Hole-Mediated PhotoRedox Catalysis: Tris(p-Substituted)biarylaminium Radical Cations as Tunable, Precomplexing and Potent Photooxidants

Abstract

<p>Electrochemically-mediated Photoredox Catalysis emerged as a powerful synthetic technique in recent years, overcoming fundamental limitations of electrochemistry and photoredox catalysis in the single electron transfer activation of small organic molecules. However, the mechanism of how photoexcited radical ion species with ultrashort (picosecond-order) lifetimes could ever undergo productive photochemistry has eluded synthetic chemists. We report tri(<i>para</i>-substituted)biarylamines as a tunable class of electroactivated photocatalysts that become superoxidants in their photoexcited states, even able to oxidize molecules (such as dichlorobenzene and trifluorotoluene) beyond the solvent window limits of cyclic voltammetry. Furthermore, we demonstrate that precomplexation not only permits the excited state photochemistry of tris(<i>para</i>-substituted)biarylaminium cations, but enables and rationalizes the surprising photochemistry of their <i>higher-order</i> doublet (D<i><sub>n</sub></i>) excited states.</p>

Photocatalyst-Free, Visible-Light-Mediated C(sp3)–H Arylation of Amides via a Solvent-Caged EDA Complex

“…Further work is ongoing to establish the factors that dictate EDA complexation when certain arenes are similarly electron rich. We are excited by future implications of "preassembly" as an emerging control element in photochemistry and photocatalysis, as well as opportunities to use solvation, in addition to dispersion interactions, 41 hydrogen bonding, 42 and microheterogeneous solutions, 43 to exert control of photochemical outcomes through confined spaces.…”

Emerging Activation Modes and Techniques in Visible-Light-Photocatalyzed Organic Synthesis

“…Interestingly, multiple instances have been reported where quenching of a higher excited D-state is deemed responsible for extreme oxidation/reduction events. 49 59 61 This ‘anti-Kasha’ behavior can be discerned from conventional lowest excited-state mechanisms by comparison of the calculated D 1 energy with the potential necessary for the observed redox step. Alternatively, one could irradiate the radical ion with such a wavelength so that only a selectively excitation from D 0 to D 1 is possible.…”

“…When bimolecular reactivity is nonetheless observed, precomplexation of the substrate with the ground-state radical anion/cation catalyst is most likely to be involved. 59 61 Unfortunately, finding experimental evidence for this ground-state preassembly remains difficult, as its effect on UV-vis and EPR spectra is often absent. 49 59 However, when investigating the mechanism of the tri(biphenyl-4-yl)amine (T p BPA) catalyzed N -arylation of pyrazoles (similar to Scheme 26 ), the Barham group found spectroscopic evidence of precomplexation between the T p BPA radical cation and arene substrates.…”

“…49 59 However, when investigating the mechanism of the tri(biphenyl-4-yl)amine (T p BPA) catalyzed N -arylation of pyrazoles (similar to Scheme 26 ), the Barham group found spectroscopic evidence of precomplexation between the T p BPA radical cation and arene substrates. 61 The generation of extreme oxidants or reductants in an intimate catalyst-substrate precomplex might furthermore allow redox processes at potentials which would normally not be tolerated by the solvent. 52b…”

Tris(4-bromophenyl)aminium Hexachloroantimonate as a “Waste-Utilized”-Type Initiator-Promoted C–H Chlorination via C–H Activation Relay: Synthesis of Chlorinated Pyrroles

“…We have been engaged in the research of radical cation salts for a long time, and the counterions also exert significant effects on the reactivity of radical cation salts. 2 For example, triarylaminium radical cation salts with different counterions (PF 6 − , BF 4 − , ClO 4 − , SbCl 6 − , and so on) exhibit inequable oxidizability, 3,4 and in some certain conditions, the SbCl 6…”