Acridinium-Based Photocatalysts: A Sustainable Option in Photoredox Catalysis

Abstract

The emergence of visible light photoredox catalysis has enabled the productive use of lower energy radiation, leading to highly selective reaction platforms. Polypyridyl complexes of iridium and ruthenium have served as popular photocatalysts in recent years due to their long excited state lifetimes and useful redox windows, leading to the development of diverse photoredox-catalyzed transformations. The low abundances of Ir and Ru in the earth's crust and, hence, cost make these catalysts nonsustainable and have limited their application in industrial-scale manufacturing. Herein, we report a series of novel acridinium salts as alternatives to iridium photoredox catalysts and show their comparability to the ubiquitous [Ir(dF-CF3-ppy)2(dtbpy)](PF6).

Direct C−H Carbamoylation of Nitrogen‐Containing Heterocycles

“…We envisioned the mechanistic scenario depicted in Scheme for the visible‐light‐promoted carbamoylation of heterocycles. Blue‐LED‐light activation of an appropriate photocatalyst—in our case 9‐mesityl‐3,6‐di‐ tert ‐butyl‐10‐phenylacridin‐10‐ium tetrafluoro‐borate 1 —would permit the latter to reach its excited state 1* ( E 1/2 red [PC*/PC ‐ ]=+2.08 V vs. SCE in CH 3 CN; SCE=saturated calomel electrode), which is sufficiently oxidizing for SET to occur from the oxamate salt 2 through reductive quenching ( E 1/2 red =+1.76 V vs. SCE in DMSO/H 2 O 1:1 for 8 , see Supporting Information). Loss of carbon dioxide would then generate the desired nucleophilic carbamoyl radical 3 that should rapidly add to a protonated heteroarene 4 to provide intermediate 5 .…”

“…Loss of carbon dioxide would then generate the desired nucleophilic carbamoyl radical 3 that should rapidly add to a protonated heteroarene 4 to provide intermediate 5 . Simultaneously, the photocatalyst 1 .− ( E 1/2 ox [PC − /PC]=+0.59 V vs. SCE in CH 3 CN) could be oxidized by persulfate ( E 1/2 ox =+1.75 V vs. SCE in CH 3 CN) to afford the sulfate dianion and the highly oxidizing sulfate radical anion, regenerating photocatalyst 1 . The latter anion ( E 1/2 ox =+2.36 V vs. SCE in CH 3 CN) is able to oxidize 5 through hydrogen‐atom transfer (HAT), yielding the desired product 6 .…”

Visible‐Light‐Enabled Direct Decarboxylative N‐Alkylation

“…We recently described new acridine photocatalysts A1 and A2 for the direct visible-light-driven decarboxylation of ab road range of carboxylic acids to produce alkyl radicals by ap hotoinduced proton-coupled electron transfer (PCET) process in the acridine-carboxylic acid hydrogen-bond complex ( Figure 1); [10] that is,b yadifferent mechanism to that observed for structurally related N-alkyl and N-aryl acridinium salts. [11] Thed irectional character of the acridinecarboxylic acid interaction in combination with the PCET enabled at riple catalytic biointerfaced conversion of esters into alkenes that was not possible with photoredox catalysts operating by the photoinduced oxidation of carboxylate anions. [10] It was therefore hypothesized that the acridine-catalyzed, visible-light-driven direct decarboxylation of carboxylic acids may be compatible with readily oxidizable anilines,a nd may be coupled with ac arbon-nitrogen bond-forming catalytic cycle,t hus resulting in the direct decarboxylative alkylation (DDA) of anilines.T he distinct mechanism of acridinecatalyzed photodecarboxylation was further expected to enable the decarboxylative N-alkylation of ab road range of anilines,i ncluding N-alkyl anilines,d iaryl amines,a nd Nheterocycles,w hich is currently not possible by the decarboxylation of activated carboxylic acid derivatives.Advantageously,acridines A1 and A2 are readily accessible on agram scale in one and two steps from inexpensive precursors.…”

Continuous Heterogeneous Photocatalysis in Serial Micro‐Batch Reactors

“…[1,6] Visible-light photoredox catalysis (PRC) [7,8] generally employs costly ruthenium and iridium catalysts or organic molecules prone to degradation. [9,10] Difficulties associated with the separation and recycling of these catalysts render heterogeneous or easily recyclable catalysts,such as modified transition-metal catalysts [11][12][13] and semiconductors, [14][15][16] highly desirable for PRC.G raphitic carbon nitrides (g-C 3 N 4 ), ac lass of metal-free polymers,a re among the most promising materials for heterogeneous PRC. [17,18] Combining the advantages of continuous photochemistry and g-C 3 N 4 catalysis to provide an efficient and sustainable process is conceptually attractive but challenging practically.…”