Proton‐Coupled Electron Transfer (PCET) with 1,4‐Bisguanidino‐Benzene Derivatives: Comparative Study and Use in Acid‐Initiated C‐H Activation

Abstract

Proton‐coupled electron transfer (PCET) is of key importance in modern synthetic chemistry. Redox‐active guanidines were established by our group as valuable alternatives to toxic high‐potential benzoquinones in a variety of different PCET reactions. In this work, the PCET reactivity of a series of 1,4‐bisguanidino‐benzenes varying in their redox potentials and proton affinities is evaluated. The relevant redox and protonation states are fully characterized, and the compounds sorted with respect to their PCET reactivity by comparative PCET experiments supplemented by quantum‐chemical calculations. Depending on the studied reactions, the driving force is either electron transfer or proton transfer; thereby the influence of both processes on the overall reactivity could be assessed. Then, two of the PCET reagents are applied in representative oxidative aryl‐aryl coupling reactions, namely the intramolecular coupling of 3,3’’‐4,4’’‐tetramethoxy‐o‐terphenyl to give the corresponding triphenylene, the intermolecular coupling of N‐ethylcarbazole to give N,N’‐diethyl‐3,3’‐bicarbazole, and in the oxidative lactonization of 2‐[(4‐methoxyphenyl)methyl]‐benzoic acid. Under mild conditions, the reactions proceed fast and efficient. Only small amounts of acid are needed, in clear contrast to the corresponding coupling reactions with traditional high‐potential benzoquinones such as DDQ or chloranil requiring a large excess of a strong acid.

Connecting Organic Redox‐Active Building Blocks through Mild Noncatalytic C−H Activation

“…By coordination of transition metals, it should be possible to further vary the electronic structure and communication between the two redox-active units; here we could resort to the previous work on other GFAs, especially GFA2. [55,66,68,86]…”

“…Due to their strong Brønsted basicity in their reduced state, they act as oxidizing reagents if electron transfer is coupled with proton transfer. Hence, efficient oxidative aryl‐aryl coupling reactions and other selective oxidation reactions were reported [61–66] . They were also applied as redox‐active ligands in a variety of transition‐metal complexes [55,67,68] .…”

“…Hence, efficient oxidative aryl-aryl coupling reactions and other selective oxidation reactions were reported. [61][62][63][64][65][66] They were also applied as redox-active ligands in a variety of transition-metal complexes. [55,67,68] IET processes between the redox-active guanidine ligand and the metal atom in these complexes could be triggered by various means, such as temperature or solvent polarity changes, [69][70][71][72] co-ligand substitution [73] or addition, [74,75] coordination to a secondary coordination site, [76] or hydrogen-bonding.…”

Redox‐Active Dendrimer‐Like Oligoguanidines and Their Use in a Proton‐Coupled Electron Transfer Reaction

“…Especially in aryl-aryl coupling reactions, that are of enormeous importance in modern synthetic chemistry, [50,51] redox-active guanidines outperform the traditionally used, toxic benzoquinone derivatives like 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tetrachloro-1,4-benzoquinone (CA). [18,33,39,40]…”

“…Moreover, GFA compounds like GFA1, [32] GFA2 [33] and GFA3 [19] are currently quite intensively used in proton-coupled electron transfer (PCET) reactions [34,35] in their oxidized dicationic state, e. g. in oxidative aryl-aryl coupling reactions and other oxidations. [18,[36][37][38][39][40] According to Figure 1b, the dication (GFA3) 2 + takes up two electrons and two protons to give the reduced, twofold-protonated molecule (GFA3 + 2H) 2 + . [37] The reduced (protonated) GFA3 is quickly re-oxidized by dioxygen in the presence of a catalyst, and therefore could by used as redox-catalyst in selective oxidation reactions with dioxygen as terminal oxidant.…”