Probing Intramolecular Electron Transfer in Redox Tag Processes

Abstract

Herein, we show that redox tag-guided intermolecular formal [2 + 2] cycloaddition can be used as a probe to investigate intramolecular single-electron transfer (SET) mechanisms. The efficacy of intramolecular SET can be evaluated in association with concomitant carbon–carbon bond formation and/or cleavage, leading to cycloaddition or cross-metathesis. Experimental and theoretical results suggest that the intramolecular SET is under both thermodynamic and kinetic control and can also occur through bonds, not only through space.

Cationic DABCO-Based Catalyst for Site-Selective C–H Alkylation via Photoinduced Hydrogen-Atom Transfer

“…Instead, if the catalyst has a suitable aromatic moiety as a redox mediator on its N -substituent, the oxidation preferentially occurs at the aromatic ring to form the transient intermediate A . Subsequent intramolecular electron transfer from the nitrogen atom to the aromatic ring furnishes B as the active radical species, which abstracts the hydrogen atom from the C–H bond of the substrate . The resultant nucleophilic radical reacts with the electron-deficient alkene to form a new C–C bond, along with the generation of a carbon-centered radical, which subsequently undergoes the SET from the reduced form of acridinium catalyst ( Mes-Acr • ) and protonation to afford the alkylated product.…”

Radical cation Diels–Alder reactions of arylidene cycloalkanes

“…We have developed radical cation cycloadditions using (photo)electrochemical single-electron transfer in lithium per- chlorate (LiClO 4 )/nitromethane (CH 3 NO 2 ) solution [36][37][38][39][40][41][42][43][44]. During the course of our studies, we found that the TiO 2 photoelectrochemical approach was more beneficial than simple electrochemistry in most cases, probably because both single-electron oxidation and reduction are made possible at the same surface [45].…”

Design of a Photocatalytic [2+2] Cycloaddition Reaction Using Redox‐Tag Strategy

“…Therefore, it is important to consider not only electron transfer between the photocatalyst and substrate, but also intra‐ and intermolecular electron transfers, while designing a photocatalytic process. Since direct observation of electron transfer is not easy, synthetic outcomes [9] and theoretical calculations [10] are suitable tools for investigating the electron transfer process.…”

“…It is also suggested that the success of intramolecular SET is determined by the design of the substrate. The substituent on the aromatic ring and the length of the alkyl chain between the aromatic ring and enol ether (Table 1) serve as controlling factors [9] . Since the aromatic ring serves as the electron donor in intramolecular SET an appropriate electron density in the aromatic ring needs to be achieved.…”

“…In addition, our previous study suggested that the intramolecular SET proceeded through bonds, not through space, with a long chain length inhibiting successful intramolecular electron transfer. [9] We call such a reaction design based on intramolecular SET a Redox-Tag strategy which was effectively applied to the radical cation Diels-Alder reaction and the vinylcyclopropane rearrangement. [12] With the Redox-Tag concept in hand, we herein report new [2 + 2] cycloaddition reactions using a TiO 2 photocatalyst.…”