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.
Highlights
Flavin-mediated photocatalytic oxidations are established in synthetic chemistry
A series of 5-dFls 1–5 was synthesised with different substituents in position 5 (H, Me, iPr, CF3, and Ph-) via two different routes depending on the substituent (Fig. 2 and Supplementary Notes 1 and 2)
Depending on the reaction conditions for the following step, the corresponding products 1–4 were obtained. As this synthetic method did not allow the synthesis of the phenyl derivative 5, route B was developed. In this three-component microwave-assisted procedure, adapted from Tu et al.[49], compound 5 was obtained from a reaction of benzaldehyde with N-butyl-3,4-dimethoxyaniline (7) and N-methylbarbituric acid (9)
Summary
Depending on the reaction conditions for the following step, the corresponding products 1–4 were obtained As this synthetic method did not allow the synthesis of the phenyl derivative 5, route B was developed. In this three-component microwave-assisted procedure, adapted from Tu et al.[49], compound 5 was obtained from a reaction of benzaldehyde with N-butyl-3,4-dimethoxyaniline (7) and N-methylbarbituric acid (9). Following this procedure, the phenyl derivative could be isolated in both the oxidised (5ox) and fully reduced form (5red), which reflects the low sensitivity of the a Flavins: XO Riboflavin (RF) : X = H
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