Reactivity of Superbasic Carbanions Generated via Reductive Radical‐Polar Crossover in the Context of Photoredox Catalysis

• Published in: Angewandte Chemie International Edition 2024-04, Vol.63 (18), p.e202400815-n/a
• Main Authors: Grotjahn, Sascha, Graf, Christina, Zelenka, Jan, Pattanaik, Aryaman, Müller, Lea, Kutta, Roger Jan, Rehbein, Julia, Roithová, Jana, Gschwind, Ruth M., Nuernberger, Patrick, König, Burkhard
• Format: Article
• Language: English
• Subjects: Acetonitrile; Aldehydes; Basicity; Carbanions; Catalysis; Crossovers; Dimethylformamide; Electron transfer; Esters; Intermediates; Isotope effects; Ketones; Mode of action; Organometallic compounds; Photocatalysis; Photoredox catalysis; Protons; Reaction mechanisms; Solvents
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202400815

Photocatalytic reactions involving a reductive radical‐polar crossover (RRPCO) generate intermediates with carbanionic reactivity. Many of these proposed intermediates resemble highly reactive organometallic compounds. However, conditions of their formation are generally not tolerated by their isolated organometallic versions and often a different reactivity is observed. Our investigations on their nature and reactivity under commonly used photocatalytic conditions demonstrate that these intermediates are indeed best described as free, superbasic carbanions capable of deprotonating common polar solvents usually assumed to be inert such as acetonitrile, dimethylformamide, and dimethylsulfoxide. Their basicity not only towards solvents but also towards electrophiles, such as aldehydes, ketones, and esters, is comparable to the reactivity of isolated carbanions in the gas‐phase. Previously unsuccessful transformations thought to result from a lack of reactivity are explained by their high reactivity towards the solvent and weakly acidic protons of reaction partners. An intuitive explanation for the mode of action of photocatalytically generated carbanions is provided, which enables methods to verify reaction mechanisms proposed to involve an RRPCO step and to identify the reasons for the limitations of current methods. Photocatalytic reactions with a reductive radical‐polar crossover (RRPCO) involve intermediates with carbanionic reactivity. These are best described as free carbanions. Reactions with such carbanions depend on the balance between their nucleophilicity and basicity. Deprotonation of reaction partners and common organic solvents such as acetonitrile, dimethylformamide, and dimethylsulfoxide is the main competing reaction to nucleophilic addition.