A Unified Method for Oxidative and Reductive Decarboxylative Arylation with Orange Light-Driven Ir/Ni Metallaphotoredox Catalysis.

Carboxylic acids and their derivatives are powerful building blocks in dual Ir/Ni metallaphotoredox methods of decarboxylative arylation due to their abundance as feedstock compounds. However, the library of accessible carboxylic acids is limited by trends in radical stability, often necessitating the development of specific systems for challenging substrates. Herein, we disclose the application of a new Ir(III) photocatalyst and low-energy orange light Ir/Ni metallaphotoredox system with broad applicability in activating both native carboxylic acids and redox-active esters (RAEs).

Orange Light-Driven C(sp)-C(sp) Cross-Coupling via Spin-Forbidden Ir(III) Metallaphotoredox Catalysis.

We report the development and characterization of a library of Ir(III) photocatalysts capable of undergoing spin-forbidden excitation (SFE) under orange light irradiation (595 nm). These catalysts were successfully applied to the construction of synthetically valuable C(sp)-C(sp) bonds inaccessible with existing methods of low-energy light-driven dual nickel/photoredox catalysis, demonstrating the synthetic utility of this photocatalyst family. The photocatalysts are capable of accessing both oxidatively and reductively activated coupling partners, illustrated through deaminative arylation and potassium alkyl trifluoroborate cross-coupling reactions with aryl halides.

Metallaphotoredox-Mediated Decarboxylative Cross-Coupling of α-Oxy Morpholine Carboxylic Acids and (Hetero)Aryl Halides.

A decarboxylative C(sp)-C(sp) cross-coupling reaction of α-oxy carboxylic acids using dual nickel/photoredox catalysis has been developed for the synthesis of complex morpholines and other saturated heterocycles, affording direct entry to scaffolds of interest in drug discovery. This chemistry can be applied to the coupling of an array of (hetero)aryl halides and α-heteroatom acids, providing C(sp)-C(sp)-coupled products in modest to excellent yields and enabling access to intermediates that can be further derivatized to multivector architectures.

Overcoming Photochemical Limitations in Metallaphotoredox Catalysis: Red-Light-Driven C-N Cross-Coupling.

Aryl amination is an essential transformation for medicinal, process, and materials chemistry. In addition to classic Buchwald-Hartwig amination conditions, blue-light-driven metallaphotoredox catalysis has emerged as a valuable tool for C-N cross-coupling. However, blue light suffers from low penetration through reaction media, limiting its scalability for industrial purposes.

Ni/Photoredox-Catalyzed C(sp)-C(sp) Cross-Coupling of Alkyl Pinacolboronates and (Hetero)Aryl Bromides.

Utilizing quinoline as a mild, catalytic additive, broadly applicable conditions for the Ni/photoredox-catalyzed C(sp)-C(sp) cross-coupling of (hetero)aryl bromides and alkyl pinacolboronate esters were developed, which can be applied to both batch and flow reactions. In addition to primary benzylic nucleophiles, both stabilized and nonstabilized secondary alkyl boronic esters are effective coupling partners. Density functional theory calculations suggest that alkyl radical generation occurs from an alkyl-B(pin)-quinoline complex, which may proceed via an energy transfer process.

The Development of Reaction Cascades to Synthesize Dimeric Coccinellid Alkaloids.

Over the course of the past decade, our group has been intensely interested in achieving the laboratory synthesis of varied members of the coccinellid alkaloid family of natural products. These compounds, produced by varied species of ladybugs throughout the world as defensive agents, include several polycyclic members that can formally be considered as either monomeric or dimeric with architectures that contain between 3 and 7 ring systems along with an array of stereocenters. As a result of their fascinating structures, many groups have achieved syntheses of varied monomeric members using a variety of synthetic strategies and tactics.

Anti-Markovnikov Hydroamination of Unactivated Alkenes with Primary Alkyl Amines.

We report here a photocatalytic method for the intermolecular anti-Markovnikov hydroamination of unactivated olefins with primary alkyl amines to selectively furnish secondary amine products. These reactions proceed through aminium radical cation (ARC) intermediates and occur at room temperature under visible light irradiation in the presence of an iridium photocatalyst and an aryl thiol hydrogen atom donor. Despite the presence of excess olefin, high selectivities are observed for secondary over tertiary amine products, even though the secondary amines are established substrates for ARC-based olefin amination under similar conditions.

Catalytic Ring Expansions of Cyclic Alcohols Enabled by Proton-Coupled Electron Transfer.

We report here a catalytic method for the modular ring expansion of cyclic aliphatic alcohols. In this work, proton-coupled electron transfer activation of an allylic alcohol substrate affords an alkoxy radical intermediate that undergoes subsequent C-C bond cleavage to furnish an enone and a tethered alkyl radical. Recombination of this alkyl radical with the revealed olefin acceptor in turn produces a ring-expanded ketone product.

Decarboxylative Intramolecular Arene Alkylation Using N-(Acyloxy)phthalimides, an Organic Photocatalyst, and Visible Light.

An intramolecular arene alkylation reaction has been developed using the organic photocatalyst 4CzIPN, visible light, and N-(acyloxy)phthalimides as radical precursors. Reaction conditions were optimized via high-throughput experimentation, and electron-rich and electron-deficient arenes and heteroarenes are viable reaction substrates. This reaction enables access to a diverse set of fused, partially saturated cores which are of high interest in synthetic and medicinal chemistry.

Organocatalyzed, Visible-Light Photoredox-Mediated, One-Pot Minisci Reaction Using Carboxylic Acids via N-(Acyloxy)phthalimides.

An improved, one-pot Minisci reaction has been developed using visible light, an organic photocatalyst, and carboxylic acids as radical precursors via the intermediacy of in situ-generated N-(acyloxy)phthalimides. The conditions employed are mild, demonstrate a high degree of functional group tolerance, and do not require a large excess of the carboxylic acid reactant. As a result, this reaction can be applied to drug-like scaffolds and molecules with sensitive functional groups, enabling late-stage functionalization, which is of high interest to medicinal chemistry.

Catalytic intermolecular hydroaminations of unactivated olefins with secondary alkyl amines.

The intermolecular hydroamination of unactivated alkenes with simple dialkyl amines remains an unsolved problem in organic synthesis. We report a catalytic protocol for efficient additions of cyclic and acyclic secondary alkyl amines to a wide range of alkyl olefins with complete anti-Markovnikov regioselectivity. In this process, carbon-nitrogen bond formation proceeds through a key aminium radical cation intermediate that is generated via electron transfer between an excited-state iridium photocatalyst and an amine substrate.

A strategy for complex dimer formation when biomimicry fails: total synthesis of ten coccinellid alkaloids.

Although dimeric natural products can often be synthesized in the laboratory by directly merging advanced monomers, these approaches sometimes fail, leading instead to non-natural architectures via incorrect unions. Such a situation arose during our studies of the coccinellid alkaloids, when attempts to directly dimerize Nature's presumed monomeric precursors in a putative biomimetic sequence afforded only a non-natural analogue through improper regiocontrol. Herein, we outline a unique strategy for dimer formation that obviates these difficulties, one which rapidly constructs the coccinellid dimers psylloborine A and isopsylloborine A through a terminating sequence of two reaction cascades that generate five bonds, five rings, and four stereocenters.

Total syntheses of dalesconol A and B.

[Image: see text] : Use of a carefully designed acyclic intermediate provided the means to execute a cascade-based construction which formed the entire core of the polyketide-derived dalesconols in a single flask. A number of additional and carefully controlled synthetic operations completed an expeditious synthesis of both of these highly bioactive natural products as well as structural congenors.