Unusual synthesis of 1-(4-fluorobenzyl)-N-(1-(1-(4-fluorobenzyl)-6-isopropoxy-1H-benzo[d]imidazol-2-yl)piperidin-4-yl)-6-isopropoxy-1H-benzo[d]imidazol-2-amine

The title compound was formed while attempting the synthesis of 1-(4-fluorobenzyl)-6isopropoxy-N-(1-methylpiperidin-4-yl)-1H-benzo[d]imidazol-2-amine, a candidate antihistamine. Its formation involves an unusual mechanism where 1-(4-fluorobenzyl)-2-chloro-6isopropoxy-1H-benzo[d]imidazole reacts with 1-methylpiperidinamine presumably through Ndemethylation followed by self-catalyzed N-diarylation. Apparently the formation of the desired 1-(4-fluorobenzyl)-6-isopropoxy-N-(1-methylpiperidin-4-yl)-1H-benzo[d]imidazol-2-amine is rate limiting and, once formed, it reacts faster with 1-methylpiperidinamine to yield the title compound. A plausible explanation for this unusual behavior is proposed.