Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

Bis(imino)aryl NCN pincer cobalt complexes: synthesis and disproportionation

Treatment of iPr[NCN]Br (2,6-(2,6-iPr2C6H3C[double bond, length as m-dash]N)2C6H3Br) with nBuLi in THF and the subsequent addition of 1 equiv. of CoCl2, CoCl2(Ph3P)2, and CoBr2 gave pincer Co(ii) complexes {iPr[NCN]Co(μ-Cl)}2 (1d), iPr[NCN]CoClPh3P (1d-Ph3P), and iPr[NCN]CoBr2·Li(THF)4 (1d-LiBr) respectively in moderate yields, whereas the slow addition of in situ prepared iPr[NCN]Li to CoCl2 in THF afforded an unexpected mixed-valence cobalt(i/ii) complex κ2C,N,η6-iPr[NCN]Co-κN-CoCl3·Li(THF)4 (2d). Complex 2d was probably formed via a disproportionation reaction of the iPr[NCN]Co(ii) species with excess CoCl2 during the reaction. Nevertheless, addition of CoCl2 to in situ formed 1d-THF at room temperature did not lead to 2d but gave a trinuclear Co(ii) complex {iPr[NCN]Co(μ-Cl)(μ-Br/Cl)}2Co (1d-CoCl2) in moderate yield. Similar reactions using ligands containing small ortho groups in the imine moieties R[NCN]Br (2,6-(2,6-Me2C6H3C[double bond, length as m-dash]N)2C6H3Br, Me[NCN]Br; 2,6-(2,6-Et2C6H3C[double bond, length as m-dash]N)2C6H3Br, Et[NCN]Br; 2,6-(2,4,6-Me3C6H2C[double bond, length as m-dash]N)2C6H3Br, Mes[NCN]Br) and CoBr2, regardless of the reactant addition sequence, afforded mixed-valence cobalt(i/ii) complexes {κ2C,N,η6-R[NCN]Co-κN-CoBr(μ-Br)}2 (Me[NCN] (2a), Et[NCN] (2b), and Mes[NCN] (2c)), suggesting that the bulkiness of the ortho-groups in the imine moieties of the ligands plays an important role in the disproportionation reaction. In the presence of PMe3, Co(ii) complexes κ2C,N-R[NCN]CoBr(PMe3)2 (3a-d) and a bisligated cobalt(ii) complex κ3N,C,N-κ2C,N-iPr[NCN]2CoPMe3 (4d) can be prepared respectively in high yields. The molecular structures of 1d-LiBr, 1d-CoCl2, 2b, 2d, 3a, and 4d were confirmed by X-ray crystallographic analysis and the detailed mechanisms of the disproportionation reaction were proposed.

Visible-Light Controlled Divergent Catalysis Using a Bench-Stable Cobalt(I) Hydride Complex

While the use of visible light in conjunction with transition metal catalysis offers powerful opportunities to switch between on/‐off states of catalytic activity, the next frontier would be the ability to switch the actual function of the catalyst and resulting products. Here we report such an example of multi‐dimensional catalysis. Featuring an easily prepared, bench‐stable cobalt(I) hydride complex in conjunction with pinacolborane, we can switch the reaction outcome between two widely employed transformations, olefin migration and hydroboration, with visible light as the trigger.

Carbonylative, Catalytic Deoxygenation of 2,3-Disubstituted Epoxides with Inversion of Stereochemistry: An Alternative Alkene Isomerization Method

Reactions facilitating inversion of alkene stereochemistry are rare, sought-after transformations in the field of modern organic synthesis. Although a number of isomerization reactions exist, most methods require specific, highly activated substrates to achieve appreciable conversion without side product formation. Motivated by stereoinvertive epoxide carbonylation reactions, we developed a two-step epoxidation/deoxygenation process that results in overall inversion of alkene stereochemistry. Unlike most deoxygenation systems, carbon monoxide was used as the terminal reductant, preventing difficult postreaction separations, given the gaseous nature of the resulting carbon dioxide byproduct. Various alkyl-substituted cis- and trans-epoxides can be reduced to trans- and cis-alkenes, respectively, in >99:1 stereospecificity and up to 95% yield, providing an alternative to traditional, direct isomerization approaches.

