Single-Electron Transmetalation: Synthesis of 1,1-Diaryl-2,2,2-trifluoroethanes by Photoredox/Nickel Dual Catalytic Cross-Coupling

Decarboxylative Aminomethylation of Aryl- and Vinylsulfonates through Combined Nickel- and Photoredox-Catalyzed Cross-Coupling

A mild approach for the decarboxylative aminomethylation of aryl sulfonates by the combination of photoredox and nickel catalysis through C-O bond cleavage is described for the first time. A wide range of aryl triflates as well as aryl mesylates, tosylates and alkenyl triflates afford the corresponding products in good to excellent yields.

Photochemical Nickel-Catalyzed C-H Arylation: Synthetic Scope and Mechanistic Investigations

An iridium photocatalyst and visible light facilitate a room temperature, nickel-catalyzed coupling of (hetero)aryl bromides with activated α-heterosubstituted or benzylic C(sp³)–H bonds. Mechanistic investigations on this unprecedented transformation have uncovered the possibility of an unexpected mechanism hypothesized to involve a Ni–Br homolysis event from an excited-state nickel complex. The resultant bromine radical is thought to abstract weak C(sp³)–H bonds to generate reactive alkyl radicals that can be engaged in Ni-catalyzed arylation. Evidence suggests that the iridium photocatalyst facilitates nickel excitation and bromine radical generation via triplet–triplet energy transfer.

Dual Catalysis Strategies in Photochemical Synthesis

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Brønsted acids, organocatalysts, enzymes, and transition metal complexes.

Photoredox Catalysis in Organic Chemistry

In recent years, photoredox catalysis has come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In a general sense, these approaches rely on the ability of metal complexes and organic dyes to convert visible light into chemical energy by engaging in single-electron transfer with organic substrates, thereby generating reactive intermediates. In this Perspective, we highlight the unique ability of photoredox catalysis to expedite the development of completely new reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the construction of challenging carbon–carbon and carbon–heteroatom bonds.

Single-Electron Transmetalation via Photoredox/Nickel Dual Catalysis: Unlocking a New Paradigm for sp(3)-sp(2) Cross-Coupling

The important role of transition metal-catalyzed cross-coupling in expanding the frontiers of accessible chemical territory is unquestionable. Despite empowering chemists with Herculean capabilities in complex molecule construction, contemporary protocols are not without their Achilles' heel: Csp(3)-Csp(2)/sp(3) coupling. The underlying challenge in sp(3) cross-couplings is 2-fold: (i) methods employing conventional, bench-stable precursors are universally reliant on extreme reaction conditions because of the high activation barrier of transmetalation; (ii) circumvention of this barrier invariably relies on use of more reactive precursors, thereby sacrificing functional group tolerance, operational simplicity, and broad applicability. Despite the ubiquity of this problem, the nature of the transmetalation step has remained unchanged from the seminal reports of Negishi, Suzuki, Kumada, and Stille, thus suggesting that the challenges in Csp(3)-Csp(2)/sp(3) coupling result from inherent mechanistic constraints in the traditional cross-coupling paradigm. Rather than submitting to the limitations of this conventional approach, we envisioned that a process rooted in single-electron reactivity could furnish the same key metalated intermediate posited in two-electron transmetalation, while demonstrating entirely complementary reactivity patterns. Inspired by literature reports on the susceptibility of organoboron reagents toward photochemical, single-electron oxidative fragmentation, realization of a conceptually novel open shell transmetalation framework was achieved in the facile coupling of benzylic trifluoroborates with aryl halides via cooperative visible-light activated photoredox and Ni cross-coupling catalysis. Following this seminal study, we disclosed a suite of protocols for the cross-coupling of secondary alkyl, α-alkoxy, α-amino, and α-trifluoromethylbenzyltrifluoroborates. Furthermore, the selective cross-coupling of Csp(3) organoboron moieties in the presence of Csp(2) organoboron motifs was also demonstrated, highlighting the nuances of this approach to transmetalation. Computational modeling of the reaction mechanism uncovered useful details about the intermediates and transition-state structures involved in the nickel catalytic cycle. Most notably, a unique dynamic kinetic resolution process, characterized by radical homolysis/recombination equilibrium of a Ni(III) intermediate, was discovered. This process was ultimately found to be responsible for stereoselectivity in an enantioselective variant of these cross-couplings. Prompted by the intrinsic limitations of organotrifluoroborates, we sought other radical feedstocks and quickly identified alkylbis(catecholato)silicates as viable radical precursors for Ni/photoredox dual catalysis. These hypervalent silicate species have several notable benefits, including more favorable redox potentials that allow extension to primary alkyl systems incorporating unprotected amines as well as compatibility with less expensive Ru-based photocatalysts. Additionally, these reagents exhibit an amenability to alkenyl halide cross-coupling while simultaneously expanding the aryl halide scope. In the process of exploring these reagents, we serendipitously discovered a method to effect thioetherification of aryl halides via a H atom transfer mechanism. This latter discovery emphasizes that this robust cross-coupling paradigm is "blind" to the origins of the radical, opening opportunities for a wealth of new discoveries. Taken together, our studies in the area of photoredox/nickel dual catalysis have validated single-electron transmetalation as a powerful platform for enabling conventionally challenging Csp(3)-Csp(2) cross-couplings. More broadly, these findings represent the power of rational design in catalysis and the strategic use of mechanistic knowledge and manipulation for the development of new synthetic methods.

