Bimetallic C–C Bond-Forming Reductive Elimination from Nickel

Fundamental Principles in Catalysis from Mechanistic Studies of Oxidative Decarboxylative Coupling Reactions

ConspectusOxidative decarboxylative coupling (ODC) reactions have been recognized as powerful alternatives to traditional cross-coupling reactions due to the ability to generate (hetero)biaryl structures from simple and readily available carboxylic acid precursors. These reactions, however, are underdeveloped due to the requirement for ortho-nitrobenzoate coupling partners and silver salts as oxidants. Our research program has focused on the development of new catalytic ODC reactions, as well as mechanistic studies of these reactions to uncover the origin of these synthetic limitations. As the framework for these studies, we explored two key ODC reactions developed in our group: (1) a Ni-catalyzed decarboxylative arylation reaction that relies on silver as the oxidant and (2) a Cu-catalyzed decarboxylative thiolation reaction capable of operating under aerobic conditions. Our findings, disclosed in this Account, have uncovered the importance of the ortho-substituent and revealed that Ag-based oxidants are also responsible for mediating the decarboxylation and transmetalation steps.Systematic exploration of the decarboxylation of a series of well-defined Ag-benzoate complexes allowed us to probe the importance of the ortho-nitro group in the decarboxylation step. Kinetic measurements of a large series of differently substituted benzoates were found to correlate with the field effect (F) of the ortho-substituent, revealing this feature to be responsible for the enhanced reactivity of these favored benzoates.Our studies of the Ni-catalyzed decarboxylative arylation reaction uncovered an unexpected redox transmetalation step in this system. Synthesis and isolation of the proposed nickelacycle and Ag-aryl intermediates enabled direct study of the fundamental coupling steps. Catalytic and stoichiometric reactions of these complexes, paired with DFT calculations, supported a redox transmetalation step in which the Ag-aryl intermediate transfers the aryl ligand from AgI to NiII with concomitant oxidation to generate a NiIII-bis(aryl) intermediate.Finally, detailed mechanistic studies of our Cu-catalyzed decarboxylative thiolation reaction demonstrated how this catalyst system is able to use O2 as the terminal oxidant. Kinetic studies paired with the synthesis and reactivity of well-defined copper intermediates revealed decarboxylation from a CuI-benzoate resting state, despite the oxidizing reaction conditions which could support higher oxidation state intermediates. We also identified the intermediacy of diphenyl disulfide (PhSSPh) formed from the thiophenol (PhSH) coupling partner under the aerobic Cu-catalyzed conditions. The reaction of PhSSPh with the catalyst proceeds via oxidative transfer of the PhS fragment to CuI that is analogous to that of the redox transmetalation observed in Ni-catalyzed decarboxylative arylation.These studies combined suggest significant implications for ODC reactions more broadly. For example, it appears that silver is important for mediating the decarboxylation step when the catalyst is unable (as is the case with nickel) but is not needed when the catalyst is an efficient decarboxylation mediator on its own (as with copper). Furthermore, silver plays an additional role in mediating an oxidative transfer of the coupling partner in arylation reactions but is not required when the coupling partner is itself oxidizing (such as PhSSPh). We anticipate that these mechanistic insights will facilitate the development of new ODC reactions that operate under milder conditions and with broader substrate scopes.

Synthesis of Iron(IV) Alkynylide Complexes and Their Reactivity to Form 1,3-Diynes

The isolation of thermally unstable and highly reactive organoiron(IV) complexes is a challenge for synthetic chemists. In particular, the number of examples where the C-based ligand is not part of the chelating ligand remains scarce. These compounds are of interest because they could pave the way to designing catalytic cycles of bond forming reactions proceeding via organoiron(IV) intermediates. Herein, we report the synthesis and characterization, including single-crystal X-ray diffraction, of a family of alkynylferrates(III) and Fe(IV) alkynylide complexes. The alkynylferrates(III) are formed by transmetalation of the Fe(III) precursor [(N3N')FeIII] (N3N'3- is tris(N-tert-butyldimethylsilyl-2-amidoethyl)amine) with lithium alkynylides, and their further one-electron oxidation enables the synthesis of the corresponding Fe(IV) alkynylides. The electronic structure of this family of organometallic Fe(III) and Fe(IV) complexes has been thoroughly investigated by spectroscopic methods (EPR, NMR, 57Fe Mössbauer, X-Ray absorption (XAS) and emission (XES) spectroscopies) and theoretical calculations. While alkynylferrates(III) are sluggish to engage into C-C bond forming processes, the Fe(IV) alkynylides react to afford 1,3-diynes at room temperature. A bimolecular reductive elimination from a bimetallic Fe(IV) intermediate to form the 1,3-diynes is proposed based on the mechanistic investigations performed.

