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Mechanistic Insight into Formation of Tetraarylazadipyrromethenes
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Dr Michael Hall
Grossi M, Palma A, McDonnell SO, Hall MJ, Rai DK, Muldoon J, O'Shea DF
Journal of Organic Chemistry
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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. In spite of the practical simplicity, the reaction pathway(s) for these conversions are lengthy and unclear. In order to gain insight into the steps involved, 15N labelling experiments with MS and NMR analysis were utilized for conversion of 4-nitro-1,3-diphenylbutan-1-one 1 into tetraphenylazadipyrromethene 2 with 15NH4OAc. To permit examination of later stages of the reaction sequence to 2, the 15N labelled potential intermediate 3,5-diphenyl-1H-pyrrol-2-amine 10 was synthesized. A study of the dimerization pathway utilizing 15N labelled 10 revealed an unprecedented nitrogen rearrangement in the final stages of the pathway involving a ring opening/closing of a pyrrole ring. Utilizing 15N labelling 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, the other a reaction of 7 with 2,4-diphenylpyrrole 8.
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