Rhodium(III) Catalyzed Regioselective and Stereospecific Allylic Arylation in Water by β-Fluorine Elimination of the Allylic Fluoride: Toward the Synthesis of Z-Alkenyl-Unsaturated Amides

A direct coupling of arylboronic acids with allylic fluorides was carried out in water without additives using a rhodium(III) catalyst (Cp*RhCl₂)₂. The transformation proceeded with excellent γ-selectivity to afford major allyl-aryl coupling products (Z) γ-substituted α,β-unsaturated amides. The reactions of α-chiral allylic fluorides took place with excellent α-to-γ chirality transfer to give allylated arenes with a stereogenic center at the benzylic and allylic position.

Migratory Hydrogenation of Terminal Alkynes by Base/Cobalt Relay Catalysis

Migratory functionalization of alkenes has emerged as a powerful strategy to achieve functionalization at a distal position to the original reactive site on a hydrocarbon chain. However, an analogous protocol for alkyne substrates is yet to be developed. Herein, a base and cobalt relay catalytic process for the selective synthesis of (Z)-2-alkenes and conjugated E alkenes by migratory hydrogenation of terminal alkynes is disclosed. Mechanistic studies support a relay catalytic process involving a sequential base-catalyzed isomerization of terminal alkynes and cobalt-catalyzed hydrogenation of either 2-alkynes or conjugated diene intermediates. Notably, this practical non-noble metal catalytic system enables efficient control of the chemo-, regio-, and stereoselectivity of this transformation.

One simple Ir/hydrosilane catalytic system for chemoselective isomerization of 2-substituted allylic ethers

Here, we describe one simple Ir/hydrosilane catalytic system for chemoselective isomerization of 2-substituted allylic ethers. This facile strategy shows high efficiency towards a variety of substrates, including derivatives from bioactive molecules. The substituent at the α position of the olefins is supposed to be critical in retarding the alkene hydrosilylation process and leading the reaction to go through the isomerization pathway.

Solid-State Molecular Organometallic Catalysis in Gas/Solid Flow (Flow-SMOM) as Demonstrated by Efficient Room Temperature and Pressure 1-Butene Isomerization

The use of solid–state molecular organometallic chemistry (SMOM–chem) to promote the efficient double bond isomerization of 1-butene to 2-butenes under flow–reactor conditions is reported. Single crystalline catalysts based upon the σ-alkane complexes [Rh(R₂PCH₂CH₂PR₂)(η²η²-NBA)][BAr^F₄] (R = Cy, ^tBu; NBA = norbornane; Ar^F = 3,5-(CF₃)₂C₆H₃) are prepared by hydrogenation of a norbornadiene precursor. For the ^tBu-substituted system this results in the loss of long-range order, which can be re-established by addition of 1-butene to the material to form a mixture of [Rh(^tBu₂PCH₂CH₂P^tBu₂)(cis-2-butene)][BAr^F₄] and [Rh(^tBu₂PCH₂CH₂P^tBu₂)(1-butene)][BAr^F₄], in an order/disorder/order phase change. Deployment under flow-reactor conditions results in very different on-stream stabilities. With R = Cy rapid deactivation (3 h) to the butadiene complex occurs, [Rh(Cy₂PCH₂CH₂PCy₂)(butadiene)][BAr^F₄], which can be reactivated by simple addition of H₂. While the equivalent butadiene complex does not form with R = ^tBu at 298 K and on-stream conversion is retained up to 90 h, deactivation is suggested to occur via loss of crystallinity of the SMOM catalyst. Both systems operate under the industrially relevant conditions of an isobutene co-feed. cis:trans selectivites for 2-butene are biased in favor of cis for the ^tBu system and are more leveled for Cy.

Cobalt-Catalyzed Z to E Isomerization of Alkenes: An Approach to (E)-β-Substituted Styrenes

An efficient cobalt-catalyzed Z to E isomerization of β-substituted styrenes using the amido-diphosphine ligand was developed, delivering the (E)-isomers with good functional tolerance and high stereoselectivity. The reaction could be scaled up to gram-scale with a catalyst loading of 0.1 mol %, using a mixture of (Z)- and (E)-alkene as the starting material. Preliminary mechanistic studies indicated that cobalt(I)-hydride and a benzylic-cobalt species were probably involved in the reaction, as supported by experiments and DFT calculations.

