Walking Metals for Remote Functionalization

Pd-Catalyzed Migratory 1,1-Cycloannulation Reaction of Alkenes

Here, we report a novel strategy for the preparation of diverse heterocycles via a Pd-catalyzed migratory 1,1-cycloannulation reaction (MCAR) of alkenes. Starting from readily available alkenyl amines and alkenyl alcohols, this approach allows the formation of a wide range of five- to seven-membered azaheterocycles and oxaheterocycles with high efficiency and good functional group tolerance. The key to the realization of this reaction is the use of 4-iodophenol or 2-iodophenol derivatives where the phenolic hydroxyl group plays a critical role in controlling the direction of migration and the ring-size of the heterocycles through the formation of a quinone methide intermediate.

Facile construction of distal and diversified tertiary and quaternary stereocenters

Exploration of novel chiral pharmaceutical candidates is motivation to immersive efforts among synthetic chemists. Achieving skeletal construction and chiral diversity in a highly efficient manner is a momentous goal in the chemical society. Unfortunately, current methods for chiral induction focus primarily on a specific site. Herein, we realized the asymmetric chain-walking arylation of alkenyl alcohols for the construction of multisite tertiary and quaternary stereocenters in high yields and enantioselectivity. This new operation-friendly strategy carries substantial potential for future industrialization.

Borenium-Catalyzed "Boron Walking" for Remote Site-Selective Hydroboration

Remote functionalization through progressive olefin isomerization enables site-selective modification at a distal position, diversifying the synthetic approaches. However, the developed protocols have long relied on transition metal catalysis. Transition metal catalysts are deemed irreplaceable, albeit facing challenges in metal residue and catalyst poisoning. In this work, we present a pioneering approach that employs a borenium ion as a catalyst for site-selective, remote borylation, eliminating the need for metal catalysts. As the reaction progresses, borylation isomers at different positions emerge, gradually and ultimately converging into the predominant α-borylation product. This process is akin to a "walking" of a boron moiety along a carbon skeleton toward an aryl terminus. Detailed mechanistic studies and DFT calculations substantiate the borenium-catalyzed, stepwise migration via a reversible B-H insertion/elimination sequence. This remote borylation exhibits good functional group compatibility, complementing those methods reliant on transition metals. Furthermore, this metal-free protocol permits the convenient synthesis of silyl-remote-boryl compounds, demonstrating an opposite regioselectivity to that observed in transition-metal-catalyzed tandem silylation-borylation reactions. This discovery therefore contributes to site-selective, remote difunctionalization via sequential C-B and C-Si derivatizations, exemplified by the synthesis of amino-remote-alcohol bioactive molecules.

Coordination-Assisted Ni-Catalyzed Regio- and Enantioselective 1,2-Borylalkylation of Unactivated Alkenes

Herein we successfully utilize various directing groups to achieve a ligand-enabled nickel-catalyzed 1,2-borylalkylation of unactivated alkenes. A β-amino alcohol was employed as the ligand for non-asymmetric 1,2-borylalkylation of unactivated alkenes, while a bulky chiral diamine ligand was used to achieve the asymmetric 1,2-borylalkylation of allyl amides.

Palladium-catalyzed remote internal C(sp3)-H bond chlorination of alkenes

C(sp3)-Cl bonds are present in numerous biologically active molecules and can also be used as a site for diversification by substitution or cross-coupling reactions. Herein, we report a remote internal site-selective C(sp3)-H bond chlorination of alkenes through sequential alkene isomerization and hydrochlorination, enabling the synthesis of both benzylic and tertiary chlorides with excellent site-selectivity. This transformation offers exciting possibilities for the late-stage chlorination of derivatives of natural products and pharmaceuticals. We also demonstrate the regioconvergent synthesis of a single alkyl chloride from unrefined mixtures of isomeric alkenes, which can be extracted directly from petrochemical sources.

Iridium-Catalyzed Enantioselective Vinylogous and Bisvinylogous Allenylic Substitution

Compared to the widely explored enol silanes, the applicability of their extended variants especially as bisvinylogous nucleophiles in enantioselective catalysis has been sparse. Herein, we describe the first enantioselective vinylogous and bisvinylogous allenylic substitution using silyl dienol and trienol ethers, respectively, as a nucleophile. With racemic allenylic alcohols as the electrophile, these enantioconvergent reactions are cooperatively catalyzed by an Ir(I)/(phosphoramidite,olefin) complex and Lewis acidic La(OTf)3 and display remarkable regio- and diastereoselectivity in most cases. The ability of such extended silyl enol ethers in distant functionalization and creation of remote stereocenters is evident from the resulting γ- and ε-allenylic unsaturated ketones, bearing δ- and ζ-stereocenters, respectively, which are obtained in moderate to high yields with good to excellent enantioselectivity. The synthetic utility of these unsaturated carbonyls bearing an allene moiety is demonstrated with several transformations, including controlled reductions and stereoselective olefinations, which lead to products with desired degrees of unsaturation.

Multifunctionalization of Alkenyl Alcohols via a Sequential Relay Process

Aryl-substituted aliphatic amines are widely recognized as immensely valuable molecules. Consequently, the development of practical strategies for the construction of these molecules becomes increasingly urgent and critical. Here, we have successfully achieved multifunctionalization reactions of alkenyl alcohols in a sequential relay process, which enables transformation patterns of arylamination, deuterated arylamination, and methylenated arylamination to the easy access of multifarious arylalkylamines. Notably, a novel functionalization mode for carbonyl groups has been developed to facilitate the processes of deuterium incorporation and methylene introduction, thereby providing new means for the diverse transformations of carbonyl groups. This methodology displays a wide tolerance toward functional groups, while also exhibiting good applicability across various skeletal structures of alkenols and amines.

