Unveiling Extreme Photoreduction Potentials of Donor-Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides

Abstract

Since the seminal work of Zhang in 2016, donor-acceptor cyanoarene-based fluorophores, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), have been widely applied in photoredox catalysis, and used as excellent metal-free alternatives to noble metal Ir- and Ru-based photocatalysts. However, all the reported photoredox reactions involving this chromophore family are based on harnessing the energy from a single visible light photon, with a limited range of redox potentials from -1.92 V to +1.79 V. Here, we document the unprecedented discovery that this family of fluorophores can undergo consecutive photoinduced electron transfer (ConPET) to achieve very high reduction potentials. One of the newly synthesized catalysts, 2,4,5-tri(9H-carbazol-9-yl)-6-(ethyl(phenyl)amino)isophthalonitrile (3CzEPAIPN), possesses a long-lived (12.95 ns) excited radical anion form, 3CzEPAIPN<sup>•</sup><sup>−</sup>*, which can be used to activate reductively recalcitrant aryl chlorides (E<sub>red </sub>≈ -1.9 to -2.9 V) under mild conditions. The resultant aryl radicals can be engaged in synthetically valuable aromatic C-B, C-P, and C-C bond formation to furnish arylboronates, arylphosphonium salts, arylphosphonates, and spirocyclic cyclohexadienes, respectively.

CdS Quantum Dots as Potent Photoreductants for Organic Chemistry Enabled by Auger Recombination

“…To explore whether the aryl radicals formed could be used for C-C and C-X bond formation, we examined trapping reagents to furnish products 6, 7, and 8 in moderate yields, comparable to those reported using other aryl radical trapping conditions. 15,24,38 Consistent with the propensity of aryl radicals to undergo HAT with weak C-H bonds present on amine reductants and DMPU, we observed that employing DMSO as solvent and sodium formate as the terminal reductant for aryl radical trapping generally improved the selectivity for the desired products over hydrodehalogenation. 12 We anticipate that further improvements could be made with additional optimization: the use of DMSO instead of DMPU was required for selective aryl radical functionalization but led to lower conversion, especially for electron-rich aryl electrophiles.…”

“…19,21,22 Electrochemistry-primed photoredox catalysis requires a more complex apparatus, leading to challenges in vessel design and scalability. Moreover, the optimal organocatalysts for some strategies require multi-step syntheses and purification, 13,23,24 and can be expensive to use in significant quantities. Therefore, while these strategies have enabled redox-initiated transformations of substrates far beyond the potentials accessible by traditional photocatalysis, these early reports underscore the need for improved photocatalysts with additional stability and access to new mechanisms of reactivity.…”

“…26, In addition to their high photostability, QDs exhibit tunable, size-dependent optical and redox properties; are made in single-step syntheses with no chromatography from abundant precursors; 62 reversibly bind to many molecules at once (typically 1 -5 ligands/nm 2 of QD surface are found for closely related CdSe QDs [63][64][65] ) through common organic functional groups (-CO 2 H, -PO 3 H, -SH, -NH 2 ); can become charged with many electrons at once without decomposing; 66,67 and undergo many electronic processes with no direct analogue in smallmolecules. 68 Inspired by reports of conPET-type photoreduction mechanisms operative within commonly used photocatalyst systems, 12,24,69 we envisioned that QDs could achieve a similar mode of reactivity, while also addressing the catalyst stability and availability challenges of organocatalyst-mediated photoreductions. In particular, we considered that Auger recombination processes, a family of electronic events inaccessible to small-molecule photocatalysts which generate excited charge carriers from carrier recombination of trion or biexciton states, [70][71][72][73] could be used to drive energetically demanding photoreductions of organic molecules (Figure 1C).…”

Understanding Ir(III) Photocatalyst Structure–Activity Relationships: A Highly Parallelized Study of Light-Driven Metal Reduction Processes

“…A linear relationship with respect to light intensity (Figure 9D) indicated a single-photon process across the measured light intensities. 41 A coordination equilibrium between D and the Zn(II) ions was discovered (Supporting Information, pp S53 and S54); therefore, Sn(0) data is presented for simplicity, although both reactions follow the same trends. Complete data sets for Sn(0) and Zn(0) deposition utilizing Ir1−Ir22 are presented in the Supporting Information (Figures S7−S12).…”

Solvated Electrons! The Missing Link in Highly Reducing Photocatalysis

“…10 The concept of a two-photon excitation process, commonly purported as a consecutive photoelectron transfer (conPET) pathway, has been reported with numerous notable photocatalysts such as PDI, DCA, anthraquinone, Rhodamine 6G, benzo[ghi]perylene (BPI), 4-DPAIPN, 3-CzEPAIPN, Mes-Acr, and Deazaflavin. [11][12][13][14][15][16][17][18][19] As a common benchmark reaction, photoredox C(sp 2 )-X bond activation in aryl bromides and chlorides, Birch reduction, and sulfonamide cleavage has showcased the extreme photoreducing ability of radical photocatalysts.…”