Β-hydride Elimination Reaction Research Articles

Kumada-Tamao-Corriu Type Reaction of Aromatic Bromo- and Iodoamines with Grignard Reagents.

The first example of the Kumada-Tamao-Corriu type reaction of unprotected bromoanilines with Grignard reagents is described. The method uses a palladium source and a newly designed Buchwald-type ligand as the catalytic system. Secondary and tertiary bromo- and iodoamines were also successfully coupled to alkyl Grignard reagents. The products of the competitive β-hydride elimination reaction were successfully reduced using a highly efficient electron-deficient phosphine ligand (BPhos). Mechanistic considerations allowed us to establish that the less electron-rich phosphine ligands stabilize the transition state much better than the electron-rich ones; hence, they increase the reaction yield and reduce the amount of β-hydride elimination products. The developed method proved to be tolerant of many functional groups and can be applied to many different aromatic bromo- and iodoamines. Multigram synthesis of p-toluidine from 4-bromoaniline was achieved with a palladium catalyst loading of only 0.03 mol%.

Mechanistic Studies of Carbonyl Allylation Mediated by (NHC)CuH: Isoprene Insertion, Allylation, and β-Hydride Elimination.

The ability of Cu-H complexes to undergo selective insertion of unsaturated hydrocarbons under mild conditions has rendered them valuable, versatile catalysts. The direct formation of Cu allyl intermediates from unfunctionalized 1,3-dienes and transient Cu hydrides is an appealing strategy for upgrading conjugated diene feedstocks. However, empirical mechanistic studies of the underlying elementary steps and characterization of key intermediates in Cu-H catalysis are sparse. Using [(NHC)CuH]2 (NHC = N-heterocyclic carbene), we examined the steric effects of NHC ligands on two key elementary steps of CuH-catalyzed carbonyl allylation: the insertion of a diene into the Cu-H bond to produce a Cu-allyl complex, and the formation of C-C bonds from stoichiometric allylations of ketones and aldehydes. The resulting allyl and homoallylic alkoxide complexes have been characterized by NMR spectroscopy and single-crystal X-ray diffraction. Employing isolable (NHC)Cu-allyl complexes, we further evaluated the roles of the ligand size, electronic properties of carbonyl substrates, coordinating groups within the substrate, and solvent on the regioselectivity, diastereoselectivity, and relative rate of the C-C bond formation step. In contrast to the clean allylation of ketones, allylation of aldehydes provided a rare example of a formal β-hydride elimination reaction from a secondary homoallylic alkoxide species. Mechanistic studies of key elementary steps provide insights for a range of catalytic reactions of dienes mediated by hydride complexes.

Single-Site Heterogeneous Organometallic Ir Catalysts Embedded on Graphdiyne: Structural Manipulation Beyond the Carbon Support.

Accurate control over the coordination circumstances of single-atom catalysts (SACs) is decisive to their intrinsic activity. Here, two single-site heterogeneous organometallic catalysts (SHOCs), Cp*Ir-L/GDY (L = OH- and Cl- ; Cp* = pentamethylcyclopentadienyl), with the fine-tuned local coordination and electronic structure of Ir sites, are constructed by anchoring Cp*Ir complexes on graphdiyne (GDY) matrix via a one-pot procedure. The spectroscopic studies and theoretical calculations indicate that the Ir atoms in Cp*Ir-Cl/GDY and Cp*Ir-OH/GDY have a much higher oxidation state than Ir in the SAC Ir/GDY. As a proof-of-principle demonstration, the GDY-supported SHOCs are used for formic acid dehydrogenation, which display a fivefold enhancement of catalytic activity compared with SAC Ir/GDY. The kinetic isotope effect and in situ Fourier-transform infrared studies reveal that the rate-limiting step is the β-hydride elimination process, and Cp* on the Ir site accelerates the β-hydride elimination reaction. The GDY-supported SHOCs integrate the merits of both SACs and molecular catalysts, wherein the isolated Ir anchored on GDY echoes with SACs' behavior, and the Cp* ligand enables precise structural and electronic regulation like molecular catalysts. The scheme of SHOCs adds a degree of freedom in accurate regulation of the local structure, the electronic property, and therefore the catalytic performance of single-atom catalysts.

Palladium Complexes Based on Ylide-Functionalized Phosphines (YPhos): Broadly Applicable High-Performance Precatalysts for the Amination of Aryl Halides at Room Temperature.

Palladium allyl, cinnamyl, and indenyl complexes with the ylide‐substituted phosphines Cy3P+−C−(R)PCy2 (with R=Me (L1) or Ph (L2)) and Cy3P+−C−(Me)PtBu2 (L3) were prepared and applied as defined precatalysts in C−N coupling reactions. The complexes are highly active in the amination of 4‐chlorotoluene with a series of different amines. Higher yields were observed with the precatalysts in comparison to the in situ generated catalysts. Changes in the ligand structures allowed for improved selectivities by shutting down β‐hydride elimination or diarylation reactions. Particularly, the complexes based on L2 (joYPhos) revealed to be universal precatalysts for various amines and aryl halides. Full conversions to the desired products are reached mostly within 1 h reaction time at room temperature, thus making L2 to one of the most efficient ligands in C−N coupling reactions. The applicability of the catalysts was demonstrated for aryl chlorides, bromides and iodides together with primary and secondary aryl and alkyl amines, including gram‐scale applications also with low catalyst loadings of down to 0.05 mol %. Kinetic studies further demonstrated the outstanding activity of the precatalysts with TOF over 10.000 h−1.