Intramolecular Agostic Interactions and Dynamics of a Methyl Group at a Preorganized Dinickel(II) Site

Alkyl nickel intermediates relevant to catalytic processes often feature agostic stabilization, but relatively little is known about the situation in oligonickel systems. The dinickel(I) complex K[LNiI2], which is based on a compartmental pyrazolato-bridged ligand L3- with two β-diketiminato chelate arms, or its masked version, the dihydride complex [KL(NiII-H)2] that readily releases H2, oxidatively add methyl tosylate to give diamagnetic [LNiII2(CH3)] (1) with d(Ni···Ni) ≈ 3.7 Å. Structural characterization shows that the methyl group in 1 is bound to one NiII and exhibits an intramolecular agostic interaction with the more distant NiII. This is supported spectroscopically (viz., a ν(C-H) stretch at 2658 cm-1 and lowered 1JC-H of 114 Hz) and by DFT calculations, including topological analysis of the computed electron density for 1. NMR spectroscopy reveals very fast hopping of the CH3 group between the two NiII ions, which according to DFT has a minute barrier of 4 kcal mol-1 and proceeds via a planar CH3 moiety in the transition state (Walden-like inversion). The alkylidene group in K[LNi2(μ-CHSi(Me3)3)], obtained from the reaction of [KL(Ni-H)2] with N2CHSiMe3, is symmetrically bridging. This work provides new insight into the stabilization and dynamics of alkyl ligands at dinickel sites with a constrained metal···metal distance.

Late-Stage C(sp2)-C(sp3) Diversification via Nickel Oxidative Addition Complexes

Herein, we describe nickel oxidative addition complexes (Ni-OACs) of drug-like molecules as a platform to rapidly generate lead candidates with enhanced C(sp3) fraction. The potential of Ni-OACs to access new chemical space has been assessed not only in C(sp2)-C(sp3) couplings but also in additional bond formations without recourse to specialized ligands and with improved generality when compared to Ni-catalyzed reactions. The development of an automated diversification process further illustrates the robustness of Ni-OACs, thus offering a new gateway to expedite the design-make-test-analyze (DMTA) cycle in drug discovery.

Ni-Catalyzed Regio- and Enantioselective Homoallylic Coupling: Synthesis of Chiral Branched 1,5-Dienes Featuring a Quaternary Stereogenic Center and Mechanistic Analysis

Asymmetric synthesis of small molecules comprising quaternary stereogenic carbon centers represents a challenging objective. Here regio- and enantioselective synthesis of chiral 1,5-dienes featuring quaternary stereocenters is reported via nickel-promoted by reductive homoallylic coupling. The developed methodology features an atypical preference for the formation of unusual branched regioisomers (rr >20 : 1) in a sterically challenging allylic substitution event and furnishes the products with enantiomeric ratios of up to 98 : 2 and with high chemo- and E-selectivity. A range of experimental evidences suggest that zinc plays a dual role to generate electrophilic and nucleophilic Ni(II)-allyl intermediates empowering a unique formal bimetallic cross-electrophile manifold in two separate kinetic regimes.

Oxidative Addition and β-Hydride Elimination by a Macrocyclic Dinickel Complex: Observing Bimetallic Elementary Reactions

The dinickel(I) complex Ni2 (tBu PONNOPONNO), featuring a planar macrocyclic diphosphoranide ligand tBu PONNOPONNO, offers a unique architectural platform for observing bimetallic elementary reactions. Oxidative addition reactions of alkyl halides produce dinickel(II) complexes of the type Ni2 (μ-R)(μ-X)(tBu PONNOPONNO). However, when R=Et β-hydride elimination is observed to form a dinickel monohydride, with the rate dependent on the nature of X. DFT studies suggest a new mechanism for bimetallic β-hydride elimination, where the rate dependence arises from the steric pressure imposed by the X group on the opposing trans face of the dinickel macrocycle. This work enhances understanding of bimetallic elementary reactions, particularly β-hydride elimination, which have not been well-explored for dinuclear systems.

