Bio-inspired catalyst can reduce arenes without the need for harsh conditions

Photoredox catalysis driven by visible light has improved chemical synthesis by enabling milder reaction conditions and unlocking distinct reaction mechanisms. Despite the transformative impact, visib...

The cross-electrophile coupling between aryl and alkyl halides is an important synthetic tool in the creation of Csp2–Csp3 bonds from abundant building blocks. These couplings have traditionally relied on thermally driven, reductive couplings using metallic Zn/Mn to generate reactive low-valent nickel species. Recent work has expanded this reactivity to visible light-driven methodologies utilizing a range of photocatalysts and terminal reductants. However, early photocatalyzed approaches required the use of precious metal photocatalysts and stoichiometric amounts of a costly silane. The work described herein expands photoredox catalyzed cross-electrophile coupling with a focus on the use of organic photocatalysts and an inexpensive, readily available sacrificial electron source (triethanolamine). To overcome limitations in substrate scope arising from redox incompatibilities between photocatalyst and substrate, we introduce two sets of conditions that minimize unwanted substrate-specific side reactivity. These synthetic protocols enable cross-electrophile couplings of challenging aryl bromide substrates, such as unprotected bromoindoles and 2-bromopyrimidines. We found that the propensity of aryl halide reagents to undergo side reactions is correlated with electronic parameters: the C–Br 13C chemical shift of aryl bromides is a robust predictor for this reactivity and enables facile reaction condition selection.

The development of N,N-diaryl dihydrophenazine organic photoredox catalysts (PCs) has enabled numerous examples of organocatalyzed atom transfer radical polymerization (O-ATRP) of methyl methacrylate (MMA) monomer to polymers with low dispersity (Đ < 1.30) and near-unity initiator efficiency (I* ∼ 100%), as well as small molecule synthesis. In this work, we investigate the influence of core substitution (CS) by alkyl, aryl, and heteroatom groups on singlet excited state reduction potential (ES1°*). We observe that a highly reducing ES1°* is in part a result of a locally excited (LE)-dominated hybridized local and charge transfer (HLCT) excited state in CS PCs, which is influenced by the identity of the core substituent. Additionally, the PCs that possess a LE-dominated HLCT character maintain a relatively oxidizing PC radical cation oxidation potential (E1/2) for deactivation in O-ATRP compared to fully LE PCs reported in prior work. For example, a thiophenol core substituted (heteroatom CS, HetCS) PC shows the most negative ES1°* (−2.07 V vs SCE), more LE character (Stokes shift = 124 nm), and has an oxidizing PC radical cation (E1/2 = 0.30 V vs SCE). The CS PCs with improved properties, including more negative ES1°*, perform best in O-ATRP of MMA with the HetCS PC showing the best control in both DMAc (Đ = 1.08, I* = 89%) and EtOAc (Đ = 1.06, I* = 97%). Additionally, the HetCS PC was found to mediate the controlled polymerization of n-butyl acrylate (n-BA) (Đ = 1.24, I* = 97%), which has remained challenging in O-ATRP without supplemental deactivation strategies. An aryl CS PC was found to have moderate control as low as 1 ppm PC, indicating facilitation of low PC loadings (Đ = 1.33, I* = 69%). The relationship between excited state character, ES1°*, and polymerization control observed in this work provides a foundation for increasing the utility of phenazine PCs across photoredox catalysis.

Chemistry that can break carbon–fluorine bonds in persistent molecules called PFAS could help to clean up the environment. Testing in real-world settings must come next.

In an effort to keep undergraduate chemical education at pace with contemporary interest in photochemical methodologies, we have identified a straightforward system for which steady-state photochemical kinetics may can be tracked via in situ UV–vis spectroscopy: an aqueous solution of methylene blue and ascorbic acid. Under red-light illumination, methylene blue forms a highly oxidizing excited state which is reduced by ascorbic acid, resulting in the colorless leucomethylene blue and the loss of methylene blue spectral features. Using a laser diode of steady output power, the quantum yield of the photoreaction was experimentally determined to be Φrxn = 0.106─indicating that about one in ten absorbed photons results in a productive reaction in this system. Φrxn is a critical metric in developing efficient photochemical reactions. The reaction was then used as a chemical actinometer to measure the power of an inexpensive red laser pointer and LED. These facile experiments can be employed in analytical and physical chemistry courses to expose students to important photochemical concepts through the familiar lens of UV–vis spectroscopy.

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