Photoreduction of Anthracenes Catalyzed by peri‐Xanthenoxanthene: a Scalable and Sustainable Birch‐Type Alternative

Abstract

The typical Birch reduction transforms arenes into 1,4‐cyclohexadienes using alkali metals, an alcohol as a proton source, and an amine as solvent. Capitalizing on the strong photoreductive properties of peri‐xanthenoxanthene (PXX), herein we report the photocatalyzed “Birch‐type” reduction of acenes employing visible blue light irradiation at room temperature in the presence of air. Upon excitation at 405 or 460 nm in the presence of a mixture of N,N‐diisopropylethylamine (DIPEA) and trifluoromethanesulfonimide (HNTf2) in DMSO, PXX photocatalyzes the selective reduction of full‐carbon acene derivatives (24 – 75%). Immobilization of PXX onto polydimethylsiloxane (PDMS) beads (PXX‐PDMS) allowed the use of the catalyst in heterogeneous batch reactions, yielding 9‐phenyl‐9,10‐dihydroanthracene in high yields (68%). The catalyst could be easily recovered and reused, with no notable catalytic performance drop observed after five reaction cycles. Integration of the PXX‐PDMS beads into a microreactor enabled the reduction of acenes under continuous flow conditions, thereby validating the sustainability and scalability of this heterogeneous phase approach.

Heterogeneous Organocatalysts for Light-Driven Reactions in Continuous Flow

“…The substrate was pumped through a T-mixer, along with air, directly into the photoreactor; the best results (yield 100%) were obtained employing a loading of 4.5 mg•g −1 of TMPyP@SiO2, a concentration of 0.01 (mol L −1 ) of the starting material, a flow of liquid and gas of 0.1 mL min −1 and 0.2 mL min −1 , respectively, and two cartridges (Figure 23). Concerning the silicon-based support, recently, Bonifazi and co-workers reported the photoreduction of anthracenes mediated by peri-xanthenoxanthene to afford the Birch adduct, where the reaction was tested in continuous flow by supporting the photocatalyst PXX onto polydimethylsiloxane (PDMS) microparticles [89]. Anthracene was selected as the benchmark substrate due to its affordable reduction potential (−2.0 V vs. SCE).…”

“…Interestingly, working in homogeneous conditions requires a very diluted concentration of the catalyst (around 2% w/v or less) to avoid selfaggregation. The authors investigated the scope extension and studied the mechanism of reaction; thus, to explore the potential of the catalyst, from a more sustainable point of view, they moved to prepare the heterogeneous analogue and test it in the benchmark Concerning the silicon-based support, recently, Bonifazi and co-workers reported the photoreduction of anthracenes mediated by peri-xanthenoxanthene to afford the Birch adduct, where the reaction was tested in continuous flow by supporting the photocatalyst PXX onto polydimethylsiloxane (PDMS) microparticles [89]. Anthracene was selected as the benchmark substrate due to its affordable reduction potential (−2.0 V vs. SCE).…”