Pyrazine Radical Cations as a Catalyst for the Aerobic Oxidation of Amines

Abstract

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.

Fluorescent Hybrid Porous Polymers as Sustainable Heterogeneous Photocatalysts for Cross-Dehydrogenative Coupling Reactions

“…Once the activity of these HPPs as oxygen photosensitizers was demonstrated, we decided to assess the generality of this reactivity in another model reaction, namely, the oxidative homocoupling of benzylic amines, as we have previously studied it using phenazine radical cations and POSS-based hybrid porous polymers as catalysts under thermal conditions. To our delight, the tested amines 4 could also be converted into the corresponding imines 5 with excellent yields under the established photocatalytic conditions (Table ).…”

Non‐emissive RuII Polypyridyl Complexes as Efficient and Selective Photosensitizers for the Photooxidation of Benzylamines

“…In additional experiments, full conversion to the desired products was obtained for substrates 1 e , 1 f , 1 g and 1 h , when the photocatalyst loading was increased to 1 mol % (entries 5–8 in Table ). By contrast, the application of this protocol to other challenging substrates such as α‐methylbenzylamine ( 1 k ), n ‐hexylamine ( 1 l ) or 1,2,3,4‐tetrahydroquinoline ( 1 m ) failed to produce the expected imines in agreement with the results reported for other catalytic systems (entries 11, 12 and 13 in Table ). Consequently, the presence of either steric hindrance on the α‐C as in 1 k , or nonactivated (non‐benzylic) α‐H, as in 1 l and 1 m , is strongly detrimental for this oxidation.…”

Phenazine Radical Cations as Efficient Homogeneous and Heterogeneous Catalysts for the Cross‐Dehydrogenative Aza‐Henry Reaction

“…Recently, our group reported a novel synthetic methodology for the synthesis of the elusive 1,4functional tetramethylpyrazines (Scheme 1,A) and used them as catalysts for the aerobic oxidative homocoupling of primary amines, thanks to the reversible redox behavior of these compounds and the remarkable stability of their radical-cationic form. [62] We explored further the exploitation of stable radical cations as oxidation catalysts with the development of a series of 5,10-disubstitued phenazines (Scheme 1,B) which showed remarkable activities for the oxidative homo and cross coupling of amines. [63] Interestingly, even though the strong redox properties of phenazines and the stability of their open-shell derivatives are relatively well-known, there are very few additional reports on their use as redox catalysts.…”