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

Abstract

Compounds with two or more redox‐active units, connected by a bridge that supports electronic coupling between the units, have been studied for a long time. Redox reactions could result in mixed‐valence compounds, in which the electronic structure depends on the degree of electronic coupling. Here we report the synthesis of the first molecule in which two redox‐active guanidines are connected through a bridge. The compound is synthesized in an unprecedented, simple one‐step procedure involving an atom‐economically favorable, non‐catalytic C−H activation reaction at room temperature from novel 1,2,4‐trisguanidino‐benzene molecules and 1,4‐benzenedithiol. We then apply similar protocols to introduce other substituents in 5‐position of a 1,2,4‐trisguanidino‐benzene molecule, demonstrating the scope of these exceptional C−H activation reactions. Finally, we report the synthesis of a hexakisguanidino‐biphenyl by direct coupling of two trisguanidino‐benzene units through C−H activation. Its oxidation leads to the first mixed‐valence compound with charge delocalization between two redox‐active guanidino units. The redox states of the resulting novel compounds are systematically studied.

Intramolecular Through‐Space Double‐Electron Transfer Between A Pair of Redox‐Active Guanidine Units Aligned by Dithiolate Bridges

“…The imino N=C bond lengths in the three In Scheme 2, a mechanism for this peculiar reaction is proposed, that is in accordance with the results presented in this work and in a previous study. [62] Nucleophilic attack at the C atom in position 5 of the C 6 ring is supported by proton transfer to the guanidino group in position 4. The SÀ H bond cleavage and the NÀ H bond formation presumably proceed in one step, as sketched in Scheme 2.…”

“…Moreover, the E 1/2 values are close to that measured for GFA2 (E 1/2 = À 0.56 V). [62] From the two E 1/2 values, the Gibbs free energy for disproportionation of (GFA4) 2 + into GFA4 and (GFA4) 4 + could be estimated. With the two formula ΔG = À 2 F • ΔE 0 (and ΔE 0 � ΔE 1/2 ) and ΔG = À RT lnK, one obtains ΔG = 15.4 kJ mol À 1 and K = 2.08 • 10 À 3 (298 K).…”

“…[67][68][69][70][71][72] Very recently, we showed that oxidation of 1,2,4-triguanidino-benzenes (e. g. GFA1, Scheme 1a) leads to the activation of the CÀ H group in 5-position of the C 6 ring, and provides the key for derivatisation and aryl-aryl coupling reactions. [62] Using this new synthetic tool, we synthesized first GFA2 and then GFA3 (see Scheme 1b), with two triguanidino groups connected by a benzenedithiolate bridge. The analysis showed that the electronic coupling through the dithiolate bridge is weak.…”

“…[59] Our group developed guanidino-functionalized aromatic molecules (GFAs) as a new, versatile class of redox-active organic molecules. [60,61] One example of relevance for this work, 1,2,4-tris(N,N'-dimethylethylene-guanidino)-benzene (GFA1), [62] is sketched in Scheme 1a. For this compound, one-and twoelectron oxidation occur at the same potential; from cyclic voltammetry measurements an E 1/2 value of À 0.52 V (E ox = À 0.46 V) in CH 2 Cl 2 was obtained.…”

“…For this compound, one-and twoelectron oxidation occur at the same potential; from cyclic voltammetry measurements an E 1/2 value of À 0.52 V (E ox = À 0.46 V) in CH 2 Cl 2 was obtained. Benzene derivatives with three, [62] four [60,63] or six [64] guanidino groups were prepared, and the redox potential was found to shift continuously to lower values with increasing number of guanidino substituents. A number of studies demonstrated the influence of the redox potential on modifications of the aromatic core, the guanidino groups and additional substituents at the aromatic core.…”