Enabling the Cross-Coupling of Tertiary Organoboron Nucleophiles through Radical-Mediated Alkyl Transfer

Catalytic tert-alkylation of enamides via C-C bond cleavage under photoredox conditions

Efficient C-C bond cleavage is recognized as a persistent challenge in the field of synthetic methodology. In this study, we found that tertiary alkyl radicals are smoothly formed from tertiary alkylated dienones (BHT adducts) via SET, using PDI as a photocatalyst. Resulting tert-alkyl radicals could be applied to the tert-alkylation of enamides. The driving force of this C-C bond cleavage reaction is the mesolytic cleavage of the BHT adducts. The mechanistic study revealed that PDI anion radical is the key active species during the catalytic cycle.

Ru-Mg promoted reductive cross-coupling of allyl bromides and alkenes to synthesize 1,7-octadienes with an all-carbon quaternary center

A Ru-promoted reductive cross-coupling of allyl bromides and electron-deficient alkenes to provide terminal 1,7-octadienes with magnesium as a reductant is reported herein. This approach enables the facile construction of a series of complex terminal 1,7-octadienes with an all-carbon quaternary center under mild reaction conditions, and the synthetic utility of the current method has been demonstrated by a gram scale synthesis. Preliminary mechanism investigations suggested that a radical pathway might not be involved in this transformation.

At the Speed of Light: The Systematic Implementation of Photoredox Cross-Coupling Reactions for Medicinal Chemistry Research

The adoption of new and emerging techniques in organic synthesis is essential to promote innovation in drug discovery. In this Perspective, we detail the strategy we used for the systematic deployment of photoredox-mediated, metal-catalyzed cross-coupling reactions in AbbVie's medicinal chemistry organization, focusing on topics such as assessment, evaluation, implementation, and accessibility. The comprehensive evaluation of photoredox reaction setups and published methods will be discussed, along with internal efforts to build expertise and photoredox high-throughput experimentation capabilities. We also highlight AbbVie's academic-industry collaborations in this field that have been leveraged to develop new synthetic strategies, along with discussing the internal adoption of photoredox cross-coupling reactions. The work described herein has culminated in robust photocatalysis and cross-coupling capabilities which are viewed as key platforms for medicinal chemistry research at AbbVie.

Nickel-Electrocatalytic Decarboxylative Arylation to Access Quaternary Centers

There is a pressing need, particularly in the field of drug discovery, for general methods that will enable direct coupling of tertiary alkyl fragments to (hetero)aryl halides. Herein a uniquely powerful and simple set of conditions for achieving this transformation with unparalleled generality and chemoselectivity is disclosed. This new protocol is placed in context with other recently reported methods, applied to simplify the routes of known bioactive building blocks molecules, and scaled up in both batch and flow. The role of pyridine additive as well as the mechanism of this reaction are interrogated through Cyclic Voltammetry studies, titration experiments, control reactions with Ni(0) and Ni(II)-complexes, and ligand optimization data. Those studies indicate that the formation of a BINAPNi(0) is minimized and the formation of an active pyridine-stabilized Ni(I) species is sustained during the reaction. Our preliminary mechanistic studies ruled out the involvement of Ni(0) species in this electrochemical cross-coupling, which is mediated by Ni(I) species via a Ni(I)-Ni(II)-Ni(III)-Ni(I) catalytic cycle.

Chemoselective Reactions of Functionalized Sulfonyl Halides

Chemoselective transformations of functionalized sulfonyl fluorides and chlorides are surveyed comprehensively. It is shown that sulfonyl fluorides provide an excellent selectivity control in their reactions. Thus, numerous conditions are tolerated by the SO2 F group - from amide and ester formation to directed ortho-lithiation and transition-metal-catalyzed cross-couplings. Meanwhile, sulfur (VI) fluoride exchange (SuFEx) is also compatible with numerous functional groups, thus confirming its title of "another click reaction". On the contrary, with a few exceptions, most transformations of functionalized sulfonyl chlorides typically occur at the SO2 Cl moiety.

