Transition metal salts dependent intramolecular cyclization of a diimine

1,4-Bis(N-tosylethylamino)-2,3-diphenyl-1,4-diaza-1,3-butadiene (L1) was separated from a condensation reaction of benzil with N-tosyl-ethylenediamine. It was shown that the intramolecular cyclization of L1 has much dependence on the transition metal salts. The transformation of L1 in the absence of any transition metal salts gave product 1-tosylaminoethyl-2-tosylaminomethyl-4,5-diphenyl-1H- imidazole (L2) at a temperature above 100 °C. Similar reactions assisted by Ni(CH3COO)2·4H2O, MnCl2·4H2O, Mn(CH3COO)2·4H2O, NiCl2·6H2O or NiSO4·7H2O could also give L2 at a much lower temperature and a much higher yield. However, the presence of Cu(CH3COO)2·H2O in a similar transformation reaction led to the formation of a bicyclic derivative 1-tosyl-5,6-diphenyl-1H-2,3-dihydroimidazo[1,2-a]imidazole (L3). But both L2 and L3 were not detected when Co(CH3COO)2·4H2O was used in a similar reaction and all attempts to transform L2 into L3 failed.