Reductions of esters, acyl halides, alkyl halides, and sulfonate esters with sodium borohydride in polyethylene glycols : Scope and limitation of the reaction

Sodium borohydride in ethylene glycol oligomers (PEGs) has been explored as a novel reducing system for esters, acyl chlorides, alkyl halides, and sulfonate esters. The selectivity of the system is exemplified by its inertness toward nitrogen-containing functional groups such as amides, azides, nitriles, and nitroalkanes. Both hydroxy groups of the oligomeric diols have been established to be necessary for the above reducing system. The nature of the reductant formed by NaBH4 in excess PEG 400 is discussed. Furthermore, an alkoxyborohydride, Na[(PEG)2BH2], can be prepared by reaction of 1 mol of NaBH4 and 2 mol of PEG 400. In THF the reagent reduces halides and tosylates rapidly to hydrocarbons in good yields.