Remote Catalytic C(sp3)-H Alkylation via Relayed Carbenoid Transfer upon Olefin Chain Walking

Transition metal carbenes have emerged as versatile intermediates for various types of alkylations. While reactions of metal carbene species with alkenes have been extensively studied, most examples focus on cyclopropanation and allylic C-H insertion. Herein, we present the first example of a catalytic strategy for the carbene-involved regioselective remote C-H alkylation of internal olefins by synergistically combining two iridium-mediated reactivities of olefin chain walking and carbenoid migratory insertion. The present method, utilizing sulfoxonium ylides as a bench-stable robust carbene precursor, was found to be effective for a series of olefins tethered with alkyl chains, heteroatom substituents, and complex biorelevant moieties. Combined experimental and computational studies revealed that reversible iridium hydride-mediated olefin chain walking proceeds to lead to a terminal alkyl-Ir intermediate, which then forms a carbenoid species for the final migratory insertion, resulting in regioselective terminal-alkylated products.

Methoxide-Enabled Zirconium-Catalyzed Migratory Alkene Hydrosilylation

A zirconocene dichloride-catalyzed alkene hydrosilylation is reported that can be applied to non-activated and conjugated terminal and internal alkenes. It involves a catalytic Zr-walk process and leads to a selective conversion to the linear product. Lithium methoxide serves as mild catalyst activating agent, which significantly increases the applicability and operational simplicity in comparison to earlier zirconium(II)-based protocols. Supported by additional experiments and calculations, a mechanism via zirconium(IV) intermediates is proposed. Due to the benign nature and ready-availability of the zirconium catalyst, the reaction is an attractive alternative to established alkene hydrosilylation methods.

Regio- and Enantioselective Nickel-Catalyzed Ipso- and Remote Hydroamination Utilizing Organic Azides as Amino Sources for the Synthesis of Primary Amines

Primary amines serve as key synthetic precursors to most other N-containing compounds, which are important in organic and medicinal chemistry. Herein, we present a NiH-catalyzed mild ipso- and remote hydroamination technique that utilizes organic azides as deprotectable primary amine sources. This strategy offers a highly flexible platform for the efficient construction of α-chiral branched primary amines, as well as linear primary amines.

Energy-Transfer Enabled 1,4-Aryl Migration

Functional group translocation is undoubtedly a pivotal synthetic transformation in organic chemistry. Numerous types of reactions involving radical 1,2-aryl or 1,4-aryl migration via electron transfer mechanism have been extensively investigated. Nevertheless, energy-transfer enabled 1,4-arylation remains unknown. Herein we disclose that an unprecedented di-π-ethane rearrangement featuring 1,4-aryl migration facilitated by energy transfer catalysis under visible light conditions. The newly developed mild protocol exhibits tolerance towards diverse functional groups and enables the migration of a multitude of aromatic rings, encompassing both electron-withdrawing and electron-rich functional groups. The open-shell strategy has also found successful application in the modification of several drugs. Large-scale experiments, continuous-flow experiment, and versatile manipulation of products have demonstrated the robustness and potential utility of this synthetic method. Preliminary mechanistic studies have supported the involvement of radical species in this di-π-ethane rearrangement and have also provided evidence for the energy transfer mechanism.

Ni-catalysed remote C(sp3)-H functionalization using chain-walking strategies

The dynamic translocation of a metal catalyst along an alkyl side chain - often coined as 'chain-walking' - has opened new retrosynthetic possibilities that enable functionalization at unactivated C(sp3)-H sites. The use of nickel complexes in chain-walking strategies has recently gained considerable momentum owing to their versatility for forging sp3 architectures and their redox promiscuity that facilitates both one-electron or two-electron reaction manifolds. This Review discusses the relevance and impact that these processes might have in synthetic endeavours, including mechanistic considerations when appropriate. Particular emphasis is given to the latest discoveries that leverage the potential of Ni-catalysed chain-walking scenarios for tackling transformations that would otherwise be difficult to accomplish, including the merger of chain-walking with other new approaches such as photoredox catalysis or electrochemical activation.

Regiodivergent Radical-Relay Alkene Dicarbofunctionalization

Herein, we report a regiodivergent 1,n-dicarbofunctionalization of unactivated olefins enabled by a Ni-catalyzed radical relay that forges both C(sp3)-C(sp3) and C(sp2)-C(sp3) linkages, even at long-range. Initial studies support an intertwined scenario resulting from the merger of an atom-transfer radical addition (ATRA) and a chain-walking event, with site-selectivity being dictated by a judicious choice of the ligand backbone.

Palladium-Catalyzed Enantioselective Migratory Hydroamidocarbonylation of Amide-Linked Alkenes to Access Chiral α-Alkyl Succinimides

A Pd-catalyzed asymmetric isomerization-hydroamidocarbonylation of amide-containing alkenes was developed, affording a variety of chiral α-alkyl succinimides in moderate to good yields with high enantioselectivities. The key to success was introducing bulky 1-adamentyl P-substitution and 2,3,5,6-tetramethoxyphenyl group into the rigid P-chirogenic bisphosphine ligand to create stronger steric hinderance and deeper catalytic pocket. By this approach, regio- or stereo-convergent synthesis of enantiomeric succinimides from the mixture of olefin isomers was achieved.

Palladium-Catalyzed Isomerization/Hydrocarbonylation of Electron-Deficient Alkenes for the Construction of Lactones and Lactams

An efficient palladium-catalyzed isomerization/hydrocarbonylation of electron-deficient alkenes was developed, affording a variety of benzofuran-2(3H)-ones, dihydrofuran-2(3H)-ones, indolin-2-ones, and isochroman-3-ones in high yields. The reaction exhibits excellent regioselectivity, high atom economy, and a broad range of substrates. The late-stage modification of bioactive molecules and scaled-up reaction demonstrated the practicability of this methodology. Furthermore, the method was employed as the key step for the synthesis of an insulin degrading enzyme (IDE) activator.

