Reductive Elimination Step Research Articles

Theoretical study on the mechanism of Ni−Al bimetallic catalyzed dual C−H cyclization of amides and alkynes

The reaction mechanism of Ni−Al bimetallic catalyzed dual C−H cyclization of amides and internal alkynes has been studied theoretically. Four reactions were considered with N-phenylformamide (S1) and (2Ph)-N,N-bis(1-phenylethyl)formamide (S2) as amides, second phosphine oxide (SPO) and PMe3 as ligands, respectively. For two reactions with SPO as ligand and the reaction of S2 with PMe3, the reaction mechanism mainly includes formyl C(sp2)−H bond activation, phenyl C(sp2)−H or benzylic methyl C(sp3)−H bond activation, release of alkene, alkyne coordination and insertion, isomerization, and reductive elimination steps generating the cyclization product. The reaction of S1 with PMe3 proceeds via formyl C(sp2)−H bond activation, isomerization, and reductive elimination steps giving the alkyne addition product. In addition, the influence of substituent in SPO on reaction mechanism and the regio-selectivity of phenylpropyne in reaction have been calculated in detail.

Iridium-catalyzed hydroacylation reactions of C1-substituted oxabenzonorbornadienes with salicylaldehyde: an experimental and computational study.

An experimental and theoretical investigation on the iridium-catalyzed hydroacylation of C1-substituted oxabenzonorbornadienes with salicylaldehyde is reported. Utilizing commercially available [Ir(COD)Cl]2 in the presence of 5 M KOH in dioxane at 65 °C, provided a variety of hydroacylated bicyclic adducts in up to a 95% yield with complete stereo- and regioselectivity. The mechanism and origins of selectivity in the iridium-catalyzed hydroacylation reaction has been examined at the M06/Def2TZVP level of theory. The catalytic cycle consists of three key steps including oxidative addition into the aldehyde C–H bond, insertion of the olefin into the iridium hydride, and C–C bond-forming reductive elimination. Computational results indicate the origin of regioselectivity is involved in the reductive elimination step.

Mechanism and Stereochemistry of Rhodium-Catalyzed [5 + 2 + 1] Cycloaddition of Ene-Vinylcyclopropanes and Carbon Monoxide Revealed by Visual Kinetic Analysis and Quantum Chemical Calculations.

Previously, we developed a rhodium-catalyzed [5 + 2 + 1] cycloaddition of ene-vinylcyclopropanes (ene-VCPs) and carbon monoxide to synthesize eight-membered carbocycles. The efficiency of this reaction can be appreciated from its application in the synthesis of several natural products. Herein we report the results of a 15-year investigation into the mechanism of the [5 + 2 + 1] cycloaddition by applying visual kinetic analysis and high-level quantum chemical calculations at the DLPNO-CCSD(T)//BMK level. According to the kinetic measurements, the resting state of the catalyst possesses a dimeric structure (with two rhodium centers) whereas the active catalytic species is monomeric (with one rhodium center). The catalytic cycle consists of cyclopropane cleavage (the turnover-limiting step), alkene insertion, CO insertion, reductive elimination, and catalyst transfer steps. Other reaction pathways have also been considered but then have been ruled out. The steric origin of the diastereoselectivity (cis versus trans) was revealed by comparing the alkene insertion transition states. In addition, how the double-bond configuration of the VCPs (Z versus E) affects the substrate reactivity and the origins of chemoselectivity ([5 + 2 + 1] versus [5 + 2]) were also investigated. The present study will provide assistance in understanding other carbonylative annulations catalyzed by transition metals.

Palladium/Xu-Phos-catalyzed enantioselective cascade Heck/remote C(sp2) –H alkylation reaction

<h2>Summary</h2> Over the past 30 years, Heck-type difunctionalization of unactivated alkenes involving C−H bond activation has emerged as a powerful strategy for the construction of synthetically valuable spirocycles; however, the development of asymmetric version has largely lagged behind. Herein, we demonstrate a robust Heck-type difunctionalization of a broad range of unactivated alkenes enabled by the first palladium/Xu-Phos-catalyzed tandem Heck/remote C–H bond alkylation. Moreover, both enantiomers of the product can be efficiently prepared using the same enantiomer of a chiral ligand via a position of the phenyl ring-dependent enantiodivergent synthesis. The salient features of this methodology include operational simplicity, high chemo- and enantio-selectivities, and broad substrate scope. In addition, we first revealed that the C(sp²)–H activation, alkene insertion, and C−I reductive elimination steps are reversible by experiments.