Nickel-Carbon Bond Oxygenation with Green Oxidants via High-Valent Nickel Species

Described herein is the synthesis of the Ni^II complex (^tBuMe₂tacn)Ni^II(cycloneophyl) (^tBuMe₂tacn = 1-tert-butyl-4,7-dimethyl-1,4,7-triazacyclononane, cycloneophyl = -CH₂CMe₂-o-C₆H₄-) and its reactivity with dioxygen and peroxides. The new ^tBuMe₂tacn ligand is designed to enhance the oxidatively induced bond-forming reactivity of high-valent Ni intermediates. Tunable chemoselectivity for Csp²-O vs Csp²-Csp³ bond formation was achieved by selecting the appropriate solvent and reaction conditions. Importantly, the use of cumene hydroperoxide and meta-chloroperbenzoic acid suggests a heterolytic O-O bond cleavage upon reaction with (^tBuMe₂tacn)Ni^II(cycloneophyl). Mechanistic studies using isotopically labeled H₂O₂ support the generation of a high-valent Ni-oxygen species via an inner-sphere mechanism and subsequent reductive elimination to form the Csp²-O bond. Kinetic studies of the exceptionally fast Csp²-O bond-forming reaction reveal a first-order dependence on both (^tBuMe₂tacn)Ni^II(cycloneophyl) and H₂O₂, and thus an overall second-order reaction. Eyring analysis further suggests that the oxidation of the Ni^II complex by H₂O₂ is the rate-determining step, which can be modulated by the presence of coordinating solvents. Moreover, computational studies fully support the conclusions drawn from experimental results. Overall, this study reveals for the first time the ability to control the oxidatively induced C-C vs C-O bond formation reactions at a Ni center. Importantly, the described system merges the known organometallic reactivity of Ni with the biomimetic oxidative transformations resembling oxygenases and peroxidases, and involving high-valent metal-oxygen intermediates, which is a novel approach that should lead to unprecedented oxidative catalytic transformations.

A Redox Transmetalation Step in Nickel-Catalyzed C-C Coupling Reactions

Ni-catalyzed C-H functionalization reactions are becoming efficient routes to access a variety of functionalized arenes, yet the mechanisms of these catalytic C-C coupling reactions are not well understood. Here, we report the catalytic and stoichiometric arylation reactions of a nickel(II) metallacycle. Treatment of this species with silver(I)-aryl complexes results in facile arylation, consistent with a redox transmetalation step. Additionally, treatment with electrophilic coupling partners generates C-C and C-S bonds. We anticipate that this redox transmetalation step may be relevant to other coupling reactions that employ silver salts as additives.

Ligand-free nickel catalyzed perfluoroalkylation of arenes and heteroarenes

We describe a "ligand-free" Ni-catalyzed perfluoroalkylation of heteroarenes to produce a diverse array of trfiluoromethyl, pentafluoroethyl and heptafluoropropyl adducts. Catalysis proceeds at room temperature via a radical pathway. The catalytic protocol is distinguished by its simplicity, and its wide scope demonstrates the potential in the late-stage functionalization of drug analogues and peptides.

Mechanistic Investigation of the Nickel-Catalyzed Metathesis between Aryl Thioethers and Aryl Nitriles

Functional group metathesis is an emerging field in organic chemistry with promising synthetic applications. However, no complete mechanistic studies of these reactions have been reported to date, particularly regarding the nature of the key functional group transfer mechanism. Unraveling the mechanism of these transformations would not only allow for their further improvement but would also lead to the design of novel reactions. Herein, we describe our detailed mechanistic studies of the nickel-catalyzed functional group metathesis reaction between aryl methyl sulfides and aryl nitriles, combining experimental and computational results. These studies did not support a mechanism proceeding through reversible migratory insertion of the nitrile into a Ni-Ar bond and provided strong support for an alternative mechanism involving a key transmetalation step between two independently generated oxidative addition complexes. Extensive kinetic analysis, including rate law determination and Eyring analysis, indicated the oxidative addition complex of aryl nitrile as the resting state of the catalytic reaction. Depending on the concentration of aryl methyl sulfide, either the reductive elimination of aryl nitrile or the oxidative addition into the C(sp²)-S bond of aryl methyl sulfide is the turnover-limiting step of the reaction. NMR studies, including an unusual ³¹P-²H HMBC experiment using deuterium-labeled complexes, unambiguously demonstrated that the sulfide and cyanide groups exchange during the transmetalation step, rather than the two aryl moieties. In addition, Eyring and Hammett analyses of the transmetalation between two Ni(II) complexes revealed that this central step proceeds via an associative mechanism. Organometallic studies involving the synthesis, isolation, and characterization of all putative intermediates and possible deactivation complexes have further shed light on the reaction mechanism, including the identification of a key deactivation pathway, which has led to an improved catalytic protocol.

