Bond-forming Reductive Elimination Research Articles

Cobalt-Catalyzed Intramolecular Hydroacylation Involving Cyclopropane Cleavage.

A simple cobalt-diphosphine catalyst has been found to efficiently promote intramolecular cyclization of ortho-cyclopropylvinyl- and cyclopropylidenemethyl-substituted benzaldehydes into benzocyclooctadienone and benzocycloheptadienone derivatives, respectively. This ring-opening hydroacylation likely involves aldehyde C-H oxidative addition, olefin insertion, cyclopropane cleavage by β-carbon elimination, and C-C bond-forming reductive elimination, as was supported by mechanistic experiments and DFT calculations.

Correlation between the C-C Cross-Coupling Activity and C-to-Ni Charge Transfer Transition of High-Valent Ni Complexes.

High-valent Ni complexes have proven to be good platforms for diverse cross-coupling reactions that are otherwise difficult to be achieved with conventional low-valent catalysts. However, their reductive elimination (RE) activities are still significantly variable by up to 5 orders of magnitude, depending on the supporting ligand and oxidation state of the Ni center. To elucidate frontier molecular orbitals (FMOs) that determine the RE activity of the Ni center, the electronic structures of cycloneophyl (CH2C(CH3)2-o-C6H4) NiIII and NiIV complexes have been characterized by utilizing various transition metal-based spectroscopic techniques such as electronic absorption, magnetic circular dichroism, electron paramagnetic resonance, resonance Raman, and X-ray absorption spectroscopies. In combination with density functional theory computations, the spectroscopic analyses have shown that the energies of the C-to-Ni charge-transfer (CT) electronic transitions are strongly correlated to the rates of C-C bond-forming RE reaction. This correlation suggests that the kinetic barrier of the RE reaction is determined by energy cost for internal CT (ICT) from the coordinated carbon moiety to the Ni center, and that FMOs involved in the RE reaction and the C-to-Ni CT electronic transitions are essentially identical. This FMO determination has led us to discover that photoexcitation to the C-to-Ni CT excited states accelerates the C-C cross-coupling reaction by up to 105 times, as the CT electronic transition can substitute for the rate-determining ICT step of the RE reaction at the ground electronic state.

Correction: Pd-Catalyzed oxidative isomerization of propargylic acetates: highly efficient access to α-acetoxyenones via alkenyl Csp2–O bond-forming reductive elimination from PdIV

Correction for 'Pd-Catalyzed oxidative isomerization of propargylic acetates: highly efficient access to α-acetoxyenones via alkenyl Csp2-O bond-forming reductive elimination from PdIV' by Jun Li et al., Chem. Commun., 2016, 52, 10644-10647, DOI: 10.1039/C6CC04463H.

Oxidatively Induced Aryl–CF3 Coupling at Diphosphine Nickel Complexes

This communication describes the synthesis of a series of diphosphine NiII(Ph)(CF3) complexes and studies of their reactivity toward oxidatively induced Ph–CF3 bond-forming reductive elimination. T...

Cross-Coupling of Aryl Trifluoromethyl Sulfones with Arylboronates by Cooperative Palladium/Rhodium Catalysis.

The Suzuki-Miyaura arylation of aryl trifluoromethyl sulfones via C-SO2 bond cleavage has been developed by means of cooperative palladium/rhodium catalysis. A series of aryl trifluoromethyl sulfones and arylboronic acid neopentylglycol esters are converted to the corresponding biaryls. Mechanistic investigations suggest that (1) the rhodium catalyst mediates the transfer of the aryl ring from arylboronate to palladium, resulting in the acceleration of the transmetalation step, and (2) the C-C bond-forming reductive elimination step is the turnover-limiting step.

Aryl-Fluoride Bond-Forming Reductive Elimination from Nickel(IV) Centers.

The treatment of pyridine- and pyrazole-ligated NiII σ-aryl complexes with Selectfluor results in C(sp2)-F bond formation under mild conditions. With appropriate design of supporting ligands, diamagnetic NiIV σ-aryl fluoride intermediates can be detected spectroscopically and/or isolated during these transformations. These studies demonstrate for the first time that NiIV σ-aryl fluoride complexes participate in challenging C(sp2)-F bond-forming reductive elimination to yield aryl fluoride products.

