Oxidative Addition Of Aryl Halides Research Articles

Mapping the mechanisms of oxidative addition in cross-coupling reactions catalysed by phosphine-ligated Ni(0).

The complexes of first-row transition metals can undergo elementary reactions by multiple pathways due to their propensity to undergo both one- and two-electron redox steps. Classic and recent studies of the oxidative addition of aryl halides to Ni(0)-a common step in widely practised cross-coupling processes-have yielded contradictory conclusions about stepwise, radical versus concerted mechanisms, but such information is crucial to the design of catalysts based on earth-abundant metals. Here we show that the oxidative addition of aryl halides to Ni(0) ligated by monophosphines occurs by both mechanisms and delineate how the branching of radical and non-radical pathways depends on the electronic properties of both the ligand and reactant arene as well as the identity of the halide. The one-electron pathway occurs by outer-sphere electron transfer to form an aryl radical rather than the often-proposed halogen atom transfer.

Ligand Combination Approach in Pd‐Catalyzed Direct C−H Arylation of Benzothiazole and Benzoxazole

AbstractThe use of ligand combination approach in the Pd catalyzed direct C−H arylation of benzothiazole and benzoxazole is demonstrated. The combination of two ligands phenanthroline monohydrate (Phen.H2O) and triphenylphosphine (PPh3) with Pd‐catalyst have offered the best catalytic properties in the C−H bond arylation reactions than either of two ligands employed separately in the reactions. This protocol exhibits a wide substrate scope affording C2 arylated benzothiazole and benzoxazole derivatives in excellent yields. Mechanistic studies including kinetic isotope effect (KIE), H/D exchange and competition experiments show that the C−H bond activation of benzothiazole and benzoxazole is very facile under reaction conditions. Furthermore, different mechanistic pathways originated from the catalytically active species such as Pd(η2‐dba)L (dba=dibenzylidene acetone) and Pd(PPh3)2 were investigated using computational methods. The computational results suggest that the generation of catalytically active species from Pd2(dba)3 is endoergic process and Phen ligand can initiate this step. On the other hand, the presence of PPh3 could be crucial for the oxidative addition of aryl halides which is the rate‐determining step of the reaction.

Understanding Formation and Roles of NiII Aryl Amido and NiIII Aryl Amido Intermediates in Ni-Catalyzed Electrochemical Aryl Amination Reactions.

Ni-catalyzed electrochemical aryl amination (e-amination) is an attractive, emerging approach to building C-N bonds. Here, we report in-depth experimental and computational studies that examined the mechanism of Ni-catalyzed e-amination reactions. Key NiII-amine dibromide and NiII aryl amido intermediates were chemically synthesized and characterized. The combination of experiments and DFT calculations suggest (1) there is coordination of an amine to the NiII catalyst before the cathodic reduction and oxidative addition steps, (2) a stable NiII aryl amido intermediate is produced from the cathodic half-reaction, a critical step in controlling the selectivity between cross-coupling and undesired homo-coupling reaction pathways, (3) the diazabicycloundecene additive shifts the aryl halide oxidative addition mechanism from a NiI-based pathway to a Ni0-based pathway, and (4) redox-active bromide in the supporting electrolyte functions as a redox mediator to promote the oxidation of the stable NiII aryl amido intermediate to a NiIII aryl amido intermediate. Subsequently, the NiIII aryl amido intermediate undergoes facile reductive elimination to provide a C-N cross-coupling product at room temperature. Overall, our results provide new fundamental understandings about this e-amination reaction and guidance for further development of other Ni-catalyzed electrosynthetic reactions such as C-C and C-O cross-couplings.

Experimental and computational insights into the mechanism of the copper(i)-catalysed sulfonylative Suzuki-Miyaura reaction.

A mechanistic study into the copper(i)-catalysed sulfonylative Suzuki-Miyaura reaction, incorporating sulfur dioxide, is described. Utilising spectroscopic and computational techniques, an exploration into the individual components of the competing catalytic cycles is delineated, including identification of the resting state catalyst, transmetalation of arylboronic acid onto copper(i), the sulfur dioxide insertion process, and the oxidative addition of aryl halide to CuI. Studies also investigated prominent side-reactions which were uncovered, including a competing copper(ii)-catalysed mechanism. This led to an additional proposed and connected CuI/CuII/CuIII catalytic cycle to account for by-product formation.

