Pyridine‐Enhanced Head‐to‐Tail Dimerization of Terminal Alkynes by a Rhodium–N‐Heterocyclic‐Carbene Catalyst

A general regioselective rhodium‐catalyzed head‐to‐tail dimerization of terminal alkynes is presented. The presence of a pyridine ligand (py) in a Rh–N‐heterocyclic‐carbene (NHC) catalytic system not only dramatically switches the chemoselectivity from alkyne cyclotrimerization to dimerization but also enhances the catalytic activity. Several intermediates have been detected in the catalytic process, including the π‐alkyne‐coordinated Rh^I species [RhCl(NHC)(η²‐HC?CCH₂Ph)(py)] ( 3 ) and [RhCl(NHC){η²‐C(tBu)?C(E)CH?CHtBu}(py)] ( 4 ) and the Rh^III–hydride–alkynyl species [RhClH{? C?CSi(Me)₃}(IPr)(py)₂] ( 5 ). Computational DFT studies reveal an operational mechanism consisting of sequential alkyne C? H oxidative addition, alkyne insertion, and reductive elimination. A 2,1‐hydrometalation of the alkyne is the more favorable pathway in accordance with a head‐to‐tail selectivity.     

Theoretical Study on the Mechanism of Ni‐Catalyzed Alkyl–Alkyl Suzuki Cross‐Coupling

Ni‐catalyzed cross‐coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/ L1 ( L1 =trans‐N,N′‐dimethyl‐1,2‐cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a Ni^I–Ni^III catalytic cycle with three main steps: transmetalation of [Ni^I( L1 )X] (X=Cl, Br) with 9‐borabicyclo[3.3.1]nonane (9‐BBN)R₁ to produce [Ni^I( L1 )(R₁)], oxidative addition of R₂X with [Ni^I( L1 )(R₁)] to produce [Ni^III( L1 )(R₁)(R₂)X] through a radical pathway, and C? C reductive elimination to generate the product and [Ni^I( L1 )X]. The transmetalation step is rate‐determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol^?1). On the other hand, the cross‐coupling of alkyl chlorides can be catalyzed by Ni/ L2 ( L2 =trans‐N,N′‐dimethyl‐1,2‐diphenylethane‐1,2‐diamine) because the activation barrier of transmetalation with L2 is lower than that with L1 . Importantly, the Ni⁰–Ni^II catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [Ni^II( L1 )(R₁)(R₂)] is very difficult.     

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

A cross‐hydroalkenylation/rearrangement cascade (HARC), using a cyclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni(allyl)]BAr^F catalysts (NHC=N‐heterocyclic carbenes). By controlling the (NHC)Ni^IIH relative insertion reactivity with cyclopropene and alkyne, a broad scope of cyclopentadienes was obtained with highly selectively. The structural features of the new (NHC)Ni^II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni^II‐assisted vinylcyclopropane rearrangement reactivity.     

Carbon–Carbon Cross‐Coupling Reactions Catalyzed by a Two‐Coordinate Nickel(II)–Bis(amido) Complex via Observable NiI,NiII, and NiIII Intermediates

Recently, the development of more sustainable catalytic systems based on abundant first‐row metals, especially nickel, for cross‐coupling reactions has attracted significant interest. One of the key intermediates invoked in these reactions is a Ni^III–alkyl species, but no such species that is part of a competent catalytic cycle has yet been isolated. Herein, we report a carbon–carbon cross‐coupling system based on a two‐coordinate Ni^II–bis(amido) complex in which a Ni^III–alkyl species can be isolated and fully characterized. This study details compelling experimental evidence of the role played by this Ni^III–alkyl species as well as those of other key Ni^I and Ni^II intermediates. The catalytic cycle described herein is also one of the first examples of a two‐coordinate complex that competently catalyzes an organic transformation, potentially leading to a new class of catalysts based on the unique ability of first‐row transition metals to accommodate two‐coordinate complexes.     

Carbon–Carbon Cross‐Coupling Reactions Catalyzed by a Two‐Coordinate Nickel(II)–Bis(amido) Complex via Observable NiI,NiII, and NiIII Intermediates

Recently, the development of more sustainable catalytic systems based on abundant first‐row metals, especially nickel, for cross‐coupling reactions has attracted significant interest. One of the key intermediates invoked in these reactions is a Ni^III–alkyl species, but no such species that is part of a competent catalytic cycle has yet been isolated. Herein, we report a carbon–carbon cross‐coupling system based on a two‐coordinate Ni^II–bis(amido) complex in which a Ni^III–alkyl species can be isolated and fully characterized. This study details compelling experimental evidence of the role played by this Ni^III–alkyl species as well as those of other key Ni^I and Ni^II intermediates. The catalytic cycle described herein is also one of the first examples of a two‐coordinate complex that competently catalyzes an organic transformation, potentially leading to a new class of catalysts based on the unique ability of first‐row transition metals to accommodate two‐coordinate complexes.     

