Ni-catalyzed cross-electrophile coupling between vinyl/aryl and alkyl sulfonates: synthesis of cycloalkenes and modification of peptides

One-Pot Chlorination and Cross-Electrophile Coupling of Alcohols with Aryl Chlorides

Although alkyl alcohols and aryl chlorides are the two most abundant substrate pools for cross-electrophile coupling, methods to couple them remain limited. Herein we demonstrate a simple procedure for the in situ deoxychlorination of alcohols followed by XEC with aryl chlorides. A broad substrate scope can be achieved by tuning the rate of the reaction via halide exchange. Key to success is the identification of 1-chloro-N,N,2-trimethyl-1-propenylamine as a mild, noninterfering halogenation reagent.

Nickel-Catalyzed Cross-Electrophile Coupling of Aryl Triflates with Alkyl Halides: Mechanism-Informed Design of More General Conditions

Aryl triflates make up a class of aryl electrophiles that are available in a single step from the corresponding phenol. Despite the known reactivity of nickel complexes for aryl C-O bond activation of phenol derivatives, nickel-catalyzed cross-electrophile coupling using aryl triflates has proven challenging. Herein, we report a method to form C(sp2)-C(sp3) bonds by coupling aryl triflates with alkyl bromides and chlorides using phenanthroline (phen) or pyridine-2,6-bis(N-cyanocarboxamidine) (PyBCamCN)-ligated nickel catalysts. The scope of the reaction is demonstrated with 38 examples (61 ± 14% average yield). Mechanistic studies provide a rationale for the conditions used and a roadmap for further applications of cross-electrophile coupling. First, the rate of alkyl radical generation is controlled by maintaining the majority of alkyl halide as the alkyl chloride, which is unreactive, and utilizing a dynamic halide exchange process to adjust the concentration of reactive alkyl bromide or iodide. Second, the challenge of using electron-rich aryl triflates appears to be due to off-cycle transmetalation to form unproductive aryl zinc reagents. The optimal PyBCamCN ligand together with LiCl avoids this deleterious transmetalation step.

Cross-Electrophile Coupling: Principles, Methods, and Applications in Synthesis

Cross-electrophile coupling (XEC), defined by us as the cross-coupling of two different σ-electrophiles that is driven by catalyst reduction, has seen rapid progression in recent years. As such, this review aims to summarize the field from its beginnings up until mid-2023 and to provide comprehensive coverage on synthetic methods and current state of mechanistic understanding. Chapters are split by type of bond formed, which include C(sp3)-C(sp3), C(sp2)-C(sp2), C(sp2)-C(sp3), and C(sp2)-C(sp) bond formation. Additional chapters include alkene difunctionalization, alkyne difunctionalization, and formation of carbon-heteroatom bonds. Each chapter is generally organized with an initial summary of mechanisms followed by detailed figures and notes on methodological developments and ending with application notes in synthesis. While XEC is becoming an increasingly utilized approach in synthesis, its early stage of development means that optimal catalysts, ligands, additives, and reductants are still in flux. This review has collected data on these and various other aspects of the reactions to capture the state of the field. Finally, the data collected on the papers in this review is offered as Supporting Information for readers.

Nickel-Catalyzed Reductive Alkenylation of Enol Derivatives: A Versatile Tool for Alkene Construction

