Acyl Radical Chemistry via Visible-Light Photoredox Catalysis

Abstract

Visible light photoredox catalysis has enabled easy access to acyl radicals under mild reaction conditions. Reactive acyl radicals, generated from various acyl precursors such as aldehydes, α-ketoacids, carboxylic acids, anhydrides, acyl thioesters, acyl chlorides, or acyl silanes, can undergo a diverse range of synthetically useful transformations, which were previously difficult or inaccessible. This review summarizes such recent progress of visible-light-driven acyl radical generation using transition metal photoredox catalysts, metallaphotocatalysts, hypervalent iodine catalysts or organic photocatalysts.

Photochemistry of a 9‐Dithianyl‐Pyronin Derivative: A Cornucopia of Reaction Intermediates Lead to Common Photoproducts

“…In pathway A , carboxylic acid 13 could simply be formed by a dark autoxidation of aldehyde 11 in the presence of oxygen via radical photoinitiation (Scheme ) . The resulting acyl radical could efficiently couple with ground‐state oxygen to form peroxy acid 18 , which would react with another molecule of aldehyde and rearrange to give a Criegee‐like intermediate, which disproportionates into two molecules of acid 13 . Our previous work suggested that pyronin‐9‐carboxylates, analogous to xanthene‐9‐carboxylic acid, could release carbon monoxide (CO) upon irradiation via a putative α‐lactone intermediate ( 19 , Scheme ), observed, for example, in atmospheric photochemistry of methacrolein .…”

Combined Photoredox and Carbene Catalysis for the Synthesis of Ketones from Carboxylic Acids**

“…When Az‐6 was employed instead of Az‐1 , modest enantioselectivity was observed (Scheme ). To the best of our knowledge, this preliminary result is a novel enantioselective acyl‐like radical–radical coupling for the formation of ketones bearing an α‐stereogenic center, thus differentiating this work from other acyl radical processes . Efforts to increase the selectivity and scope of this process are currently underway.…”

“…). However, carboxylic acid‐derived acyl radicals have primarily been employed in Giese‐type additions to activated alkenes . Limited reports describe the coupling of an acyl radical with an alkyl radical, thus presenting an opportunity to explore and develop new reactivity.…”

Electrochemistry and Photoredox Catalysis: A Comparative Evaluation in Organic Synthesis

“…In a similar way, some transformations currently seem very challenging via an electrochemical approach. Examples are the usage of carbazoles due to electropolymerization (Section 2.4) [84,88,89,90,91] and the difficulty of employing aldehydes as acyl radical precursors [3] compared to the photochemical approach [198].…”