Photochemical reaction of 2,4,4,6-tetrasubstituted 1,4-dihydropyridines in deaerated media: photocolouration and photorearrangement accompanying dehydrogenation

Abstract

The photochemical colour change of 2,2,4,6-tetraphenyl-1,4-dihydropyridine (1) in deaerated solutions from colourless to violet, which has been reported as a photochromic phenomenon, was found to be an irreversible change and to give 2,3,4,6-tetraphenylpyridine (7) as a main product. Spiro-[2,6-diphenyl-1,4-dihydropyridine-4,9′-fluorene](2), 4-p-bromophenyl-2,4,6-triphenyl-1,4-dihydropyridine (3), and 2,4,6-triphenyl-4-p-tolyl-1,4-dihydropyridine (4) also showed a similar photocolour change in deaerated solutions and gave 1,3-diphenyl-2-azatriphenylene (8), 3-p-bromophenyl-2,4,6-triphenyl-pyridine (9), and 2,3,6-triphenyl-4-p-tolylpyridine (11), respectively. These reactions proceed through di-π-methane rearrangement accompanying dehydrogenation. 4-Benzyl-2,4,6-triphenyl-1,4-dihydro-pyridine (5) and 4-benzyl-1-methyl-2,4,6-triphenyl-1,4-dihydropyridine (6) showed very faint colouration on irradiation. Compound (5) gave 2,4,6-triphenylpyridine (12) produced by elimination of the benzyl group and a hydrogen. For a compound (6) products have not been isolated.