ConspectusKetone-to-alkene transformations are essential in organic synthesis, and transition-metal-catalyzed cross-coupling reactions involving enol derivatives have become powerful tools to achieve this goal. While substantial progress has been made in nucleophile-electrophile reactions, recent developments in nickel-catalyzed reductive alkenylation reactions have garnered increasing attention. These methods accommodate a broad range of functional groups such as aldehyde, ketone, amide, alcohol, alkyne, heterocycles, and organotin compounds, providing an efficient strategy to access structurally diverse alkenes. This Account primarily highlights the contributions from our laboratory to this growing field while also acknowledging key contributions from other researchers.Our early efforts in this area focused on coupling radical-active substrates, such as α-chloroboronates. This method follows the conventional radical chain mechanism, resulting in facile access to valuable allylboronates. Encouraged by these promising results, we subsequently expanded the substrate scope to encompass radical-inactive compounds. By developing new strategies for controlling cross-selectivity, we enabled the coupling of Csp3 electrophiles (e.g., alcohols and sulfonates), Csp2 electrophiles (e.g., bromoalkenylboronates and acyl fluorides), and heavier group-14 electrophiles like chlorosilanes and chlorogermanes with alkenyl triflates. These advances have provided efficient synthetic routes to a wide range of valuable products, including aliphatic alkenes, enones, dienylboronates, and silicon- and germanium-containing alkenes. Notably, these methods are particularly effective for synthesizing functionalized cycloalkenes, which are traditionally challenging to obtain through conventional methods involving alkenyl halide or organometallic couplings. We have also extended the scope of enol derivatives from triflates to acetates. These compounds are among the most accessible, stable, cost-effective, and environmentally friendly reagents, while their application in cross-coupling has been hampered by low reactivity and selectivity challenges. We showcased that by the use of a Ni(I) catalyst, alkenyl acetates could undergo reductive alkylation with a broad range of alkyl bromides, yielding diverse cyclic and acyclic aliphatic alkenes.Furthermore, our work has demonstrated that reductive coupling of enol derivatives with alkenes provides a highly appealing alternative for alkene synthesis. Particularly, this approach offers opportunity to address the regioselectivity challenges encountered in conventional alkene transformations. For instance, achieving regioselective hydrocarbonation of aliphatic 1,3-dienes has been a longstanding challenge in synthetic chemistry. By using a phosphine-nitrile ligand, we developed a nickel-catalyzed reductive alkenylation of 1,3-dienes with alkenyl triflates, delivering a diverse array of 1,4-dienes with high 1,2-branch selectivity (>20:1) while preserving the geometry of the C3-C4 double bond. Additionally, our investigations laid the foundation for enantioselective reductive alkenylation methodologies, offering new pathways for constructing enantioenriched diketones as well as complex carbo- and heterocyclic compounds. The introduced alkenyl functionality can be further diversified, enhancing molecular diversity and complexity.

Ni-Catalyzed Asymmetric Reductive Arylation of α-Substituted Imides

α-Aryl imides are common structural motifs in bioactive molecules and proteolysis-targeting chimeras designed for targeted protein degradation. An asymmetric Ni-catalyzed reductive cross-coupling of imide electrophiles and (hetero)aryl halides has been developed to synthesize enantioenriched α-arylglutarimides from simple starting materials. Judicious selection of electrophile pairs allows for coupling of both electron-rich and electron-deficient (hetero)aryl halides in good yields and enantioselectivities.

Claisen Self-Condensation of Lactones in the Synthesis of Ionizable Lipids

The Claisen self-condensation of lactones can be carried out safely and efficiently under Mukaiyama conditions, in the presence of TiCl4 and triethylamine. The primary Claisen products can be elaborated to various derivatives or converted directly into dihydroxyketones. Such compounds are valuable educts for the synthesis of ionizable lipids for the delivery of nucleic acid therapeutics and can now be accessed through a concise, economical, scalable route that avoids more technically challenging reaction sequences.

Cross-selective Deoxygenative Coupling of Aliphatic Alcohols: Installation of Methyl Groups including Isotopic Labels by Nickel Catalysis

Nickel-catalyzed cross-electrophile coupling reactions of two aliphatic alcohol derivatives remain a challenge. Herein, we report a nickel-catalyzed reductive methylation reaction of aliphatic mesylates with methyl tosylate. This reaction provides straightforward access to compounds bearing aliphatic methyl groups from alkyl alcohol derivatives. Isotopically labelled substrates and reagents can be employed in the reaction to provide perdeuterated and 13C-labelled products. This transformation can be achieved by employing stoichiometric Mn reductant or electrochemically. Additionally, mechanistic experiments show that alkyl iodides are key intermediates in the transformation which undergo a stereoablative reaction via radical intermediates.

