Strongly Reducing, Visible-Light Organic Photoredox Catalysts as Sustainable Alternatives to Precious Metals

Abstract

Photoredox catalysis is a versatile approach for the construction of challenging covalent bonds under mild reaction conditions, commonly using photoredox catalysts (PCs) derived from precious metals. As such, there is need to develop organic analogues as sustainable replacements. Although several organic PCs have been introduced, there remains a lack of strongly reducing, visible-light organic PCs. Herein, we establish the critical photophysical and electrochemical characteristics of both a dihydrophenazine and a phenoxazine system that enables their success as strongly reducing, visible-light PCs for trifluoromethylation reactions and dual photoredox/nickel-catalyzed C-N and C-S cross-coupling reactions, both of which have been historically exclusive to precious metal PCs.

Keywords: organocatalysis; photocatalysis; photochemistry; radicals; sustainable chemistry.

Figure 1

Photophysical and electrochemical properties of precious metal and organic photoredox catalysts. (A) UV-vis absorption spectra of PCs 1–4 in N,N-dimethylacetamide (DMA). (B) Structures of precious metal and organic PCs. (C) Values enclosed in parentheses are from density functional theory (DFT) calculations. All experimental values for PCs 3 and 4 were measured in DMA at room temperature. [a] Triplet excited state reduction potential, E⁰*, in units of V vs. SCE. [b] Ground state oxidation potential [c] Triplet energy (E_triplet), estimated from the fluorescence wavelength of the charge transfer singlet state. [d] Maximum absorption wavelength (λ_max,abs); molar absorptivity (ε_max,abs) at λ_max,abs. [e] Emission maximum wavelength (λ_max,em). [f] Triplet excited state lifetime (τ_triplet). [g] Quantum yield (Φ_triplet) of charge transfer triplet excited state (³CT*), and metal-ligand charge transfer triplet state (³MLCT*). [h] λ_max,abs and ε_max,abs were measured in this work in DMA. All other values were obtained from ref. [8a,9b], and were measured in acetonitrile, except for the λ_max,em, which was measured in alcoholic solvent at 77K. [i] λ_max,abs and ε_max,abs were measured in this work in DMA solvent for [Ru(bpy)₃](PF₆)₂. All other values were obtained from ref. [8b,9a], and were measured in acetonitrile. See Supporting Information for details.

Figure 2

Photophysical properties of organic PCs. (A) Photograph showing solvatochromic shifts in the emission of 4 when irradiated with 365 nm light in solvents of increasing polarity; from left to right: 1-hexene, benzene, dioxane, ethyl acetate, pyridine, and DMF. (B) Normalized emission spectra of 4 in solvents of varying polarity. Transient absorption spectra of 3 (C) and 4 (D) in DMA at room temperature. Cyclic voltammetry (CV) experiments with various scan rates for (E) 3 and (F) 4. See Supporting Information for details.

Figure 3

Photoredox catalyzed transformations using organic PCs 3 and 4. (A) Radical trifluoromethylations using PC 3 on alkenes, five-membered heteroarenes, arenes, and cross-addition on alkenes. (B) Dual organic photoredox and nickel catalyzed C-N cross-coupling reaction scope. (C) Dual organic photoredox and nickel catalyzed C-S cross-coupling scope. Data reported as isolated yields. Values in parentheses are the ratio of Z : E : β-hydride elimination product. ^aReaction was also conducted using sunlight for 1 week (67% yield for trifluoromethylation, 83% yield for C-N coupling, 94% yield for C-S coupling). ^bCF₃CF₂I was used instead of CF₃I. ^cReaction time 6 hours. ^dReaction was also conducted on a larger 10 mmol scale (73% yield for trifluoromethylation, 53% yield for C-N coupling, 98% yield for C-S coupling). ^eReaction was also conducted at reduced catalyst loading of 0.25 mol %, instead of standard 1.0 mol % (69% yield for trifluoromethylation, 53% yield for C-N coupling, 98% yield for C-S coupling after 24 hours). ^fPerformed without HCOOK. ^gReaction performed with 10 mol % pyrrolidine as the ligand and reduced nickel loading to 1.0 mol %. ^hReaction catalyzed by PC 4. ⁱReaction catalyzed by PC 3.