Olefin functionalization/isomerization enables stereoselective alkene synthesis

Iron-Catalyzed Tunable Alkene Migratory Silylation and Transposition

The example of iron-catalyzed alkenes migratory silylation and transposition has been demonstrated, affording a tunable approach to synthesize thermodynamically stable allylsilanes and internal alkenes with high efficiency and regioselectivity. These reactions showcase several advantageous features, including good functional group tolerance, excellent regioselectivity, a broad substrate scope, scalability to gram-scale synthesis, and late-stage functionalization of bio-relevant molecules. Furthermore, the relay catalytic mechanism of the migratory silylation, involving both iron-silyl and iron-hydride intermediates, provides valuable insights into iron-catalyzed coupling reactions, opening new avenues for the development of novel transformations under iron catalysis.

Proton-Modulated Nickel Hydride Electrocatalysis for the Hydrogenation of Unsaturated Bonds and Olefin Isomerization

Transition-metal hydrides stand as indispensable intermediates in both energy conversion and organic synthesis. Their electrochemical generation represents a compelling sustainable approach, enabling precise control over the reactivity and expanding the scope of electrocatalytic hydrogenation and isomerization. However, a major challenge in Ni-catalyzed electrochemical hydrogenation is the competing hydrogen evolution reaction (HER), which has led to various innovative strategies aimed at circumventing Ni-H formation. Here, we pursued an alternative approach by designing a bifunctional ligand with a pendant amine moiety to promote Ni-H formation. This design enabled selective (semi)hydrogenation of a diverse range of substrates, including terminal and internal alkynes, alkenes, and aldehydes, achieving an unprecedented substrate scope. Remarkably, we also demonstrated tunable positional selectivity for olefin isomerization by employing different types of proton sources. Our hydrogenation and isomerization method also exhibits excellent functional group tolerance, streamlining access to pharmaceuticals and their derivatives. Computational studies revealed the crucial, noninnocent role of the proton source in modulating metal hydride selectivity, either through hydrogen bonding, direct protonation of the pendant amine, or facilitation of protodemetalation.

Ligand-Enabled Nondirected and Regioselective Arylation of Internal Alkenes with Simple Arenes

Regioselective functionalization of internal alkenes has become a highly efficient approach for preparing stereochemically defined multi-substituted olefins. Unlike traditional methods that require directing groups, activating groups, or active chemical bonds (e.g., halide, pseudo halide, organometallic reagent, etc.), there remains a strong demand for nondirected and selective functionalization of unactivated alkenes with simple coupling partners, both in academic research or industrial applications. Herein, we report the development of a pyridone-oxazoline (Pyoox) type ligand that combines the features of both pyridone and pyridine-oxazoline in assisting Pd-catalyzed olefination. This ligand enables the activation of simple (hetero) arenes and internal alkenes within a single reaction system. A nondirected and regioselective arylation from simple raw materials has been achieved, providing a straightforward route to various trisubstituted olefins in moderate to excellent yields, with excellent regio-/stereocontrol. Experimental and computational studies on mechanisms offer insight into the distinctive properties and performance of this ligand-promoted catalysis. The synthetic utility of this method is further demonstrated by the simplified synthesis and late-stage diversification of bioactive molecules.

Facile Access to Quaternary Carbon Centers via Ni-Catalyzed Arylation of Alkenes with Organoborons

Quaternary carbon centers are widespread structural motifs, thus representing extensive interest in organic synthesis. We describe here an efficient nickel-catalyzed intermolecular, Markovnikov-selective arylation of minimally functionalized alkenes with stable organoborons, affording a broad range of cyclic or acyclic quaternary carbon centers under mild conditions. The utilization of the diimine ligand is critical for high reactivity and chemoselectivity. Furthermore, using a bulky chiral diimine as the ligand for the Ni catalyst, quaternary carbon stereocenters can be readily prepared with high levels of enantiocontrol. Mechanism studies suggest that, before protonation, a rare nickel shift from alkyl nickel to aryl nickel might occur.