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes.

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp3 )-C and C(sp3 )-O bond-forming reactions at high-valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral NiIV complex ligated by tris(pyrazolyl)borate and a cationic octahedral NiIII complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp3 )-C(sp2 ) bond-forming reductive elimination occurs from both centers, but the NiIII complex reacts up to 300-fold faster than the NiIV , depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO- ), the NiIV complex exclusively undergoes C(sp3 )-OAc bond formation, while the NiIII analogue forms the C(sp3 )-C(sp2 ) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M-C(sp3 ) bonds, and thus their relative reactivity towards outer-sphere SN 2-type bond-forming reactions.

Regio- and Diastereoselective Iron-Catalyzed [4+4]-Cycloaddition of 1,3-Dienes.

A family of single-component iron precatalysts for the [4+4]-cyclodimerization and intermolecular cross-[4+4]-cycloaddition of monosubstituted 1,3-dienes is described. Cyclooctadiene products were obtained with high regioselectivity, and catalyst-controlled access to either cis- or trans-diastereomers was achieved using 4-substituted diene substrates. Reactions conducted either with single-component precatalysts or with iron dihalide complexes activated in situ proved compatible with common organic functional groups and were applied on multigram scale (up to >100 g). Catalytically relevant, S = 1 iron complexes bearing 2-(imino)pyridine ligands, (RPI)FeL2 (RPI = [2-(2,6-R2-C6H3-N═CMe)-C5H4N] where R = iPr or Me, L2 = bis-olefin), were characterized by single-crystal X-ray diffraction, Mößbauer spectroscopy, magnetic measurements, and DFT calculations. The structural and spectroscopic parameters are consistent with an electronic structure description comprised of a high spin iron(I) center ( SFe = 3/2) engaged in antiferromagnetically coupling with a ligand radical anion ( SPI = -1/2). Mechanistic studies conducted with these single-component precatalysts, including kinetic analyses, 12C/13C isotope effect measurements, and in situ Mößbauer spectroscopy, support a mechanism involving oxidative cyclization of two dienes that determines regio- and diastereoselectivity. Topographic steric maps derived from crystallographic data provided insights into the basis for the catalyst control through stereoselective oxidative cyclization and subsequent, stereospecific allyl-isomerization and C-C bond-forming reductive elimination.

Csp3-Csp3 Bond-Forming Reductive Elimination from Well-Defined Copper(III) Complexes.

Carbon-carbon bond-forming reductive elimination from elusive organocopper(III) complexes has been considered the key step in many copper-catalyzed and organocuprate reactions. However, organocopper(III) complexes with well-defined structures that can undergo reductive elimination are extremely rare, especially for the formation of Csp3-Csp3 bonds. We report herein a general method for the synthesis of a series of [alkyl-CuIII-(CF3)3]- complexes, the structures of which have been unequivocally characterized by NMR spectroscopy, mass spectrometry, and X-ray crystal diffraction. At elevated temperature, these complexes undergo reductive elimination following first-order kinetics, forming alkyl-CF3 products with good yields (up to 91%). Both kinetic studies and DFT calculations indicate that the reductive elimination to form Csp3-CF3 bonds proceeds through a concerted transition state, with a Δ H⧧ = 20 kcal/mol barrier.

Aryl–CF3 Coupling from Phosphinoferrocene-Ligated Palladium(II) Complexes

This article describes a detailed investigation of ligand effects on Ph–CF3 coupling from phosphinoferrocene-ligated PdII(Ph)(CF3) complexes. This study reveals that increasing the size of the phosphine substituents results in an enhanced rate of Ph–CF3 coupling, with (DtBPF)Pd(Ph)(CF3) (DtBPF = 1,1′-bis(di-tert-butylphosphino)ferrocene) being the most reactive complex. The mechanism of Ph–CF3 bond formation from both (DtBPF)Pd(Ph)(CF3) and (DiPrPF)Pd(Ph)(CF3) (DiPrPF = 1,1′-bis(diisopropylphosphino)ferrocene) was interrogated experimentally and computationally. These studies implicate a pathway involving concerted Ph–CF3 bond-forming reductive elimination from the four-coordinate PdII centers. An alternative pathway involving α-fluoride elimination and subsequent PhF2C–F coupling from PdII(CF2Ph)(F) intermediates was also evaluated but was ruled out based on DFT as well as the independent synthesis and reactivity studies of (DiPrPF)Pd(CF2Ph)(F).

