Photoredox/Nickel-Catalyzed Single-Electron Tsuji-Trost Reaction: Development and Mechanistic Insights

Photoinduced 1,5-HAT-enabled 1,7-hydrosulfonylation of allylic ethers and amides

Herein, we report a photoinduced 1,7-hydrosulfonylation of allylic ethers and amides via a sequential Pd-mediated 1,5-HAT process and Pd-catalyzed allylic nucleophilic attack of arylsulfonates. This rationally designed synthetic protocol allows for facile construction of a series of structurally novel allylic sulfonated scaffolds, and features mild conditions, cheap and readily available raw materials and functional group compatibility.

Advances in Computational Methods for Modeling Photocatalytic Reactions: A Review of Recent Developments

Photocatalysis is a fascinating process in which a photocatalyst plays a pivotal role in driving a chemical reaction when exposed to light. Its capacity to harness light energy triggers a cascade of reactions that lead to the formation of intermediate compounds, culminating in the desired final product(s). The essence of this process is the interaction between the photocatalyst's excited state and its specific interactions with reactants, resulting in the creation of intermediates. The process's appeal is further enhanced by its cyclic nature-the photocatalyst is rejuvenated after each cycle, ensuring ongoing and sustainable catalytic action. Nevertheless, comprehending the photocatalytic process through the modeling of photoactive materials and molecular devices demands advanced computational techniques founded on effective quantum chemistry methods, multiscale modeling, and machine learning. This review analyzes contemporary theoretical methods, spanning a range of lengths and accuracy scales, and assesses the strengths and limitations of these methods. It also explores the future challenges in modeling complex nano-photocatalysts, underscoring the necessity of integrating various methods hierarchically to optimize resource distribution across different scales. Additionally, the discussion includes the role of excited state chemistry, a crucial element in understanding photocatalysis.

Photoinduced Platinum-Catalyzed Reductive Allylation of α-Diketones with Allylic Carbonates

A unique process for the photoinduced platinum-catalyzed reductive allylation of α-diketones with allylic carbonates has been developed. This allylation reaction was found to proceed selectively at the more electron-deficient carbonyl group of the diketone to afford an α-keto homoallylic alcohol. Such products could be further derivatized by transformation of the remaining carbonyl group. A mechanistic investigation suggests that a ketyl radical generated in response to photoirradiation reacts with a (π-allyl)platinum complex to form a C-C bond.

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.

Regioselective Formal β-Allylation of Carbonyl Compounds Enabled by Cooperative Nickel and Photoredox Catalysis

The Tsuji-Trost reaction between carbonyl compounds and allylic precursors has been widely used in the synthesis of natural products and pharmaceutical compounds. As the α-C-H bond is far more acidic than the β-C-H bond, carbonyl compounds undergo highly regioselective allylation at the α-position and their β-allylation is therefore highly challenging. This innate α-reactivity conversely hampers diversity, especially if the corresponding β-allylation product is targeted. Herein, we present a formal intermolecular β-C-C bond formation reaction of a broad range of aldehydes and ketones with different allyl electrophiles through cooperative nickel and photoredox catalysis. β-Selectivity is achieved via initial transformation of the aldehydes and ketones to their corresponding silyl enol ethers. The overall transformation features mild conditions, excellent regioselectivity, wide functional group tolerance and high reaction efficiency. The introduced facile and regioselective β-allylation of carbonyl compounds proceeding through cooperative catalysis allows the preparation of valuable building blocks that are difficult to access from aldehydes and ketones using existing methodology.

Mechanistic explorations on the decarboxylative allylation of amino esters via dual photoredox and palladium catalysis

Mechanistic studies reveal that the decarboxylative allylation of amino esters via dual photoredox and palladium catalysis occurs via oxidation giving π-allyl-Pd(II) species and carboxylate, which is oxidized by *Ir(III)-catalyst offering benzyl radicals. The alkylated product is formed via an S_N2 pathway. Single-electron transfer between Pd(I)-species and Ir(II)-catalysis restores both catalysts.