trans‐Carbocarbonation of Internal Alkynes through a Formal anti‐Carbopalladation/C−H Activation Cascade

An intramolecular Pd‐catalyzed cascade reaction is presented that consists of a formal anti‐carbopalladation of a C?C triple bond followed by C?H activation. As a result, oligocyclic ring systems with an embedded tetrasubstituted double bond are formed. The key to success in affording the trans geometry of the emerging double bond are alkyne units with residues that must not undergo β‐hydride elimination (e.g., t‐butyl or silyl groups). Silyl groups proved to be a perfect handle to further convert the tetrasubstituted alkenes. The evaluation of kinetic data with a deuterium‐labeled compound and X‐ray analyses of trapped intermediates provided additional insight into the catalytic cycle.     

Transition‐Metal‐Ion‐Mediated Polymerization of Dopamine: Mussel‐Inspired Approach for the Facile Synthesis of Robust Transition‐Metal Nanoparticle–Graphene Hybrids

Inspired by the high transition‐metal‐ion content in mussel glues, and the cross‐linking and mechanical reinforcement effects of some transition‐metal ions in mussel threads, high concentrations of nickel(II), cobalt(II), and manganese(II) ions have been purposely introduced into the reaction system for dopamine polymerization. Kinetics studies were conducted for the Ni²⁺–dopamine system to investigate the polymerization mechanism. The results show that the Ni²⁺ ions could accelerate the assembly of dopamine oligomers in the polymerization process. Spectroscopic and electron microscopic studies reveal that the Ni²⁺ ions are chelated with polydopamine (PDA) units, forming homogeneous Ni²⁺–PDA complexes. This facile one‐pot approach is utilized to construct transition‐metal‐ion–PDA complex thin coatings on graphene oxide, which can be carbonized to produce robust hybrid nanosheets with well‐dispersed metallic nickel/metallic cobalt/manganese(II) oxide nanoparticles embedded in PDA‐derived thin graphitic carbon layers. The nickel–graphene hybrid prepared by using this approach shows good catalytic properties and recyclability for the reduction of p ‐ nitrophenol.     

Tertiary Carbinamine Synthesis by Rhodium‐Catalyzed [3+2] Annulation of N‐Unsubstituted Aromatic Ketimines and Alkynes

A convenient and waste‐free synthesis of indene‐based tertiary carbinamines by rhodium‐catalyzed imine/alkyne [3+2] annulation is described. Under the optimized conditions of 0.5–2.5 mol % [{(cod)Rh(OH)}₂] (cod=1,5‐cyclooctadiene) catalyst, 1,3‐bis(diphenylphosphanyl)propane (DPPP) ligand, in toluene at 120 °C, N‐unsubstituted aromatic ketimines and internal alkynes were coupled in a 1:1 ratio to form tertiary 1H‐inden‐1‐amines in good yields and with high selectivities over isoquinoline products. A plausible catalytic cycle involves sequential imine‐directed aromatic C? H bond activation, alkyne insertion, and a rare example of intramolecular ketimine insertion into a Rh^I–alkenyl linkage.     

“(Diphosphine)Nickel”‐Catalyzed Negishi Cross‐Coupling: An Experimental and Theoretical Study

The use of a strongly donating “(bis‐dialkylphosphine)Ni” fragment promotes the catalytic coupling of a large range of ArCl and ArZnCl derivatives under mild conditions. Stoichiometric mechanistic investigations and DFT calculations prove that a Ni⁰/Ni^II cycle is operative in this system.     

Improved molecular weight control in ring‐opening metathesis polymerization reactions in organic and aqueous media using N‐heterocyclic carbene–ruthenium arene/alkyne catalyst systems

A binary catalytic system, RuCl₂(N‐heterocyclic carbene)(p‐cymene)/alkyne, was developed for improved molecular weight control in ring‐opening metathesis polymerization (ROMP) reactions of norbornene derivatives in organic and aqueous media. Monometallic ruthenium arene compounds were activated using aryl and aliphatic terminal alkynes to form highly active metathesis species. The effects of alkyne structure and concentration on the overall catalytic activity were systematically investigated. The catalytic activity of the metathesis active species can be tuned by varying alkyne substituents. Also, the initiation rate of the ROMP reaction can be tuned by increasing the alkyne‐to‐Ru ratio. ROMP polymers with a wide range of molecular weights (91–832 kDa) were isolated in organic media, whereas polymers with a molecular weight range of 110–280 kDa with average particle sizes of 150–250 nm were isolated in aqueous media. Copyright © 2016 John Wiley & Sons, Ltd.     

