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Twofold Oxidized and Twofold Protonated Redox‐Active Guanidine: An Ultimate Intermediate in Proton‐Coupled Electron‐Transfer Reactions
Author(s) -
Wild Ute,
Hübner Olaf,
Greb Lutz,
Enders Markus,
Kaifer Elisabeth,
Himmel HansJörg
Publication year - 2018
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201801378
Subject(s) - chemistry , protonation , redox , reagent , proton coupled electron transfer , electron transfer , counterion , photochemistry , proton , guanidine , radical ion , solvent , hydrogen bond , inorganic chemistry , ion , organic chemistry , molecule , physics , quantum mechanics
Due to their high Brønsted basicity and redox properties, guanidino‐functionalized aromatics (GFAs) are privileged proton‐coupled electron‐transfer (PCET) reagents. Herein we show that the GFA 1,2,4,5‐tetrakis(tetramethylguanidino)‐benzene ( 1 ), after being oxidized to the green dicationic state ( 1 2+ ), can bind one or two protons, resulting in the blue monoprotonated trication ( 1 +H) 3+ and the orange diprotonated tetracation ( 1 +2H) 4+ , realizing the first purely organic PCET reagent that can be diprotonated in its oxidized state. With trifluoroacetate counterions, low‐barrier hydrogen‐bonding is observed, allowing to shift protons reversibly in dependence of the temperature and solvent polarity between 1 2+ and trifluoroacetate. The protonation of 1 2+ has dramatic effects on its oxidation capabilities.