Mechanistic and preparative studies of Electrochemical Deoxygenation and Debromination

The indirect deoxygenation of a range of secondary and tertiary alcohols has been achieved via the electrochemical reduction of the corresponding 3,5- bis(trifluoromethyl)benzoates in up to 93% isolated yield. The reactions were carried out using graphite electrodes in NMP as the solvent, at relatively low potentials (-1.4 V vs Ag/Ag+) and up to 79% current efficiencies. Mechanistic studies were performed and are consistent with an ErCi mechanism in which the first step involves the reversible formation of a radical anion, followed by fragmentation to 3,5- bis(trifluoromethyl)benzoate and the corresponding alkyl radical. Subsequent electrochemical reduction of the radical would be expected to form a carbanion. Protonation, presumably by trace water would give the observed alkane. Electrochemical deoxygenation of the benzylic vicinal diols was also observed. These generally resulted in elimination to form the alkene product. The more stable trans alkene was formed irrespective of the stereochemistry of the starting diol, consistent with a stepwise mechanism in which -elimination from the carbanion yields the alkene. The optimized conditions of electrochemical deoxygenation were then successfully applied to electrochemical protodebromination. Primary, secondary, tertiary, and aromatic bromides were debrominated to give an excellent yield of the debrominated product. The mechanistic steps were found to be similar to those observed for the deoxygenation. This provides an additional method for the indirect deoxygenation of alcohols via bromides.