Nickel-Catalyzed Three-Component Cross-Electrophile Coupling of 1,3-Dienes with Aldehydes and Aryl Bromides

We herein report a Ni-catalyzed three-component cross-electrophile coupling of 1,3-dienes with aldehydes and aryl bromides using manganese metal as the reducing agent. This efficient protocol accomplishes dicarbofunctionalization of 1,3-dienes to synthesize diverse structural 1,4-disubstituted homoallylic alcohols by forming two new C-C bonds in one time. Mechanistic study suggests that an allyl-nickel(I) species is involved in the catalytic cycle.

Selective Benzylic CH-Borylations by Tandem Cobalt Catalysis

Metal-catalyzed C-H activations are environmentally and economically attractive synthetic strategies for the construction of functional molecules as they obviate the need for pre-functionalized substrates and minimize waste generation. Great challenges reside in the control of selectivities, the utilization of unbiased hydrocarbons, and the operation of atom-economical dehydrocoupling mechanisms. An especially mild borylation of benzylic CH bonds was developed with the ligand-free pre-catalyst Co[N(SiMe3 )2 ]2 and the bench-stable and inexpensive borylation reagent B2 pin2 that produces H2 as the only by-product. A full set of kinetic, spectroscopic, and preparative mechanistic studies are indicative of a tandem catalysis mechanism of CH-borylation and dehydrocoupling via molecular CoI catalysts.

Oxidative C–C/C–X coupling in organometallic nickel complexes: insights from DFT

NiIII and NiIV-center complexes prefer direct reductive elimination than reacting through five-coordinate intermediates. 32+ complex in the presence of Cl− undergoes Cl–Csp2 elimination preferably over Cl–Csp3 and Csp3–Csp2 elimination.

A Process Chemistry Benchmark for sp2-sp3 Cross Couplings

As sp²-sp³ disconnections gain acceptance in the medicinal chemist's toolbox, an increasing number of potential drug candidates containing this motif are moving into the pharmaceutical development pipeline. This raises a new set of questions and challenges around the novel, direct methodologies available for forging these bonds. These questions gain further importance in the context of process chemistry, where the focus is the development of scalable processes that enable the large-scale delivery of clinical supplies. In this paper, we describe our efforts to apply a wide variety of standard, photo-, and electrochemical sp²-sp³ cross-coupling methods to a pharmaceutically relevant intermediate and optimize each through a combination of high throughput and mechanistically guided experimentation. With data regarding the performance, benefits, and limitations of these novel methods, we evaluate them against a more traditional two-step palladium-catalyzed process. This work reveals trends and similarities between these sp²-sp³ bond-forming methods and suggests a path forward for further refinements.

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

Reductive C-C Coupling from Molecular Au(I) Hydrocarbyl Complexes: A Mechanistic Study

Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C-C bond formation. In this study, we investigate C-C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes including [Au2(μ-CH3)(PMe2Ar')2][NTf2], [Au2(μ-CH3)(XPhos)2][NTf2], and [Au2(μ-CH3)(tBuXPhos)2][NTf2] {Ar' = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H2-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For [Au2(μ-CH3)(XPhos)2][NTf2] and [Au2(μ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au2(μ-CH3)(PMe2Ar')2][NTf2] increases as the steric bulk of the phosphine substituent Ar' decreases. Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the mechanism of C-C coupling likely involves (1) cleavage of [Au2(μ-CH3)(PMe2Ar')2][NTf2] to form Au(PR2Ar')(NTf2) and Au(CH3)(PMe2Ar'), (2) phosphine migration from a second equivalent of [Au2(μ-CH3)(PMe2Ar')2][NTf2] aided by binding of the Lewis acidic [Au(PMe2Ar')]+, formed in step 1, to produce [Au2(CH3)(PMe2Ar')][NTf2] and [Au2(PMe2Ar')]+, and (3) recombination of [Au2(CH3)(PMe2Ar')][NTf2] and Au(CH3)(PMe2Ar') to eliminate ethane.