Macrocyclization of polymers via ring‐closing metathesis and azide/alkyne‐“click”‐reactions: An approach to cyclic polyisobutylenes

The end‐to‐end cyclization of telechelic polyisobutylenes (PIB's) toward cyclic polyisobutylenes is reported, using either ring‐closing metathesis (RCM) or the azide/alkyne‐“click”‐reaction. The first approach uses bisallyl‐telchelic PIB's (M_n = 1650, 3680, 9770 g mol^?1) and Grubbs 1st‐, 2nd‐, and 3rd‐generation catalyst leading to cyclic PIB's in 60–80% yield, with narrow polydispersities (M_w/M_n = 1.25). Azide/alkyne‐“click”‐reactions of bisalkyne‐telechelic PIB's (M_n = 3840 and 9820 g mol^?1) with excess of 1,11‐diazido‐undecane leads to the formation of mixtures of linear/cyclic PIB's under formation of oligomeric cycles. Subsequent reaction of the residual azide‐moieties in the linear PIB's with excess of alkyne‐telechelic PEO enables the chromatographic removal of the resulting linear PEO‐PIB‐block copolymers by column chromatography. Thus pure cyclic PIB's can be obtained using this double‐“click”‐method, devoid of linear contaminants. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 671–680, 2010     

Discovery of Highly Selective Alkyne Semihydrogenation Catalysts Based on First‐Row Transition‐Metallated Porous Organic Polymers

Five different first‐row transition metal precursors (V^III, Cr^III, Mn^II, Co^II, Ni^II) were successfully incorporated into a catechol porous organic polymer (POP) and characterized using ATR‐IR and XAS analysis. The resulting metallated POPs were then evaluated for catalytic alkyne hydrogenation using high‐throughput screening techniques. All POPs were unexpectedly found to be active and selective catalysts for alkyne semihydrogenation. Three of the metallated POPs (V, Cr, Mn) are the first of their kind to be active single‐site hydrogenation catalysts. These results highlight the advantages of using a POP platform to develop new catalysts which are otherwise difficult to achieve through traditional heterogeneous and homogeneous routes.     

Coordination‐Induced Spin‐State Switching of an Aminyl‐Radical‐Bridged Nickel(II) Porphyrin Dimer between Doublet and Sextet States

A bis(Ni^II‐porphyrinyl)aminyl radical with meso‐C₆F₅ groups was prepared as a spin‐delocalized stable aminyl radical with a doublet spin state. Upon addition of pyridine, both Ni^II centers became hexacoordinated by accepting two axial pyridines, which triggered a spin‐state change of the Ni^II centers from diamagnetic (S=0) to paramagnetic (S=1). The resulting high‐spin Ni^II centers interact with the aminyl radical ferromagnetically to give rise to an overall sextet state (S=5/2). Importantly, this coordination‐induced spin‐state switching can be conducted in a reversible manner, in that washing of the high‐spin radical with aqueous hydrochloric acid regenerates the original doublet radical in good yield.     

Enantioselective Divergent Synthesis of (−)‐cis‐α‐ and (−)‐cis‐γ‐Irone by Using Wilkinson’s Catalyst

A simple, efficient synthesis is reported for (?)‐cis‐α‐ and (?)‐cis‐γ‐irone, two precious constituents of iris oils, in ≥99 % diastereomeric and enantioselective ratios. The two routes diverge from a common intermediate prepared from (?)‐epoxygeraniol. Of general interest in this approach is the installation of the enone moiety of irones through a NHC?Au^I‐catalyzed Meyer–Schuster‐like rearrangement of a propargylic benzoate and the use of Wilkinson’s catalyst for the stereoselective hydrogenation of a prostereogenic exocyclic double bond to secure the critical cis stereochemistry of the alkyl groups at C2 and C6 of the irones. The stereochemical aspects of this reaction are rationally supported by DFT calculation of the conformers of the substrates undergoing the hydrogenation and by a modeling study of the geometry of the rhodium η² complexes involved in the diastereodifferentiation of the double bond faces. Thus, computational investigation of the η² intermediates formed in the catalytic cycle of prostereogenic alkene hydrogenation by using Wilkinson’s catalyst could be highly predictive of the stereochemistry of the products.     

Access to Multifunctionalized Benzofurans by Aryl Nickelation of Alkynes: Efficient Synthesis of the Anti‐Arrhythmic Drug Amiodarone

An unconventional nickel‐catalyzed reaction was developed for the synthesis of multifunctionalized benzofurans from alkyne‐tethered phenolic esters. The transformation involves the generation of a nucleophilic vinyl Ni^II species by the regioselective syn‐aryl nickelation of an alkyne, which then undergoes an intramolecular cyclization with phenol ester to yield highly functionalized 1,1‐disubstituted alkenes with 3‐benzofuranyl and (hetero)aryl substituents. The methodology can be used for the late‐stage benzofuran incorporation of various drug molecules and natural products, such as 2‐propylvaleric acid, gemfibrozil, biotin, and lithocholic acid. Furthermore, this arylative cyclization method was successfully applied for the efficient synthesis of the anti‐arrhythmic drug amiodarone.     

