Visible-light-induced radical decarboxylative functionalization of carboxylic acids and their derivatives has recently received considerable attention as a novel and efficient method to create CC and CX bonds. Generally, this visible-light-promoted decarboxylation process can smoothly occur under mild reaction conditions with a broad range of substrates and an excellent functional-group tolerance. The radical species formed from the decarboxylation step can participate in not only single photocatalytic transformations, but also dual-catalytic cross-coupling reactions by combining photoredox catalysis with other catalytic processes. Recent advances in this research area are discussed herein.

Der radikalischen decarboxylierenden Funktionalisierung von Carbonsäuren und ihren Derivaten mithilfe von sichtbarem Licht wird seit einiger Zeit erhöhte Aufmerksamkeit zuteil, da sie ein neuartiges und effizientes Verfahren zur Knüpfung von C-C- und C-X-Bindungen ist. Dieses Verfahren lässt sich unter milden Reaktionsbedingungen auf eine breite Palette von Substraten anwenden und toleriert eine Vielzahl funktioneller Gruppen. Die bei der Decarboxylierung gebildeten Radikale können nicht nur für photokatalytische Einzelumsetzungen, sondern auch für duale katalytische Kreuzkupplungen genutzt werden. Für Letzteres wird die Photoredoxkatalyse mit anderen katalytischen Verfahren verknüpft. Die jüngsten Fortschritte in diesem Forschungsbereich werden im Folgenden vorgestellt.

A series of 3-oxoglutaric acid derivatives have been hydrogenated in different solvents in the presence of [RuCl(benzene)(S)-SunPhos]Cl (SunPhos=(2,2,2′,2′-tetramethyl-[4,4′-bibenzo[d][1,3]dioxole]-5,5′-diyl)bis(diphenylphosphine)). Unlike simple β-keto acid derivatives, these advanced analogues can be readily hydrogenated in uncommon solvents such as THF, CH₂Cl₂, acetone, and dioxane with high enantioselectivities. Two possible catalytic cycles have been proposed to explain the different reactivities of these 1,3,5-tricarbonyl substrates in the tested solvents. The C-2 and C-4 substituents had notable but irregular influence on the reactivity and enantioselectivity of the reactions. More pronounced solvent effects were observed: the ee values increased from around 20 % in EtOH or THF to 90 % in acetone. Inversion of the product configuration was observed when the solvent was changed from EtOH to THF or acetone, and a mixed solvent system can lead to better enantioselectivity than a single solvent.

Application of catalytic asymmetric hydrogenation of ethyl 2-(benzo[b]thiophen-5-yl)-2-oxoacetate for the formal synthesis of cognitive enhancer T-588 is described. The hydrogenation was conducted under the catalysis of Ru-SunPhos system with HCl as additive. Ethyl 2-(benzo[b]thiophen-5-yl)-2-hydroxy-acetate was obtained with 98.6% ee, which could be easily upgraded to 99.5% ee after a simple recrystallization and served as the key intermediate in the synthesis of T-588.

An efficient asymmetric hydrogenation of α-hydroxy ketones was reported with the catalyst prepared from [RuCl₂(benzene)]₂ and SunPhos, chiral terminal 1,2-diols were obtained in up to 99% ee. This Ru-catalyzed asymmetric hydrogenation reaction of α-hydroxy ketones represents a new route for the synthesis of chiral terminal 1,2-diols.