Versatile cobalt-catalyzed regioselective chain-walking double hydroboration of 1,n-dienes to access gem-bis(boryl)alkanes

Double hydroboration of dienes is the addition of a hydrogen and a boryl group to the two double bonds of a diene molecule and represents a straightforward and effective protocol to prepare synthetically versatile bis(boryl)alkanes, provided that this reaction occurs selectively. However, this reaction can potentially yield several isomeric organoboron products, and it still remains a challenge to control the regioselectivity of this reaction, which allows the selective production of a single organoboron product, in particular, for a broad scope of dienes. By employing a readily available cobalt catalyst, here we show that this double hydroboration yields synthetically useful gem-bis(boryl)alkanes with excellent regioselectivity. In addition, the scope of dienes for this reaction is broad and encompasses a wide range of conjugated and non-conjugated dienes. Furthermore, mechanistic studies indicate that this cobalt-catalyzed double hydroboration occurs through boryl-directed chain-walking hydroboration of alkenylboronates generated from anti-Markovnikov 1,2-hydroboration of 1,n-diene.

Control of regioselectivity in the double hydroboration of dienes to obtain a single organoboron compound is a considerable synthetic challenge. Here, the authors show a cobalt-catalyzed chain-walking double hydroboration of 1,n-dienes to access gem-bis(boryl)alkanes with regioselective control.

Homogeneous vs. heterogeneous: mechanistic insights into iron group metal-catalyzed reductions from poisoning experiments

Iron group catalysts constitute a promising alternative to well-established noble metal catalysts in reduction reactions. This review advocates the use of kinetic poisoning experiments to distinguish between homotopic and heterotopic mechanisms.

Synthesis of Isomerically Pure (Z)-Alkenes from Terminal Alkynes and Terminal Alkenes: Silver-Catalyzed Hydroalkylation of Alkynes

Alkenes are an important class of compounds common among biologically active molecules and often are used as intermediates in organic synthesis. Many alkenes exist in two stereoisomeric forms (E and Z), which have different structures and different properties. The selective formation of the two isomers is an important synthetic goal that has long inspired the development of new synthetic methods. However, the efficient synthesis of diastereopure, thermodynamically less stable, Z-alkenes is still challenging. Here, we demonstrate an efficient synthesis of diastereopure Z-alkenes (Z:E > 300:1) through a silver-catalyzed hydroalkylation of terminal alkynes, using alkylboranes as coupling partners. We also describe the exploration of the substrate scope, which reveals the broad functional group compatibility of the new method. Preliminary mechanistic studies suggest that a 1,2-metalate rearrangement of the silver borate intermediate is the key step responsible for the stereochemical outcome of the reaction.

Efficient Cobalt Catalyst for Ambient-Temperature Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted Borylation Mechanism, and a New Synthetic Route to Amides

N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl₂ in the presence of ^PyEtPDI afforded the six-coordinate Co(II) salt, [(^PyEtPDI)CoCl][Cl]. Upon adding 2 equiv of NaEt₃BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (κ⁴-N,N,N,N-^PyEtIP^CHMeN^EtPy)Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (κ⁴-N,N,N,N-^PyEtIP^CHMeN^EtPy)Co was found to catalyze the dihydroboration of nitriles using HBPin with turnover frequencies of up to 380 h^-1 at ambient temperature. Stoichiometric addition experiments revealed that HBPin adds across the Co-N_amide bond to generate a hydride intermediate that can react with additional HBPin or nitriles. Computational evaluation of the reaction coordinate revealed that the B-H addition and nitrile insertion steps occur on the antiferromagnetically coupled triplet spin manifold. Interestingly, formation of the borylimine intermediate was found to occur following BPin transfer from the borylated chelate arm to regenerate (κ⁴-N,N,N,N-^PyEtIP^CHMeN^EtPy)Co. Borylimine reduction is in turn facile and follows the same ligand-assisted borylation pathway. The independent hydroboration of alkyl and aryl imines was also demonstrated at 25 °C. With a series of N,N-diborylamines in hand, their addition to carboxylic acids allowed for the direct synthesis of amides at 120 °C, without the need for an exogenous coupling reagent.

