Photoredox/Nickel Dual Catalysis for the C(sp3 )-C(sp3 ) Cross-Coupling of Alkylsilicates with Alkyl Halides

Homogeneous Organic Reductant Based on 4,4'-tBu2-2,2'-Bipyridine for Cross-Electrophile Coupling

The synthesis of a new homogeneous reductant based on 4,4'-tBu2-2,2'-bipyridine, tBu-OED4, is reported. tBu-OED4 was prepared on a multigram scale in two steps from inexpensive and commercially available starting materials, with no chromatography required for purification. tBu-OED4 has a reduction potential of -1.33 V (vs Ferrocenium/Ferrocene) and is soluble in a range of common organic solvents. We demonstrate that tBu-OED4 can facilitate Ni/Co dual-catalyzed C(sp2)-C(sp3) cross-electrophile coupling reactions and is highly functional group tolerant. tBu-OED4 is expected to be a valuable addition to the set of homogeneous reductants available for organic transformations.

3-Chloropropylbis(catecholato)silicate as a Bifunctional Reagent for the One-Pot Synthesis of Tetrahydroquinolines from o-Bromosulfonamides

Bis(catecholato)silicate salts are easily accessible reagents that can be used to install alkyl fragments through photoredox-enabled cross-coupling. These reagents can incorporate various functional groups including pendant alkyl halides. A halogenated organosilicate reagent was leveraged to develop a one-pot synthesis of tetrahydroquinolines from o-bromosulfonamides, where the bifunctional reagent participates in a nickel/photoredox cross-coupling followed by intramolecular nucleophilic substitution. The functional group tolerance of this cross-coupling strategy allowed for the preparation of a series of substituted tetrahydroquinolines.

Tunable and Practical Homogeneous Organic Reductants for Cross-Electrophile Coupling

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air stability compared to current homogeneous reductants for metal-mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of -0.85 V versus ferrocene, which is significantly milder than conventional reductants used in XEC. All of the new reductants can facilitate C(sp²)-C(sp³) Ni-catalyzed XEC reactions and are compatible with complex substrates that are relevant to medicinal chemistry. The reductants span a range of nearly 0.5 V in reduction potential, which allows for control over the rate of electron transfer events in XEC. Specifically, we report a new strategy for controlled alkyl radical generation in Ni-catalyzed C(sp²)-C(sp³) XEC. The key to our approach is to tune the rate of alkyl radical generation from Katritzky salts, which liberate alkyl radicals upon single electron reduction, by varying the redox potentials of the reductant and Katritzky salt utilized in catalysis. Using our method, we perform XEC reactions between benzylic Katritzky salts and aryl halides. The method tolerates a variety of functional groups, some of which are particularly challenging for most XEC transformations. Overall, we expect that our new reductants will both replace conventional homogeneous reductants in current reductive transformations due to their stability and relatively facile synthesis and lead to the development of novel synthetic methods due to their tunability.

Metallaphotoredox: The Merger of Photoredox and Transition Metal Catalysis

The merger of photoredox catalysis with transition metal catalysis, termed metallaphotoredox catalysis, has become a mainstay in synthetic methodology over the past decade. Metallaphotoredox catalysis has combined the unparalleled capacity of transition metal catalysis for bond formation with the broad utility of photoinduced electron- and energy-transfer processes. Photocatalytic substrate activation has allowed the engagement of simple starting materials in metal-mediated bond-forming processes. Moreover, electron or energy transfer directly with key organometallic intermediates has provided novel activation modes entirely complementary to traditional catalytic platforms. This Review details and contextualizes the advancements in molecule construction brought forth by metallaphotocatalysis.

Visible-Light-Mediated Z-Stereoselective Monoalkylation of β,β-Dichlorostyrenes by Photoredox/Nickel Dual Catalysis

Metal-catalyzed alkylation of 1,1-dihalovinyl moiety commonly suffers from both a lack of stereoselectivity and the overreaction leading to the dialkylation product. The methodology described herein features a new pathway to alkylate stereoselectively β,β-dichlorostyryl substrates to provide the Z-trisubstituted olefin only with fair to good yields. This cross-coupling reaction bears on the smooth and photoinduced formation of a C-centered radical that engages in a nickel-catalyzed organometallic cycle to form the key Csp2–Csp3 bond.

A Parisian Vision of the Chemistry of Hypercoordinated Silicon Derivatives

Less than ten years of acquaintance with hypercoordinated silicon derivatives in our lab is described in this account. Martin's spirosilane derivatives open new opportunities as ligands and as agents for the activation of small molecules and bis-catecholato silicates have proven to be exquisite radical precursors in photoredox conditions for broad synthetic applications.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

Recent Advances in C(sp3)–C(sp3) Cross-Coupling via Metalla­photoredox Strategies

Transition-metal-catalyzed carbon–carbon cross-coupling reactions represent a significant achievement in modern synthetic chemistry and they have become indispensable tools for the construction of organic molecules. Despite the important progress in this area, methods for coupling two C(sp3)-hybridized alkyl fragments remain elusive. To date, existing methods have largely relied on using organometallic reagents as the nucleophilic coupling partners, thereby inevitably limiting the compatibility of functional groups. Although cross-electrophile coupling may alleviate the pain somewhat, it is necessary to add a stoichiometric amount of a reductant to complete the catalytic cycle. Recently, the emergence of photoredox-mediated single-electron transmetalation has evoked an ideal paradigm for selectively manipulating C(sp3)–C(sp3) cross-coupling with the unprecedented application of native C(sp3) functionalities instead of organometallic reagents, thus opening a new window for C(sp3)–C(sp3) bond creation. This short review highlights the recent advances in this exciting subfield.