Programmable late-stage functionalization of bridge-substituted bicyclo[1.1.1]pentane bis-boronates

Modular functionalization enables versatile exploration of chemical space and has been broadly applied in structure-activity relationship (SAR) studies of aromatic scaffolds during drug discovery. Recently, the bicyclo[1.1.1]pentane (BCP) motif has increasingly received attention as a bioisosteric replacement of benzene rings due to its ability to improve the physicochemical properties of prospective drug candidates, but studying the SARs of C2-substituted BCPs has been heavily restricted by the need for multistep de novo synthesis of each analogue of interest. Here we report a programmable bis-functionalization strategy to enable late-stage sequential derivatization of BCP bis-boronates, opening up opportunities to explore the SARs of drug candidates possessing multisubstituted BCP motifs. Our approach capitalizes on the inherent chemoselectivity exhibited by BCP bis-boronates, enabling highly selective activation and functionalization of bridgehead (C3)-boronic pinacol esters (Bpin), leaving the C2-Bpin intact and primed for subsequent derivatization. These selective transformations of both BCP bridgehead (C3) and bridge (C2) positions enable access to C1,C2-disubstituted and C1,C2,C3-trisubstituted BCPs that encompass previously unexplored chemical space.

New Copper Complexes with N,O-Donor Ligands Based on Pyrazole Moieties Supported by 3-Substituted Acetylacetone Scaffolds

The new 3-monosubstituted acetylacetone ligands, 3-(phenyl(1H-pyrazol-1-yl)methyl)pentane-2,4-dione (HLacPz) and 3-((3,5-dimethyl-1H-pyrazol-1-yl)(phenyl)methyl)pentane-2,4-dione (HLacPzMe), were synthesized and used as supporting ligands for new copper(II) and copper(I) phosphane complexes of the general formulae [Cu(HLacX)2(LacX)2] and [Cu(PPh3)2(HLacX)]PF6 (X = Pz (pyrazole) or PzMe (3,5-dimethylpyrazole)), respectively. In the syntheses of the Cu(I) complexes, the triphenylphosphine coligand (PPh3) was used to stabilize copper in the +1 oxidation state, avoiding oxidation to Cu(II). All compounds were characterized by CHN analysis, 1H-NMR, 13C-NMR, FT-IR spectroscopy, and electrospray ionization mass spectrometry (ESI-MS). The ligands HLacPz (1) and HLacPzMe (2) and the copper complex [Cu(PPh3)2(HLacPz)]PF6 (3) were also characterized by X-ray crystallography. The reactivity of these new compounds was investigated and the new compounds 4-phenyl-4-(1H-pyrazol-1-yl)butan-2-one (7) and 4-(3,5-dimethyl-1H-pyrazol-1-yl)-4-phenylbutan-2-one (8) were obtained in basic conditions via the retro-Claisen reaction of related 3-monosubstituted acetylacetone, providing efficient access to synthetically useful ketone compounds. Compound 8 was also characterized by X-ray crystallography.

Divergent 1,2-carboallylation of terminal alkynes enabled by metallaphotoredox catalysis with switchable triplet energy transfer

We report a metallaphotoredox strategy for stereodivergent three-component carboallylation of terminal alkynes with allylic carbonates and alkyl trifluoroborates. This redox-neutral dual catalytic protocol utilizes commercially available organic photocatalyst 4CzIPN and nickel catalysts to trigger a radical addition/alkenyl-allyl coupling sequence, enabling straightforward access to functionalized 1,4-dienes in a highly chemo-, regio-selective, and stereodivergent fashion. This reaction features a broad substrate generality and a tunable triplet energy transfer control with pyrene as a simple triplet energy modulator, offering a facile synthesis of complex trans- and cis-selective skipped dienes with the same set of readily available substrates.