Redox activity of a dissymmetric ligand bridging divalent ytter-bium and reactive nickel fragments

The reaction of a reactive nickel dimethyl 1 bearing a redox-active, dissymmetric ligand, which is obtained by deprotonation of 2-pyrimidin-2-yl-1H-benzimidazole (Hbimpm) with a divalent lanthanide complex, Cp*₂Yb(OEt₂), affords an unprecedented, trimeric 2 with C(sp³)-C(sp³) bond formation between two ligands in an exo position. Meanwhile, the transient, dimeric species 3 can be isolated with the same ligand coupling fashion, however, with a drastic distorsion angle of the bimpm ligand and reactive NiMe₂ fragment, revealing the possible mechanism of this rearrangement. A much more stable dimeric congener, 5, with an exo ligand coupling, is synthesized in the presence of 18-crown-6, which captures the potassium counter ion. The C-C coupling formation between two bimpm ligands results from the effective electron transfer from divalent lanthanide fragments. Without the divalent lanthanide, the reductive coupling occurs on a different carbon of the ligand, nicely showing the modulation of the spin density induced by the presence of the lanthanide ion. The electronic structures of these complexes are investigated by magnetic study (SQUID), indicating a ²F_7/2 ground state for each ytterbium in all the heterometallics. This work firstly reports ligand coupling reactivity in a redox-active, yet dissymmetric system with divalent organolanthanides, and the reactive nickel moiety can impact the intriguing transition towards a stable homoleptic, trinulear lanthanide species.

Dialkyl Ether Formation at High-Valent Nickel

In this article, we investigated the I2-promoted cyclic dialkyl ether formation from 6-membered oxanickelacycles originally reported by Hillhouse. A detailed mechanistic investigation based on spectroscopic and crystallographic analysis revealed that a putative reductive elimination to forge C(sp3)-OC(sp3) using I2 might not be operative. We isolated a paramagnetic bimetallic NiIII intermediate featuring a unique Ni2(OR)2 (OR = alkoxide) diamond-like core complemented by a μ-iodo bridge between the two Ni centers, which remains stable at low temperatures, thus permitting its characterization by NMR, EPR, X-ray, and HRMS. At higher temperatures (>-10 °C), such bimetallic intermediate thermally decomposes to afford large amounts of elimination products together with iodoalkanols. Observation of the latter suggests that a C(sp3)-I bond reductive elimination occurs preferentially to any other challenging C-O bond reductive elimination. Formation of cyclized THF rings is then believed to occur through cyclization of an alcohol/alkoxide to the recently forged C(sp3)-I bond. The results of this article indicate that the use of F+ oxidants permits the challenging C(sp3)-OC(sp3) bond formation at a high-valent nickel center to proceed in good yields while minimizing deleterious elimination reactions. Preliminary investigations suggest the involvement of a high-valent bimetallic NiIII intermediate which rapidly extrudes the C-O bond product at remarkably low temperatures. The new set of conditions permitted the elusive synthesis of diethyl ether through reductive elimination, a remarkable feature currently beyond the scope of Ni.

Mechanisms of Nickel-Catalyzed Coupling Reactions and Applications in Alkene Functionalization

Nickel complexes exhibit distinct properties from other group 10 metals, including a small nuclear radius, high paring energy, low electronegativity, and low redox potentials. These properties enable Ni catalysts to accommodate and stabilize paramagnetic intermediates, access radical pathways, and undergo slow β-H elimination. Our research program investigates how each of these fundamental attributes impact the catalytic properties of Ni, in particular in the context of alkene functionalization.Alkenes are versatile functional groups, but stereoselective carbofunctionalization reactions of alkenes have been underdeveloped. This challenge may derive from the difficulty of controlling selectivity via traditional two-electron migratory insertion pathways. Ni catalysts could lead to different stereodetermining steps via radical mechanisms, allowing access to molecular scaffolds that are otherwise difficult to prepare. For example, an asymmetric alkene diarylation reaction developed by our group relies upon the radical properties of Ni(III) intermediates to control the enantioselectivity and give access to a library of chiral α,α,β-triarylethane molecules with biological activity.Mechanistic studies on a two-component reductive 1,2-difunctionalization reaction have shed light on the origin of the cross-electrophile selectivity, as C sp² and C sp³ electrophiles are independently activated at Ni(I) via two-electron and radical pathways, respectively. Catalyst reduction has been identified to be the turnover-limiting step in this system. A closer investigation of the radical formation step using a (Xantphos)Ni(I)Ar model complex reveals that Ni(I) initiates radical formation via a concerted halogen-abstraction pathway.The low redox potentials of Ni have allowed us to develop a reductive, trans-selective diene cyclization, wherein a classic two-electron mechanism operates on a Ni(I)/Ni(III) platform, accounting for the chemo- and stereoselectivity. This reaction has found applications in the efficient synthesis of pharmaceutically relevant molecules, such as 3,4-dimethylgababutin.The tendency of Ni to undergo one-electron redox processes prompted us to explore dinuclear Ni-mediated bond formations. These studies provide insight into Ni-Ni bonding and how two metal centers react cooperatively to promote C-C, C-X, and N-N bond forming reductive elimination.Finally, isolation of β-agostic Ni and Pd complexes has allowed for X-ray and neutron diffraction characterization of these highly reactive molecules. The bonding parameters serve as unambiguous evidence for β-agostic interactions and help rationalize the slower β-H elimination at Ni relative to Pd. Overall, our research has elucidated the fundamental properties of Ni complexes in several contexts. Greater mechanistic understanding facilitates catalyst design and helps rationalize the reactivity and selectivity in Ni-catalyzed alkene functionalization reactions.