C-S-Selective Stille-Coupling Enables Stereodefined Alkene Synthesis

A palladium-catalyzed highly C-S-selective Stille cross-coupling between aryl thianthrenium salts and tri- or tetrasubstituted alkenyl stannanes is described. Herein, critical challenges including site- and chemoselectivity control are well addressed through C-H thianthrenation and C-S alkenylation, thereby providing an expedient access to stereodefined tri- and tetrasubstituted alkenes in a stereoretentive fashion. Indeed, the palladium-catalyzed Stille-alkenylation of poly(pseudo)halogenated arenes displays privileged capability to differentiate C-S over C-I, C-Br, C-Cl bonds, as well as oxygen-based triflates (C-OTf), tosylates (C-OTs), carbamates and sulfamates under mild reaction conditions. Sequential and multiple cross-couplings via selective C-X functionalization should be widely applicable for increasing functional molecular complexity. Modular installation of stereospecific alkene motifs into pharmaceuticals illustrated the synthetic application of the present protocol in drug discovery.

(Z)-Selective Isomerization of 1,1-Disubstituted Alkenes by Scandium-Catalyzed Allylic C-H Activation

The isomerization of 1,1-disubstituted alkenes through 1,3-hydrogen shift is an atom-efficient route for synthesizing trisubstituted alkenes, which are important moieties in many natural products, pharmaceuticals, and organic materials. However, this reaction often encounters regio- and stereoselectivity challenges, typically yielding E/Z-mixtures of the alkene products or thermodynamically favored (E)-alkenes. Herein, we report the (Z)-selective isomerization of 1,1-disubstituted alkenes to trisubstituted (Z)-alkenes via the regio- and stereospecific activation of an allylic C-H bond. The key to the success of this unprecedented transformation is the use of a sterically demanding half-sandwich scandium catalyst in combination with a bulky quinoline compound, 2-tert-butylquinoline. Deuterium-labeling experiments and density functional theory (DFT) calculations have revealed that 2-tert-butylquinoline not only facilitates the C═C bond transposition through hydrogen shuttling but also governs the regio- and stereoselectivity due to the steric hindrance of the tert-butyl group. This protocol enables the synthesis of diverse (Z)-configured acyclic trisubstituted alkenes and endocyclic trisubstituted alkenes from readily accessible 1,1-disubstituted alkenes. It offers an efficient and selective route for preparing a new family of synthetically challenging (Z)-trisubstituted alkenes with broad substrate scope, 100% atom efficiency, high regio- and stereoselectivity, and an unprecedented reaction mechanism.

Alkene Isomerization Catalyzed by a Mn(I) Bisphosphine Borohydride Complex

An additive-free manganese-catalyzed isomerization of terminal alkenes to internal alkenes is described. This reaction is implementing an inexpensive nonprecious metal catalyst. The most efficient catalyst is the borohydride complex cis-[Mn(dippe)(CO)2(κ2-BH4)]. This catalyst operates at room temperature, with a catalyst loading of 2.5 mol %. A variety of terminal alkenes is effectively and selectively transformed into the respective internal E-alkenes. Preliminary results show chain-walking isomerization at an elevated temperature. Mechanistic studies were carried out, including stoichiometric reactions and in situ NMR analysis. These experiments are flanked by computational studies. Based on these, the catalytic process is initiated by the liberation of "BH3" as a THF adduct. The catalytic process is initiated by double bond insertion into an M-H species, leading to an alkyl metal intermediate, followed by β-hydride elimination at the opposite position to afford the isomerization product.

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.

Controlling Reactivity and Selectivity in the Mizoroki-Heck Reaction: High Throughput Evaluation of 1,5-Diaza-3,7-diphosphacyclooctane Ligands

We report a high throughput evaluation of the Mizoroki-Heck reaction of diverse olefin coupling partners. Comparison of different ligands revealed the 1,5-diaza-3,7-diphosphacyclooctane (P2N2) scaffold to be more broadly applicable than common "gold standard" ligands, demonstrating that this family of readily accessible diphosphines has unrecognized potential in organic synthesis. In particular, two structurally related P2N2 ligands were identified to enable the regiodivergent arylation of styrenes. By simply altering the phosphorus substituent from a phenyl to tert-butyl group, both the linear and branched Mizoroki-Heck products can be obtained in high regioisomeric ratios. Experimental and computational mechanistic studies were performed to further probe the origin of selectivity, which suggests that both ligands coordinate to the metal in a similar manner but that rigid positioning of the phosphorus substituent forces contact with the incoming olefin in a π-π interaction (for P-Ph ligands) or with steric clash (for P-tBu ligands), dictating the regiocontrol.

