Halide Noninnocence and Direct Photoreduction of Ni(II) Enables Coupling of Aryl Chlorides in Dual Catalytic, Carbon-Heteroatom Bond-Forming Reactions.

Recent mechanistic studies of dual photoredox/Ni-catalyzed, light-driven cross-coupling reactions have found that the photocatalyst (PC) operates through either reductive quenching or energy transfer cycles. To date, reports invoking oxidative quenching cycles are comparatively rare and direct observation of such a quenching event has not been reported. However, when PCs with highly reducing excited states are used (e.g., Ir(ppy)3), photoreduction of Ni(II) to Ni(I) is thermodynamically feasible. Recently, a unified reaction system using Ir(ppy)3 was developed for forming C-O, C-N, and C-S bonds under the same conditions, a prospect that is challenging with PCs that can photooxidize these nucleophiles. Herein, in a detailed mechanistic study of this system, we observe oxidative quenching of the PC (Ir(ppy)3 or a phenoxazine) via nanosecond transient absorption spectroscopy. Speciation studies support that a mixture of Ni-bipyridine complexes forms under the reaction conditions, and the rate constant for photoreduction increases when more than one ligand is bound. Oxidative addition of an aryl iodide was observed indirectly via oxidation of the resulting iodide by Ir(IV)(ppy)3. Intriguingly, the persistence of the Ir(IV)/Ni(I) ion pair formed in the oxidative quenching step was found to be necessary to simulate the observed kinetics. Both bromide and iodide anions were found to reduce the oxidized form of the PC back to its neutral state. These mechanistic insights inspired the addition of a chloride salt additive, which was found to alter Ni speciation, leading to a 36-fold increase in the initial turnover frequency, enabling the coupling of aryl chlorides.

Impact of Alkyl Core Substitution Kinetics in Diaryl Dihydrophenazine Photoredox Catalysts on Properties and Performance in O-ATRP.

Organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization method mediated by organic photoredox catalysts (PCs) for producing polymers with well-defined structures. While N,N-diaryl dihydrophenazine PCs have successfully produced polymers with low dispersity (D < 1.3) in O-ATRP, low initiator efficiencies (I* ~ 60-80%) indicate an inability to achieve targeted molecular weights and have been attributed to the addition of radicals to the PC core. In this work, we measure the rates of alkyl core substitution (AkCS) to gain insight into why PCs differing in N-aryl group connectivity exhibit differences in polymerization control. Additionally, we evaluate how PC properties evolve during O-ATRP when a non-core-substituted PC is used. PC 1 with 1-naphthyl groups in the N-aryl position resulted in faster AkCS (k 1 = 1.21 ± 0.16 × 10-3 s-1, k 2 = 2.04 ± 0.11 × 10-3 s-1) and better polymerization control at early reaction times as indicated by plots of molecular weight (number average molecular weight = M n) vs conversion compared to PC 2 with 2-naphthyl groups (k 1 = 6.28 ± 0.38 × 10-4 s-1, k 2 = 1.15 ± 0.07 × 10-3 s-1). The differences in rates indicate that N-aryl connectivity can influence polymerization control by changing the rate of AkCS PC formation. The rate of AkCS increased from the initial to the second substitution, suggesting that PC properties are modified by AkCS. Increased PC radical cation (PC•+) oxidation potentials (E 1/2 = 0.26-0.27 V vs SCE) or longer triplet excited-state lifetimes (τ T1 = 1.4-33 µs) for AkCS PCs 1b and 2b compared to parent PCs 1 and 2 (E 1/2 = 0.21-0.22 V vs SCE, τ T1 = 0.61-3.3 µs) were observed and may explain changes to PC performance with AkCS. Insight from evaluation of the formation, properties, and performance of AkCS PCs will facilitate their use in O-ATRP and in other PC-driven organic transformations.

Effects of the Chalcogenide Identity in N-Aryl Phenochalcogenazine Photoredox Catalysts.

Phenochalcogenazines such as phenoxazines and phenothiazines have been widely employed as photoredox catalysts (PCs) in small molecule and polymer synthesis. However, the effect of the chalcogenide in these catalysts has not been fully investigated. In this work, a series of four phenochalcogenazines is synthesized to understand how the chalcogenide impacts catalyst properties and performance. Increasing the size of the chalcogenide is found to distort the PC structure, ultimately impacting the properties of each PC. For example, larger chalcogenides destabilize the PC radical cation, possibly resulting in catalyst degradation. In addition, PCs with larger chalcogenides experience increased reorganization during electron transfer, leading to slower electron transfer. Ultimately, catalyst performance is evaluated in organocatalyzed atom transfer radical polymerization and a photooxidation reaction for C(sp2)-N coupling. Results from these experiments highlight that a balance of PC properties is most beneficial for catalysis, including a long-lived excited state, a stable radical cation, and a low reorganization energy.

Radical Addition to N,N-Diaryl Dihydrophenazine Photoredox Catalysts and Implications in Photoinduced Organocatalyzed Atom Transfer Radical Polymerization.

Photoinduced organocatalyzed atom transfer radical polymerization (O-ATRP) is a controlled radical polymerization methodology catalyzed by organic photoredox catalysts (PCs). In an efficient O-ATRP system, good control over molecular weight with an initiator efficiency (I* = M n,theo/M n,exp × 100%) near unity is achieved, and the synthesized polymers possess a low dispersity (D). N,N-Diaryl dihydrophenazine catalysts typically produce polymers with low dispersity (D < 1.3) but with less than unity molecular weight control (I* ~ 60-80%). This work explores the termination reactions that lead to decreased control over polymer molecular weight and identifies a reaction leading to radical addition to the phenazine core. This reaction can occur with radicals generated through reduction of the ATRP initiator or the polymer chain end. In addition to causing a decrease in I*, this reactivity modifies the properties of the PC, ultimately impacting polymerization control in O-ATRP. With this insight in mind, a new family of core-substituted N,N-diaryl dihydrophenazines is synthesized from commercially available ATRP initiators and employed in O-ATRP. These new core-substituted PCs improve both I* and D in the O-ATRP of MMA, while minimizing undesired side reactions during the polymerization. Further, the ability of one core-substituted PC to operate at low catalyst loadings is demonstrated, with minimal loss of polymerization control down to 100 ppm (weight average molecular weight [M w] = 10.8 kDa, D = 1.17, I* = 104% vs M w = 8.26, D = 1.10, I* = 107% at 1000 ppm) and signs of a controlled polymerization down to 10 ppm of the catalyst (M w = 12.1 kDa, D = 1.36, I* = 107%).