Photoinduced Chemo-, Site- and Stereoselective α-C(sp3 )-H Functionalization of Sulfides

The ubiquity of sulfur-containing molecules in biologically active natural products and pharmaceuticals has long attracted synthetic chemists to develop efficient strategies towards their synthesis. The strategy of direct α-C(sp³ )-H modification of sulfides provides a streamlining access to complex sulfur-containing molecules. Herein, we report a photoinduced chemo-, site- and stereoselective α-C(sp³ )-H functionalization of sulfides using isatins as the photoredox reagent and coupling partner catalyzed by a chiral gallium(III)-N,N'-dioxide complex. The reaction proceeds through a verified single-electron transfer (SET) mechanism with high efficiency, excellent functional group tolerance, as well as a broad substrate scope. Importantly, this cross-coupling protocol is highly selective for the direct late-stage functionalization of methionine-related peptides, regardless of the inherent structural similarity and complexity of diverse residues.

Switchable, Reagent-Controlled Diastereodivergent Photocatalytic Carbocyclisation of Imine-Derived α-Amino Radicals

A reagent‐controlled stereodivergent carbocyclisation of aryl aldimine‐derived, photocatalytically generated, α‐amino radicals possessing adjacent conjugated alkenes, affording either bicyclic or tetracyclic products, is described. Under net reductive conditions using commercial Hantzsch ester, the α‐amino radical species underwent a single stereoselective cyclisation to give trans‐configured amino‐indane structures in good yield, whereas using a substituted Hantzsch ester as a milder reductant afforded cis‐fused tetracyclic tetrahydroquinoline frameworks, resulting from two consecutive radical cyclisations. Judicious choice of the reaction conditions allowed libraries of both single and dual cyclisation products to be synthesised with high selectivity, notable predictability, and good‐to‐excellent yields. Computational analysis employing DFT revealed the reaction pathway and mechanistic rationale behind this finely balanced yet readily controlled photocatalytic system.

Photoredox Catalysis of Aromatic β-Ketoesters for in Situ Production of Transient and Persistent Radicals for Organic Transformation

Radical formation is the initial step for conventional radical chemistry. Reported herein is a unified strategy to generate radicals in situ from aromatic β-ketoesters by using a photocatalyst. Under visible-light irradiation, a small amount of photocatalyst fac-Ir(ppy)₃ generates a transient α-carbonyl radical and persistent ketyl radical in situ. In contrast to the well-established approaches, neither stoichiometric external oxidant nor reductant is required for this reaction. The synthetic utility is demonstrated by pinacol coupling of ketyl radicals and benzannulation of α-carbonyl radicals with alkynes to give a series of highly substituted 1-naphthols in good to excellent yields. The readily available photocatalyst, mild reaction conditions, broad substrate scope, and high functional-group tolerance make this reaction a useful synthetic tool.

One Amine-3 Tasks: Reductive Coupling of Imines with Olefins in Batch and Flow

Owing to their wide range of biological properties, γ-aminobutyric acid derivatives (GABA) have been extensively studied and found noteworthy industrial applications. However, atom-economical and efficient processes for their production are scarce and would greatly benefit from further investigations. Herein, we demonstrate that an iridium-based photocatalyst promotes the direct reductive cross-coupling of imines with olefins upon irradiation with visible light to give GABA derivatives in good yields and selectivities. We also stress the enabling triple role of tributylamine additive in this process, discuss the advantages of strategies based on proton-coupled electron transfer (PCET) and demonstrate the scale-up of this reaction in continuous flow.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

Reductive Amination by Photoredox Catalysis and Polarity-Matched Hydrogen Atom Transfer

The excitation of a Ru^II photosensitizer in the presence of ascorbic acid leads to the reduction of iminium ions to electron-rich α-aminoalkyl radical intermediates, which are rapidly converted into reductive amination products by thiol-mediated hydrogen atom transfer (HAT). As a result, the reductive amination of carbonyl compounds with amines by photoredox catalysis proceeds in good to excellent yields and with broad substrate scope and good functional group tolerance. The three key features of this work are 1) the rapid interception of electron-rich α-aminoalkyl radical intermediates by polarity-matched HAT in a photoredox reaction, 2) the method of reductive amination by photoredox catalysis itself, and 3) the application of this new method for temporally and spatially controlled reactions on a solid support, as demonstrated by the attachment of a fluorescent dye on an activated cellulose support by photoredox-catalyzed reductive amination.

Photoredox-Controlled Mono- and Di-Multifluoroarylation of C(sp3 )-H Bonds with Aryl Fluorides

A controllable mono- and di-multifluoroarylation of acyclic and cyclic N-aryl amines with aryl fluorides by photocatalyzed dual C(sp³ )-H/C(sp² )-F functionalization has been developed, providing new access to a wide array of valuable α-fluoroarylated amines. In addition, the one-pot consecutive hetero-di-multifluoroarylation of N-aryl pyrrolidines and N,N-dimethylanilines was achieved with high to excellent diastereoselectivity. This new defluorinative C(sp³ )-C(sp² ) coupling is distinguished by a broad scope, good regioselectivity, and mild conditions as well as gram-scale and late-stage applicability, and thus constitutes a significant advance in the arylation of unactivated C(sp³ )-H bonds with aryl fluorides.