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 electrgenerated 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 beta carbon atom) can achieve an antiperiplanar arrangement with the leaving group (trialkylamine on the alpha carbon atom).