A Modular Synthesis of Unsymmetrical Tetraarylazadipyrromethenes

A stepwise route to unsymmetrical tetraarylazadipyrromethenes by a condensation of 2,4-diaryl-5-nitroso-pyrroles with 2,4-diarylpyrroles is described. This modular building-block approach allows
for the introduction of up to four different aryl substituents on the azadipyrromethene and is tolerant
of a varied substituent set. An efficient synthesis of the 2,4-diarylpyrroles building blocks from
1,3-diaryl-4-nitro-butan-1-ones by nitro hydrolysis to a keto-aldehyde and subsequent ammonia
condensation reaction has been achieved. The facile conversion of 2,4-diarylpyrroles into their
α-nitroso analogues by their reaction with sodium nitrite generated the second building block
required for the synthesis.