Oxidative Addition of Aryl Halides to a Ni(I)-Bipyridine Complex.

The oxidative addition of aryl halides to bipyridine- or phenanthroline-ligated nickel(I) is a commonly proposed step in nickel catalysis. However, there is a scarcity of complexes of this type that both are well-defined and undergo oxidative addition with aryl halides, hampering organometallic studies of this process. We report the synthesis of a well-defined Ni(I) complex, [(CO2Etbpy)NiICl]4 (1). Its solution-phase speciation is characterized by a significant population of monomer and a redox equilibrium that can be perturbed by π-acceptors and σ-donors. 1 reacts readily with aryl bromides, and mechanistic studies are consistent with a pathway proceeding through an initial Ni(I) → Ni(III) oxidative addition to form a Ni(III) aryl species. Such a process was demonstrated stoichiometrically for the first time, affording a structurally characterized Ni(III) aryl complex.

DFT reveals the support effects in Pd nanoclusters over defect-ridden graphene for the oxidative addition of bromobenzene

Oxidative addition of aryl halides is an elementary and the rate limiting step common to several cross-coupling reactions. Metal complex-based homogeneous catalysts are conventionally used for catalysing such reactions. In search for heterogeneous catalysts, graphene has been widely used as a support for immobilising catalytically active Pd nanoparticles or clusters. In this study, we investigated the activities of small Pdn (n = 3, 4) clusters immobilised over pristine and defected graphenes for the oxidative addition of bromobenzene in the presence of water solvation. We compared the adsorption energetics and activation barriers of oxidative addition for a series of graphene supports and contrasted them with homogeneous free Pdn catalysts. Our theoretical investigations reveal the defected graphene supports to act as charge donors thereby drastically reducing the activation barriers of oxidative addition of bromobenzene when compared to free Pdn clusters. Double vacancy defected graphene and B-doped graphene were concluded as the best support materials providing stronger traps for small Pdn (n = 3, 4) clusters and reducing the oxidative addition barriers, making them potential heterogeneous catalysts for cross-coupling reactions.

Mechanistic insight into deep holes from interband transitions in Palladium nanoparticle photocatalysts

SummaryUtilizing hot electrons generated from localized surface plasmon resonance is of widespread interest in the photocatalysis of metallic nanoparticles. However, hot holes, especially generated from interband transitions, have not been fully explored for photocatalysis yet. In this study, a photocatalyzed Suzuki-Miyaura reaction using mesoporous Pd nanoparticle photocatalyst served as a model to study the role of hot holes. Quantum yields of the photocatalysts increase under shorter wavelength excitations and correlate to “deeper” energy of the holes from the Fermi level. This work suggests that deeper holes in the d-band catalyze the oxidative addition of aryl halide R-X onto Pd0 at the nanoparticles' surface to form R-PdII-X complex, thus accelerating the rate-determining step of the catalytic cycle. The hot electrons do not play a decisive role. In the future, catalytic mechanisms induced by deep holes should deserve as much attention as the well-known hot electron transfer mechanism.

Copper(I) activation of C-X bonds: bimolecular vs. unimolecular reaction mechanism.

Experimental kinetic studies and DFT calculations show that the oxidative addition of aryl halides (Ar-X) to complexes [Cu(NHC)R] follow different paths depending on the nature of X. For X = Br a concerted addition leads to cis-[Cu(NHC)XRAr] from which the usual C-C coupled product Ar-R eliminates. However, for X = I trans-[Cu(NHC)IRAr] is formed instead, leading to the elimination of R-I in a metathesis reaction. This behaviour is accounted for by a change in the reaction mechanism for Ar-I, which involves two molecules of copper(I) complex, the second one stabilising the incipient iodide formed in the C-I breaking (oxidative addition) and C-I forming (reductive elimination) processes.