Nickel-catalyzed γ-alkylation of cyclopropyl ketones with unactivated primary alkyl chlorides: balancing reactivity and selectivity via halide exchange

A novel method was developed for synthesizing γ-alkyl ketones via nickel-catalyzed cross-electrophile coupling of cyclopropyl ketones and non-activated primary alkyl chlorides. High reactivity and selectivity can be achieved with sodium iodide as a crucial cocatalyst that generates a low concentration of alkyl iodide via halide exchange, thus avoiding the formation of alkyl dimers. This reaction possessed excellent regioselectivity and high step economy circumventing in situ or pregenerated organometallics.

A radical 1,4-aryl migration enables nickel-catalysed remote cross-electrophile coupling of β-bromo amino acid esters with vinyl triflates

A radical 1,4-aryl migration enabling a cross-electrophile coupling reaction toward remote transalkylation of N-benzyl alanine has been developed. In this strategy, with the occurrence of a radical-mediated Turce-Smiles rearrangement, key α-aminoalkyl radicals are generated. The as-formed α-aminoalkyl radical serves as a robust coupling partner for cross-electrophilic coupling with vinyl triflates, affording a series of olefin-tethered amino acid motifs.

Nickel-Catalyzed Reductive Arylation of α-Bromo Sulfoxide

A nickel-catalyzed cross-electrophile coupling of aryl iodides with α-bromo sulfoxide to access a diverse array of aryl benzyl sulfoxides has been discovered. These reactions occurred under mild conditions with excellent functional group tolerance so that optically enriched sulfoxides could be coupled with aryl iodides, generating corresponding sulfoxides with excellent stereochemical integrity. Furthermore, the scalability of this transformation was demonstrated. Initial mechanistic studies revealed that the reaction undergoes a radical pathway.

Total Synthesis of (+)-Aberrarone

The structurally intriguing diterpene (+)-aberrarone has been assembled in only 12 steps from the commercially available (S,S)-carveol without protecting group manipulations. This concise synthesis features a Cu-catalyzed asymmetric hydroboration to generate the chiral methyl group, a Ni-catalyzed reductive coupling to link two fragments, and a Mn-mediated radical cascade cyclization to construct the triquinane system.

A Nickel-Catalyzed Cross-Electrophile Coupling Reaction of 1,3-Dimesylates for Alkylcyclopropane Synthesis: Investigation of Stereochemical Outcomes and Radical Lifetimes

Understanding mechanistic details of the nickel-catalyzed coupling reactions of Csp3 alcohol derivatives is key to developing selective reactions of this widely prevalent functional group. In this manuscript, we utilize a combination of experimental data and DFT studies to define the key intermediates, stereochemical outcome, and competing pathways of a nickel-catalyzed cross-electrophile coupling reaction of 1,3-dimesylates. Stereospecific formation of a 1,3-diiodide intermediate is achieved in situ by the Grignard reagent. The overall stereoablative stereochemical outcome is due to a nickel-catalyzed halogen atom abstraction with a radical rebound that is slower than epimerization of the alkyl radical. Finally, lifetimes of this alkyl radical intermediate are compared to radical clocks to enhance the understanding of the lifetime of the secondary alkyl radical.

Stereospecific Nickel-Catalyzed Cross-Electrophile Coupling Reaction of Alkyl Mesylates and Allylic Difluorides to Access Enantioenriched Vinyl Fluoride-Substituted Cyclopropanes

Cross-electrophile coupling reactions involving direct C-O bond activation of unactivated alkyl sulfonates or C-F bond activation of allylic gem-difluorides remain challenging. Herein, we report a nickel-catalyzed cross-electrophile coupling reaction between alkyl mesylates and allylic gem-difluorides to synthesize enantioenriched vinyl fluoride-substituted cyclopropane products. These complex products are interesting building blocks with applications in medicinal chemistry. Density functional theory (DFT) calculations demonstrate that there are two competing pathways for this reaction, both of which initiate by coordination of the electron-deficient olefin to the low-valent nickel catalyst. Subsequently, the reaction can proceed by oxidative addition of the C-F bond of the allylic gem-difluoride moiety or by directed polar oxidative addition of the alkyl mesylate C-O bond.

