Electrochemical reduction of tetraalkylammonium tetrafluoroborates at carbon cathodes in dimethylformamide

The tetraethylammonium or tetra-n-butylammonium cation can be reduced at a glassy carbon cathode in dimethylformamide to form an ethyl or n-butyl carbanion. Abstraction of a proton from either (a) residual water present in the solvent-supporting electrolyte system or (b) the parent tetraalkylammonium cation by an electrogenerated ethyl or n-butyl carbanion yields ethane or n-butane respectively. Each of these proton transfer reactions gives rise to a base-initiated Hofmann elimination in which a tetraalkylammonium cation decomposes to afford an alkene and a trialkylamine: on the one hand, the hydroxide ion resulting from the deprotonation of water by an electrogenerated alkyl carbanion attacks a tetraalkylammonium cation; on the other hand, the Hofmann elimination is a direct consequence of the deprotonation of a tetraalkylammonium cation by an alkyl carbanion. Our studies indicate that the tetraalkylammonium cation undergoes a more facile Hofmann elimination than does the tetra-n-butylammonium cation; we attribute this difference to the size of the alkyl group, which affects the ease with which the proton (to be removed from the β carbon atom) can achieve an antiperiplanar arrangement with the leaving group (trialkylamine on the α carbon atom).