Unique regioselectivity of the Pd-catalysed cross-dehydrogenative coupling reaction of simple polyaromatic hydrocarbons with polyfluoroarenes.

A Pd-catalysed cross-dehydrogenative coupling reaction introduced two polyfluoroarenes into simple polyaromatic hydrocarbons at sterically favourable positions. An investigation of the reaction mechanism revealed that the unique regioselectivity was determined by the reductive elimination step rather than the C-H bond cleavage step.

Mechanistic study of the bismuth mediated fluorination of arylboronic esters and further rational design.

Density functional theory (DFT) calculations have been performed to gain insight into the catalytic mechanism of the bismuth redox catalyzed fluorination of arylboronic esters to deliver the widely used arylfluoride compounds (Science 2020, 367, 313–317). The study reveals that the whole catalysis can be characterized via three stages: (i) transmetallation generates the Bi(iii) intermediate 5, capitalizing on the use of KF as an activator. (ii) 5 then reacts with the electrophilic fluorination reagent 1-fluoro-2,6-dichloropyridinium 4via oxidative addition to give the Bi(v) intermediate IM4A. (iii) IM4A undergoes a reductive elimination step to yield aryl fluoride compounds and regenerates the bismuth catalyst for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible. The transmetallation step, with a barrier of 25.4 kcal mol−1, is predicted to be the rate-determining step (RDS) during the whole catalytic cycle. Furthermore, based on a mechanistic study, new catalysts with the framework of tethered bis-anionic ligands were designed, which will help to improve current catalytic systems and develop new bismuth mediated fluorination of arylboronic esters.

Dinickel Catalyzed Vinylidene-Alkene Cyclization Reactions.

A dinickel catalyst promotes reductive cyclization reactions of 1,1-dichloroalkenes containing pendant olefins. The reactions can be conducted with a Zn reductant or electrocatalytically using a carbon working electrode. Mechanistic studies are consistent with the intermediacy of a Ni2(vinylidene) species, which adds to the alkene and generates a metallacyclic intermediate. β-Hydride elimination followed by C-H reductive elimination forms the cyclization product. The proposed dinickel metallacycle is structurally characterized and its stoichiometric conversion to product is demonstrated. Spin polarized, unrestricted DFT calculations are used to further examine the cyclization mechanism. These computational models reveal that both nickel centers function cooperatively to mediate the key oxidative addition, migratory insertion, β-hydride elimination, and reductive elimination steps.

Acceleration Mechanisms of C–H Bond Functionalization Catalyzed by Electron-Deficient CpRh(III) Complexes

A rhodium(III) complex bearing a 1,3-bis(ethoxycarbonyl)-substituted or an unsubstituted cyclopentadienyl ligand (CpE or Cp) significantly accelerates a variety of oxidative C–H bond functionalization reactions. However, the driving force of the acceleration compared with a conventionally used Cp*Rh(III) complex has not been elucidated. Herein, we performed density functional theory (DFT) calculations of the rhodium(III)-catalyzed oxidative C–H bond olefination and annulation reactions using Cp*, Cp, and CpE ligands, which revealed that the CpERh(III) complex stabilizes transition states of not only a C–H bond activation step but also rate-determining reductive elimination and insertion steps by strong orbital interactions. For the sterically demanding substrates, the less sterically hindered CpRh(III) complex can stabilize the transition states of the reductive elimination step more than the CpERh(III) complex. Moreover, the whole reaction pathways were calculated to elucidate the mechanism and selectivity of the oxidative [4 + 2] and [2 + 2 + 2] annulation reactions under cationic and neutral conditions, respectively.