Nickel Catalyzed Cross-Coupling of Aryl and Alkenyl Triflates with Alkyl Thiols

We report a nickel catalyzed C-S cross-coupling of aryl and alkenyl triflates with alkyl thiols. A variety of the corresponding thioethers were synthesized using an air-stable nickel precatalyst under mild reaction conditions with short reaction times. A broad substrate scope, including pharmaceutically relevant compounds, could be demonstrated.

Hydroxy-directed iridium-catalyzed enantioselective formal β-C(sp2)-H allylic alkylation of α,β-unsaturated carbonyls

Hydroxy-directed iridium-catalyzed enantioselective formal β-C(sp2)-H allylic alkylation of kojic acid and structurally related α,β-unsaturated carbonyl compounds is developed. This reaction, catalyzed by an Ir(i)/(P,olefin) complex, utilizes the nucleophilic character of α-hydroxy α,β-unsaturated carbonyls, to introduce an allyl group at its β-position in a branched-selective manner in good to excellent yield with uniformly high enantioselectivity (up to >99.9 : 0.1 er). To the best of our knowledge, this report represents the first example of the use of kojic acid in a transition metal catalyzed highly enantioselective transformation.

Reductive Cross-Coupling of Unreactive Electrophiles

Transition-metal-catalyzed reductive coupling of electrophiles has emerged as a powerful tool for the construction of molecules. While major achievements have been made in the field of cross-couplings between organic halides and pseudohalides, an increasing number of reports demonstrates reactions involving more readily available, low-cost, and stable, but unreactive electrophiles. This account summarizes the recent results in our laboratory focusing on this topic. These findings typically include deoxygenative C-C coupling of alcohols, reductive alkylation of alkenyl acetates, reductive C-Si coupling of chlorosilanes, and reductive C-Ge coupling of chlorogermanes.The reductive deoxygenative coupling of alcohols with electrophiles is synthetically appealing, but the potential of this chemistry remains to be disclosed. Our initial study focused on the reaction of allylic alcohols and aryl bromides by the combination of nickel and Lewis acid catalysis. This method offers a selectivity that is opposite to that of the classic Tsuji-Trost reactions. Further investigation on the reaction of benzylic alcohols led to the foundation of a dynamic kinetic cross-coupling strategy with applications in the nickel-catalyzed reductive arylation of benzylic alcohols and cobalt-catalyzed enantiospecific reductive alkenylation of allylic alcohols. The titanium catalysis was later established to produce carbon radicals directly from unactivated tertiary alcohols via C-OH cleavage. The development of their coupling reactions with carbon fragments delivers new methods for the construction of all-carbon quaternary centers. These reactions have shown high selectivity for the functionalization of tertiary alcohols, leaving primary and secondary alcohols intact. Alkenyl acetates are inexpensive, stable, and environmentally friendly and are considered the most attractive alkenyl reagents. The development of reductive alkylation of alkenyl acetates with benzyl ammoniums and alkyl bromides offers mild approaches for the conversion of ketones into aliphatic alkenes.Extensive studies in this field have enabled us to extend the cross-electrophile coupling from carbon to silicon and germanium chemistry. These reactions harness the ready availability of chlorosilanes and chlorogermanes but suffer from the challenge of their low reactivity toward transition metals. Under reductive nickel catalysis, a broad range of alkenyl and aryl electrophiles couple well with vinyl- and hydrochlorosilanes. The use of alkyl halides as coupling partners led to the formation of functionalized alkylsilanes. The C-Ge coupling seems less substrate-dependent, and various common chlorogermanes couple well with aryl, alkenyl, and alkyl electrophiles. In general, functionalities such as Grignard-sensitive groups (e.g., acid, amide, alcohol, ketone, and ester), acid-sensitive groups (e.g., ketal and THP protection), alkyl fluoride and chloride, aryl bromide, alkyl tosylate and mesylate, silyl ether, and amine are tolerated. These methods provide new access to organosilicon and organogermanium compounds, some of which are challenging to obtain otherwise.

