On the Mechanism of Dehydrogenation of the 2,4,5-Triphenyl-4,5-dihydroimidazole over Solid Catalysts

Abstract

Abstract 1) Amarine has been completely dehydrogenated, without the presence of oxygen, by refluxing solutions in xylene, toluene, benzene or acetone with Raney nickel, whereas no dehydrogenation of isoamarine occurred at all under the same conditions. Amarine was also dehydrogenated without the presence of oxygen by refluxing solutions with platinum black, palladium black or active carbon. The dehydrogenation of amarine, carried out by refluxing a solution in benzene with sulfur powder, was accompanied by the evolution of hydrogen sulfide. 2) Amarine was adsorbed with active carbon from a solution in benzene at room temperature, whereas hardly any isoamarine was adsorbed under the same conditions, the difference in the dehydrogenation behavior of these cis- and trans-isomers being very marked. 3) Considering the results obtained, the dehydrogenation of amarine by refluxing solutions with solid catalysts may reasonably be considered cis-dehydrogenation, the mechanism being, as follows; an amarine molecule is adsorbed on solid catalysts with the side of the plane of an imidazole ring, to which cis 4- and 5-hydrogen atoms are attached, and these hydrogen atoms, which tend to move from the imidazole ring, are eliminated under the effect of solid catalysts. Isoamarine, which has a trans-conformation unfavorable to adsorption, is not subjected to dehydrogenation with solid catalysts.