On the Mechanism of Decomposition of Geminal Diamines

The kinetically preferred mode of decomposition of a series of geminal diamines 3a-g, generated by the in situ reduction of the corresponding amidines, has been determined through the reductive trapping of the iminium cations formed. Each of the geminal diamines has been shown to break down with preferential expulsion of the less basic amine leaving group. This preference is shown to be independent of pH in the range 2-11 and regardless of whether the reaction involves the formation of an aromatic or aliphatic iminium cation. The effects of para substituents on the partitioning of the geminal diamine were analyzed in terms of “effective charges” on the two nitrogens in the ground and transition states. The derived transition state features a symmetrical charge distribution in which the developing positive charge on the nitrogen of the incipient iminium cation is balanced by the depletion of positive charge on the nitrogen of the departing amine.