Visible-light-promoted radical cascade alkylation/cyclization: access to alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-ones

A new protocol is herein described for the direct generation of alkylated indolo/benzoimidazo[2,1-a]isoquinolin-6(5H)-one derivatives by using Hantzsch esters as alkylation radical precursors using a photoredox/K₂S₂O₈ system. This oxidative alkylation of active alkenes involves a radical cascade cyclization process and a sequence of Hantzsch ester single electron oxidation, C-C bond cleavage, alkylation, arylation and oxidative deprotonation.

Investigating Oxidative Addition Mechanisms of Allylic Electrophiles with Low-Valent Ni/Co Catalysts Using Electroanalytical and Data Science Techniques

The catalysis by a π-allyl-Co/Ni complex has drawn significant attention recently due to its distinct reactivity in reductive Co/Ni-catalyzed allylation reactions. Despite significant success in reaction development, the critical oxidative addition mechanism to form the π-allyl-Co/Ni complex remains unclear. Herein, we present a study to investigate this process with four catalysis-relevant complexes: Co(^MeBPy)Br₂, Co(^MePhen)Br₂, Ni(^MeBPy)Br₂, and Ni(^MePhen)Br₂. Enabled by an electroanalytical platform, Co(I)/Ni(I) species were found responsible for the oxidative addition of allyl acetate. Kinetic features of different substrates were characterized through linear free-energy relationship (Hammett-type) studies, statistical modeling, and a DFT computational study. In this process, a coordination-ionization-type transition state was proposed, sharing a similar feature with Pd(0)-mediated oxidative addition in Tsuji-Trost reactions. Computational and ligand structural analysis studies support this mechanism, which should provide key information for next-generation catalyst development.

Exploiting photoredox catalysis for carbohydrate modification through C–H and C–C bond activation

Photoredox catalysis has recently emerged as a powerful synthetic platform for accessing complex chemical structures through non-traditional bond disconnection strategies that proceed through free-radical intermediates. Such synthetic strategies have been used for a range of organic transformations; however, in carbohydrate chemistry they have primarily been applied to the generation of oxocarbenium ion intermediates in the ubiquitous glycosylation reaction. In this Review, we present more intricate light-induced synthetic strategies to modify native carbohydrates through homolytic C-H and C-C bond cleavage. These strategies allow access to glycans and glycoconjugates with profoundly altered carbohydrate skeletons, which are challenging to obtain through conventional synthetic means. Carbohydrate derivatives with such structural motifs represent a broad class of natural products integral to numerous biochemical processes and can be found in active pharmaceutical substances. Here we present progress made in C-H and C-C bond activation of carbohydrates through photoredox catalysis, focusing on the operational mechanisms and the scope of the described methodologies.

Synthesis of gem-Difluorinated 1,3-Dienes via Synergistic Cu/Pd-Catalyzed Borodifluorovinylation of Alkynes

gem-Difluoroalkenes (=CF₂), which normally act as metabolically stable bioisosteres for carbonyl groups (C═O), are widely applied in agrochemicals and pharmaceuticals and are also used as building blocks in organic synthesis. Herein, an example of Cu/Pd-catalyzed borodifluorovinylation was achieved using alkynes, difluoroethylene bromide, and B₂pin₂ as chemical feedstocks, providing the corresponding conjugated gem-difluoroalkene scaffold with good functional group compatibility. Moreover, an array of fluorinated synthons can be obtained through further transformations.

Synthesis of Multisubstituted Allylic Alcohols via a Nickel-Catalyzed Cross-Electrophile Ring-Opening Reaction

Herein we report a nickel-catalyzed cross-electrophile ring-opening reaction of vinyl epoxides wherein aryl iodides, alkyl iodides, and benzyl chlorides can all serve as the electrophilic coupling partners, providing a new approach to preparing multisubstituted allylic alcohols. This new method features broad substrate scope (76 examples), good step-economy, and high L/B- and E/Z selectivity as well as mild reaction conditions.

