Reductions and Revisions

My most recent work in the lab has been to reduce the an ester to a primary alcohol. In other words, we’re attempting to alter the oxidation state of a particular portion of the compound. To this end, we’re using the reducing agent diisobutylaluminum hydride, or Dibal-H, for the reaction. This reagent is a bit dangerous to work with it, because it is pyrophoric, meaning it spontaneously ignites in air at room temperature. This requires that we make a 1.0 molar solution of Dibal-H in a solvent, which will be added to the starting material.

In all four runs, it appears that the ester is being partially and/or fully reduced to the alcohol after 30 minutes of stirring at -78C (a temperature achieved using a dry ice and acetone bath). Although the desired product is forming,  issues arose during the workup. In this portion of the reaction, any remaining, unreacted reagents are quenched. For this particular reaction, a small amount of methanol was added directly to the reaction to quench Dibal-H. A saturated solution of Rochelle’s salt, sodium potassium tartrate, was then added to serve as a ligand for aluminum and break any resulting emulsions. Unfortunately, in each run it was difficult to separate the product from the aqueous phase during extraction. Alcohols are polar molecules that are water soluble, so this presents a challenge to getting a high yield.  This reaction remained to have a low yield (10-20%) despite changing the scale of the reaction, pH, reaction time, and polarity of the extraction solvent. At this point, it was deemed wise to avoid the reduction altogether and revise our current synthesis route.