Photochemical Povarov-type Reactions: Electron Donor-Acceptor Photoactivation by Visible Light.

This report investigates the mechanism of photochemical Povarov-type reactions of N,N-dialkylanilines and maleimides in polar solvents (DMF or dioxane) in the presence of light. Fundamental aspects of the electron donor-acceptor (EDA) photoactivation pathway proposed to underpin this chemistry are examined through integrated experimental and computational studies. This approach provided evidence supporting the involvement of an EDA complex in facilitating this chemistry via a reaction mechanism that does not involve a triplet manifold. Most notably, our findings indicate that relying solely on UV-vis absorption spectroscopic data to either account for or predict reactivity in synthetic experiments may not always provide the complete picture. More specifically, this relates to considering UV-vis absorption spectroscopic data, calculated values for association constants (KEDA) and molar extinction coefficients (ε), with the reactivity observed in associated synthetic reactions in practice.

Extending Photocatalyst Activity through Choice of Electron Donor.

Sacrificial additives are commonly employed in photoredox catalysis as a convenient source of electrons, but what occurs after electron transfer is often overlooked. Tertiary alkylamines initially form radical cations following electron transfer, which readily deprotonate to form strongly reducing, neutral α-amino radicals. Similarly, the oxalate radical anion (C2O4•-) rapidly decomposes to form CO2•- (E0 ≈ -2.2 V vs SCE). We show that not only are these reactive intermediates formed under photoredox conditions, but they can also impact the desired photochemistry, both positively and negatively. Photoredox systems using oxalate as an electron donor are able to engage substrates with greater energy demands, extending reactivity past the energy limits of single and multiphoton transition metal catalysts. Furthermore, oxalate offers better chemoselectivity than the commonly employed triethylamine when reducing substrates with moderate energy requirements.

Reinterpreting the Fate of Iridium(III) Photocatalysts─Screening a Combinatorial Library to Explore Light-Driven Side-Reactions.

Photoredox catalysts are primarily selected based on ground and excited state properties, but their activity is also intrinsically tied to the nature of their reduced (or oxidized) intermediates. Catalyst reactivity often necessitates an inherent instability, thus these intermediates represent a mechanistic turning point that affords either product formation or side-reactions. In this work, we explore the scope of a previously demonstrated side-reaction that partially saturates one pyridine ring of the ancillary ligand in heteroleptic iridium(III) complexes. Using high-throughput synthesis and screening under photochemical conditions, we identified different chemical pathways, ultimately governed by ligand composition. The ancillary ligand was the key factor that determined photochemical stability. Following photoinitiated electron transfer from a sacrificial tertiary amine, the reduced intermediate of complexes containing 1,10-phenanthroline derivatives exhibited long-term stability. In contrast, complexes containing 2,2'-bipyridines were highly susceptible to hydrogen atom transfer and ancillary ligand modification. Detailed characterization of selected complexes before and after transformation showed differing effects on the ground and excited state reduction potentials dependent on the nature of the cyclometalating ligands and excited states. The implications of catalyst stability and reactivity in chemical synthesis was demonstrated in a model photoredox reaction.