Radical-triggered translocation of C-C double bond and functional group

Multi-site functionalization of molecules provides a potent approach to accessing intricate compounds. However, simultaneous functionalization of the reactive site and the inert remote C(sp3)-H poses a formidable challenge, as chemical reactions conventionally occur at the most active site. In addition, achieving precise control over site selectivity for remote C(sp3)-H activation presents an additional hurdle. Here we report an alternative modular method for alkene difunctionalization, encompassing radical-triggered translocation of functional groups and remote C(sp3)-H desaturation via photo/cobalt dual catalysis. By systematically combining radical addition, functional group migration and cobalt-promoted hydrogen atom transfer, we successfully effectuate the translocation of the carbon-carbon double bond and another functional group with precise site selectivity and remarkable E/Z selectivity. This redox-neutral approach shows good compatibility with diverse fluoroalkyl and sulfonyl radical precursors, enabling the migration of benzoyloxy, acetoxy, formyl, cyano and heteroaryl groups. This protocol offers a resolution for the simultaneous transformation of manifold sites.

Cobalt-Catalyzed Asymmetric Migratory Nozaki-Hiyama-Kishi Coupling

Selective functionalization of ubiquitous C-H bonds based on 1,n-metal migration provides an attractive and sustainable route to access complex molecules from readily available precursors. Herein, we report a Co-catalyzed asymmetric reductive migratory Nozaki-Hiyama-Kishi (NHK) coupling between two readily available electrophiles, aryl (pseudo)halides and aldehydes, via an unprecedented through-space aryl-to-alkenyl 1,4-cobalt/hydride shift. The judicious choice of ligands is crucial for selectivity, leading to either ipso- or migratory NHK products with exquisite control of regio-, E/Z-, and enantioselectivity. Enabled by a ligand relay catalytic strategy, this platform has been further extended to aryl-to-aryl asymmetric migratory NHK coupling. These high-value NHK adducts, including α-chiral allylic alcohols and benzyl alcohols, are readily convertible to a variety of useful synthons.

Asymmetric dihydroboration of allenes enabled by ligand relay catalysis

Catalytic asymmetric hydroboration of unsaturated bonds has been recognized as the most straightforward method for the construction of chiral organoboron compounds. Although catalytic asymmetric hydroboration of alkenes has been well-developed, enantioselective hydroboration of allenes still remains rare probably due to the challenges in controlling the enantio-, stereo-, and regioselectivity. Additionally, the hydroboration products might go through over-borohydride, making the catalytic asymmetric dihydroboration of allenes challenging. Here, we report a cobalt-catalyzed asymmetric dihydroboration of allenes using a ligand relay strategy with two simple ligands. This protocol shows excellent enantio-, stereo-, and regioselectivity with positive functional group compatibilities in the construction of chiral 1,4-diboronate products. The applications of this reaction are demonstrated by various product derivatizations, gram-scale reactions, and the preparation of artigenin analogues. Mechanistic studies indicate that the achiral ligand controls the first hydroboration of allenes, and the chiral oxazoline iminopyridine ligand is responsible for the subsequent isomerization and asymmetric hydroboration.

Alkene Isomerisation Catalysed by a Superbasic Sodium Amide

Typically catalysed by transition metals, alkene isomerisation is a powerful methodology for preparation of internal olefins. In contrast, the use of more earth abundant main group reagents is limited to activated substrates, requiring high temperatures and excess stoichiometric amounts. Opening a new avenue for progressing this field, here we report applications of bulky sodium amide NaTMP (TMP=2,2,6,6-tetramethylpiperidide) when partnered with tridentate Lewis donor PMDETA (N,N,N',N'',N''-pentamethyldiethylenetriamine) in catalytic alkene isomerisation of terminal olefins under mild reaction conditions. An array of distinct olefins could successfully be isomerised, including unactivated olefins, allylamines, and allylethers, showing the high activity of this partnership. In-depth mechanistic insights provided by X-ray crystallography, real-time nuclear magnetic resonance (NMR) monitoring, and density functional theory (DFT) calculations have unveiled the crucial role of in situ-generated TMP(H) in facilitating efficient isomerisation, and the choice of alkali-metal. Additionally, theoretical studies shed light on the observed E/Z selectivity, particularly accounting for the selective formation of Z-vinyl ethers. The versatility of our method is further demonstrated through the isomerisation of unactivated cycloalkenes, which undergo hydrogen isotope exchange to produce deuterated compounds.

Regio-, stereo-, and enantioselective ipso- and migratory defluorinative olefin cross-coupling to access highly functionalized monofluoroalkenes

Monofluoroalkenes serve as nonhydrolyzable mimetics of amides and are frequently encountered in drug candidates. Herein we report a regio-, enantio-, and stereoselective NiH-catalyzed ipso- and migratory defluorinative olefin cross-coupling employing readily available olefins and gem-difluoroalkenes under mild conditions. This approach enables the efficient synthesis of a broad array of structurally diverse monofluoroalkenes bearing a tertiary allylic stereogenic center. Mechanistically, the challenging migratory defluorinative olefin cross-coupling process is successfully realized through a ligand relay catalytic strategy, enabling the formal C(sp3)-H/C(sp2)-F activation with high levels of regio-, stereo-, and enantiocontrol.