Nickel-catalyzed electrochemical carboxylation of unactivated aryl and alkyl halides with CO2

Electrochemical catalytic reductive cross couplings are powerful and sustainable methods to construct C−C bonds by using electron as the clean reductant. However, activated substrates are used in most cases. Herein, we report a general and practical electro-reductive Ni-catalytic system, realizing the electrocatalytic carboxylation of unactivated aryl chlorides and alkyl bromides with CO2. A variety of unactivated aryl bromides, iodides and sulfonates can also undergo such a reaction smoothly. Notably, we also realize the catalytic electrochemical carboxylation of aryl (pseudo)halides with CO2 avoiding the use of sacrificial electrodes. Moreover, this sustainable and economic strategy with electron as the clean reductant features mild conditions, inexpensive catalyst, safe and cheap electrodes, good functional group tolerance and broad substrate scope. Mechanistic investigations indicate that the reaction might proceed via oxidative addition of aryl halides to Ni(0) complex, the reduction of aryl-Ni(II) adduct to the Ni(I) species and following carboxylation with CO2.

Cover Feature: Computational Study of Bridge Splitting, Aryl Halide Oxidative Addition to PtII, and Reductive Elimination from PtIV: Route to Pincer‐PtII Reagents with Chemical and Biological Applications (Chem. Eur. J. 62/2021)

Density functional theory studies of bridge-splitting of [PtIIR2(SEt2)]2 (R = 4-tolyl) by bromoarenes containing nitrogen donor groups, subsequent intramolecular coordination assisted oxidative addition, and reductive elimination elucidate the mechanism for formation of mer-PtII(N,C,N)Br pincers via PtIV, where the pincers have applications in fundamental and applied chemistry. While the research covers the breaking of bridges, it has built a collaborative bridge between Utrecht and Hobart that has stood for over 30 years. More information can be found in the Full Paper by A. J. Canty et al. (DOI: 10.1002/chem.202102687).

Computational Study of Bridge Splitting, Aryl Halide Oxidative Addition to PtII , and Reductive Elimination from PtIV : Route to Pincer-PtII Reagents with Chemical and Biological Applications.

Density functional theory computation indicates that bridge splitting of [PtII R2 (μ-SEt2 )]2 proceeds by partial dissociation to form R2 Pta (μ-SEt2 )Ptb R2 (SEt2 ), followed by coordination of N-donor bromoarenes (L-Br) at Pta leading to release of Ptb R2 (SEt2 ), which reacts with a second molecule of L-Br, providing two molecules of PtR2 (SEt2 )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH2 )2 C6 H3 Br (pz=pyrazol-1-yl) and 2,6-(Me2 NCH2 )2 C6 H3 Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via PtII Tol2 (L-N,Br) and reductive elimination from PtIV intermediates gives mer-PtII (L-N,C,N)Br and Tol2 . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH2 )2 C6 H3 Br, a stable PtIV product, fac-PtIV Me2 {2,6-(pzCH2 )2 C6 H3 -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable PtIV Tol2 {L-N,C,N}Br undergoing facile Tol2 reductive elimination. The mechanisms reported herein enable the synthesis of PtII pincer reagents with applications in materials and bio-organometallic chemistry.

Cooperative Bond Activation and Facile Intramolecular Aryl Transfer of Nickel–Aluminum Pincer‐type Complexes

AbstractPincer‐type nickel–aluminum complexes were synthesized using two equivalents of the phosphinoamide, [PhNCH2PiPr2]−. The Ni0–AlIII complexes, {(MesPAlP)Ni}2(μ‐N2) and {(MesPAlP)Ni}2(μ‐COD), where MesPAlP is (Mes)Al(NPhCH2PiPr2)2, were structurally characterized. The (PAlP)Ni system exhibited cooperative bond cleavage mediated by the two‐site Ni–Al unit, including oxidative addition of aryl halides, H2 activation, and ortho‐directed C−H bond activation of pyridine N‐oxide. One intriguing reaction is the reversible intramolecular transfer of the mesityl ring from the Al to the Ni site, which is evocative of the transmetalation step during cross‐coupling catalysis. The aryl‐transfer product,(THF)Al(NPhCH2PiPr2)2Ni(Mes), is the first example of a first‐row transition metal–aluminyl pincer complex. The addition of a judicious donor enables the Al metalloligand to convert reversibly between the alane and aluminyl forms via aryl group transfer to and from Ni, respectively. Theoretical calculations support a zwitterionic Niδ−–Alδ+ electronic structure in the nickel–aluminyl complex.

Cooperative Bond Activation and Facile Intramolecular Aryl Transfer of Nickel-Aluminum Pincer-type Complexes.