1 Introduction

2 Nickel/Photoredox-Catalyzed C(sp3)–C(sp3) Cross-Coupling

3 Palladium/Photoredox-Catalyzed C(sp3)–C(sp3) Cross-Coupling

4 Copper/Photoredox-Catalyzed C(sp3)–C(sp3) Cross-Coupling

5 Direct C(sp3)–H Alkylation via Metallaphotoredox-Mediated Hydrogen­ Atom Transfer

6 Conclusion and Perspectives

Practical and scalable synthesis of bench-stable organofluorosilicate salts

Silanes have enjoyed significant success as synthetic tools in the last few decades. In many of the reactions that use silanes, a pentacoordinate silicate is proposed as the reactive intermediate. Despite this, there is no general method to synthesize pentacoordinate fluorosilicates and use them as reagents instead of organo- or alkoxysilanes. Herein, we report the first practical synthesis of organotetrafluorosilicates. The method is tolerant of a number of different functional groups including electrophiles with preferential attack of the fluoride on the silane rather than the electrophile. This transformaton is generally high yielding, even at the mole scale. Furthermore, we demonstrate that organotetrafluorosilicates are both more reactive than the corresponding trialkoxysilanes and more stable under solvolytic conditions. Organotetrafluorosilicates can be used as substrates for a variety of coupling reactions, oxidations, and radical reactions. Overall, organotetrafluorosilicates represent a new platform on which to develop challenging transformations.

Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy

Alkyl radicals are key intermediates in organic synthesis. Their classic generation from alkyl halides has a severe drawback due to the employment of toxic tin hydrides to the point that "flight from the tyranny of tin" in radical processes was considered for a long time an unavoidable issue. This review summarizes the main alternative approaches for the generation of unstabilized alkyl radicals, using photons as traceless promoters. The recent development in photochemical and photocatalyzed processes enabled the discovery of a plethora of new alkyl radical precursors, opening the world of radical chemistry to a broader community, thus allowing a new era of photon democracy.

Radical coupling of β-ketoesters and amides promoted by Brønsted/Lewis acids

Recent advances in photocatalysis have enabled radical methods with complementary chemoselectivity to established two electron bond forming approaches. While this radical strategy has previously been limited to substrates with favorable redox potentials, Brønsted/Lewis acid activation has emerged as a means of facilitating otherwise difficult reductions. We report herein our investigations into the Lewis acid-promoted redox activation of β-ketocarbonyls in a model photocatalytic radical alkylation reaction. Rapid evaluation of substrates and reactions conditions was achieved by high throughput experimentation using 96-well plate photoreactors.

Alkyl Carbon-Carbon Bond Formation by Nickel/Photoredox Cross-Coupling

The union of photoredox and nickel catalysis has resulted in a renaissance in radical chemistry as well as in the use of nickel-catalyzed transformations, specifically for carbon-carbon bond formation. Collectively, these advances address the longstanding challenge of late-stage cross-coupling of functionalized alkyl fragments. Empowered by the notion that photocatalytically generated alkyl radicals readily undergo capture by Ni complexes, wholly new feedstocks for cross-coupling have been realized. Herein, we highlight recent developments in several types of alkyl cross-couplings that are accessible exclusively through this approach.

Ni/Co-Catalyzed Homo-Coupling of Alkyl Tosylates

A direct reductive homo-coupling of alkyl tosylates has been developed by employing a combination of nickel and nucleophilic cobalt catalysts. A single-electron-transfer-type oxidative addition is a pivotal process in the well-established nickel-catalyzed coupling of alkyl halides. However, the method cannot be applied to the homo-coupling of ubiquitous alkyl tosylates due to the high-lying σ*(C-O) orbital of the tosylates. This paper describes a Ni/Co-catalyzed protocol for the activation of alkyl tosylates on the construction of alkyl dimers under mild conditions.

Metal-free desilylative C–C bond formation by visible-light photoredox catalysis

A novel metal-free desilylative C–C bond formation from simple organosilanes by visible-light acridinium photoredox catalysis is presented.

Metal-, photocatalyst-, and light-free late-stage C–H alkylation of N-heteroarenes with organotrimethylsilanes using persulfate as a stoichiometric oxidant

Minisci C–H alkylation of N-heteroarenes with readily available benzylsilanes and heteroatom substituted silanes was developed.

Silyl-mediated photoredox-catalyzed Giese reaction: addition of non-activated alkyl bromides

The emergence of photoredox catalysis has enabled the discovery of mild and efficient conditions for the generation of a variety of radical reaction platforms. Herein is disclosed the development of a conjugate addition reaction of non-activated alkyl bromides to Michael acceptors under visible-light photoredox catalysis. Optimization of the reaction was achieved using high-throughput experimentation (HTE) tools to enable the identification of mild, general and practical reaction conditions. A diverse set of alkyl bromides was successfully added to cyclic or acyclic α,β-unsaturated esters and amides. The features of this transformation allowed also access to a key intermediate of Vorinostat®, an HDAC inhibitor used to fight cancer and HIV.

Palladium/Light Induced Radical Alkenylation and Allylation of Alkyl Iodides Using Alkenyl and Allylic Sulfones

Alkenylation and allylation of alkyl iodides with alkenyl and allyl sulfones, respectively, took place under Pd/photoirradiation system. The initial alkyl radical, derived from a single electron transfer between Pd(0) and RI, underwent the title transformations. Pd(0) was regenerated through a reductive elimination of PhSO₂PdI, which is formed by the combination of the sulfonyl radical and the palladium radical. The addition of water was effective, presumably by pushing the equilibrium through hydrolysis of PhSO₂I.