Carbon(sp3)-nitrogen bond-forming reductive elimination from phosphine-ligated alkylpalladium(II) amide complexes: A DFT study

DFT methods were employed to investigate C(sp3)-N bond-formation via reductive elimination from alkylpalladium(II) amide complexes. The hemi-lability of an ortho-methoxy substituent is computed to have minimal impact on reductive elimination barriers. In general, for both anilide and phosphine substituents, their steric impact is more substantial than electronic/Hammett factors. β-Hydrogen elimination is competitive with reductive elimination while β-methyl elimination is much less favorable. For phosphine-ligated Pd(II) amide complexes, the solvent impact on reductive elimination free energy barriers is small, and overall the substituent effects on either the phosphine or anilide ligand are subtle.

Pd-Catalyzed Alkene Diamination Reactions of Nitrogen Electrophiles: Synthesis of Cyclic Guanidines and Ureas Bearing Dialkylaminomethyl Groups.

The Pd-catalyzed coupling of N-allylguanidines or N-allylureas with O-benzoylhydroxylamine derivatives affords cyclic guanidines or cyclic ureas bearing dialkylaminomethyl groups. The desired products are obtained in good yield, and substrates bearing substituents at the allylic position are transformed with moderate diastereoselectivity. The mechanism of these reactions appears to involve anti-aminopalladation of the alkene, followed by a rare sp3C-sp3N bond-forming reductive elimination from an alkylpalladium complex that contains β-hydrogen atoms.

Carbon–Oxygen Bond Forming Reductive Elimination from Cycloplatinated(IV) Complexes

The synthesis of the two cycloplatinated(IV) complexes [PtMe(OAc)2(C∧N)(H2O)] (C∧N = 2-phenylpyridinate (2a), benzo[h]quinolate, 2b) by reaction of [PtMe(C∧N)(SMe2)] with PhI(OAc)2 is described. Complexes 2 undergo carbon–oxygen bond forming reductive elimination instead of C–C reductive elimination to produce MeOAc as protected methanol. The kinetics and mechanism of both Pt–O bond formation and C–O reductive elimination have been experimentally and theoretically investigated. The results suggest that formation of methyl acetate proceeds via nucleophilic attack of the dissociated acetate ligand at the methyl group carbon in a cationic five-coordinate intermediate cycloplatinated(IV) complex.

Copper-Catalysed Aminoboration of Vinylarenes with Hydroxylamine Esters-A Computational Mechanistic Study.

An in-depth computational probe of the copper-mediated formal aminoboration of β-alkylstyrenes with bis(pinacolato)diboron B2 pin2 and an archetype hydroxylamine ester by a dppbz-ligated {P^P}CuI boryl catalyst (dppbz≡{P^P}≡1,2-bis(diphenylphosphino)benzene) is presented. This first comprehensive computational study of the copper-mediated formal aminoboration utilising an electrophilic strategy has identified the most accessible pathway for productive catalysis. The mechanistic picture derived from smooth energy profiles acquired by employing a reliable computational protocol applied to a realistic catalyst model conforms to all available experimental data. The high degree of regio- and stereoselectivity achieved in syn-borylcupration and Umpolung electrophilic amination is instrumental to the exclusive generation of the (syn)-β-aminoalkylborane product. On the one hand, syn-borylcupration furnishes exclusively β-borylalkylcopper nucleophile upon boryl addition onto the vinylarene β-carbon. Its subsequent approach by the hydroxylamine electrophile to deliver the product with the release of {P^P}CuI benzoate favours a stepwise stereoretentive SN 2-type oxidative addition/N-C bond-forming reductive elimination sequence. The copper benzoate species represents the catalyst resting state, and its transformation into the catalytically active borylcopper species upon salt metathesis with Li(OtBu) base and transmetallation with B2 pin2 is turnover limiting. Electronically modified β-alkylstyrenes featuring a para-CF3 substituted phenyl ring render the borylcupration faster, and more electron-rich hydroxylamine agents decelerate the electrophilic amination.