Pincer-type nickel-aluminum complexes were synthesized using two equivalents of the phosphinoamide, [PhNCH2 Pi Pr2 ]- . The Ni0 -AlIII complexes, {(Mes PAlP)Ni}2 (μ-N2 ) and {(Mes PAlP)Ni}2 (μ-COD), where Mes PAlP is (Mes)Al(NPhCH2 Pi Pr2 )2 , were structurally characterized. The (PAlP)Ni system exhibited cooperative bond cleavage mediated by the two-site Ni-Al unit, including oxidative addition of aryl halides, H2 activation, and ortho-directed C-H bond activation of pyridine N-oxide. One intriguing reaction is the reversible intramolecular transfer of the mesityl ring from the Al to the Ni site, which is evocative of the transmetalation step during cross-coupling catalysis. The aryl-transfer product,(THF)Al(NPhCH2 Pi Pr2 )2 Ni(Mes), is the first example of a first-row transition metal-aluminyl pincer complex. The addition of a judicious donor enables the Al metalloligand to convert reversibly between the alane and aluminyl forms via aryl group transfer to and from Ni, respectively. Theoretical calculations support a zwitterionic Niδ- -Alδ+ electronic structure in the nickel-aluminyl complex.

Oxidative Addition of Aryl and Alkyl Halides to a Reduced Iron Pincer Complex.

The two-electron oxidative addition of aryl and alkyl halides to a reduced iron dinitrogen complex with a strong-field tridentate pincer ligand has been demonstrated. Addition of iodobenzene or bromobenzene to (3,5-Me2MesCNC)Fe(N2)2 (3,5-Me2MesCNC = 2,6-(2,4,6-Me-C6H2-imidazol-2-ylidene)2-3,5-Me2-pyridine) resulted in rapid oxidative addition and formation of the diamagnetic, octahedral Fe(II) products (3,5-Me2MesCNC)Fe(Ph)(N2)(X), where X = I or Br. Competition experiments established the relative rate of oxidative addition of aryl halides as I > Br > Cl. A linear free energy of relative reaction rates of electronically differentiated aryl bromides (ρ = 1.5) was consistent with a concerted-type pathway. The oxidative addition of alkyl halides such as methyl-, isobutyl-, or neopentyl halides was also rapid at room temperature, but substrates with more accessible β-hydrogen positions (e.g., 1-bromobutane) underwent subsequent β-hydride elimination. Cyclization of an alkyl halide containing a radical clock and epimerization of neohexyl iodide-d2 upon oxidative addition to (3,5-Me2MesCNC)Fe(N2)2 are consistent with radical intermediates during C(sp3)-X bond cleavage. Importantly, while C(sp2)-X and C(sp3)-X oxidative addition produces net two-electron chemistry, the preferred pathway for obtaining the products is concerted and stepwise, respectively.

Confirmation of Suzuki-Miyaura Cross-Coupling Reaction Mechanism through Synthetic Architecture of Nanocatalysts.

Despite widespread use of heterogeneous Pd catalysts in Suzuki-Miyaura coupling reactions, detailed roles of Pd, especially the nature of its active species, are still a topic of controversial debate. While some studies showed an active surface of Pd nanoparticles or nanoclusters acting heterogeneously, others claimed soluble Pd species leached from the metallic Pd to be active species which are homogeneous in nature. Besides, within the homogeneous mechanism, how the Pd leaches and promotes the cross-coupling reaction is then another question that needs to be addressed. It could be envisioned that if the soluble Pd species and solid-phase Pd are physically separated, the mechanism of Suzuki-Miyaura coupling could then be confirmed through examining the catalytic activity in different reaction regions. Herein we use microporous Stöber silica as a membrane to separate the soluble Pd species from solid Pd and conduct size-selective reactions which allow the passage of leaching Pd species, but not of reactants or products larger than the membrane aperture. With this strategy, we have been able to differentiate the surface reaction from the solution cross-coupling. We find that the leached Pd species are the only genuine catalytic intermediate in the cross-coupling reactions. We also confirm that oxidative addition of aryl halides to the solid Pd leads to leaching of the soluble Pd species which is necessary to promote Suzuki-Miyaura reactions.

Mechanistic Studies of the Oxidative Addition of Aryl Halides to Ni(0) Centers Bearing Phosphine Ligands.

