A Toolbox Approach To Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor–Acceptor Cyanoarenes

Abstract

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor−acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors' molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts' redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely organic photoredox catalysts were evaluated in three challenging, mechanistically distinct classes of benchmark reactions (either requiring balanced, highly oxidizing or strongly reducing properties) to demonstrate their enormous potential as customizable photocatalysts, that outperform and complement prevailing typical best photocatalysts.

Der “Persistent Radical Effect” in der organischen Chemie

“…Cyanoarene sind elektronenarme Arene,d ie über einen Elektronentransfer reduziert werden kçnnen. [79] Zum Beispiel kann durch die photoinduzierte Oxidation eines Fluoren-basierten Polymers mit Dicyanobenzol das entsprechende Radikalkation zusammen mit dem Dicyanobenzol-Radikalanion erzeugt werden. Die beiden ionischen Spezies dissoziieren und der getrennte Ladungszustand weist eine lange Lebensdauer ( % 30 ns) auf.…”

Photochemical Hydroacylation of Michael Acceptors Utilizing an Aldehyde as Photoinitiator

“…To solve these problems, photo-organocatalysis came into the spotlight:i ti sm ore sustainable, eco-friendly,a nd gradually becomes equallye ffective as entirely organic catalysts are utilized for the same transformations. [2] Over the last few years, we have initiatedaprogram with the aim to contribute to this domain by developing ap hoto-organocatalytic protocol,u tilizing commonh ousehold lamps and ac ommercially available small organic molecule as the photocatalyst. [3] One of these applications was the green photochemical CÀHa ctivation of aldehydes, leading to the hydroacylation of electron-deficient olefins (Scheme 1d).…”

Organophotoredox‐Mediated Amide Synthesis by Coupling Alcohol and Amine through Aerobic Oxidation of Alcohol

“…Allylic alcoholc oupled successfully with ap rimary amine (Scheme 2, compound 27). Aliphatic alcohols were less favorable substrates for amide formations (Scheme 2, compounds [28][29][30].…”