Back electron transfer

Some photochemical synthesis methods are hampered by back electron transfer, an unproductive relaxation pathway. To prevent this, scientists have identified strategies to promote productive reaction steps: these include leaving groups, redox auxiliaries, and electrostatic interactions.

For a photochemical reaction to result in a high-yielding product, competing pathways must be minimized to favor the productive pathway. In photochemical synthesis based on electron transfer one of those competing pathways is back electron transfer.

Back electron transfer can hamper the progress of the forward reaction, for example in photochemical methods where a charge transfer complex is formed and excited that consists of an electron donor and an acceptor.¹⁾ Figure 1 illustrates how back electron transfer competes with forward reaction progress that leads to the desired product. Upon irradiation of a charge transfer complex, a radical ion pair can form upon single electron transfer (SET). Back electron transfer can then regenerate the electron donor (D) and the electron acceptor (A). To promote forward reaction progress, several strategies have been employed; a suitable leaving group (green, Figure 1) solves this issue: losing a leaving group or a redox auxiliary leads to a radical which can now react further to form the desired product.

Leaving groups circumvent back electron transfer

Previously, charge transfer complex-based chemistries typically relied on a stoichiometric use of the donor and acceptor, which usually led to stoichiometric waste and a poor atom economy. Additionally, back electron transfer, as a competing relaxation pathway, hampered these methods. Using a leaving group to prevent back electron transfer has proven useful in developing synthetically useful charge transfer complex-based chemistries. Further developments have resulted in elegant catalytic strategies: either the donor²⁾ or the acceptor³⁾ was applied in catalytic amounts (Figure 2). The system requires a reductive turnover of the donor or an oxidative turnover of the acceptor so that they can be used in catalytic amounts. The turnover can either be introduced via an external reductant or oxidant or by an inherent reductive or oxidative step during the reaction. Finally, a radical (A^•, D^•) is generated from the corresponding donor or acceptor, which then reacts with a radical acceptor to generate the product.

Electrostatics as a tool

Back electron transfer was investigated for a charge transfer mediated formal [2+2] photocycloaddition. In this reaction, there was no leaving group to circumvent back electron transfer – hence, a slow reaction was observed. Kalow and co-workers studied this formal [2+2] photocycloaddition in cucurbit[8]uril (CB[8]) as a host. They observed that an azastilbene, 4,4’-stilbenedicarboxylate and the CB[8] ternary complex reacted selectively in a cross-photocyclization whereas other substrates did not react (Figure 3).⁴⁾ They found that after photoexcitation of the charge transfer complex the electron transfer process is slower. This is the case because the process takes place in the Marcus inverted region. This is a result of Marcus‘ theory: whereas more exergonic reactions usually have higher reaction rates, electron transfer should, according to Marcus‘ theory, become slower in the negative ΔG° domain.

Electrostatic interactions between the substrates stabilize the ground state and thus, the productive cyclization reaction competes with the back electron transfer relaxation pathway.

Although charge transfer complexes from several substrates were investigated, only one complex resulted in productive cyclization reaction, namely the complex stabilized by electrostatics in the ground state. Transient absorption spectroscopy revealed that for this complex, back electron transfer was slower and the radical ion pair had a longer lifetime (successful pair: t = 40 ps; unsuccessful pairs: t = 0.3 – 10 ps). Because the forward photocyclization reaction is slow it benefits from the use of a CB[8] host that protects the radical ion pair from side reactions.

Another way to suppress back electron transfer

These reactions are related to the use of charge transfer based chemistries without the need for a photocatalyst. However, back electron transfer is important for photocatalysis. In a recent example (Figure 4)⁵⁾, Leonori and coworkers minimized the back electron transfer. This generated α-anilinoalkyl radicals that are synthetically applied in various reactions. The forward reaction outcompeted back electron transfer because a tertiary amine additive facilitated single electron transfer and subsequent deprotonation.

The mechanism

While the photochemical community has recognized that back electron transfer is important, there is limited information on the mechanism of this significant pathway. Shigeto and coworkers investigated the back electron transfer mechanism from the radical ion pair, a benzene-1,2-dicarbonitrile (DBC) radical anion and pyrene dimer radical cation (Figure 5).⁶⁾ Herein, the solvent acetonitrile (MeCN) acts as a charge mediator. In the proposed mechanism, the DBC radical anion transfers an electron to the solvent (top, Figure 5). Subsequently, the charges of the resulting dimer anion of acetonitrile and the dimer pyrene radical cation recombine to form pyrene and acetonitrile (bottom, Figure 5). More information regarding the mechanism of back electron transfer and how it can be avoided will surely provide valuable tools in the development of photochemical synthesis strategies based on electron transfer.

AUF EINEN BLICK

Manche photochemischen Synthesen scheitern daran, dass der angeregte Zustand nicht weiter reagiert, sondern relaxiert.

Back electron transfer ist ein solcher unproduktiver Reaktionspfad.

Manchmal gelingt es Forschenden jedoch, diesen zu umgehen: durch Abgangsgruppen und Radikalfänger, elektrostatische Effekte oder Additive.

Dies ist der letzte Beitrag zur Serie Blickpunkt Synthese von Johannes Walker und Line Næsborg. Ab jetzt schreiben im Wechsel Alicia Casitas, Universität Marburg, und Larissa von Krbek, Universität Bonn.

Korrigendum: Beim letzten Beitrag aus der Blickpunkt-Reihe, dem Blickpunkt Anorganik von Birkel et al. [Nachr. Chem. 2024, 72(12), 50], wurde in der Printversion der Coautor Niels Kubitza nicht genannt. Die Redaktion bedauert diesen Fehler.

Der Autor

Line Næsborg has been a junior group leader and Liebig fellow at the University of Münster since 2020. Her research revolves around the development of photochemical methods using micelle hosts that enable different pathways and outcomes compared to organic solvents.

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Wissenschaft + ForschungBlickpunkt Synthese