Discovery and characterization of an acridine radical photoreductant

Abstract

Summary: Photoinduced electron transfer (PET) is a phenomenon wherein the absorption of light by a chemical species provides an energetic driving force for an electron transfer reaction. 1 – 4 This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics, and photosensitive materials. In recent years, research in the area of photoredox catalysis has leveraged PET for the catalytic generation of both neutral and charged organic free radical species. These technologies have enabled a wide range of previously inaccessible chemical transformations and have seen widespread utilization in both academic and industrial settings. These reactions are often catalyzed by visible-light absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium, or copper. 5 , 6 While a wide variety of closed shell organic molecules have been shown to behave as competent electron transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as an excited state donor or acceptor. This is perhaps somewhat unsurprising in light of previously reported doublet excited state lifetimes for neutral organic radicals, which are typically several orders of magnitude shorter than singlet lifetimes for known transition metal photoredox catalysts. 7 – 11 Herein we document the discovery, characterization, and reactivity of a neutral acridine radical with a maximum excited state oxidation potential of −3.36 V vs. SCE: significantly more reducing than elemental lithium and marking it as one of the most potent chemical reductants reported. 12 Spectroscopic, computational, and chemical studies indicate that the formation of a twisted intramolecular charge transfer species enables the population of higher energy doublet excited states, leading to the observed potent photoreductant behavior. We demonstrate that this catalytically-generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and bodes well for the adoption of this system in additional organic transformations requiring dissolving metal reductants.

Tandem Photoredox Catalysis: Enabling Carbonylative Amidation of Aryl and Alkylhalides

“…[13] Recently our group disclosed amethod for the in situ generation of asecond highly reducing iridium photocatalyst from [Ir(ppy) 2 (dtb-bpy)] + via a2-photon tandem photoredox cycle [14] (Figure 2a)a nd Nicewicz uncovered similar behaviour with organophotoredox catalysts. [15] This strategy has been successfully applied to the reductive protodehalogena-tion of unactivated organohalides and showcases the ability of common photoredox catalysts to yield potent excited state donors via multiphoton excitation.…”

Visible-Light Photoredox-Catalyzed C–O Bond Cleavage of Diaryl Ethers by Acridinium Photocatalysts at Room Temperature

“…Since the complex procedure for synthesis of PC 3, PC 4-PC 9, with a similar skeleton to that of PC 3 but with different substituents on the 9-aryl ring, by a two-step synthetic procedure, 52 were investigated ( Table 1, entries 7-12). Notably, the aryl rings with electron-withdrawing groups instead of mesitylene, typically used in other acridinium catalysts, [44][45][46][47][48][49][50][51][52][53][54] resulted in noticeably high e ciencies. To our delight, PC 9 with 2'-Cl and 4'-F on the 9-aryl ring resulted in 80% of 3a with 71% of 4a (entry 12).…”

“…Inspired by the signi cant contributions of Fukuzumi, [44][45][46] Nicewicz, [47][48][49] Sparr, [50][51] et al [52][53][54] on acridinium PCs, we investigated the use of a new acridinium catalyst. Notably, an aryl ring with electronwithdrawing groups, instead of mesitylene, typically used in other acridinium catalysts, [44][45][46][47][48][49][50][51][52][53][54] on the 9position resulted in obviously higher e ciency. Furthermore, with this method, the model of 4-O-5 lignin linkages afforded phenol and 2-methoxyphenol in high e ciency.…”

Tandem Photoredox Catalysis: Enabling Carbonylative Amidation of Aryl and Alkylhalides

“…Multiphoton excitation catalysis has emerged as a new strategy to overcome the thermodynamic limitations of visible light photoredox catalysis to generate high energy photoreductants . Recently our group disclosed a method for the in situ generation of a second highly reducing iridium photocatalyst from [Ir(ppy) 2 (dtb‐bpy)] + via a 2‐photon tandem photoredox cycle (Figure a) and Nicewicz uncovered similar behaviour with organophotoredox catalysts . This strategy has been successfully applied to the reductive protodehalogenation of unactivated organohalides and showcases the ability of common photoredox catalysts to yield potent excited state donors via multiphoton excitation.…”