Synthesis of Vicinal Carbocycles by Intramolecular Nickel-Catalyzed Conjunctive Cross-Electrophile Coupling Reaction

A nickel-catalyzed intramolecular conjunctive cross-electrophile coupling reaction has been established. This method enables the synthesis of 3,5-vicinal carbocyclic rings found in numerous biologically active compounds and natural products. We provide mechanistic experiments that indicate this reaction proceeds through alkyl iodides formed in situ, initiates at the secondary electrophilic center, and proceeds through radical intermediates.

Recent Progress on Transition-Metal-Mediated Reductive C(sp3)–O Bond Radical Addition and Coupling Reactions

In this short review, we summarize the recent developments on thermo-driven C(sp3)–O bond radical scission methods and their applications in the construction of C(sp3)–C bonds via conjugate addition with activated double bonds and reductive coupling mediated by economic 3d metals, in particular nickel. We have arranged the review based on three approaches for C(sp3)–O bond radical scission (vide infra). After generating the radical intermediates, their subsequent transformation into C(sp3)–C bonds enabled by C(sp3)–O cross-electrophile coupling with carbon electrophiles is discussed in detail.

1 Introduction

2 Direct Single-Electron Transfer to a C(sp3)–O Bond

3 Radical Scission of Activated C(sp3)–O Bonds via Single-Electron Transfer to Protecting Groups

4 In Situ Activation of Alcohols

5 Summary and Outlook

Conformational Flexibility as a Tool for Enabling Site-Selective Functionalization of Unactivated sp3 C-O Bonds in Cyclic Acetals

A dual catalytic manifold that enables site-selective functionalization of unactivated sp³ C–O bonds in cyclic acetals with aryl and alkyl halides is reported. The reaction is triggered by an appropriate σ*–p orbital overlap prior to sp³ C–O cleavage, thus highlighting the importance of conformational flexibility in both reactivity and site selectivity. The protocol is characterized by its excellent chemoselectivity profile, thus offering new vistas for activating strong σ sp³ C–O linkages.

Nickel-Catalyzed Reductive C(sp2 )-Si Coupling of Chlorohydrosilanes via Si-Cl Cleavage

We report here a new method for the synthesis of organohydrosilanes from phenols and ketones. This method is established through reductive C-Si coupling of chlorohydrosilanes via unconventional Si-Cl cleavage. The reaction offers access to aryl- and alkenylhydrosilanes with a scope that is complementary to those of the established methods. Electron-rich, electron-poor, and ortho-/meta-/para-substituted (hetero)aryl electrophiles, as well as cyclic and acyclic alkenyl electrophiles, were coupled successfully. Functionalities, including Grignard-sensitive groups (e.g., primary amine, amide, phenol, ketone, ester, and free indole), acid-sensitive groups (e.g., ketal and THP protection), alkyl-Cl, pyridine, furan, thiophene, Ar-Bpin, and Ar-SiMe₃ , were tolerated. Gram-scale reaction, incorporation of -Si(H)R₂ into complex biologically active molecules, and derivatization of formed organohydrosilanes are demonstrated.

Nickel-Catalyzed Decarboxylative Coupling of Redox-Active Esters with Aliphatic Aldehydes

The addition of alkyl fragments to aliphatic aldehydes is a highly desirable transformation for fragment couplings, yet existing methods come with operational challenges related to the basicity and instability of the nucleophilic reagents commonly employed. We report herein that nickel catalysis using a readily available bioxazoline (BiOx) ligand can catalyze the reductive coupling of redox-active esters with aliphatic aldehydes using zinc metal as the reducing agent to deliver silyl-protected secondary alcohols. This protocol is operationally simple, proceeds under mild conditions, and tolerates a variety of functional groups. Initial mechanistic studies suggest a radical chain pathway. Additionally, alkyl tosylates and epoxides are suitable alkyl precursors to this transformation providing a versatile suite of catalytic reactions for the functionalization of aliphatic aldehydes.