Selective 1,2-Hydroarylation(Alkenylation) of gem-Difluoroalkenes to Access (-CF2 H) Motifs

An emerging class of C-C coupling transformations that furnish drug-like building blocks involves catalytic hydrocarbonation of alkenes. However, despite notable advances in the field, hydrocarbon addition to gem-difluoroalkenes without additional electronic activation remains largely unsuccessful. This owes partly to poor reactivity and the propensity of difluoroalkenes to undergo defluorinative side reactions. Here, we report a nickel catalytic system that promotes efficient 1,2-selective hydroarylation and hydroalkenylation, suppressing defluorination and providing straightforward access to a diverse assortment of prized organofluorides bearing difluoromethyl-substituted carbon centers. In contrast to radical-based pathways and reactions triggered by hydrometallation via a nickel-hydride complex, our experimental and computational studies support a mechanism in which a catalytically active nickel-bromide species promotes selective carbonickelation with difluoroalkenes followed by alkoxide exchange and hydride transfer, effectively overcoming the difluoroalkene's intrinsic electronic bias.

Divergent and Selective Light Alkene Cross-Coupling

Light olefins are abundantly manufactured in the petroleum industry and thus represent ideal starting materials for modern chemical synthesis. Selective and divergent transformations of feedstock light olefins to value-added chemicals are highly sought-after but remain challenging. Herein we report an exceptionally regioselective carbonickelation of light alkenes followed by in situ trapping with three types of nucleophiles, namely a reductant, base, or Grignard reagent. This protocol enables efficient 1,2-hydrofunctionalization, dicarbofunctionalization, and branched-selective Heck-type cross-coupling of light alkenes with aryl and alkenyl reagents to streamline access to diverse alkyl arenes and complex alkenes. Harnessing bulky N-heterocyclic carbene ligands with acenaphthyl backbones for nickel catalysts is crucial to attain high reactivity and selectivity. This strategy provides a rare, modular, and divergent platform for upgrading feedstock alkenes and is expected to find broad applications in medicinal chemistry and industrial processes.

Machine Learning-Guided Development of Trialkylphosphine Ni(I) Dimers and Applications in Site-Selective Catalysis

Owing to the unknown correlation of a metal's ligand and its resulting preferred speciation in terms of oxidation state, geometry, and nuclearity, a rational design of multinuclear catalysts remains challenging. With the goal to accelerate the identification of suitable ligands that form trialkylphosphine-derived dihalogen-bridged Ni^(I) dimers, we herein employed an assumption-based machine learning approach. The workflow offers guidance in ligand space for a desired speciation without (or only minimal) prior experimental data points. We experimentally verified the predictions and synthesized numerous novel Ni^(I) dimers as well as explored their potential in catalysis. We demonstrate C-I selective arylations of polyhalogenated arenes bearing competing C-Br and C-Cl sites in under 5 min at room temperature using 0.2 mol % of the newly developed dimer, [Ni^(I)(μ-Br)PAd₂(n-Bu)]₂, which is so far unmet with alternative dinuclear or mononuclear Ni or Pd catalysts.

Evidence for Ruthenium(II) Peralkene Complexes as Catalytic Species during the Isomerization of Terminal Alkenes in Solution

The isomerization (chain-walking) reaction of terminal to internal alkenes is catalyzed by part-per-million amounts of practically any Ru source when the reaction is carried out with a neat terminal alkene. Here, we provide evidence that the soluble starting Ru sources evolve to catalytically active peralkene Ru(II) species under reaction conditions. These species may also explain the isomerization products found during other Ru-catalyzed alkene processes, i.e., alkene metathesis reactions. A Finke-Watzky mechanism for catalyst formation is consistent with the evidence obtained.

