Deazaflavin reductive photocatalysis involves excited semiquinone radicals

Abstract

Flavin-mediated photocatalytic oxidations are established in synthetic chemistry. In contrast, their use in reductive chemistry is rare. Deazaflavins with a much lower reduction potential are even better suited for reductive chemistry rendering also deazaflavin semiquinones as strong reductants. However, no direct evidence exists for the involvement of these radical species in reductive processes. Here, we synthesise deazaflavins with different substituents at C5 and demonstrate their photocatalytic activity in the dehalogenation of p-halogenanisoles with best performance under basic conditions. Mechanistic investigations reveal a consecutive photo-induced electron transfer via the semiquinone form of the deazaflavin as part of a triplet-correlated radical pair after electron transfer from a sacrificial electron donor to the triplet state. A second electron transfer from the excited semiquinone to p-halogenanisoles triggers the final product formation. This study provides first evidence that the reductive power of excited deazaflavin semiquinones can be used in photocatalytic reductive chemistry.

Photocatalytic Oxidative [2+2] Cycloelimination Reactions with Flavinium Salts: Mechanistic Study and Influence of the Catalyst Structure

“…A self‐constructed Time Correlated Single Photon Counting (TCSPC) setup [16h,30] was used to record emission decay data at single detection wavelength. A quartz cuvette with four optical windows of the dimension 2 mm×10 mm was used.…”

“…The ns to ms transient absorption spectroscopy was recorded by a streak camera setup as described previously [16h,21a,30–31] . In brief, the third harmonic of a Nd : YAG laser (10 Hz, Surelite II, Continuum) pumping an Optical Parametric Oscillator (OPO, Continuum) tuned to 355/460 nm (10 mJ, ca .…”

“…The Sub‐ps Pump/Supercontinuum‐Probe Spectroscopy were carried out using UV/Vis pump‐supercontinuum probe spectrometer at 1 kHz repetition frequency as described in ref [16h] . In brief, a Ti‐sapphire amplifier system (Coherent Libra) was used to generate 800 nm with 1.2 mJ pulses at 1 kHz.…”

Molecular Editing of Flavins for Catalysis

“…64 In 2020, König, Cibulka, Kutta, and co-workers reported deazaflavin 24, which was successfully applied in one-electron reductions under irradiation resulting in dehalogenation of aryl halide substrates (Scheme 18A). 65 These results are remarkable since their benchmark substrate 4-bromoanisole is very resistant towards reduction: E red = -2.75 V (vs. SCE). In a very detailed spectro-electrochemical study, it was shown that the active catalyst species is the photoexcited semiquinone 25* (Scheme 18B), which is a very strong reductant E* = -3.3 V (vs. SCE).…”

Tuning Riboflavin Derivatives For Photodynamic Inactivation of Pathogens

“…[5][6][7][8] This photoexcitation process has been exploited in a number of photocatalytic applications, whereby modification of the flavin chromophore can enable the formation of high potential oxidative and reductive intermediates to afford useful synthetic methodology. [9][10][11] Another application in which photoexcitation is utilised to generate reactive intermediates is within photodynamic inactivation (PDI) of pathogens. In this technique, a photosensitiser (PS) such as riboflavin, is employed to generate a burst of reactive oxygen species (ROS) in the vicinity of a particular pathogen in order to cause irreversible damage that leads to inactivation.…”