The oxidative addition of aryl halides is a common entry point in catalytic cycles for cross-coupling and related reactions. In the case of phosphine-supported nickel(0) fragments, the formation of reactive Ni(ii)-aryl products often competes with the production of Ni(i) species. Here, recent advances in the mechanistic understanding of these reactions are highlighted. In particular, the denticity of the supporting ligand has a significant influence on the outcome of the reaction.

Oxidative Addition of Aryl Halides to a Triphosphine Ni(0) Center to Form Pentacoordinate Ni(II) Aryl Species.

Oxidative addition of aryl halides to Ni(0) is a ubiquitous elementary step in cross-coupling and related reactions, usually producing a square-planar Ni(II)–aryl intermediate. Here we show that a triphosphine ligand supports oxidative addition at a tris-ligated Ni(0) center to cleanly form stable five-coordinate Ni(II)–aryl compounds. Kinetic and computational studies support a concerted, two-electron mechanism rather than radical halogen abstraction. These results support the idea that oxidative addition to triphosphine Ni(0) species may be more generally involved in Ni/phosphine catalytic systems.

Unusual Electronic Effects of Ancillary Ligands on the Perfluoroalkylation of Aryl Iodides and Bromides Mediated by Copper(I) Pentafluoroethyl Complexes of Substituted Bipyridines.

Several perfluoroalkylcopper compounds have been reported previously that serve as reagents or catalysts for the perfluoroalkylation of aryl halides. However, the relationships between the reactivity of such complexes and the electronic properties of the ancillary ligands are unknown, and such relationships are not well-known in general for copper complexes that mediate or catalyze cross coupling. We report the synthesis and characterization of a series of pentafluoroethylcopper(I) complexes ligated by bipyridine ligands possessing varied electronic properties. In contrast to the limited existing data on the reactivity of L2Cu(I)-X complexes bearing amine and pyridine-type ligands in Ullmann-type aminations with aryl halides, the reactions of aryl halides with pentafluoroethylcopper(I) complexes bearing systematically varied bipyridine ligands were faster for complexes bearing less electron-donating bipyridines than for complexes bearing more electron-donating bipyridines. Analysis of the rates of reaction and the relative populations of the neutral complexes [(R2bpy)CuC2F5] and ionic complexes [(R2bpy)2Cu][Cu(C2F5)2] formed by these reagents in solution suggests that this effect of electronics on the reaction rate results from an unusual trend of faster oxidative addition of aryl halides to [(R2bpy)CuC2F5] complexes containing less electron-donating R2bpy ligands than to those containing more electron-donating R2bpy ligands.

Intermolecular Oxidative Addition of Aryl Halides to Platinum(II) Alkyl Complexes

We report a well-defined example of intermolecular aryl halide oxidative addition (OA) to Pt(II). Complexes of the type (IMes)PtMe2(L) and (IMes′)PtMe(L) (L = SMe2, pyridine; IMes = N,N-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; IMes′ = cyclometalated IMes) undergo intermolecular OA of phenyl iodide (PhI) at 60 °C, producing toluene via reductive elimination from a proposed Pt(IV) phenyl species. Isolation of a model Pt(IV) OA product provides evidence for a Pt(II)/Pt(IV) pathway. The OA of PhI is not limited to Pt(II) IMes complexes; analogous reactions also proceed with phosphine-ligated Pt(II) dialkyl complexes, demonstrating that this reaction is feasible for a variety of electron-rich Pt(II) complexes bearing labile ligands.

Supramolecular Host-Selective Activation of Iodoarenes by Encapsulated Organometallics

Supramolecular hosts offer defined microenvironments that facilitate selective host-guest interactions, enabling reactivity that would otherwise be challenging in bulk solution. While impressive rate enhancements and selectivities have been reported, similar reactivity can often be accessed through modifications of reaction conditions even in the absence of the host. We report here an oxidative addition of aryl halides across the metal centers in Cu(I) and Pd(II) organometallics that is assisted by the presence of a supramolecular host, realized via electrostatic stabilization and increased local substrate concentrations. When reaction conditions were screened to assess background reactivity, alternative reactivity (typically decomposition) resulted, indicating that encapsulation led to host-selective reaction trajectories.