Regime Switch in the Dual-Catalyzed Coupling of Alkyl Silicates with Aryl Bromides

Metallaphotoredox catalyzed cross-coupling of an arylbromide (Ar-Br) with an alkyl bis(catecholato)silicate (R-Si⊖ ) has been analyzed in depth using a continuum of analytical techniques (EPR, fluorine NMR, electrochemistry, photophysics) and modeling (micro-kinetics and DFT calculations). These studies converged on the impact of four control parameters consisting in the initial concentrations of the iridium photocatalyst ([Ir]0 ), nickel precatalyst ([Ni]0 ) and silicate ([R-Si⊖ ]0 ) as well as light intensity I0 for an efficient reaction between Ar-Br and R-Si⊖ . More precisely, two regimes were found to be possibly at play. The first one relies on an equimolar consumption of Ar-Br with R-Si⊖ smoothly leading to Ar-R, with no side-product from R-Si⊖ and a second one in which R-Si⊖ is simultaneously coupled to Ar-Br and degraded to R-H. This integrative approach could serve as a case study for the investigation of other metallaphotoredox catalysis manifolds of synthetic significance.

A Paradigm Shift in Catalysis: Electro- and Photomediated Nickel-Catalyzed Cross-Coupling Reactions

ConspectusTransition-metal catalyzed cross-coupling reactions are fundamental reactions in organic chemistry, facilitating strategic bond formations for accessing natural products, organic materials, agrochemicals, and pharmaceuticals. Redox chemistry enables access to elusive cross-coupling mechanisms through single-electron processes as an alternative to classical two-electron strategies predominated by palladium catalysis. The seminal reports of Baran, MacMillan, Doyle, Molander, Weix, Lin, Fu, Reisman, and others in merging redox perturbation (photochemical, electrochemical, and purely chemical) with catalysis are pivotal to the current resurgence and mechanistic understanding of first-row transition metal-based catalysis. The hallmark of this redox platform is the systematic modulation of transition-metal oxidation states by a photoredox catalyst or at a heterogeneous electrode surface. Electrocatalysis and photocatalysis enhance transition metal catalysis' capacity for bond formation through electron- or energy-transfer processes that promote otherwise challenging elementary steps or elusive mechanisms. Cross-coupling conditions promoted by electrocatalysis and photocatalysis are mild, and bond formation proceeds with exceptionally high chemoselectivity and wide functional group tolerance. The interfacing of abundant first-row transition-metal catalysis with electrocatalysis and photocatalysis has brought about a paradigm shift in cross-coupling technology as practitioners are quickly applying these tools in synthesizing fine chemicals and pharmaceutically relevant motifs. In particular, the merger of Ni catalysis with electro- and photochemistry ushered in a new era for carbon-carbon and carbon-heteroatom cross-couplings with expanded generality compared to their thermally driven counterparts. Over the past decade, we have developed enabling photo- and electrochemical methods throughout our combined research experience in industry (BMS, AstraZeneca) and academia (Professor Baran, Scripps Research) in cross-disciplinary collaborative environments. In this Account, we will outline recent progress from our past and present laboratories in photo- and electrochemically mediated Ni-catalyzed cross-couplings. By highlighting these cross-coupling methodologies, we will also compare mechanistic features of both electro- and photochemical strategies for forging C(sp2)-C(sp3), C(sp3)-C(sp3), C-O, C-N, and C-S bonds. Through these side-by-side comparisons, we hope to demystify the subtle differences between the two complementary tools to enact redox control over transition metal catalysis. Finally, building off the collective experience of ourselves and the rest of the community, we propose a tactical user guide to photo- and electrochemically driven cross-coupling reactions to aid the practitioner in rapidly applying such tools in their synthetic designs.

Substrate Plasticity Enables Group-Selective Transmetalation: Catalytic Stereospecific Cross-Couplings of Tertiary Boronic Esters

Activation of enantiomerically enriched tertiary alkylboronic esters with adamantyllithium generated in situ enables stereoretentive boron-to-copper transmetalation. The resulting alkylcopper species can undergo cross-coupling reactions with an array of electrophiles to furnish synthetically useful compounds bearing quaternary stereocenters. DFT calculations of the transmetalation process provide insights for reactivity and selectivity.