Nickel(II)‐Mediated Reversible Thiolate/Disulfide Conversion as a Mimic for a Key Step of the Catalytic Cycle of Methyl‐Coenzyme M Reductase

According to the well‐accepted mechanism, methyl‐coenzyme M reductase (MCR) involves Ni‐mediated thiolate‐to‐disulfide conversion that sustains its catalytic cycle of methane formation in the energy saving pathways of methanotrophic microbes. Model complexes that illustrate Ni‐ion mediated reversible thiolate/disulfide transformation are unknown. In this paper we report the synthesis, crystal structure, spectroscopic properties and redox interconversions of a set of Ni^II complexes comprising a tridentate N₂S donor thiol and its analogous N₄S₂ donor disulfide ligands. These complexes demonstrate reversible Ni^II‐thiolate/Ni^II‐disulfide (both bound and unbound disulfide‐S to Ni^II) transformations via thiyl and disulfide monoradical anions that resemble a primary step of MCR's catalytic cycle.     

Exploring Bis(cyclometalated) Ruthenium(II) Complexes as Active Catalyst Precursors: Room‐Temperature Alkene–Alkyne Coupling for 1,3‐Diene Synthesis

Described is the development of a new class of bis(cyclometalated) ruthenium(II) catalyst precursors for C? C coupling reactions between alkene and alkyne substrates. The complex [(cod)Ru(3‐methallyl)₂] reacts with benzophenone imine or benzophenone in a 1:2 ratio to form bis(cyclometalated) ruthenium(II) complexes ( 1 ). The imine‐ligated complex 1 a promoted room‐temperature coupling between acrylic esters and amides with internal alkynes to form 1,3‐diene products. A proposed catalytic cycle involves C? C bond formation by oxidative cyclization, β‐hydride elimination, and C? H bond reductive elimination. This Ru^II/Ru^IV pathway is consistent with the observed catalytic reactivity of 1 a for mild tail‐to‐tail methyl acrylate dimerization and for cyclobutene formation by [2+2] norbornene/alkyne cycloaddition.     

General and Mild Ni0‐Catalyzed α‐Arylation of Ketones Using Aryl Chlorides

A general methodology for the α‐arylation of ketones using a nickel catalyst has been developed. The new well‐defined [Ni(IPr*)(cin)Cl] ( 1 c ) pre‐catalyst showed great efficiency for this transformation, allowing the coupling of a wide range of ketones, including acetophenone derivatives, with various functionalised aryl chlorides. This cinnamyl‐based Ni–N‐heterocyclic carbene (NHC) complex has demonstrated a different behaviour to previously reported NHC‐Ni catalysts. Preliminary mechanistic studies suggest a Ni⁰/Ni^II catalytic cycle to be at play.     

Nickel‐Catalyzed Methylation of Aryl Halides with Deuterated Methyl Iodide

A nickel‐catalyzed methylation of aryl halides with cheap and readily available CH₃I or CD₃I is described. The reaction is applicable to a wide range of substrates and allows installation of a CD₃ group under mild reaction conditions without deuterium scrambling to other carbon atoms. Initial mechanistic studies on the stoichiometric and catalytic reactions of the isolated [(dppp)Ni(C₆H₄‐4‐CO₂Et)Br] [dppp=1,3‐bis(diphenylphosphanyl)propane] suggest that a Ni⁰/Ni^II catalytic cycle is favored.     

[Bis(3,5‐dimethylpyrazol‐1‐yl‐κN2)hydro(pyrazol‐1‐yl‐κN2)borato][(3,5‐dimethylpyrazol‐1‐yl‐κN2)dihydro(pyrazol‐1‐yl‐κN2)borato]nickel(II)

The title compound, [Ni(C₈H₁₂BN₄)(C₁₃H₁₈BN₆)] or Bp′Tp′Ni^II, where Bp′ is (3,5‐dimethylpyrazol‐1‐yl)dihydro(pyrazol‐1‐yl)borate and Tp′ is bis(3,5‐dimethylpyrazol‐1‐yl)hydro(pyrazol‐1‐yl)borate, contains a divalent Ni^II centre bound by the chelating N atoms of the polysubstituted pyrazolylborate ligands. It is shown to lack a strong agostic B—H...Ni interaction, implying that the sixth coordination site is unoccupied in the solid state. This square‐pyramidal complex is the only known crystal structure where the Ni^II centre is pentacoordinated while bonded exclusively to pyrazolyl units. This is of interest with respect to electrochemical and catalytic properties.