Pd-Catalyzed C(sp2)-H/C(sp2)-H Coupling of Limonene

Limonene undergoes a regioselective Pd(II)-catalyzed C(sp2)-H/C(sp2)-H coupling with acrylic acid esters and amides, α,β-unsaturated ketones, styrenes, and allyl acetate, affording novel 1,3-dienes. DFT computations gave results in accord with the experimental results and allowed for the formulation of a plausible mechanism. The postfunctionalization of one of the coupled products was achieved via a large-scale Sonogashira reaction conducted under micellar catalysis.

Alkene 1,3-Difluorination via Transient Oxonium Intermediates

The 1,3-difunctionalization of unactivated alkenes is an under-explored transformation that leads to moieties that are otherwise challenging to prepare. Herein, we report a hypervalent iodine-mediated 1,3-difluorination of homoallylic (aryl) ethers to give unreported 1,3-difluoro-4-oxy groups with moderate to excellent diastereoselectivity. The transformation proceeds through a different mode of reactivity for 1,3-difunctionalization, in which a regioselective addition of fluoride opens a transiently formed oxonium intermediate to rearrange an alkyl chain. The optimized protocol is scalable and shown to proceed well with a variety of functional groups and substitution on the alkenyl chain, hence providing ready access to this fluorinated, conformationally controlled moiety.

Catalytic Regio- and Enantioselective Remote Hydrocarboxylation of Unactivated Alkenes with CO2

The catalytic regio- and enantioselective hydrocarboxylation of alkenes with carbon dioxide is a straightforward strategy to construct enantioenriched α-chiral carboxylic acids but remains a big challenge. Herein we report the first example of catalytic highly enantio- and site-selective remote hydrocarboxylation of a wide range of readily available unactivated alkenes with abundant and renewable CO2 under mild conditions enabled by the SaBOX/Ni catalyst. The key to this success is utilizing the chiral SaBOX ligand, which combines with nickel to simultaneously control both chain-walking and the enantioselectivity of carboxylation. This process directly furnishes a range of different alkyl-chain-substituted or benzo-fused α-chiral carboxylic acids bearing various functional groups in high yields and regio- and enantioselectivities. Furthermore, the synthetic utility of this methodology was demonstrated by the concise synthesis of the antiplatelet aggregation drug (R)-indobufen from commercial starting materials.

Di-π-ethane Rearrangement of Cyano Groups via Energy-Transfer Catalysis

Molecular rearrangement occupies a pivotal position among fundamental transformations in synthetic chemistry. Radical translocation has emerged as a prevalent synthetic tool, efficiently facilitating the migration of diverse functional groups. In contrast, the development of di-π-methane rearrangement remains limited, particularly in terms of the translocation of cyano functional groups. This is primarily attributed to the energetically unfavorable three-membered-ring transition state. Herein, we introduce an unprecedented di-π-ethane rearrangement enabled by energy-transfer catalysis under visible light conditions. This innovative open-shell rearrangement boasts broad tolerance toward a range of functional groups, encompassing even complex drug and natural product derivatives. Overall, the reported di-π-ethane rearrangement represents a complementary strategy to the development of radical translocation enabled by energy-transfer catalysis.

Remote Functionalization by Pd-Catalyzed Isomerization of Alkynyl Alcohols

In recent years, progress has been made in the development of catalytic methods that allow remote functionalizations based on alkene isomerization. In contrast, protocols based on alkyne isomerization are comparatively rare. Herein, we report a general Pd-catalyzed long-range isomerization of alkynyl alcohols. Starting from aryl-, heteroaryl-, or alkyl-substituted precursors, the optimized system provides access preferentially to the thermodynamically more stable α,β-unsaturated aldehydes and is compatible with potentially sensitive functional groups. We showed that the migration of both π-components of the carbon-carbon triple bond can be sustained over several methylene units. Computational investigations served to shed light on the key elementary steps responsible for the reactivity and selectivity. These include an unorthodox phosphine-assisted deprotonation rather than a more conventional β-hydride elimination in the final tautomerization event.

tert-Butyl as a Functional Group: Non-Directed Catalytic Hydroxylation of Sterically Congested Primary C-H Bonds

The tert-butyl group is a common aliphatic motif extensively employed to implement steric congestion and conformational rigidity in organic and organometallic molecules. Because of the combination of a high bond dissociation energy (~100 kcal mol-1) and limited accessibility, in the absence of directing groups, neither radical nor organometallic approaches are effective for the chemical modification of tert-butyl C-H bonds. Herein we overcome these limits by employing a highly electrophilic manganese catalyst, [Mn(CF3bpeb)(OTf)2], that operates in the strong hydrogen bond donor solvent nonafluoro-tert-butyl alcohol (NFTBA) and catalytically activates hydrogen peroxide to generate a powerful manganese-oxo species that effectively oxidizes tert-butyl C-H bonds. Leveraging on the interplay of steric, electronic, medium and torsional effects, site-selective and product chemoselective hydroxylation of the tert-butyl group is accomplished with broad reaction scope, delivering primary alcohols as largely dominant products in preparative yields. Late-stage hydroxylation at tert-butyl sites is demonstrated on 6 densely functionalized molecules of pharmaceutical interest. This work uncovers a novel disconnection approach, harnessing tert-butyl as a potential functional group in strategic synthetic planning for complex molecular architectures.

Nickel-Catalyzed Alkene Isomerization to Access Bench-Stable Enamines and Their [3 + 2] Annulation

Enamines are difficult to prepare on the bench due to their instability, which results in side reactions, decompositions, poor yields, etc. Herein, we developed a simple and effective method for making bench-stable enamines using a very low amount of nickel catalyst loading. The deuterium exchange, competitive reaction, and radical clock experiment have all been found to favor the ionic mechanism of this alkene isomerization. Scale-up and [3 + 2] annulation reaction of enamines with activated cyclopropane to deliver cyclopentane derivatives have shown the value of this method in organic synthesis.

