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

Abstract

The redox activity of molecular phenazine catalysts has been previously exploited for aerobic oxidative amine homo-and cross-coupling reactions. In this contribution, we have extended the reaction scope of this novel type of organocatalyst and used them in the cross-dehydrogenative aza-Henry coupling of isoquinolines with nitromethane under aerobic conditions. Additionally, we have designed and prepared a novel porous organic polymer by cross-linking of tetrakis(4-bromophenyl)silane and dihydrophenazine through Pd-catalyzed Buchwald-Hartwig cross-coupling. This new type of heterogeneous catalyst, apart from being robust and easily reusable, also showed outstanding catalytic activities and improved selectivity compared to its molecular counterpart. A plausible reaction mechanism was proposed based on spectroscopic and kinetic measurements.

Electron Donor–Acceptor (EDA) Complex between a Triarylamine and B(C₆F₅)₃ for the Photocatalytic Dehydrogenative Cross-Coupling of Phenols

“…Triarylaminium radical cations ( +• NAr 3 ) are exceptionally stable persistent radicals that are used in organic synthesis, materials science, and organometallic chemistry as mild yet relatively strong single electron oxidants "magic blue". 1) They are generally prepared from the corresponding neutral triarylamines (NAr 3 ) with the assistance of a silver-based oxidant 2) ; however, from an economic and environmental perspective, it would be desirable to avoid the use of a stoichiometric amount of such an expensive metal reagent, and thus, approaches to generate the radical cation more efficiently have recently been explored [3][4][5][6][7][8][9] (Chart 1a). Liu and Klussmann have reported that triarylamines can be oxidized in the presence of a stoichiometric amount of benzoyl peroxide and that the resulting triarylaminium radical cations successfully promote the difunctionalization of styrenes.…”

Organo-redox-catalysis for the difunctionalization of alkenes and oxidative Ritter reactions by C–H functionalization

“…17,24 Both the amines and the radical cations are also utilized as redox-catalysts in electrochemical 25,26 or photochemical reactions, 27,28 as well as in aerobic oxidations. 29,30 Despite this plethora of applications, we are not aware of amine-based redox catalysis in the activation of peroxides, which would open many opportunities for synthetic applications.…”

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

“…In 2019, Aprile and co-workers reported the use of POSS-based materials as catalysts for the preparation of cyclic carbonates from CO 2 and epoxides, wherein the catalysts were successfully recovered and reused for four consecutive runs . In 2020, our group reported the efficient aza-Henry reaction , using phenazine-containing POSS-based polymers as a catalyst under aerobic conditions . POSS-based materials and composites have found also a relatively broad use in light-emitting applications, including photoelectronic, sensing, and imaging devices. − Comparatively, the use of POSS-based POPs as a photocatalyst in organic synthesis remains scarce …”

“…40 In 2020, our group reported the efficient aza-Henry reaction 41,42 using phenazine-containing POSS-based polymers as a catalyst under aerobic conditions. 43 POSS-based materials and composites have found also a relatively broad use in light-emitting applications, including photoelectronic, sensing, and imaging devices. 44−47 Comparatively, the use of POSS-based POPs as a photocatalyst in organic synthesis remains scarce.…”