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

A Brief Introduction to Chemical Reaction Optimization

From the start of a synthetic chemist's training, experiments are conducted based on recipes from textbooks and manuscripts that achieve clean reaction outcomes, allowing the scientist to develop practical skills and some chemical intuition. This procedure is often kept long into a researcher's career, as new recipes are developed based on similar reaction protocols, and intuition-guided deviations are conducted through learning from failed experiments. However, when attempting to understand chemical systems of interest, it has been shown that model-based, algorithm-based, and miniaturized high-throughput techniques outperform human chemical intuition and achieve reaction optimization in a much more time- and material-efficient manner; this is covered in detail in this paper. As many synthetic chemists are not exposed to these techniques in undergraduate teaching, this leads to a disproportionate number of scientists that wish to optimize their reactions but are unable to use these methodologies or are simply unaware of their existence. This review highlights the basics, and the cutting-edge, of modern chemical reaction optimization as well as its relation to process scale-up and can thereby serve as a reference for inspired scientists for each of these techniques, detailing several of their respective applications.

Directed Photochemically Mediated Nickel-Catalyzed (Hetero)arylation of Aliphatic C-H Bonds

Site-selective functionalization of unactivated C(sp3)-H centers is challenging because of the ubiquity and strength of alkyl C-H bonds. Herein, we disclose a position-selective C(sp3)-C(sp2) cross-coupling reaction. This process engages C(sp3)-H bonds and aryl bromides, utilizing catalytic quantities of a photoredox-capable molecule and a nickel precatalyst. Using this technology, selective C-H functionalization arises owing to a 1,6-hydrogen atom transfer (HAT) process that is guided by a pendant alcohol-anchored sulfamate ester. These transformations proceed directly from N-H bonds, in contrast to previous directed, radical-mediated, C-H arylation processes, which have relied on prior oxidation of the reactive nitrogen center in reactions with nucleophilic arenes. Moreover, these conditions promote arylation at secondary centers in good yields with excellent selectivity.

Bicyclo[2.1.1]hexanes by Visible Light-Driven Intramolecular Crossed [2 + 2] Photocycloadditions

Bicyclo[2.1.1]hexanes have become increasingly popular building blocks in medicinal chemistry as bridged scaffolds that provide unexplored chemical space. We herein report a visible light-driven approach to these compounds that relies on an intramolecular crossed [2 + 2] photocycloaddition of styrene derivatives enabled by triplet energy transfer. Bicyclo[2.1.1]hexanes were obtained in good to high yields (19 examples, 61%-quantitative yield) and allowed for further functionalizations by consecutive reactions, thereby opening different pathways to decorate the aliphatic core structure.

Ligand-enabled Ni-catalyzed hydroarylation and hydroalkenylation of internal alkenes with organoborons

The transition metal-catalyzed hydrofunctionalization of alkenes offers an efficient solution for the rapid construction of complex functional molecules, and significant progress has been made during last decades. However, the hydrofunctionalization of internal alkenes remains a significant challenge due to low reactivity and the difficulties of controlling the regioselectivity. Here, we report the hydroarylation and hydroalkenylation of internal alkenes lacking a directing group with aryl and alkenyl boronic acids in the presence of a nickel catalyst, featuring a broad substrate scope and wide functional group tolerance under redox-neutral conditions. The key to achieving this reaction is the identification of a bulky 1-adamantyl β-diketone ligand, which is capable of overcoming the low reactivity of internal 1,2-disubstituted alkenes. Preliminary mechanistic studies unveiled that this reaction undergoes an Ar-Ni(II)-H initiated hydroarylation process, which is generated by the oxidative addition of alcoholic solvent with Ni(0) species and sequential transmetalation. In addition, the oxidative addition of the alcoholic solvent proves to be the turnover-limiting step.