Enantioselective Ni-Catalyzed 1,2-Borylalkynylation of Unactivated Alkenes

Enantioselective three-component difunctionalization of alkenes with boron reagents represents an attractive strategy for assembling three-dimensional chiral organoboron compounds. However, regio- and enantiocontrol comprise the pivot challenges in these transformations, which predominantly require the use of activated conjugated alkenes. Herein, by utilizing various carbonyl directing groups, including amides, sulfinamides, ketones, and esters, we succeed in realizing a nickel-catalyzed 1,2-borylalkynylation of unactivated alkenes to enable the simultaneous incorporation of a boron entity and an sp-fragment across the double bond. The products contain boryl, alkynyl, and carbonyl functional groups with orthogonal synthetic reactivities, offering three handles for further derivatization to access valuable intermediates. The utility of this ligand-enabled asymmetric protocol has been highlighted through the late-stage decoration of drug-relevant molecules.

Palladium-Catalyzed Inward Isomerization Hydroaminocarbonylation of Alkenes

In contrast to the kinetically favored outward isomerization-hydrocarbonylation of alkenes, the disfavored inward isomerization-hydrocarbonylation of alkenes remains an important challenge. Herein, we have developed a novel and effective palladium-catalyzed inward isomerization-hydroaminocarbonylation of unactivated alkenes and aniline hydrochlorides for the formation of synthetically valuable α-aryl carboxylic amides in high yields and high site-selectivities. The high efficiency of the reaction is attributed to a relay catalysis strategy, in which the Markovnikov-favored [PdH]-PtBu3 catalyst is responsible for inward isomerization, while the [PdH]-Ruphos catalyst is responsible for hydroaminocarbonylation of the resulting conjugated aryl alkenes. The reaction exhibits highly functional group tolerance and provides a new method for formal carbonylation of remote C(sp3)-H bond.

CoH-catalyzed asymmetric remote hydroalkylation of heterocyclic alkenes: a rapid approach to chiral five-membered S- and O-heterocycles

Saturated heterocycles, which incorporate S and O heteroatoms, serve as fundamental frameworks in a diverse array of natural products, bioactive compounds, and pharmaceuticals. Herein, we describe a unique cobalt-catalyzed approach integrated with a desymmetrization strategy, facilitating precise and enantioselective remote hydroalkylation of unactivated heterocyclic alkenes. This method delivers hydroalkylation products with high yields and excellent stereoselectivity, representing good efficiency in constructing alkyl chiral centers at remote C3-positions within five-membered S/O-heterocycles. Notably, the broad scope and good functional group tolerance of this asymmetric C(sp3)-C(sp3) coupling enhance its applicability.

Nickel-Catalyzed Regio- and Enantioselective Migratory Hydrocyanation of Internal Alkenes: Expanding the Scope to α,ω-Diaryl Internal Alkenes

Metal-hydride-catalyzed migratory functionalization of alkenes witnessed extensive development in the past few years. However, the asymmetric version of this reaction has remained largely underdeveloped owing to the difficulty in simultaneous control of both regio- and stereoselectivity. In addition, exploring the wider alkene substrate scope to enable more synthetically valuable applications represents another challenge in this field. In this context, a nickel-catalyzed asymmetric hydrocyanation of internal alkenes involving a chain-walking process is demonstrated. The reaction exhibits excellent regio- and enantioselectivity, proceeds under mild reaction conditions, and delivers benzylic nitriles in high yields. Even α,ω-diaryl internal alkenes, which are known to be one of the most challenging substrates of this type, could be successfully converted to the desired products with good regio- and stereoselectivity by modifying the electronic and steric effects. Theoretical calculations suggest that the η3-benzyl coordination mode and the aryl substituent (3,5-(OMe)2C6H3) on the diphosphite ligand are both key factors in regulating regio- and enantioselectivity.

Stereodivergent, Kinetically Controlled Isomerization of Terminal Alkenes via Nickel Catalysis

Because internal alkenes are more challenging synthetic targets than terminal alkenes, metal-catalyzed olefin mono-transposition (i.e., positional isomerization) approaches have emerged to afford valuable E- or Z- internal alkenes from their complementary terminal alkene feedstocks. However, the applicability of these methods has been hampered by lack of generality, commercial availability of precatalysts, and scalability. Here, we report a nickel-catalyzed platform for the stereodivergent E/Z-selective synthesis of internal alkenes at room temperature. Commercial reagents enable this one-carbon transposition of terminal alkenes to valuable E- or Z-internal alkenes via a Ni-H-mediated insertion/elimination mechanism. Though the mechanistic regime is the same in both systems, the underlying pathways that lead to each of the active catalysts are distinct, with the Z-selective catalyst forming from comproportionation of an oxidative addition complex followed by oxidative addition with substrate and the E-selective catalyst forming from protonation of the metal by the trialkylphosphonium salt additive. In each case, ligand sterics and denticity control stereochemistry and prevent over-isomerization.

Palladium-Catalyzed Remote Hydrosulfonamidation of Alkenes: Access to Primary N-Alkyl Sulfamides by the SuFEx Reaction

Herein, we establish a remote hydrosulfonamidation (HSA) of alkenes using palladium catalysis, where N-fluoro-N-(fluoro-sulfonyl)-carbamate with a sulfur(VI) fluoride moiety is demonstrated as a good amidation reagent. The anti-Markovnikov HSA reaction of terminal alkenes and the remote HSA of internal alkenes are achieved to efficiently yield primary N-alkyl-N-(fluorosulfonyl)-carbamates. In addition, this protocol enables the high-value utilization of alkane by combining the dehydrogenation process. The generated N-alkyl products exhibit a unique reactivity of sulfur(VI) fluorides, which can be directly transferred to N-alkyl sulfamides or amines via the sulfur(VI) fluoride exchange reaction, thereby streamlining their synthesis. Moreover, a (pyridyl) benzazole-type ligand proved to be vital for the excellent chemo- and regioselectivities.

