Synthesis of unsymmetrical N-(2-tert-butylphenyl)-N-(4-tert-butylphenyl)nitroxyl radical, the first stable diarylnitroxyl with vacant para-position

The title compound, first example of a stable diarylnitroxyl with vacant para-position, was best synthesized by CuCl-assisted coupling of o-tert-butylnitrosobenzene and p-tert-butylphenylboronic acid followed by N-xidation of the thus obtained unsymmetrical diarylamine. ESR investigation showed that ortho-substituted aromatic ring is removed from the conjugation plane providing unusual stability of this radical.