Photoredox Catalysis in Nickel-Catalyzed Cross-Coupling

The traditional transition metal-catalyzed cross-coupling reaction, although well suited for C(sp2)-C(sp2) cross-coupling, has proven less amenable toward coupling of C(sp3)-hybridized centers, particularly using functional group tolerant reagents and reaction conditions. The development of photoredox/Ni dual catalytic methods for cross-coupling has opened new vistas for the construction of carbon-carbon bonds at C(sp3)-hybridized centers. In this chapter, a general outline of the features of such processes is detailed.

Combination of organotrifluoroborates with photoredox catalysis marking a new phase in organic radical chemistry

Combination of organotrifluoroborates and visible-light-driven photoredox catalysis, both of which have attracted the attention of synthetic chemists, marks a new phase in the field of organic radical chemistry. We have developed photoredox-catalyzed radical reactions with organotrifluoroborates, which turn out to serve not only as a source of organic radicals but also as radical acceptors. The first part of this Perspective deals with the generation of organic radicals from organotrifluoroborates, and the latter part describes addition of the CF3 radical to alkenyltrifluoroborates. The good chemistry between organoborates and photoredox catalysis and its future will be discussed.

Single-Electron Transmetalation: Photoredox/Nickel Dual Catalytic Cross-Coupling of Secondary Alkyl β-Trifluoroboratoketones and -esters with Aryl Bromides

The first cross-coupling of secondary alkyl β-trifluoroboratoketones and -esters has been achieved through application of photoredox/nickel dual catalysis. Although the related β-trifluoroboratoamides have been effectively cross-coupled via Pd-catalysis, the corresponding ketones and esters had proven recalcitrant prior to this report. Reactions occur under mild conditions, and a variety of functional groups and sterically and electronically diverse reaction partners are tolerated.

Photoredox Catalysis Unlocks Single-Electron Elementary Steps in Transition Metal Catalyzed Cross-Coupling

Since initial reports, cross-coupling technologies employing photoredox catalysts to access novel reactivity have developed with increasing pace. In this Outlook, prominent examples from the recent literature are organized on the basis of the elementary transformation enabled by photoredox catalysis and are discussed in the context of relevant historical precedent in stoichiometric organometallic chemistry. This treatment allows mechanistic similarities inherent to odd-electron transition metal reactivity to be generalized to a set of lessons for future reaction development.

Visible Light Photoredox Cross-Coupling of Acyl Chlorides with Potassium Alkoxymethyltrifluoroborates: Synthesis of α-Alkoxyketones

A visible-light, single-electron-transfer (SET), photoredox cross-coupling for the synthesis of α-alkoxyketones has been developed. In this method, various aliphatic and aromatic acyl chlorides were successfully coupled with structurally diverse potassium alkoxymethyltrifluoroborates, producing the corresponding α-alkoxyketones with high yields. In this operationally simple and mild cross-coupling protocol, the desired ketones are obtained in one step from bench stable starting materials by a bond connection that is unique to both alkylboron chemistry and photoredox/Ni catalysis.

Magnetic nanoparticle-supported ferrocenylphosphine: a reusable catalyst for hydroformylation of alkene and Mizoroki–Heck olefination

This study was focused on regioselective, re-usable and solvent-free catalysis using Fe3O4@dop-BPPF nanomaterials.

Primary alkyl bis-catecholato silicates in dual photoredox/nickel catalysis: aryl- and heteroaryl-alkyl cross coupling reactions

Primary alkyl bis-catecholato silicates have been successfully engaged with aryl and heteroaryl bromide substrates in photoredox/nickel dual catalysis to provide cross coupling products.

Photoredox sheds new light on nickel catalysis: from carbon–carbon to carbon–heteroatom bond formation

Recent emergence of photoredox/nickel dual catalysis to generate carbon–carbon and carbon–heteroatom bonds is highlighted.