Iron-Promoted Oxidative Alkylation/Cyclization of Ynones with 4-Alkyl-1,4-dihydropyridines: Access to 2-Alkylated Indenones

An iron-promoted oxidative tandem alkylation/cyclization of ynones with 4-alkyl-substituted 1,4-dihydropyridines for the efficient synthesis of 2-alkylated indenones is described. The process occurs via oxidative homolysis of a C-C σ-bond in 1,4-dihydropyridines to generate an alkyl radical followed by the addition of C-C triple bonds in ynones and intramolecular cyclization. A wide range of alkyl radicals could be efficiently transferred to generate a series of synthetically useful 2-alkylated indenones with excellent selectivity under mild conditions.

Regio- and Enantioselective γ-Allylic Alkylation of In Situ-Generated Free Dienolates via Scandium/Iridium Dual Catalysis

An unprecedented asymmetric γ-allylic alkylation of free dienolates via Sc/Ir dual catalysis is reported, which affords a range of synthetically versatile γ-allylic crotonaldehydes in high efficiency with excellent chemo-, regio-, and enantioselectivities. The dienolates bearing no essential auxiliary groups were generated in situ by scandium triflate-mediated Meinwald rearrangement of vinyloxiranes atom-economically. With the assistance of computational density functional theory calculations, a Sc/Ir bimetallic catalytic cycle was proposed to illustrate the reaction mechanism.

Mechanisms, Challenges, and Opportunities of Dual Ni/Photoredox-Catalyzed C(sp2)-C(sp3) Cross-Couplings

The merging of photoredox and nickel catalysis has revolutionized the field of C-C cross-coupling. However, in comparison to the development of synthetic methods, detailed mechanistic investigations of these catalytic systems are lagging. To improve the mechanistic understanding, computational tools have emerged as powerful tools to elucidate the factors controlling reactivity and selectivity in these complex catalytic transformations. Based on the reported computational studies, it appears that the mechanistic picture of catalytic systems is not generally applicable, but is rather dependent on the specific choice of substrate, ligands, photocatalysts, etc. Given the complexity of these systems, the need for more accurate computational methods, readily available and user-friendly dynamics simulation tools, and data-driven approaches is clear in order to understand at the molecular level the mechanisms of these transformations. In particular, we anticipate that such improvement of theoretical methods will become crucial to advance the understanding of excited-state properties and dynamics of key species, as well as to enable faster and unbiased exploration of reaction pathways. Further, with greater collaboration between computational, experimental, and spectroscopic communities, the mechanistic investigation of photoredox/Ni dual-catalytic reactions is expected to thrive quickly, facilitating the design of novel catalytic systems and promoting our understanding of the reaction selectivity.

Regioselective Deaminative Allylation of Aliphatic Amines via Dual Cobalt and Organophotoredox Catalysis

Despite the rapid progress in C-C bond-forming reactions using Katritzky salts, their deaminative allylation remains a challenge. Inspired by the metallaphotoredox-catalyzed allylic substitution regime, here, we report the deaminative allylation of Katritzky salts via cobalt/organophotoredox dual catalysis. This cross-electrophile coupling enables regioselective allylation using a variety of allylic esters, overcoming the substrate limitations of reported protocols. Mechanistic studies indicate the involvement of a π-allyl cobalt complex as a radicalophile that mediates C-C bond formation.

Transition-Metal-Free Alkylation Strategy: A Facile Access of Alkylated Oxindoles via Alkyl Transfer

An efficient transition-metal-free alkylation/cyclization of activated alkenes using Hantzsch ester derivatives as effective alkyl reagents was described. A wide variety of valuable oxindoles were constructed in a single step with...

Alkyl-GeMe3: Neutral Metalloid Radical Precursors upon Visible-Light Photocatalysis

Single-electron transfer (SET) oxidation of ionic hypervalent complexes, representatively alkyltrifluoroborates (Alkyl-BF3-) and alkylbis(catecholato)silicates (Alkyl-Si(cat)2-), have contributed substantially to alkyl radical generation compared to alkali or alkaline earth organometallics because of their excellent activity-stability balance. Herein, we report another proposal using neutral metalloid compounds, Alkyl-GeMe3, as radical precursors. Compared to Alkyl-BF3- and Alkyl-Si(cat)2-, Alkyl-GeMe3 show comparable activity in radical addition reactions. Moreover, Alkyl-GeMe3 gives the first success of group 14 tetraalkyl nucleophiles in nickel catalyzed cross-coupling. Meanwhile, the neutral nature of these organogermanes supplemented the limination of ionic precursors in purification and derivatization. A preliminary mechanism study corresponds to the procedure that alkyl radical generates from tetraalkylgermane radical cation with the assistance of a nucleophile, which may also enlighten the development of more non-ionic alkyl radical precursors with metalloid center.