Exploiting the sp2 character of bicyclo[1.1.1]pentyl radicals in the transition-metal-free multi-component difunctionalization of [1.1.1]propellane

Strained bicyclic substructures are increasingly relevant in medicinal chemistry discovery research because of their role as bioisosteres. Over the last decade, the successful use of bicyclo[1.1.1]pentane (BCP) as a para-disubstituted benzene replacement has made it a highly valuable pharmacophore. However, various challenges, including limited and lengthy access to useful BCP building blocks, are hampering early discovery research. Here we report a single-step transition-metal-free multi-component approach to synthetically versatile BCP boronates. Radicals derived from commonly available carboxylic acids and organohalides perform additions onto [1.1.1]propellane to afford BCP radicals, which then engage in polarity-matched borylation. A wide array of alkyl-, aryl- and alkenyl-functionalized BCP boronates were easily prepared. Late-stage functionalization performed on natural products and approved drugs proceeded with good efficiency to generate the corresponding BCP conjugates. Various photoredox transformations forging C-C and C-N bonds were demonstrated by taking advantage of BCP trifluoroborate salts derived from the BCP boronates.

Visible-Light-Promoted Nickel-Catalyzed Cross-Coupling of Alkyltitanium Alkoxides with Aryl and Alkenyl Halides

Here, we report that alkyltitanium alkoxides generated in situ from Grignard reagents and Ti(OiPr)₄ undergo a photocatalyst-free nickel-catalyzed cross-coupling with organic halides upon irradiation with blue light. Mechanistic studies suggested that the reaction proceeds through radical intermediates formed by photochemical decomposition of the alkyltitanium reagents. Various aryl, heteroaryl, and vinyl halides were efficiently alkylated under the reported conditions, including those containing ester and amide groups.

Dual Nickel/Photoredox-Catalyzed Site-Selective Cross-Coupling of 1,2-Bis-Boronic Esters Enabled by 1,2-Boron Shifts

Site-selective transition-metal-catalyzed mono-deboronative cross-couplings of 1,2-bis-boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross-couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox-catalyzed mono-deboronative arylation of 1,2-bis-boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2-boron shift of primary β-boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β-aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to give trans-substituted products.

Dicarbofunctionalization of [1.1.1]Propellane Enabled by Nickel/Photoredox Dual Catalysis: One-Step Multicomponent Strategy for the Synthesis of BCP-Aryl Derivatives

Bicyclo[1.1.1]pentane (BCP) motifs as para-disubstituted aryl bioisosteres are playing an emerging role in pharmaceutical, agrochemical, and materials chemistry. The vast majority of these structures is obtained from a BCP electrophile or nucleophile, which are themselves derived from [1.1.1]propellane via cleavage of the internal C-C bond through the addition of either radicals or metal-based nucleophiles. Compared with the current stepwise approaches, a multicomponent reaction that provides direct access to complex and diverse disubstituted BCP products would be more attractive. Herein, we report a single-step, multicomponent approach to synthetically versatile arylated BCP products via nickel/photoredox catalysis. Importantly, this three-component process allows two C-C bonds to be formed in a single step and sets three quaternary centers, unprecedented in any previously reported methods. The method has been demonstrated to allow access to complex BCP architectures from aryl halide and radical precursor substrates.

IrIII/NiII-Metallaphotoredox-Catalyzed Enantioselective Decarboxylative Arylation of α-Amino Acids: Theoretical Insight of Enantio-Determining Outer-Sphere Reductive Elimination

The Ir^III/Ni^II-metallaphotoredox-catalyzed enantioselective decarboxylative arylation of α-amino acids has been systematically investigated using density functional theory calculations. The combination of oxidative quenching (Ir^III-*Ir^III-Ir^IV-Ir^III) or reductive quenching (Ir^III-*Ir^III-Ir^II-Ir^III) cycle with the nickel catalytic cycle (Ni^II-Ni^I-Ni^III-Ni^II) is possible. The favorable reaction mechanism consists of three major processes: single-electron transfer, oxidative addition, and stepwise outer-sphere reductive elimination. The rate-determining step is the oxidative addition. Unexpectedly, the enantio-determining C-C bond formation occurs via an ion-pair intermediate involved in the stepwise outer-sphere reductive elimination process, which is unusual in the Ir^III/Ni^II-metallaphotoredox catalysis. Furthermore, computational results reveal that the high enantioselectivity of this reaction is mainly dependent on the steric effect of substituents on substrates. This theoretical study provides useful knowledge for deep insights into the activity and selectivity of visible-light-mediated enantioselective metallaphotoredox dual catalysis at the molecular and atomic levels and benefits the development of asymmetric synthesis.

