Phenazine
radical cations (PhRCs) were used for the first time
as efficient metal-free catalysts for the oxidative homo- and cross-coupling
of a variety of different amines. A series of functional PhRCs were
prepared, characterized with X-ray diffraction, and their radical
character was investigated with DFT calculations. They were tested
as catalysts under neat conditions with low oxygen pressure to prepare
homo- and cross-coupled aliphatic and aromatic imines in high yields.
Although all synthesized phenazines were catalytically active, the
highest reaction rates and the best selectivity were achieved using
the 5,10-dihydro-5,10-dimethylphenazine radical cation. By means of
fluorescence, UV–vis and EPR spectroscopy, a mechanism of the
oxidative amine coupling, catalyzed by PhRCs, is proposed.
An efficient and green synthetic method for the preparation of N,N‐disubstituted tetramethylpyrazinium radical cations was developed, which does not require any solvent or strong alkylating agents. The investigation by UV/Vis and electron paramagnetic resonance (EPR) spectroscopy proved the stability of the prepared radical cation. Moreover, it was shown for the first time that the prepared N,N‐disubstituted tetramethylpyrazine can act as a powerful metal‐free catalyst for the selective oxidative homocoupling of amines to the corresponding imines under solvent‐free conditions. Furthermore, oxygen or even air can be employed as an efficient oxidative reagent.
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