Syntheses and properties of cycloamidines based on 4H-imidazoles

Abstract

Employing three different syntheses a broad spectrum of 4H-imidazoles 3a-3s has been synthesized. In the course of the two-fold aminolysis reaction leading to derivatives 3q-3s, deeply colored byproducts could be isolated and structural characterized. These novel donor-acceptor derivatives of type 7 consist of an 1H- and 4H-imidazole which are connected by a nitrogen bridge and rearrange via rapid 1.3-/1,5-hydride shifts. Using 1H NMR experiments the aminolysis product 3p shows prototropic isomers which could be detected in equilibrium for the first time. Cyclovoltammetric measurements of a series of substituted 2-aryl derivatives 3d-3i displayed two reversible single electron transfer steps with relatively small semiquinone formation constants between 102 and 4 × 103. The 4H-imidazole 3d was successfully converted into boratetraaza-pentalene 8a, which showed two well separated reduction potentials. The value of semiquinone formation constant of 8a (1.8 × 1015) is even higher than those reported for similar derivatives. 4H-imidazoles can also be employed for the efficient complexation of catalytically important metals as exemplified by copper complexes 11 and 12. Derivative 3m, which possesses an additional chelating pyridine substructure, formed a stable complex of structural composition Zn(3m)2 with diethyl zinc. © 2006 Verlag der Zeitschrift für Naturforschung.