ART─An Amino Radical Transfer Strategy for C(sp2)-C(sp3) Coupling Reactions, Enabled by Dual Photo/Nickel Catalysis

Introducing the novel concept of amino radical transfer (ART) enables the use of easily accessible and commercially available alkyl boronic esters as cross-coupling partners for aryl halides in dual photoredox/nickel catalysis mediated by visible light. Activation of otherwise photochemically innocent boronic esters by radicals generated from primary or secondary alkylamines gives rise to an outstanding functional group tolerance in a mild, fast, and air-stable reaction. As shown in more than 50 examples including unprotected alcohols, amines, and carboxylic acids, this reaction allows quick access to relevant scaffolds for organic synthesis and medicinal chemistry. In comparison with existing methods for C(sp²)-C(sp³) couplings an extraordinary generality could be realized via the ART concept, employing a single set of optimized reaction conditions. Due to its selectivity, the transformation can also be used for late-stage functionalization, as demonstrated with three exemplary syntheses of drug molecules. Furthermore, the successful one-to-one scalability of this reaction up to gram scale without the necessity of any further precautions or flow systems is demonstrated.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Directed Ni-Catalyzed Reductive Arylation of Aliphatic C–H Bonds

Herein, we describe the nickel-catalyzed reductive arylation of remote C(sp³)-H bonds with aryl electrophiles. The reaction targets secondary and tertiary C(sp³)-H bonds to deliver all-carbon quaternary centers. The success of this method relies on a novel amidyl radical precursor that tolerates reducing conditions, namely O-oxalate hydroxamic acid esters.

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.

Controlling Ni redox states by dynamic ligand exchange for electroreductive Csp3-Csp2 coupling

Cross-electrophile coupling (XEC) reactions of aryl and alkyl electrophiles are appealing but limited to specific substrate classes. Here, we report electroreductive XEC of previously incompatible electrophiles including tertiary alkyl bromides, aryl chlorides, and aryl/vinyl triflates. Reactions rely on the merger of an electrochemically active complex that selectively reacts with alkyl bromides through 1e- processes and an electrochemically inactive Ni0(phosphine) complex that selectively reacts with aryl electrophiles through 2e- processes. Accessing Ni0(phosphine) intermediates is critical to the strategy but is often challenging. We uncover a previously unknown pathway for electrochemically generating these key complexes at mild potentials through a choreographed series of ligand-exchange reactions. The mild methodology is applied to the alkylation of a range of substrates including natural products and pharmaceuticals.

Synergistic Brønsted/Lewis acid catalyzed aromatic alkylation with unactivated tertiary alcohols or di-tert-butylperoxide to synthesize quaternary carbon centers

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-butylation of electron-rich arenes using di-tert-butylperoxide. This transformation inspired the development of a synergistic Brønsted/Lewis acid catalyzed aromatic alkylation that fills a gap in the Friedel-Crafts reaction literature by employing unactivated tertiary alcohols as alkylating agents, leading to new quaternary carbon centers. Corroborated by DFT calculations, the Lewis acid serves a role in enhancing the acidity of the Brønsted acid. The use of non-allylic, non-benzylic, and non-propargylic tertiary alcohols represents an underexplored area in Friedel-Crafts reactivity.

