n grams of LiAlH₄ used (see Micovic, V.M.; Mihailovic, M.L.J.  J. Org. Chem.  1953, 18, 1190).  Following this procedure, a granular precipitate of aluminum salts forms which is easily removed by filtration and which absorbs very little of the desired product.  This procedure avoids the gelatinous precipitate and emulsions which are known to be a problem with these reductions.