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.

Pyridylphosphonium salts as alternatives to cyanopyridines in radical-radical coupling reactions

Radical couplings of cyanopyridine radical anions represent a valuable technology for functionalizing pyridines, which are prevalent throughout pharmaceuticals, agrochemicals, and materials. Installing the cyano group, which facilitates the necessary radical anion formation and stabilization, is challenging and limits the use of this chemistry to simple cyanopyridines. We discovered that pyridylphosphonium salts, installed directly and regioselectively from C–H precursors, are useful alternatives to cyanopyridines in radical–radical coupling reactions, expanding the scope of this reaction manifold to complex pyridines. Methods for both alkylation and amination of pyridines mediated by photoredox catalysis are described. Additionally, we demonstrate late-stage functionalization of pharmaceuticals, highlighting an advantage of pyridylphosphonium salts over cyanopyridines.

Cyanopyridines form dearomatized radical anions upon single-electron reduction and participate in photoredox coupling reactions. Pyridylphosphonium salts replicate that reactivity with a broader scope and increase the utility of these processes.

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

Photoredox chemistry with organic or transition metal agents has been reviewed in earlier years, but such is the pace of progress that we will overlap very little with earlier comprehensive reviews. This review first presents an overview of the area of research and then examines recent examples of C-C, C-N, C-O and C-S bond formations via radical intermediates with transition metal and organic radical promoters. Recent successes with Birch reductions are also included. The transition metal chemistry will be restricted to photocatalysts based on the most widely used metals, Ru and Ir, but includes coupling chemistries that take advantage of low-valent nickel, or occasionally copper, complexes to process the radicals that are formed. Our focus is on developments in the past 10 years (2011-2021). This period has also seen great advances in the chemistry of organic photoredox reagents and the review covers this area. The review is intended to present highlights and is not comprehensive.

Asymmetric Catalytic Vinylogous Addition Reactions Initiated by Meinwald Rearrangement of Vinyl Epoxides

The first catalytic asymmetric multiple vinylogous addition reactions initiated by Meinwald rearrangement of vinyl epoxides were realized by employing chiral N,N'-dioxide/Sc^III complex catalysts. The vinyl epoxides, as masked β,γ-unsaturated aldehydes, via direct vinylogous additions with isatins, 2-alkenoylpyridines or methyleneindolinones, provided a facile and efficient way for the synthesis of chiral 3-hydroxy-3-substituted oxindoles, α,β-unsaturated aldehydes and spiro-cyclohexene indolinones, respectively with high efficiency and stereoselectivity. The control experiments and kinetic studies revealed that the Lewis acid acted as dual-tasking catalyst, controlling the initial rearrangement to match subsequent enantioselective vinylogous addition reactions. A catalytic cycle with a possible transition model was proposed to illustrate the reaction mechanism.

Asymmetric Synthesis of Homoallylic Alcohols Featuring Vicinal Tetrasubstituted Carbon Centers via Dual Pd/Photoredox Catalysis

Dual palladium/photoredox-catalysis provides an effective method for the decarboxylative asymmetric synthesis of vicinal α,β-tri/tetra- or α,β-tetrasubstituted homoallylic alcohols using Hantzsch-type esters as radical precursors. This mild methodology capitalizes on vinyl cyclic carbonates as accessible reagents providing the target molecules in appreciable to good yields, high branch selectivity, and enantiomeric ratios of up to 94:6, making it a rare example of using prochiral electrophiles for the creation of vicinal congested carbon centers.