Modular and Facile Access to Chiral α-Aryl Phosphates via Dual Nickel- and Photoredox-Catalyzed Reductive Cross-Coupling

Chiral phosphine-containing skeletons are important motifs in bioactive natural products, pharmaceuticals, chiral catalysts, and ligands. Herein, we report a general and modular platform to access chiral α-aryl phosphorus compounds via a Ni/photoredox-catalyzed enantioconvergent reductive cross-coupling between α-bromophosphates and aryl iodides. This dual catalytic regime exhibited high efficiency and good functional group compacity. A wide variety of substrates bearing a diverse set of functional groups could be converted into chiral phosphates in good to excellent yields and enantioselectivities. The utility of the method was also demonstrated by the development of a new phosphine ligand and the synthesis of enzyme inhibitor derivatives. The detailed mechanistic studies supported a radical chain process and revealed a unique distinction compared with traditional reductive cross-coupling.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Tertiary Alkylative Suzuki–Miyaura Couplings

Suzuki–Miyaura coupling is an extremely useful way to construct Csp2–Csp2 carbon bonds. On the other hand, Csp2–Csp3 coupling reactions do not work well, and tert-alkylative Suzuki–Miyaura coupling is particularly challenging due to problematic oxidative addition and β-hydride elimination side reactions. In this short review, we will introduce recent examples of tert-alkylative Suzuki–Miyaura couplings with tert-alkyl electrophiles or -boron reagents. The review will mainly focus on catalyst and product structures and on the proposed mechanisms.

1 Introduction

2 Ni-Catalyzed tert-Alkylative Couplings

3 Pd-Catalyzed tert-Alkylative Couplings

4 Fe-Catalyzed tert-Alkylative Couplings

5 tert-Alkylative Couplings with 1-Alkenyl Borons

6 tert-Alkylative Couplings under Photoirradiation

7 Stereospecific tert-Alkylative Couplings

8 Conclusion

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.

Theoretical mechanistic study of 4CzIPN/Ni0-metallaphotoredox catalyzed enantioselective desymmetrization of cyclic meso-anhydrides

Visible-light-induced inexpensive photocatalyst and transition metal dual catalytic cross-coupling has attracted much attention for efficiently constructing various chemical bonds. The 4CzIPN/Ni⁰-metallaphotoredox catalyzed enantioselective desymmetrization of cyclic meso-anhydrides with benzyl trifluoroborates has been systematically investigated using density functional theory (DFT) calculations. A radical mechanism merging reductive quenching (PC-*PC-PC^--PC) and nickel catalytic cycles (Ni⁰-Ni^II-Ni^III-Ni^I-Ni⁰) is favourable. It consists of seven major processes: single-electron reduction of *PC by benzyl trifluoroborates to generate benzyl radical, ligand exchange, oxidative addition, radical addition, reductive elimination, reduction of Ni^I by PC^- complex via single-electron transfer (SET) process to obtain ground-state PC, and the ion exchange to afford the desired product enantio-enriched keto-acids and regenerate Ni⁰ catalyst. The oxidative addition is not only the enantio-determining step but also the rate-determining step of the catalytic cycle. In addition, we tried to disclose the origin of high enantioselectivity from both the steric and electronic effects and explain the origin of diastereoselectivity based on the proposed mechanism. Meanwhile, the difference of catalytic activity between Ni⁰ and Ni^II as the initial catalysts is caused by the different activation energy barriers based on their respective favourable reaction pathways. This study will hopefully benefit the future understanding of such photoredox-mediated dual catalyzed asymmetric synthesis.

Dual Nickel/Photoredox-Catalyzed Deaminative Cross-Coupling of Sterically Hindered Primary Amines

We report a method to activate α-3° amines for deaminative arylation via condensation with an electron-rich aldehyde and merge this reactivity with nickel metallaphotoredox to generate benzylic quaternary centers, a common motif in pharmaceuticals and natural products. The reaction is accelerated by added ammonium salts. Evidence is provided in support of two roles for the additive: inhibition of nickel black formation and acceleration of the overall reaction rate. We demonstrate a robust scope of amine and haloarene coupling partners and show an expedited synthesis of ALK2 inhibitors.