Nickel-Catalyzed Reductive C-Ge Coupling of Aryl/Alkenyl Electrophiles with Chlorogermanes

Cross-electrophile coupling has emerged as a promising tool for molecular synthesis; however, current studies have focused mainly on forging C-C bonds. We report a cross-electrophile C-Ge coupling reaction and thereby demonstrate the possibility of constructing organogermanes from carbon electrophiles and chlorogermanes. The reaction proceeds under mild conditions and offers access to both aryl and alkenyl germanes. Electron-rich, electron-poor, and ortho-/meta-/para-substituted (hetero)aryl electrophiles, as well as cyclic and acyclic alkenyl electrophiles, were coupled. Gram-scale reaction, incorporation of the -GeR₃ moiety into complex biologically active molecules, and derivatization of formed organogermanes are demonstrated.

Reductive Alkylation of Alkenyl Acetates with Alkyl Bromides by Nickel Catalysis

Catalytic alkylation of stable alkenyl C-O electrophiles is synthetically appealing, but studies to date have typically focused on the reactions with alkyl Grignard reagents. We report herein a cross-electrophile reaction of alkenyl acetates with alkyl bromides. This work has enabled a new method for the synthesis of aliphatic alkenes from alkenyl acetates to be established that can be used to add more structural complexity and molecular diversity with enhanced functionality tolerance. The method allows for a gram-scale reaction and modification of biologically active molecules, and it affords access to useful building blocks. Preliminary mechanistic studies reveal that the Ni^I species plays an essential role for the success of the coupling of these two reactivity-mismatched electrophiles.

Nickel-Catalyzed Reductive Csp2-Csp3 Cross Coupling Using Phosphonium Salts

A nickel-catalyzed reductive cross coupling with phosphonium salts and allylic C(sp³)-O bond electrophiles, which granted direct construction of the C(sp²)-C(sp³) bond, is successfully developed. The protocol features broad substrate scope, high-functional-group tolerance, and heterocycle compatibility. Notably, the much more challenging reductive cross coupling with heterocyclic thiazolylphosphonium salts has also been accomplished for the first time.

Nickel-Catalyzed Cross-Electrophile C(sp3)-Si Coupling of Unactivated Alkyl Bromides with Vinyl Chlorosilanes

Cross-electrophile C-Si coupling has emerged as a promising tool for the construction of organosilanes, but the potential of this method remains largely unexplored. Herein, we report a C(sp³)-Si coupling of unactivated alkyl bromides with vinyl chlorosilanes. The reaction proceeds under mild conditions, and it offers a new approach to alkylsilanes. Functionalities such as Grignard-sensitive groups (e.g., acid, amide, alcohol, ketone, and ester), acid-sensitive groups (e.g., ketal and THP protection), alkyl fluoride and chloride, aryl bromide, alkyl tosylate and mesylate, silyl ether, and amine were tolerated. Incorporation of the -Si(vinyl)R₂ moiety into complex molecules and the immobilization of a glass surface by formed organosilanes were demonstrated.

Cobalt-Catalyzed Enantiospecific Dynamic Kinetic Cross-Electrophile Vinylation of Allylic Alcohols with Vinyl Triflates

Asymmetric cross-electrophile coupling has emerged as a promising tool for producing chiral molecules; however, the potential of this chemistry with metals other than nickel remains unknown. Herein, we report a cobalt-catalyzed enantiospecific vinylation reaction of allylic alcohol with vinyl triflates. This work establishes a new method for the synthesis of enantioenriched 1,4-dienes. The reaction proceeds through a dynamic kinetic coupling approach, which not only allows for direct functionalization of allylic alcohols but also is essential to achieve high chemoselectivity. The use of cobalt enables the reactions to proceed with high enantiospecificity, which have failed to be realized by nickel catalysts.