Computational Mechanistic Study of Fused Phenol Formations from 1,6‐Heptadiyne Involving Carbyne Complexes

AbstractThe reaction of a tungsten carbyne complex, MeC≡WBr(CO)4, with 1,6‐heptadiyne was investigated using density functional theory (DFT) calculations. It was suggested that the plausible mechanism involves [2+2] cycloaddition of the carbyne with the diyne, subsequent insertion of CO and alkyne into a metallacyclobutadiene intermediate, and a final reductive elimination step from a metallacycloheptatrienone intermediate to produce a η5‐cyclohexadienyl complex. A related Ru‐catalyzed reaction of 1,6‐heptadiyne involving a hydroxycarbyne complex was investigated to propose a possible pathway starting from a bis(hydroxycarbyne) complex, (HOC≡)2Ru(CO)2. It was found that the biscarbyne complex undergoes carbyne‐carbyne coupling to generate a stable metallacyclopropene intermediate, which reacts with 1,6‐heptadiyne through two different pathways to produce the final η6‐arene complex. Moreover, a non‐carbyne pathway was also investigated to propose an alternative mechanism.

Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel-Catalyzed Direct Decarbonylative Borylation.

The Ni-catalyzed decarbonylative borylation of (hetero)aryl carboxylic acids with B2 cat2 has been achieved without recourse to any additives. This Ni-catalyzed method exhibits a broad substrate scope covering poorly reactive non-ortho-substituted (hetero)aryl carboxylic acids, and tolerates diverse functional groups including some of the groups active to Ni0 catalysts. The key to achieve this decarbonylative borylation reaction is the choice of B2 cat2 as a coupling partner that not only acts as a borylating reagent, but also chemoselectively activates aryl carboxylic acids towards oxidative addition of their C(acyl)-O bond to Ni0 catalyst via the formation of acyloxyboron compounds. A combination of experimental and computational studies reveals a detailed plausible mechanism for this reaction system, which involves a hitherto unknown concerted decarbonylation and reductive elimination step that generates the aryl boronic ester product. This mode of boron-promoted carboxylic acid activation is also applicable to other types of reactions.

Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel‐Catalyzed Direct Decarbonylative Borylation

AbstractThe Ni‐catalyzed decarbonylative borylation of (hetero)aryl carboxylic acids with B2cat2 has been achieved without recourse to any additives. This Ni‐catalyzed method exhibits a broad substrate scope covering poorly reactive non‐ortho‐substituted (hetero)aryl carboxylic acids, and tolerates diverse functional groups including some of the groups active to Ni0 catalysts. The key to achieve this decarbonylative borylation reaction is the choice of B2cat2 as a coupling partner that not only acts as a borylating reagent, but also chemoselectively activates aryl carboxylic acids towards oxidative addition of their C(acyl)−O bond to Ni0 catalyst via the formation of acyloxyboron compounds. A combination of experimental and computational studies reveals a detailed plausible mechanism for this reaction system, which involves a hitherto unknown concerted decarbonylation and reductive elimination step that generates the aryl boronic ester product. This mode of boron‐promoted carboxylic acid activation is also applicable to other types of reactions.

Identifying the Imperative Role of Metal-Olefin Interactions in Catalytic C-O Reductive Elimination from Nickel(II).

We present a series of experimental and computational mechanistic investigations of an unusually facile example of Ni-catalyzed C-O bond formation. Our method, originally reported in 2016, involves the formation of cyclic enol ethers from vinyl iodides bearing pendant alcohol groups. Our findings suggest that the observed reactivity arises from the coordination of the olefin in the vinyl iodide starting material and the enol ether product with Ni(0) intermediates. Density functional theory calculations reveal a plausible catalytic mechanism involving a Ni(II)/Ni(0) redox cycle featuring two-electron C-I oxidative addition and C-O reductive elimination steps. The direct formation of a η 2-enol ether Ni(0) complex from a key Ni(II) alkoxide intermediate dramatically alters the free energy (ΔG) for the vinyl C-O reductive elimination step relative to other examples of C-O reductive elimination at Ni(II). Furthermore, efficient σ-π mixing in the course of vinyl C-O reductive elimination leads to lower computed kinetic barriers (ΔG ‡) relative to those of aryl C-O reductive elimination. The conclusions drawn from these computational models are supported by synthetic organometallic experiments, whereby a vinyl-Ni(II) iodide intermediate was isolated, characterized, and proved to yield enol ether, following exposure to triethylamine. We conducted further experiments and computations, which indicated that the two-electron oxidative addition of vinyl iodides by Ni(0) depends on the formation of an η 2-vinyl iodide precomplex, based on the observation of one-electron activation of the same vinyl iodide in the presence of sterically encumbering ligands (e.g., tricyclohexylphosphine).