Positional and Geometrical Isomerisation of Alkenes: The Pinnacle of Atom Economy

Strategies to achieve spatiotemporal regulation of pre-existing alkenes via external stimuli are essential given the ubiquity of feedstock olefins in chemistry and their downstream applications. Mirroring the 1-0 switch that underpins mammalian vision through selective geometric isomerisation in retinal, strategies to manipulate 2D space by both geometric and positional isomerisation of alkenes via chemical, thermal and light-driven processes are being intensively pursued. This minireview highlights the current state of the art in activating and achieving directionality in these fundamental chemical transformations.

Controllable Isomerization of Alkenes by Dual Visible-Light-Cobalt Catalysis

We report herein that thermodynamic and kinetic isomerization of alkenes can be accomplished by the combination of visible light with Co catalysis. Utilizing Xantphos as the ligand, the most stable isomers are obtained, while isomerizing terminal alkenes over one position can be selectively controlled by using DPEphos as the ligand. The presence of the donor–acceptor dye 4CzIPN accelerates the reaction further. Transformation of exocyclic alkenes into the corresponding endocyclic products could be efficiently realized by using 4CzIPN and Co(acac)₂ in the absence of any additional ligands. Spectroscopic and spectroelectrochemical investigations indicate Co^I being involved in the generation of a Co hydride, which subsequently adds to alkenes initiating the isomerization.

E-Olefins through intramolecular radical relocation

Full control over the selectivity of carbon-carbon double-bond migrations would enable access to stereochemically defined olefins that are central to the pharmaceutical, food, fragrance, materials, and petrochemical arenas. The vast majority of double-bond migrations investigated over the past 60 years capitalize on precious-metal hydrides that are frequently associated with reversible equilibria, hydrogen scrambling, incomplete E/Z stereoselection, and/or high cost. Here, we report a fundamentally different, radical-based approach. We showcase a nonprecious, reductant-free, and atom-economical nickel (Ni)^(I)-catalyzed intramolecular 1,3-hydrogen atom relocation to yield E-olefins within 3 hours at room temperature. Remote installations of E-olefins over extended distances are also demonstrated.

Cyclometallation reactions of a three-coordinate cobalt(i) complex bearing a nonsymmetric N-heterocyclic carbene ligand

The reactions of a three-coordinate cobalt(i)–N-heterocyclic carbene complex with different organometallic reagents afford different cyclometallated cobalt–N-heterocyclic carbene complexes.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Yttrium dialkyl supported by a silaamidinate ligand: synthesis, structure and catalysis on cyclotrimerization of isocyanates

A four-coordinate yttrium dialkyl complex with a sterically demanding silaamidinate ligand exhibited high activity and excellent functional group tolerance for the catalysis of isocyanate cyclotrimerization.

Functionalized Self-Assembled Monolayers Bearing Diiminate Complexes Immobilized through Covalently Anchored Ligands

The application of synthetic organic chemistry to the surface chemistry of monolayer arrays adds a novel dimension to the power of these systems for surface modification. This paper describes the elaboration of simple functionalized monolayers into dialdimine and dialdiminate ligands tethered to the monolayer surface. These ligands are then used to coordinate metal ions in an effort to form diiminate complexes with control over their environment and orientation. Ligand anchoring is best achieved through either thiol-ene photochemistry or azide-acetylene "click" chemistry. There is an influence of ligand bulk on some surface transformations, and in some cases reactions that have been reported to be effective on simple, homogeneous monolayer surfaces are not applicable to a more complex monolayer environment. The large excess of solution reagents relative to monolayer surface functionality adds another measure of difficulty to the control of interfacial reactions. In instances where the anchoring chain includes functional groups that can directly interact with metal ions, the metalation of ligand-bearing surfaces resulted in a higher metal ion content than would have been expected from binding only to the diimine ligands.