Rapid and scalable photocatalytic C(sp2)-C(sp3) Suzuki-Miyaura cross-coupling of aryl bromides with alkyl boranes

In recent years, there has been a growing demand for drug design approaches that incorporate a higher number of sp3-hybridized carbons, necessitating the development of innovative cross-coupling strategies to reliably introduce aliphatic fragments. Here, we present a powerful approach for the light-mediated B-alkyl Suzuki-Miyaura cross-coupling between alkyl boranes and aryl bromides. Alkyl boranes were easily generated via hydroboration from readily available alkenes, exhibiting excellent regioselectivity and enabling the selective transfer of a diverse range of primary alkyl fragments onto the arene ring under photocatalytic conditions. This methodology eliminates the need for expensive catalytic systems and sensitive organometallic compounds, operating efficiently at room temperature within just 30 min. We further demonstrate the translation of the present protocol to continuous-flow conditions, enhancing scalability, safety, and overall efficiency of the method. This versatile approach offers significant potential for accelerating drug discovery efforts by enabling the introduction of complex aliphatic fragments in a straightforward and reliable manner.

Asymmetric, Remote C(sp3)-H Arylation via Sulfinyl-Smiles Rearrangement

An efficient asymmetric remote arylation of C(sp3)-H bonds under photoredox conditions is described here. The reaction features the addition radicals to a double bond followed by a site-selective radical translocation (1,n-hydrogen atom transfer) as well as a stereocontrolled aryl migration via sulfinyl-Smiles rearrangement furnishing a wide range of chiral α-arylated amides with up to >99 : 1 er. Mechanistic studies indicate that the sulfinamide group governs the stereochemistry of the product with the aryl migration being the rate determining step preceded by a kinetically favored 1,n-HAT process.

Advancements in hydrochlorination of alkenes

The hydrochlorination of alkenes has been extensively studied in research and is commonly featured in organic chemistry textbooks as an exemplification of the Markovnikov rule. However, the application of this reaction is typically limited to specific alkenes, such as highly substituted ones, styrenes, or strained systems. Conversely, monosubstituted or 1,2-disubstituted alkenes do not readily react with HCl gas or solutions of HCl gas at practical rates. The challenges associated with hydrochlorination reactions for these "non-activated" alkenes have spurred considerable research efforts over the past 30 years, which constitute the primary focus of this review. The discussion begins with classical polar hydrochlorinations, followed by metal-promoted radical hydrochlorinations, and concludes with a brief overview of recent anti-Markovnikov hydrochlorinations.

Conformational Isomerization as a Process to Determine Selectivity over Reaction Pathways: Effect of Alkene Rotation in Chain Walking via Cis Alkene Intermediates

In organic reactions, bond-forming and bond-cleaving processes are generally considered to be more important than other processes such as conformational isomerization. We report herein an example where a conformational isomerization process, propeller-like alkene rotation, is considered to determine the selectivity over the reaction pathways. The transition state with the highest energy barrier in some alkylpalladium isomerization (chain walking) events was theoretically indicated to correspond to alkene rotation, while transition states for bond-cleaving β-hydride elimination and bond-forming migratory insertion were not even observed. It was also suggested both theoretically and experimentally that the palladium chain walking over internal carbons in alkyl chains proceeds via cis alkene intermediates rather than thermodynamically more stable trans alkene intermediates, due to their relative difficulty to undergo alkene rotation.

Nickel-Catalyzed Hydrofluorination in Unactivated Alkenes: Regio- and Enantioselective C-F Bond Formation

Catalytic formation of a regio- and enantioselective C-F bond chiral center from readily available alkenes is a crucial goal, yet it continues to pose significant challenges in organic synthesis. Here, we report the regioselective formation of C-F bonds facilitated by NiH catalysis and a coordination directing strategy that enables precise hydrofluorination of both terminal and internal alkenes. Notably, we have optimized this methodology to achieve high enantioselectivity in creating aliphatic C-F stereogenic centers especially with β,γ-alkenyl substrates, using a tailored chiral Bn-BOx ligand. Another pivotal finding in our research is the identification of the (+)-nonlinear effect under optimized conditions, allowing for high enantioselectivity even with moderately enantiomerically enriched chiral ligands. Given the significant role of fluorine in pharmaceuticals and synthetic materials, this research offers essential insights into the regioselective and enantioselective formation of C-F bond chiral centers, paving the way for the efficient production of valuable fluorinated compounds.

Ligand Relay Catalysis Enables Asymmetric Migratory Hydroarylation for the Concise Synthesis of Chiral α-(Hetero)Aryl-Substituted Amines

Complementary to the design of a single structurally complex chiral ligand to promote each step in transition-metal catalysis, multiligand relay catalysis through dynamic ligand exchange with each step in the catalytic cycle promoted by its best ligand provides an attractive approach to enhance the whole reaction reactivity and selectivity. Herein, a regio- and enantioselective NiH-catalyzed migratory hydroarylation process with a simple combination of a chain-walking ligand and an asymmetric arylation ligand, producing high-value chiral α-(hetero)aryl-substituted amines and their derivatives under mild conditions, is reported. The potential synthetic applications of this transformation are demonstrated by the concise synthesis of (S)-nicotine and a CDK8 inhibitor.