(NHC)Ni(II)-Directed Insertions and Higher Substituted Olefin Synthesis from Simple Olefins

ConspectusWell-controlled olefin insertion is critical for achieving catalytic and productive bulk and fine-chemical synthesis. Developing efficient and selective methods for meeting diverse insertion demands is extremely noteworthy, as it supports numerous transformations. The challenges are related to improving catalyst performance and selectivity control and uniting previously unreactive substrate pairs to achieve higher molecular structural complexity and utility. Nickel catalysts have received persistent attention in higher substituted olefin synthesis and polymerization, and numerous new strategies have been established to fulfill the ever-changing demands. This Account focuses on the recent progress based on N-heterocyclic carbene (NHC) ligands and nickel catalysts in our laboratory in using simple terminal olefins as olefin donors or acceptors.It begins with a brief history of olefin codimerization and the major advances in hydrovinylation achieved by other research groups using ethylene as an olefin donor. It then describes problems related to the reductive elimination that can occur when both the hydrometalated alkene and NHC are on the catalyst. It emphasizes the impact of NHC catalyst generation methods on the competing reactivity. Next, it explains the principal challenges and great opportunities in using our method (with α-olefins as olefin donors and alkenyl sources) to replace intermolecular reductive hydroalkenylation reactions (which require rare and more expensive alkenyl halides and boronic acids as reactants, alongside a stoichiometric amount of metallic reagents). The Account then illustrates the potential uses of our method for solving challenging organic synthesis problems using tailor-made (NHC)Ni(II) catalysts to allow redox-neutral catalytic cycles based on high chemo- and regioselective cross-insertion controls. It shows that upon optimal steric and electronic cooperation between the NHC, olefin donor, and olefin acceptor, regiodivergent insertion and convergent synthesis can be achieved easily.In the course of our work, we uncovered several unique insights into regulating (anti-)Markovnikov hydronickelation, carbonickelation, hydrocarbonation, ring closure, 1,3-allyl shift, isomerization, and catalyst regeneration under green, neutral, and mild-temperature conditions. These insights are also outlined here, along with theoretical calculations that offer additional understandings of the insertion reactivity and selectivity differences observed between the NHC and the highly related phosphorus-based Ni(II) hydride-catalyzed cross-hydroalkenylation and cycloisomerization systems.Compared to traditional olefin and cyclic structure synthesis technology, such as olefin cross-metathesis, enyne cyclization, and cross-coupling reactions, the new catalyst systems often offer previously inaccessible product structural characteristics, substrate scope, and outcomes. In particular, the method is effective for the catalytic synthesis of unsymmetrical and functionalized 1,1-disubstituted olefins (a.k.a. gem-olefins), 1,4-dienes (a.k.a. skipped dienes), conjugated dienes, endo- and exocyclic olefins, fused and spiro rings, and aromatic products. These syntheses are variously achieved by cross-hydroalkenylation, insertion-induced rearrangement, cycloadditions, and other approaches inspired by our investigations and detailed in this Account. Cross-hydroalkenylation can be achieved with high enantioselectivity by application of carefully designed and structurally flexible C₁ and C₂ chiral NHC ligands, yielding a pool of chiral branched alkenes and 1,4-dienes directly from simple chemical feedstocks used in industry. This Account will draw further attention to green alkenylation and the related development of redox-neutral catalytic cycles.

B(C6 F5 )3 -Catalyzed E-Selective Isomerization of Alkenes

Herein, we report the B(C₆ F₅ )₃ -catalyzed E-selective isomerization of alkenes. The transition-metal-free method is applicable across a diverse array of readily accessible substrates, giving access to a broad range of synthetically useful products containing versatile stereodefined internal alkenes. The reaction mechanism was investigated by using synthetic and computational methods.