2-Fluoro-5-nitrophenyldiazonium: A Novel Sanger-Type Reagent for the Versatile Functionalization of Alcohols

As a novel Sanger-type reagent, 2-fluoro-5-nitrophenyldiazonium tetrafluoroborate enabled the versatile functionalization of primary and secondary aliphatic alcohols. Based on a mild nucleophilic aromatic substitution of the fluorine atom under unprecedented, base-free conditions, the diazonium unit on the aromatic core of the resulting aryl-alkyl ether could be employed for such diverse transformations as radical C-H activation and cyclization, as well as palladium catalyzed cross-coupling reactions.

Photochemical C-H Activation Enables Nickel-Catalyzed Olefin Dicarbofunctionalization

Alkenes, ethers, and alcohols account for a significant percentage of bulk reagents available to the chemistry community. The petrochemical, pharmaceutical, and agrochemical industries each consume gigagrams of these materials as fuels and solvents each year. However, the utilization of such materials as building blocks for the construction of complex small molecules is limited by the necessity of prefunctionalization to achieve chemoselective reactivity. Herein, we report the implementation of efficient, sustainable, diaryl ketone hydrogen-atom transfer (HAT) catalysis to activate native C-H bonds for multicomponent dicarbofunctionalization of alkenes. The ability to forge new carbon-carbon bonds between reagents typically viewed as commodity solvents provides a new, more atom-economic outlook for organic synthesis. Through detailed experimental and computational investigation, the critical effect of hydrogen bonding on the reactivity of this transformation was uncovered.

Palladium catalyzed desulfurative coupling of allyl sulfides with organoboronic acids

A palladium catalyzed desulfurative coupling of allylthioethers with organoboronic acids under mild reaction conditions is described.

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.

Organophotoredox/palladium dual catalytic decarboxylative Csp3-Csp3 coupling of carboxylic acids and π-electrophiles

A dual catalytic decarboxylative allylation and benzylation method for the construction of new C(sp³)-C(sp³) bonds between readily available carboxylic acids and functionally diverse carbonate electrophiles has been developed. The new process is mild, operationally simple, and has greatly improved upon the efficiency and generality of previous methodology. In addition, new insights into the reaction mechanism have been realized and provide further understanding of the harnessed reactivity.

Recent advances in photoredox and nickel dual-catalyzed cascade reactions: pushing the boundaries of complexity

Cascade reactions that produce multiple chemical bonds in one synthetic operation are important in the efficient construction of complex molecules. In addition, photoredox and nickel dual catalysis opens a new and powerful avenue for transition-metal-catalyzed cross-coupling reactions. By combining these two concepts, photoredox and nickel dual-catalyzed cascade reactions have been recently established, and they provide an efficient and mild method for accessing a series of valuable organic compounds.

On the Nature of C(sp3)-C(sp2) Bond Formation in Nickel-Catalyzed Tertiary Radical Cross-Couplings: A Case Study of Ni/Photoredox Catalytic Cross-Coupling of Alkyl Radicals and Aryl Halides

The merger of photoredox and nickel catalysis has enabled the construction of quaternary centers. However, the mechanism, role of the ligand, and effect of the spin state for this transformation and related Ni-catalyzed cross-couplings involving tertiary alkyl radicals in combination with bipyridine and diketonate ligands remain unknown. Several mechanisms have been proposed, all invoking a key Ni(III) species prior to undergoing irreversible inner-sphere reductive elimination. In this work, we have used open-shell dispersion-corrected DFT calculations, quasi-classical dynamics calculations, and experiments to study in detail the mechanism of carbon-carbon bond formation in Ni bipyridine- and diketonate-based catalytic systems. These calculations revealed that access to high spin states (e.g., triplet spin state tetrahedral Ni(II) species) is critical for effective radical cross-coupling of tertiary alkyl radicals. Further, these calculations revealed a disparate mechanism for the C-C bond formation. Specifically, contrary to the neutral Ni-bipyridyl system, diketonate ligands lead directly to the corresponding tertiary radical cross-coupling products via an outer-sphere reductive elimination step via triplet spin state from the Ni(III) intermediates. Implications to related Ni-catalyzed radical cross-couplings and the design of new transformations are discussed.