Theoretical Study on Ruthenium-Catalyzed Hydrocarbamoylative Cyclization of 1,6-Diyne with Dimethylformamide

The reaction mechanism of ruthenium-catalyzed hydrocarbamoylative cyclization of diynes with dimethylformamide was studied by computation of model complexes using the density functional theory (DFT) method. Based on the DFT calculations, a plausible mechanism is proposed, involving oxidative coupling of a diyne catalyzed by CpRuH (Cp = η5-C5H5), C–H bond-forming reductive elimination, dimethylformamide insertion, and β-H elimination.

Palladium-Catalyzed Hydrohalogenation of 1,6-Enynes: Hydrogen Halide Salts and Alkyl Halides as Convenient HX Surrogates.

Difficulties associated with handling H2 and CO in metal-catalyzed processes have led to the development of chemical surrogates to these species. Despite many successful examples using this strategy, the application of convenient hydrogen halide (HX) surrogates in catalysis has lagged behind considerably. We now report the use of ammonium halides as HX surrogates to accomplish a Pd-catalyzed hydrohalogenation of enynes. These safe and practical salts avoid many drawbacks associated with traditional HX sources including toxicity and corrosiveness. Experimental and computational studies support a reaction mechanism involving a crucial E-to-Z vinyl-Pd isomerization and a carbon-halogen bond-forming reductive elimination. Furthermore, rare examples of C(sp3)-Br and -Cl reductive elimination from Pd(II) as well as transfer hydroiodination using 1-iodobutane as an alternate HI surrogate are also presented.

Computational study of C(sp3)-O bond formation at a PdIV centre.

This report describes a computational study of C(sp3)-OR bond formation from PdIV complexes of general structure PdIV(CH2CMe2-o-C6H4-C,C')(F)(OR)(bpy-N,N') (bpy = 2,2'-bipyridine). Dissociation of -OR from the different octahedral PdIV starting materials results in a common square-pyramidal PdIV cation. An SN2-type attack by -OR (-OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp3)-OR bond formation. In contrast, when -OR = triflate, concerted C(sp3)-C(sp2) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and SN2 steps with different -OR follow opposing trends. The SN2 transition states exhibit "PdCO" angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally.

An Organometalllic NiIV Complex That Participates in Competing Transmetalation and C(sp2)–O Bond-Forming Reductive Elimination Reactions

This communication describes the synthesis of an organometallic NiIV complex bearing a labile trifluoroacetate (OTFA) ligand via the oxidation of a NiII precursor with PhI(OTFA)2. Intramolecular C(sp2)–O bond-forming reductive elimination from this NiIV complex is relatively slow, requiring 6 h at 70 °C to reach completion. In contrast, transmetalation with TMSCF3 occurs within just 1 h at room temperature to generate a NiIV–CF3 complex. These studies show that intermolecular reactions such as transmetalation can be competitive with intramolecular reductive elimination processes at NiIV centers.

Carbon-Carbon Bond-Forming Reductive Elimination from Isolated Nickel(III) Complexes.

This manuscript describes the design, synthesis, characterization, and reactivity studies of organometallic NiIII complexes of general structure TpNiIII(R)(R1) (Tp = tris(pyrazolyl)borate). With appropriate selection of the R and R1 ligands, the complexes are stable at room temperature and can be characterized by cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. Upon heating, many of these NiIII compounds undergo C(sp2)-C(sp2) or C(sp3)-C(sp2) bond-forming reactions that are challenging at lower oxidation states of nickel.

N-N Bond Forming Reductive Elimination via a Mixed-Valent Nickel(II)-Nickel(III) Intermediate.

Natural products containing N-N bonds exhibit important biological activity. Current methods for constructing N-N bonds have limited scope. An advanced understanding of the fundamental N-N bond formation/cleavage processes occurring at the transition-metal center would facilitate the development of catalytic reactions. Herein we present an N-N bond-forming reductive elimination, which proceeds via a mixed-valent Ni(II) -Ni(III) intermediate with a Ni-Ni bond order of zero. The discrete Ni(II) -Ni(III) oxidation states contrast with the cationic dimeric Ni analogue, in which both Ni centers are equivalent with an oxidation state of 2.5. The electronic structures of these mixed-valent complexes have implications for the fundamental understanding of metal-metal bonding interactions.