Palladium-catalysed alkene chain-running isomerization

We report a method for palladium-catalysed chain-running isomerization of terminal and internal alkenes. Using an air-stable 2,9-dimethylphenanthroline-palladium catalyst in combination with NaBAr₄ promoter, olefins are converted to the most stable double bond isomer at -30 to 20 °C. Silyl enol ethers are readily formed from silylated allylic alcohols. Fluorinated substituents are compatible with the reaction conditions, allowing the synthesis of fluoroenolates. Catalyst loading as low as 0.05% can be employed on a gram scale.

Blue-light-promoted carbon-carbon double bond isomerization and its application in the syntheses of quinolines

A blue-light-promoted carbon-carbon double bond isomerization in the absence of any photoredox catalyst is reported. It provides rapid access to a series of quinolines in good to excellent yields under simple aerobic conditions. The protocol is direct, catalyst-free and operationally convenient.

Access to stereodefined (Z)-allylsilanes and (Z)-allylic alcohols via cobalt-catalyzed regioselective hydrosilylation of allenes

Hydrosilylation of allenes is the addition of a hydrogen atom and a silyl group to a carbon–carbon double bond of an allene molecule and represents a straightforward and atom-economical approach to prepare synthetically versatile allylsilanes and vinylsilanes. However, this reaction generally produces six possible isomeric organosilanes, and the biggest challenge in developing this reaction is to control both regioselectivity and stereoselectivity. The majorities of the developed allene hydrosilylation reactions show high selectivity towards the production of vinylsilanes or branched allylsilanes. By employing a cobalt catalyst generated from readily available and bench-stable cobalt precursor and phosphine-based ligands, here we show that this reaction proceeds under mild conditions in a regioselective and stereoselective manner, and affords synthetically challenging, but valuable linear cis-allylsilanes with excellent stereoselectivity (generally cis to trans ratios: >98:2). This cobalt-catalyzed (Z)-selective allene hydrosilylation provides a general approach to access molecules containing stereodefined (Z)-alkene units.

Controlling selectivity in the hydrosilylation of allenes poses serious challenges in terms of product stereochemistry. Here the authors show that the title reaction proceeds with excellent (Z)-selectivity by use of a cobalt catalyst in the presence of phosphine-based ligands.

Cobalt-catalyzed regioselective stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes

The first transition metal-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of (E/Z)-dienes was effectively achieved with excellent E-selectivities using a cobalt catalyst.

We report the first stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes using catalysts generated from bench-stable Co(acac)₂ and phosphine ligands. A wide range of E/Z-dienes underwent this Markovnikov 1,2-hydrosilylation in a stereoconvergent manner, affording (E)-allylsilanes in high isolated yields with high stereoselectivities (E/Z = >99 : 1) and high regioselectivities (b/l up to > 99 : 1). Mechanistic studies revealed that this stereoconvergence stems from a σ–π–σ isomerization of an allylcobalt species generated by the 1,4-hydrometalation of Z-dienes. In addition, a cobalt catalyst that can only catalyze the hydrosilylation of the E-isomer of an (E/Z)-diene was identified, which allows the separation of the (Z)-isomer from an isomeric mixture of (E/Z)-dienes. Furthermore, asymmetric hydrosilylation of (E)-1-aryl-1,3-dienes was studied with Co(acac)₂/(R)-difluorphos and good enantioselectivities (er up to 90 : 10) were obtained.

Preparation of 2-Amino-5-homoallylfurans via Palladium-Catalyzed Tandem Cycloisomerization/Heck-Type Coupling of Homoallenyl Amides with Allyltrialkylsilanes

The direct access to 2-amino-5-homoallylfurans has been realized by a palladium-catalyzed tandem cycloisomerization/Heck-type coupling between homoallenyl amides and allyltrialkylsilanes, using a novel DDQ/MnO₂ combination as the efficient oxidant. The reaction exclusively affords γ-allylation products in good to excellent yields with broad substrate scope under exceptionally mild reaction conditions. It represents one of the rare examples of the Pd-catalyzed intermolecular Heck-type coupling of allytrialkylsilanes terminated by β-silyl elimination, thus complementing traditional allylation methods because of the excellent γ-selectivity.