Copper-catalyzed radical trans-selective hydroboration of ynamides with N-heterocyclic carbene boranes

Vinylboron compounds are important compounds in organic chemistry and biology. In this communication, we developed a copper(I)-catalyzed, highly regio- and stereoselective radical trans-hydroboration of ynamides with N-heterocyclic carbene (NHC)-ligated borane is reported, which leads to a series of trans-boryl enmides that can be conveniently transformed into various multi-substituted enamides. Further investigation showcased that our method is robust and scalable. The mechanism of this unique reaction is studied and discussed.

•

A Cu-catalyzed highly regio- and stereoselective radical trans-hydroboration of ynamides

•

Good to excellent yields, good functional group tolerance

•

Further investigation showcased that our method is robust and scalable

Electron Shuttle in N-Heteroaromatic Ni Catalysts for Alkene Isomerization

Simple N-heteroaromatic Ni(II) precatalysts, (L)NiMe2 (L = bipy, bipym), were used for alkene isomerization. With an original reduction method using a simple borane (HB(Cat)), a low-valent Ni center was formed readily and showed good conversion when a reducing divalent lanthanide fragment, Cp*2Yb, was coordinated to the (bipym)NiMe2 complex, a performance not achieved by the monometallic (bipy)NiMe2 analogue. Experimental mechanistic investigations and computational studies revealed that the redox non-innocence of the L ligand triggered an electron shuttle process, allowing the elusive formation of Ni(I) species that were central to the isomerization process. Additionally, the reaction occurred with a preference for mono-isomerization rather than chain-walking isomerization. The presence of the low-valent ytterbium fragment, which contributed to the formation of the electron shuttle, strongly stabilized the catalysts, allowing catalytic loading as low as 0.5%. A series of alkenes with various architectures have been tested. The possibility to easily tune the various components of the heterobimetallic catalyst reported here, the ligand L and the divalent lanthanide fragment, opens perspectives for further applications in catalysis induced by Ni(I) species.

Promoting Dinuclear-Type Catalysis in Cu1 -C3 N4 Single-Atom Catalysts

Reducing particle size in supported metal catalysts to single-atom level isolates the active metal sites and maximizes the atomic utilization efficiency. However, the large inter-atom distance, particularly in low-loading single-atom catalyst (SAC), is not favorable for a complex reaction where two (or more) reactants have to be activated. A key question is how to control the inter-atom distances to promote dinuclear-type coactivation at the adjacent metal sites. Here, it is reported that reducing the average inter-atom distance of copper SACs supported on carbon nitride (C₃ N₄ ) to 0.74 ± 0.13 nm allows these catalysts to exhibit a dinuclear-type catalytic mechanism in the nitrile-azide cycloaddition. Operando X-ray absorption fine structure study reveals a dynamic ligand exchange process between nitrile and azide, followed by their coactivation on dinuclear Cu SAC sites to form the tetrazole product. This work highlights that reducing the nearest-neighbor distance of SAC allows the mechanistic pathway to diversify from single-site to multisite catalysis.

Cobalt-Catalyzed Migration Isomerization of Dienes

A cobalt-catalyzed multipositional isomerization of conjugated dienes has been reported for the first time using an 8-oxazoline iminoquinoline ligand. This reaction is operationally simple and atom-economical using readily available starting materials with an E/Z mixture to access disubstituted 1,3-dienes with excellent yields and good E,E stereoselectivity. The mechanism via alkene insertion of cobalt hydride species and β-H elimination of a π-allyl cobalt intermediate is proposed on the basis of deuterium labeling and control experiments and density functional theory calculations.

Desymmetrizing Isomerization of Alkene via Thiazolinyl Iminoquinoline Cobalt Catalysis

We report a cobalt-catalyzed desymmetrizing isomerization of exo-cyclic alkenes to generate chiral 1-methylcyclohexene derivatives with good yields and enantioselectivities. A novel chiral thiazolinyl iminoquinoline ligand and its cobalt complex were designed and synthesized to control the establishment of tertiary or quaternary carbon centers at a remote position. This protocol is operationally simple, and a model for the stereochemical outcome has been proposed.

Radical directed regioselective functionalization of diverse alkene derivatives

Regioselective vicinal difunctionalization of diverse alkene derivatives was successfully carried out using readily available carboxylic acids.