Amide-Directed Rhodium-Catalyzed Chain-Walking Hydrothiolation of Internal Alkenes

We developed a rhodium-catalyzed chain-walking hydrothiolation process for internal alkenes, which offers a novel and efficient alternative for C(sp3)-H bond cleavage, while focusing on thiol incorporation. This method exclusively affords N,S-acetals at 36-90% yields. Regioconvergent hydrothiolation significantly improved the effectiveness of this transformation. Preliminary mechanistic investigations revealed that an amide-directing group is essential for regioselective synthesis, underlining its significance in this process.

Stereodivergent 1,3-difunctionalization of alkenes by charge relocation

Alkenes are indispensable feedstocks in chemistry. Functionalization at both carbons of the alkene-1,2-difunctionalization-is part of chemistry curricula worldwide1. Although difunctionalization at distal positions has been reported2-4, it typically relies on designer substrates featuring directing groups and/or stabilizing features, all of which determine the ultimate site of bond formation5-7. Here we introduce a method for the direct 1,3-difunctionalization of alkenes, based on a concept termed 'charge relocation', which enables stereodivergent access to 1,3-difunctionalized products of either syn- or anti-configuration from unactivated alkenes, without the need for directing groups or stabilizing features. The usefulness of the approach is demonstrated in the synthesis of the pulmonary toxin 4-ipomeanol and its derivatives.

Kinetically-Controlled Ni-Catalyzed Direct Carboxylation of Unactivated Secondary Alkyl Bromides without Chain Walking

Herein, we report the direct carboxylation of unactivated secondary alkyl bromides enabled by the merger of photoredox and nickel catalysis, a previously inaccessible endeavor in the carboxylation arena. Site-selectivity is dictated by a kinetically controlled insertion of CO2 at the initial C(sp3)-Br site by the rapid formation of Ni(I)-alkyl species, thus avoiding undesired β-hydride elimination and chain-walking processes. Preliminary mechanistic experiments reveal the subtleties of stereoelectronic effects for guiding the reactivity and site-selectivity.

Cobalt-catalyzed synthesis of amides from alkenes and amines promoted by light

Catalytic methods to couple alkene and amine feedstocks are valuable in synthetic chemistry. The direct carbonylative coupling of alkenes and amines holds promise as a perfectly atom-economical approach to amide synthesis, but general methods remain underdeveloped. Herein, we report an alkene hydroaminocarbonylation catalyzed by unmodified, inexpensive cobalt carbonyl under mild conditions and low pressure promoted by light. Silane addition after the reaction enables sequential cobalt-catalyzed amide reduction, constituting a formal alkene hydroaminomethylation. These methods exhibit exceptional scope across both alkene and amine components with high chemo- and regioselectivity and proceed efficiently even in the absence of solvent. The formation of a hydridocobalt through photodissociation of a carbonyl ligand is proposed to enable catalytic activity under mild conditions, which addresses a long-standing challenge in catalysis.

Diphosphine Ligand-Enabled Nickel-Catalyzed Chelate-Assisted Inner-Selective Migratory Hydroarylation of Alkenes

The precise control of the regioselectivity in the transition metal-catalyzed migratory hydrofunctionalization of alkenes remains a big challenge. With a transient ketimine directing group, the nickel-catalyzed migratory β-selective hydroarylation and hydroalkenylation of alkenyl ketones has been realized with aryl boronic acids using alkyl halide as the mild hydride source for the first time. The key to this success is the use of a diphosphine ligand, which is capable of the generation of a Ni(II)-H species in the presence of alkyl bromide, and enabling the efficient migratory insertion of alkene into Ni(II)-H species and the sequent rapid chain walking process. The present approach diminishes organosilanes reductant, tolerates a wide array of complex functionalities with excellent regioselective control. Moreover, this catalytic system could also be applied to the migratory hydroarylation of alkenyl azahetereoarenes, thus providing a general approach for the preparation of 1,2-aryl heteroaryl motifs with wide potential applications in pharmaceutical discovery.

Regiodivergent sp3 C-H Functionalization via Ni-Catalyzed Chain-Walking Reactions

The catalytic translocation of a metal catalyst along a saturated hydrocarbon side chain constitutes a powerful strategy for enabling bond-forming reactions at remote, yet previously unfunctionalized, sp3 C-H sites. In recent years, Ni-catalyzed chain-walking reactions have offered counterintuitive strategies for forging sp3 architectures that would be difficult to accomplish otherwise. Although these strategies have evolved into mature tools for advanced organic synthesis, it was only recently that chemists showed the ability to control the motion at which the catalyst "walks" throughout the alkyl chain. Specialized ligand backbones, additives and a judicious choice of noninnocent functional groups on the side chain have allowed the design of "a la carte" protocols that enable regiodivergent bond-forming scenarios at different sp3 C-H sites with distinct topological surface areas. Given the inherent interest in increasing the fraction of sp3 hybridized carbons in medicinal chemistry, Ni-catalyzed regiodivergent chain-walking reactions might expedite the access to target leads in drug discovery campaigns.

Catalytic Asymmetric α-Alkylation of Ketones with Unactivated Alkyl Halides

A catalytic, enantioselective method for direct α-alkylation of ketones with unactivated alkyl halides is realized by employing an α-enolizable ketone in a nickel-catalyzed C(sp3)-C(sp3) cross-coupling reaction. The key to the success is attributed to a unique bimetallic ligand. A variety of acyclic ketones and unactivated alkyl iodides can serve as suitable substrates under mild conditions to generate chiral ketones with α-quaternary carbon stereocenters in high yields with good enantioselectivities. A range of transformations based on the ketone moiety are also demonstrated to show the potential application of this method. Preliminary mechanistic studies support a dinickel-catalyzed cross-coupling mechanism.

