[Formation mechanism of 4,4-methylenebis(N,N-dimethylaniline) by the anodic oxidation of N,N-dimethylaniline]

Preparative constant-current electrolysis of N,N-dimethylaniline(100 mM, DMA) was carried out in dry acetonitrile containing Et4NClO4 to give N,N,N',N'-tetra-methylbenzidine(TMB) and 4,4'-methylenebis(N,N-dimethylaniline) (MDMA). In the presence of HCl, DMA was quantitatively oxidized to TMB. Formation mechanism of MDMA was discussed using the results of semi-empirical calculations that were PM3 and AM1. The proposed mechanism by Hand and coworker was ruled out and an alternative one is suggested as follows. DMA undergoes one-electron oxidation at the anode and gives the corresponding N,N-dimethylaniline cation (A). Aydroxy ion, instead of DMA, abstracts a proton from A and gives H2O and methyl(phenyl)aminomethyl radical (B). B reacts with DMA to give a 1-dimethylamino-4-(N-methyl-N-phenylamino)methyl-2,5-cyclohexadienyl radical (I). The radical I decomposes to N-methylaniline and a 1-dimethylamino-4-methylene-2,5-cyclohexadienyl radical (J). The radical J reacts with DMA to give a 1-dimethylamino-4-(4-dimethylaminobenzyl)-2,5-cyclohexadienyl radical (K), which is oxidized to MDMA by the anode.