Radical 1,4-Aryl Migration Enabled Remote Cross-Electrophile Coupling of α-Amino-β-Bromo Acid Esters with Aryl Bromides

We report an unprecedented, efficient nickel-catalysed radical relay for the remote cross-electrophile coupling of β-bromo-α-benzylamino acid esters with aryl bromides via 1,4-aryl migration/arylation cascades. β-Bromo-α-benzylamino acid esters are considered as unique molecular scaffolds allowing for aryl migration reactions, which are conceptually novel variants for the radical Truce-Smiles rearrangement. This reaction enables the formation of two new C(sp³ )-C(sp² ) bonds using a bench-stable Ni/bipyridine/Zn system featuring a broad substrate scope and excellent diastereoselectivity, which provides an effective platform for the remote aryl group migration and arylation of amino acid esters via redox-neutral C(sp³ )-C(sp² ) bond cleavage. Mechanistically, this cascade reaction is accomplished by combining two powerful catalytic cycles consisting of a cross-electrophile coupling and radical 1,4-aryl migration through the generation of C(sp³ )-centred radical intermediates from the homolysis of C(sp³ )-Br bonds and the switching of the transient alkyl radical into a robust α-aminoalkyl radical.

Nickel-Catalyzed Arylation of C(sp3)-O Bonds in Allylic Alkyl Ethers with Organoboron Compounds

A nickel-catalyzed cross-coupling of allylic alkyl ethers with organoboron compounds through the cleavage of the inert C(sp³)-O(alkyl) bonds is described. Several types of allylic alkyl ethers can be coupled with various boronic acids or their derivatives to give the corresponding products in good to excellent yields with wide functional group tolerance and excellent regioselectivity. The gram-scale reaction and late-stage modification of biologically active compounds further prove the practicality of this synthetic method.

Titanium: A Unique Metal for Radical Dehydroxylative Functionalization of Alcohols

The dehydroxylative functionalization of alcohols is synthetic appealing, but it remains a long-term challenge in the synthetic community. Low-valent titanium has shown the power to produce carbon radicals from alcohols via homolytic cleavage of the C–OH bonds and thus offers the potential to overcome this problem. In this perspective manuscript, we summarized the recent advance on radical dehydroxylative transformation of alcohols either promoted or catalyzed by titanium. The limitation and outlook of the studies in this field are also provided.

1 Introduction

2 Recent Developments in Dehydroxylative Functionalization of Alcohols

2.1 Stoichiometric Titanium Complexes Mediated Homolysis of Alcohols

2.2 Radical Dehydroxylative Functionalization of Alcohols by Ti Catalysis

3 Summary and Outlook

Cross-Electrophile C(sp2 )-Si Coupling of Vinyl Chlorosilanes

The cross-electrophile coupling has become a powerful tool for C-C bond formation, but its potential for forging the C-Si bond remains unexplored. Here we report a cross-electrophile Csp² -Si coupling reaction of vinyl/aryl electrophiles with vinyl chlorosilanes. This new protocol offers an approach for facile and precise synthesis of organosilanes with high molecular diversity and complexity from readily available materials. The reaction proceeds under mild and non-basic conditions, demonstrating a high step economy, broad substrate scope, wide functionality tolerance, and easy scalability. The synthetic utility of the method is shown by its efficient accessing of silicon bioisosteres, the design of new BCB-monomers, and studies on the Hiyama cross-coupling of vinylsilane products.

A Widely Applicable Dual Catalytic System for Cross-Electrophile Coupling Enabled by Mechanistic Studies