Rhodium(III)-Catalyzed Remote Hydroamidation of Internal Alkenes via Chain Walking

Hydroamination of terminal alkenes represents a powerful and well-established way to introduce nitrogenous functionality to feedstock chemicals. Remote hydroamination reactions are far less known, and represent a way to functionalize unactivated C(sp3) centers distal to the site of the alkene. These transformations commonly take place via metal hydride-mediated chain walking, and as such, regioselectivity can be challenging. The remote introduction of amides is of particular interest due to their prevalence in pharmaceuticals. Herein we report a Rh(III)-catalyzed hydroamidation procedure to functionalize the terminal position of internal alkenes, using dioxazolones as amidation reagents and i-PrOH as a hydride source. The reaction proceeds with high yield and regioselectivity, and tolerates a variety of functionality. Regioconvergent synthesis of a single linear amide from a mixture of isomeric alkenes is demonstrated. Key to the development of this reaction was determining that inorganic bases poison the catalyst, and identifying a suitable trialkylamine replacement.

Nickel-Catalyzed Migratory Cross-Coupling Reactions: New Opportunities for Selective C-H Functionalization

ConspectusMigratory cross-coupling via metal migration is a process of significant academic and industrial interest. It provides an attractive alternative for the selective installation of a functional group at remote C-H positions from simple precursors, thus enabling the direct synthesis of challenging structures not accessible with traditional cross-coupling. In particular, with the merger of 1,n-Ni/H shift and cross-coupling of nickel, the Ni-catalyzed migratory functionalization of simple precursors has undergone particularly intense development and emerged as a valuable field of research in the past few years. This Account will outline the recent progress made in this arena in terms of migration-functionalization modes, diverse functionalizations, and strategies for regio- and stereocontrol. Mechanistic studies and synthetic applications are also discussed.In detail, we systematically categorize our work into two parts based on the migration modes. In the first part, a platform is created for Ni-catalyzed migratory sp3 C-H functionalization of alkenes or alkyl halides via iterative 1,2-Ni/H shift-selective cross-coupling. The key reactive Ni(II)H species for chain-walking could be generated in situ either in a polarity-reversed fashion relying on stoichiometric reductants (X-Ni(II)-H) or in a redox-neutral fashion with the participation of nucleophilic coupling partners (FG-Ni(II)-H). One significant advantage associated with the polarity-reversed NiH system is the use of relatively stable, abundant, and safe olefin surrogates or alkyl halides instead of the sensitive organometallics required in traditional cross-coupling reactions. Another advantage is that diverse functionalizations, including carbonation and more challenging amination and thiolation could be smoothly achieved with suitable electrophiles or their precursors. Finally, to address the challenging multifaceted selectivity and reactivity issues in asymmetric migratory cross-coupling reactions, we have developed a feasible ligand relay catalytic strategy. In this dynamic ligand exchange process, one ligand promotes rapid migration while the other promotes highly regio- and stereoselective coupling. This innovative strategy overcomes the formidable challenge stemming from the difficulty of designing a single ligand to efficiently promote both steps of chain-walking and asymmetric coupling. In the second part, a new platform for Ni-catalyzed migratory sp2 C-H functionalization via 1,4-Ni/H shift-selective cross-coupling has been reported. Starting from readily available aryl or vinyl coupling partners, the in situ-generated aryl- or vinylnickel(II) species could undergo a rapid and reversible 1,4-Ni/H shift along an sp2 backbone, and subsequent selective coupling with various coupling partners would allow regio- and stereoselective access to diverse 1,4-migratory functionalization products. The key to success was the discovery of an appropriate ligand to efficiently promote both migration and subsequent selective cross-coupling. A vinyl-to-aryl 1,4-Ni/H shift successfully enables the modular ipso/ortho difunctionalization of aryl coupling partners, while an aryl-to-vinyl 1,4-Ni/H shift enables regio- and stereoselective access to functionalized trisubstituted alkenes.We hope that this Account will inspire broad interest and future development of migratory cross-coupling reactions. We strongly believe that continued efforts in this fascinating field will overcome many of the remaining challenges, including cutting-edge ligand/catalyst design to enhance reactivity and selectivity, conceptually new migration modes for additional transformations, and in-depth mechanistic studies for rational reaction design.

The Construction of Highly Substituted Piperidines via Dearomative Functionalization Reaction

Nitrogen heterocycles play a vital role in pharmaceuticals and natural products, with the six-membered aromatic and aliphatic architectures being commonly used. While synthetic methods for aromatic N-heterocycles are well-established, the synthesis of their aliphatic functionalized analogues, particularly piperidine derivatives, poses a significant challenge. In that regard, we propose a stepwise dearomative functionalization reaction for the construction of highly decorated piperidine derivatives with diverse functional handles. We also discuss challenges related to site-selectivity, regio- and diastereoselectivity, and provide insights into the reaction mechanism through mechanistic studies and density functional theory computations.

Rhodium catalyzed tunable amide homologation through a hook-and-slide strategy

Preparation of diverse homologs from lead compounds has been a common and important practice in medicinal chemistry. However, homologation of carboxylic acid derivatives, particularly amides, remains challenging. Here we report a hook-and-slide strategy for homologation of tertiary amides with tunable lengths of the inserted carbon chain. Alkylation at the α-position of the amide (hook) is followed by highly selective branched-to-linear isomerization (slide) to effect amide migration to the end of the newly introduced alkyl chain; thus, the choice of alkylation reagent sets the homologation length. The key step involves a carbon-carbon bond activation process by a carbene-coordinated rhodium complex with assistance from a removable directing group. The approach is demonstrated for introduction of chains as long as 16 carbons and is applicable to derivatized carboxylic acids in complex bioactive molecules.