Photo-Induced Ruthenium-Catalyzed C-H Benzylations and Allylations at Room Temperature

The ruthenium-catalyzed synthesis of diarylmethane compounds was realized under exceedingly mild photoredox conditions without the use of exogenous photocatalysts. The versatility and robustness of the ruthenium-catalyzed C-H benzylation was reflected by an ample scope, including multifold C-H functionalizations, as well as transformable pyrazoles, imidates and sensitive nucleosides. Mechanistic studies were indicative of a photoactive cyclometalated ruthenium complex, which also enabled versatile C-H allylations.

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.

From Esters to Ketones via a Photoredox-Assisted Reductive Acyl Cross-Coupling Strategy

A method was developed for ketone synthesis via a photoredox-assisted reductive acyl cross-coupling (PARAC) using a nickel/photoredox dual-catalyzed cross-electrophile coupling of two different carboxylic acid esters. A variety of aryl, 1°, 2°, 3°-alkyl 2-pyridyl esters can act as acyl electrophiles while N-(acyloxy)phthalimides (NHPI esters) act as 1°, 2°, 3°-radical precursors. Our PARAC strategy provides an alternative and reliable way to synthesize various sterically congested 3°-3°, 3°-2°, and aryl-3° ketones under mild and highly unified conditions, which have been otherwise difficult to access. The combined experimental and computational studies identified a Ni⁰ /Ni^I /Ni^III pathway for ketone formation.

Direct Photoexcitation of Benzothiazolines: Acyl Radical Generation and Application to Access Heterocycles

An acyl radical generation and functionalization strategy through direct photoexcitation of benzothiazolines has been developed. The formed acyl radical species can either be trapped by quinoxalin-2-ones to realize their C(3)-H functionalization or trigger a cascade radical cyclization with isonitriles to synthesise biologically important phenanthridines. The synthetic value of this protocol can be further illustrated by the modification of quinoxalin-2-ones, containing important natural products and drug-based complex molecules.

Photoredox-catalyzed multicomponent Petasis reaction in batch and continuous flow with alkyl boronic acids

Multicomponent reactions (MCRs) are ideal platforms for the generation of a wide variety of organic scaffolds in a convergent and atom-economical manner. Many strategies for the generation of highly substituted and diverse structures have been developed and among these, the Petasis reaction represents a viable reaction manifold for the synthesis of substituted amines via coupling of an amine, an aldehyde and a boronic acid (BA). Despite its synthetic utility, the inherent drawbacks associated with the traditional two-electron Petasis reaction have stimulated continuous research towards more facile and tolerant methodologies. In this regard, we present the use of free alkyl BAs as effective radical precursors in this MCR through a single-electron transfer mechanism under mild reaction conditions. We have further demonstrated its applicability to photo-flow reactors, facilitating the reaction scale-up for the rapid assembly of complex molecular structures.

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Multicomponent photocatalyzed Petasis reaction

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Boronic acids as alkyl radical precursors under mild reaction conditions

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Photo-flow chemistry application for Petasis reaction

Synthesis of chiral branched allylamines through dual photoredox/nickel catalysis

Allylamines are versatile building blocks in the synthesis of various naturally occurring products and pharmaceuticals. In contrast to terminal allylamines, the methods of synthesis of their branched congeners with internal, stereodefined double bonds are less explored. This work describes a new approach for the preparation of allylamines via cross-coupling of alkyl bromides with simple 3-bromoallylamines by merging the photoredox approach and Ni catalysis. The reaction proceeds under mild conditions, under blue light irradiation, and in the presence of an organic dye, 4CzIPN, as a photocatalyst. The scope of suitable reaction partners is broad, including alkyl bromides bearing reactive functionalities (e.g., esters, nitriles, aldehydes, ketones, epoxides) and N-protected allylamines, as well as N-allylated secondary and tertiary amines and heterocycles. The employment of non-racemic starting materials allows for rapid and easy construction of complex multifunctional allylamine derivatives without the loss of enantiomeric purity.