Direct Observation of Diastereomeric α-C-Bound Enolates during Enantioselective α-Arylations: Synthesis, Characterization, and Reactivity of Arylpalladium Fluorooxindole Complexes.

The Pd-catalyzed asymmetric α-arylation of carbonyl compounds is a valuable strategy to form benzylic stereocenters. However, the origin of the stereoselectivity of these reactions is poorly understood, and little is known about the reactivity of the putative diastereomeric arylpalladium enolate intermediates. To this end, we report the synthesis and characterization of a series of diphosphine-ligated arylpalladium fluoroenolate complexes, including complexes bearing a metal-bound, stereogenic carbon and an enantioenriched chiral diphosphine ligand. These complexes reductively eliminate to form chiral α-aryl-α-fluorooxindoles with enantioselectivities and rates that are relevant to those of the catalytic process with SEGPHOS as the ancillary ligand. Kinetic studies showed that the rate of reductive elimination is slightly slower than the rate of epimerization of the intermediate, causing the reductive elimination step to impart the greatest influence on the enantioselectivity. DFT calculations of these processes are consistent with these experimental rates and suggest that the minor diastereomer forms the major enantiomer of the product. The rates of reductive elimination from complexes containing a variety of electronically varied aryl ligands revealed the unusual trend that complexes bearing more electron-rich aryl ligands react faster than those bearing more electron-poor aryl ligands. Noncovalent Interaction (NCI) and Natural Bond Orbital (NBO) analyses of the transition-state structures for reductive elimination from the SEGPHOS-ligated complexes revealed key donor-acceptor interactions between the Pd center and the fluoroenolate fragment. These interactions stabilize the pathway to the major product enantiomer more strongly than they stabilize that to the minor enantiomer.

Di-ortho-C[sbnd]H arylation of phenylalanine: A bimetallic interaction between Pd(IV)-Ag(I)

The direct di-ortho-arylation of substituted phenylalanines has been accomplished via methoxyiminoacyl (MIA)-mediated Pd-catalyzed CH functionalization. A diverse array of phenylalanine substrates is amenable to this protocol, providing di-ortho-arylated phenylalanine derivatives with good to high efficiency. Computational results revealed that a bimetallic interaction between Pd(IV)-Ag(I) was formed through the localized orbital, which obviously favored the reductive elimination step of the arylation process.

Nickel‐Catalyzed C‐Heteroatom Cross‐Coupling Reactions under Mild Conditions via Facilitated Reductive Elimination

AbstractThe formation of C‐heteroatom bonds represents an important type of bond‐forming reaction in organic synthesis and often provides a fast and efficient access to privileged structures found in pharmaceuticals, agrochemical and materials. In contrast to conventional Pd‐ or Cu‐catalyzed C‐heteroatom cross‐couplings under high‐temperature conditions, recent advances in homo‐ and heterogeneous Ni‐catalyzed C‐heteroatom formations under mild conditions are particularly attractive from the standpoint of sustainability and practicability. The generation of NiIII and excited NiII intermediates facilitate the reductive elimination step to achieve mild cross‐couplings. This review provides an overview of the state‐of‐the‐art approaches for mild C‐heteroatom bond formations and highlights the developments in photoredox and nickel dual catalysis involving SET and energy transfer processes; photoexcited nickel catalysis; electro and nickel dual catalysis; heterogeneous photoredox and nickel dual catalysis involving graphitic carbon nitride (mpg‐CN), metal organic frameworks (MOFs) or semiconductor quantum dots (QDs); as well as more conventional zinc and nickel dual catalyzed reactions.

Nickel-Catalyzed C-Heteroatom Cross-Coupling Reactions under Mild Conditions via Facilitated Reductive Elimination.