A dual catalytic system for cross-electrophile coupling reactions between aryl halides and alkyl halides that features a Ni catalyst, a Co cocatalyst, and a mild homogeneous reductant is described. Mechanistic studies indicate that the Ni catalyst activates the aryl halide, while the Co cocatalyst activates the alkyl halide. This allows the system to be rationally optimized for a variety of substrate classes by simply modifying the loadings of the Ni and Co catalysts based on the reaction product profile. For example, the coupling of aryl bromides and aryl iodides with alkyl bromides, alkyl iodides, and benzyl chlorides is demonstrated using the same Ni and Co catalysts under similar reaction conditions but with different optimal catalyst loadings in each case. Our system is tolerant of numerous functional groups and is capable of coupling heteroaryl halides, di-ortho-substituted aryl halides, pharmaceutically relevant druglike aryl halides, and a diverse range of alkyl halides. Additionally, the dual catalytic platform facilitates a series of selective one-pot three-component cross-electrophile coupling reactions of bromo(iodo)arenes with two distinct alkyl halides. This demonstrates the unique level of control that the platform provides and enables the rapid generation of molecular complexity. The system can be readily utilized for a wide range of applications as all reaction components are commercially available, the reaction is scalable, and toxic amide-based solvents are not required. It is anticipated that this strategy, as well as the underlying mechanistic framework, will be generalizable to other cross-electrophile coupling reactions.

Ni-Catalyzed Reductive and Merged Photocatalytic Cross-Coupling Reactions toward sp3/sp2-Functionalized Isoquinolones: Creating Diversity at C-6 and C-7 to Address Bioactive Analogues

Naturally occurring isoquinolones have gained considerable attention over the years for their bioactive properties. While the late-stage introduction of various functionalities at certain positions, namely, C-3, C-4, and C-8, has been widely documented, the straightforward introduction of challenging sp3 carbon-linked acyclic aminoalkyl or aza- and oxacyclic appendages at C-6 and C-7 remains largely underexplored. Interest in 6-substituted azacyclic analogues has recently garnered attention in connection with derivatives exhibiting anticancer activity. Reported here is the first application of the versatile and recently emerging field of Ni-catalyzed reductive cross-coupling reactions to the synthesis of 6- and 7- hetero(cyclo)alkyl-substituted isoquinolones. In a second and complementary approach, a new set of C-6- and C-7-substituted positional isomers of hetero(cyclo)alkyl appendages were obtained from the merging of photocatalytic and Ni-catalyzed coupling reactions. In both cases, 6- and 7-bromo isoquinolones served as dual-purpose reacting partners with readily available tosylates and carboxylic acids, respectively.

Cross-Electrophile Coupling of Unactivated Alkyl Chlorides

Alkyl chlorides are bench-stable chemical feedstocks that remain among the most underutilized electrophile classes in transition metal catalysis. Overcoming intrinsic limitations of C(sp³)-Cl bond activation, we report the development of a novel organosilane reagent that can participate in chlorine atom abstraction under mild photocatalytic conditions. In particular, we describe the application of this mechanism to a dual nickel/photoredox catalytic protocol that enables the first cross-electrophile coupling of unactivated alkyl chlorides and aryl chlorides. Employing these low-toxicity, abundant, and commercially available organochloride building blocks, this methodology allows access to a broad array of highly functionalized C(sp²)-C(sp³) coupled adducts, including numerous drug analogues.

Nickel-Catalyzed Alkyl-Alkyl Cross-Electrophile Coupling Reaction of 1,3-Dimesylates for the Synthesis of Alkylcyclopropanes

Cross-electrophile coupling reactions of two Csp³-X bonds remain challenging. Herein we report an intramolecular nickel-catalyzed cross-electrophile coupling reaction of 1,3-diol derivatives. Notably, this transformation is utilized to synthesize a range of mono- and 1,2-disubstituted alkylcyclopropanes, including those derived from terpenes, steroids, and aldol products. Additionally, enantioenriched cyclopropanes are synthesized from the products of proline-catalyzed and Evans aldol reactions. A procedure for direct transformation of 1,3-diols to cyclopropanes is also described. Calculations and experimental data are consistent with a nickel-catalyzed mechanism that begins with stereoablative oxidative addition at the secondary center.

Reductive amidation of alkyl tosylates with isocyanates by a Ni/Co-dual catalytic system

Reductive amidation of alkyl tosylates with isocyanates using the Ni/Co-dual catalytic system is disclosed. The method proceeds efficiently under mild conditions, giving rise to the corresponding alkyl amides. Notably, the protocol can discriminate the steric environment of two alkyl tosylate moieties, enabling regioselective mono-amidation at the less-bulky site.