Electron transfer reactions. Reaction of nitrogen heterocycles with potassium

Abstract

The results of our studies on potassium-induced transformations of some selected nitrogen heterocycles are presented. The substrates under investigation include 2,3-diphenylindole (1a), 2,3-diphenyl-1-methylindole (1b), 1,2,3-triphenylindole (1c), 2,3,4,5-tetraphenylpyrrole (5a), 1,2,3,5-tetraphenylpyrrole (5b), 1-benzyl-2,3,5-triphenylpyrrole (5c), 2,4,5-triphenyloxazole (15a), 4,5-diphenyl-2-methyloxazole (15b), 2,4-diphenyl-5-methyloxazole (15c), and 2,4,5-triphenylimidazole (19). Treatment of 1a with potassium in THF gave 9H-dibenzo[a,c]carbazole (3a), whereas 1c gave a mixture of 9-phenyl-9H-dibenzo-[a,c]carbazole (3c) and 2,3-diphenylindole (1a). Under identical conditions, 1b gave only the cleavage product 1a. In contrast, when the reactions of 1a,c were carried out with potassium in THF saturated with oxygen, and with potassium superoxide in benzene containing 18-crown-6, a mixture of 2-benzamidobenzophenone (4a), the carbazoles 3a,c, and 1a was formed. Although no product was isolated on treatment of 5a with potassium in THF, the reaction of 5a with potassium in THF saturated with oxygen gave a mixture of tetraphenylpyrazine (7a), the benzoylaminostilbene 8a, the lactam 12a, benzamide (11a), and benzoic acid (14). Similar results were obtained in the reaction of 5a with potassium superoxide. The reaction of N-substituted pyrroles such as 5b,c with potassium gave the NH pyrrole 9b in each case, whereas the reaction of 5b,c with potassium in THF, saturated with oxygen, gave a mixture of 9b, the butanone 10b, the 1,4-dione 13b, the lactam 12b, the amides 11a–c, and benzoic acid (14). Attempted reactions of 5b,c with potassium superoxide did not give any isolable product; most of the starting material could be recovered unchanged in each case. A mixture of N-(1,2-diphenylethyl)benzamide (18a) and benzoic acid (14) was formed in the reaction of the oxazole 15a with potassium, whereas 15b,c, under analogous conditions, gave the N-vinylamides 17b,c and benzoic acid (14). In contrast, treatment of the imidazole 19 with potassium in THF did not give any product; however, when the reaction of 19 was carried out with potassium in THF saturated with oxygen, and with potassium superoxide, dibenzamide (21) was isolated, in each case.Radical ions have been invoked as intermediates in the transformation of the different substrates to the observed products. Cyclic voltammetric studies have been carried out to measure the reduction potentials of these radical anion intermediates. These radical anions have also been generated by pulse radiolysis in methanol, and their absorption spectra recorded. Keywords: nitrogen heterocycles, radical ions, potassium-induced transformations, pulse radiolysis, cyclic voltammetry.