High-Valent NiIII and NiIV Species Relevant to C-C and C-Heteroatom Cross-Coupling Reactions: State of the Art

Ni catalysis constitutes an active research arena with notable applications in diverse fields. By analogy with its parent element palladium, Ni catalysts provide an appealing entry to build molecular complexity via cross-coupling reactions. While Pd catalysts typically involve a M⁰/M^II redox scenario, in the case of Ni congeners the mechanistic elucidation becomes more challenging due to their innate properties (like enhanced reactivity, propensity to undergo single electron transformations vs. 2e⁻ redox sequences or weaker M–Ligand interaction). In recent years, mechanistic studies have demonstrated the participation of high-valent Ni^III and Ni^IV species in a plethora of cross-coupling events, thus accessing novel synthetic schemes and unprecedented transformations. This comprehensive review collects the main contributions effected within this topic, and focuses on the key role of isolated and/or spectroscopically identified Ni^III and Ni^IV complexes. Amongst other transformations, the resulting Ni^III and Ni^IV compounds have efficiently accomplished: i) C–C and C–heteroatom bond formation; ii) C–H bond functionalization; and iii) N–N and C–N cyclizative couplings to forge heterocycles.

Influence of initiating groups on phosphino-phenolate nickel catalyzed ethylene (co)polymerization

The influence of initiating groups on late transition metal catalyzed ethylene (co)polymerization was comprehensively studied for the first time.

Aerobic C-C and C-O bond formation reactions mediated by high-valent nickel species

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. While Ni(0), Ni(i), and Ni(ii) intermediates are most relevant in these transformations, recently Ni(iii) and Ni(iv) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni(ii) and Ni(iii) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni(ii) complexes with various other oxidants led to exclusive C-C bond formation in very good yields, the use of O₂ or H₂O₂ as oxidants led to formation of appreciable amounts of C-O bond formation products, especially for the Ni(ii) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.

Nickel(I) Aryl Species: Synthesis, Properties, and Catalytic Activity

In this work, Ni(I) aryl species that are directly relevant to cross-coupling have been synthesized. Transmetalation of (dppf)Ni^IX (dppf = 1,1'-bis(diphenylphosphino)-ferrocene, X = Cl, Br) with aryl Grignard reagents or aryl boronic acids in the presence of base produces Ni(I) aryl species of the form (dppf)Ni^I(Ar) (Ar = Ph, o-tolyl, 2,6-xylyl, 2,4,6-mesityl, 2,4,6-ⁱPr₃C₆H₂). The stability of the Ni(I) aryl species is inversely correlated to the steric bulk on the aryl ligand. The most unstable Ni(I) aryl species are the most active precatalysts for Suzuki-Miyaura reactions because they rapidly decompose to generate the active Ni(0) catalyst. This study shows that Ni(I) aryl species are initially formed in the activation of Ni(I) halide precatalysts for Suzuki-Miyaura reactions and establishes their stoichiometric and catalytic reactivity profile.

C-H Bond Trifluoromethylation of Arenes Enabled by a Robust, High-Valent Nickel(IV) Complex

The robust, high-valent Ni^IV complex [(Py)₂ Ni^IV F₂ (CF₃ )₂ ] (Py=pyridine) was synthesized and fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analysis. It reacts with aromatic compounds at 25 °C to form the corresponding benzotrifluorides in nearly quantitative yield. The monomeric and dimeric Ni^III CF₃ complexes 2⋅Py and 2 were identified as key intermediates, and their structures were unambiguously determined by EPR spectroscopy and X-ray diffraction. Preliminary kinetic studies in combination with the isolation of reaction intermediates confirmed that the C-H bond-breaking/C-CF₃ bond-forming sequence can occur both at Ni^IV CF₃ and Ni^III CF₃ centers.