Mechanistic Insight into the Formation of Tetraarylazadipyrromethenes

The tetraarylazadipyrromethene chromophore class has gained increasing attention in the past decade for a diverse set of scientific interests and applications. The most direct synthetic route available for their generation is heating of 4-nitro-1,3-diarylbutan-1-ones with an ammonium source in an alcohol solvent. Despite the practical simplicity, the reaction pathway(s) for these conversions are lengthy and unclear. To gain insight into the steps involved, N-15 labeling experiments with MS and NMR analysis were utilized for conversion of 4-nitro-1,3-diphenylbutan-1-one 1 into tetraphenylazadipyrromethene 2 with (NH4OAc)-N-15. To, permit examination, of later stages of the reaction sequence to 2, the N-15-labeled potential intermediate 3,5-diphenyl-1H-pyrrol-2-amine 10 was synthesized. A study of the dimerization pathway utilizing N-15-labeled 10 revealed an unprecedented nitrogen rearrangement in the final stages of the pathway involving a ring:opening/closing of a pyrrole ring. Utilizing N-15 labeling experiments we have shown that 2,4-diphenylpyrrole 8 can also react under the reaction conditions with 3,5-diphenyl-2H-pyrrol-2-imine 7 (from oxidation of 10) to. produce 2. Overall in the conversion of 1 into 2, two related pathways are ongoing concurrently, the first involves a dimerization of 3,5-diphenyl-2H-pyrrol-2-imine 7, and the other a reaction of 7 with 2,4-diphenylpyrrole 8.