The formation of C-heteroatom bonds represents an important type of bond-forming reaction in organic synthesis and often provides a fast and efficient access to privileged structures found in pharmaceuticals, agrochemical and materials. In contrast to conventional Pd- or Cu-catalyzed C-heteroatom cross-couplings under high-temperature conditions, recent advances in homo- and heterogeneous Ni-catalyzed C-heteroatom formations under mild conditions are particularly attractive from the standpoint of sustainability and practicability. The generation of NiIII and excited NiII intermediates facilitate the reductive elimination step to achieve mild cross-couplings. This review provides an overview of the state-of-the-art approaches for mild C-heteroatom bond formations and highlights the developments in photoredox and nickel dual catalysis involving SET and energy transfer processes; photoexcited nickel catalysis; electro and nickel dual catalysis; heterogeneous photoredox and nickel dual catalysis involving graphitic carbon nitride (mpg-CN), metal organic frameworks (MOFs) or semiconductor quantum dots (QDs); as well as more conventional zinc and nickel dual catalyzed reactions.

Theoretical mechanistic study of nickel‐catalyzed anti‐Markovnikov hydroarylation of alkenes

AbstractTransition metal‐catalyzed hydroarylation of alkenes can provide various Markovnikov products, but so‐called anti‐Markovnikov additions to alkenes are rarely reported. Here, a divalent nickel hydride complex catalyzed anti‐Markovnikov reductive cross‐coupling of styrene and aryl iodide has been investigated using density functional theory. The reaction occurs via the regio‐determining hydronickelation, the oxidative addition of the CI bond to Ni(II) center, and the rate‐determining reductive elimination of hydroarylation product. The electronic processes were investigated with a fragment molecular orbital analysis to understand the origin of anti‐Markovnikov selectivity and successive oxidative addition and reductive elimination steps.

Ni(COD)(DMFU): A Heteroleptic 16-Electron Precatalyst for 1,2-Diarylation of Alkenes

AbstractElectron-deficient olefin (EDO) ligands are known to promote a variety of nickel-catalyzed cross-coupling reactions, presumably by accelerating the reductive elimination step and preventing undesired β-hydride elimination. While there is a growing body of experimental and computational evidence elucidating the beneficial effects of EDO ligands, significant gaps remain in our understanding of the underlying coordination chemistry of the Ni–EDO species involved. In particular, most procedures rely on in situ assembly of the active catalyst, and there is a paucity of preligated Ni–EDO precatalysts. Herein, we investigate the 16-electron, heteroleptic nickel complex, Ni(COD)(DMFU), and examine the performance of this complex as a precatalyst in 1,2-diarylation of alkenes.

Article Palladium/Xu-Phos-Catalyzed Enantioselective Cascade Heck/Remote C( &lt;i&gt;sp&lt;sup&gt;2&lt;/sup&gt;&lt;/i&gt;)−H Alkylation Reaction

Heck-type C-H bond activation of unactivated alkenes has emerged as a powerful strategy for the construction of synthetically valuable spirocycles over past 30 years, however, the development of asymmetric version has lagged largely behind. Herein we demonstrate a robust Heck-type reaction of a broad range of unactivated alkenes enabled by the first palladium/Xu-Phos-catalyzed tandem Heck/remote C−H bond alkylation. Moreover, both enantiomers of the product can be efficiently prepared using the same enantiomer of a chiral ligand via a position of the phenyl ring-dependent enantiodivergent synthesis. The salient features of this methodology include operational simplicity, high chemo- and enantioselectivity, broad substrate scope. In addition, we first revealed that the C(sp2)-H activation, alkene insertion and C−I reductive elimination steps are reversible by experiments.

Pyridylic anions are soft nucleophiles in the palladium-catalyzed C(sp3)-H allylation of 4-alkylpyridines.

We report a mild palladium-catalyzed method for the selective allylation of 4-alkylpyridines in which highly basic pyridylic anions behave as soft nucleophiles. This method exploits alkylidene dihydropyridines, which are semi-stable intermediates readily formed using a ‘soft-enolization’ approach, in a new mechanistic manifold for decarboxylative allylation. Notably, the catalytic generation of pyridylic anions results in a substantially broader functional group tolerance compared to other pyridine allylation methods. Experimental and theoretical mechanistic studies strongly suggest that pyridylic anions are indeed the active nucleophiles in these reactions, and that they participate in an outer-sphere reductive elimination step. This finding establishes a new pKa boundary of 35 for soft nucleophiles in transition metal-catalyzed allylations.