The reactivity, selectivity and structure of 2-butanol on clean and oxygen-covered Au/Pd(100) alloys

The surface chemistry of 2-butanol is studied on clean and oxygen-saturated Au/Pd(100) alloys using a combination of temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy (RAIRS) as a function of alloy composition. The alloys are synthesized by evaporating gold on a Pd(100) substrate and heating to various temperatures to obtain a range of alloy compositions. It is found that 2-butanol initially forms 2-butoxide species both on clean and oxygen-saturated Au/Pd(100) alloys and can react either to reform 2-butanol or undergo a β-hydride elimination reaction to form 2-butanone. No 2-butanone is formed on alloys with palladium coverages below ~0.5 ML, and the 2-butanone yield increases linearly with palladium coverage at high coverages as the proportion of Pd‒Pd bridge sites on the alloy increases. Some ethylene is formed on alloys with palladium coverages between ~0.4 and 0.6 ML. Saturating the gold-palladium alloy with oxygen for palladium coverages above ~0.5 ML, where oxygen can dissociatively adsorb on the surface at pressures attainable in ultrahigh vacuum, increases the selectivity to 2-butanone formation to 100% for palladium coverages above ~0.75 ML, when Pd‒Pd bridge sites are present, while the selectivity decreases for palladium coverages of ~0.6 ML, when both isolated and bridge sites are present. This indicates that alloy catalysts containing just bridge sites at gold coverages below ~0.35 ML should be the most selective, while still being able to dissociatively adsorb oxygen

Hybrid Reaction Systems for the Synthesis of Alkylated Compounds Based upon Cu-Catalyzed Coupling of Radicals and Organometallic Species.

A transition metal catalyzed alkylation with an alkyl halide is one of the most difficult reactions to achieve, because of the difficult oxidative addition of an alkyl-halogen bond to a metal, and the tendency of the resulting alkylmetal intermediate to undergo a β-hydride elimination reaction to give an olefin. In this review, we discuss hybrid reaction systems involving Cu catalyzed combination of radicals and organometallic species, which enable facile alkylation reactions to construct C-C and C-heteroatom bonds. This paper highlights recent progress in arylation, alkenylation, alkynylation, cyclization, addition and introduction of heteroatoms via these hybrid reaction systems.

On the electron flow sequence driving the hydrometallation of acetylene by lithium hydride.

The sequence of the electronic flow driving the hydrometallation of acetylene by lithium hydride (and that of the opposite β-hydride elimination reaction from the alkenyl metal intermediate), was examined within the perspective provided by the bonding evolution theory (BET). The analysis was based on the application of catastrophe theory to the changes of the electron localization function topology along the intrinsic reaction coordinate. The description of the electronic processes occurring on the process was represented in terms of topological structural stability domains (SSDs) and the associated elementary bifurcation catastrophes. Within such a framework of representation, the "evolution" of the system through the different SSDs reveals the key chemical events driving the transformation, including the large polarization effect as a consequence of Pauli repulsion between ions of the positive cationic metal on the hydride domain, the activation of the CC triple bond to attack the cationic center, and the agostic stabilizing interactions involving the hardest cationic metal, followed by the attack of the hydride center. These results contribute to emphasizing the intrinsic value and usefulness of using topological-based approaches and associated tools to increase our knowledge and understanding of the subtleties underlying the electronic flow as nuclei evolve along the reaction coordinate, providing detailed and complementary insights in comparison to other interpretative tool such those based on orbital-based representations, concerning the intimate nature of the electronic rearrangement of key mechanistic processes in chemistry. Graphical abstract The sequence of the electron flow (indicated by letters a and b) along the intrinsic reaction path for the hydrometallation of acetylene by lithium hydride to yield ethenyl lithium via a four-membered transition structure (TS), as determined within the bonding evolution theory to provide the key chemical events driven the changes in the key bonding patterns. Blue arrow Main event on the side HC ≡ CH + LiH → TS, red arrow the TS → HLiC=CH2 pathway, green arrows relative motion of nuclei along the imaginary frequency at the position of TS on the intrinsic reaction coordinate.

Dimethylberyllium + CO2 → Fire! A DFT and ab Initio Study into the Photon Emission Observed in a Gas Phase Carbon Dioxide Activation Reaction

Dimethylberyllium, Me2Be, is known to ignite when the neat compound is reacted with CO2. In this contribution, we present evidence from DFT and ab initio calculations demonstrating that, while the two-stage gas phase carboxylation of Me2Be to yield beryllium acetate is strongly exothermic, it is not sufficiently so to result in the formation of excited states, as required by a combustion process. The reaction, however, will liberate sufficient heat to drive endothermic unimolecular decomposition reactions. In the case of the reaction of diethylberyllium Et2Be, this results in the formation of beryllium hydride via a β-hydride elimination reaction, and potentially of Be atoms. Pyrolysis of Me2Be, which lacks β-hydrogen atoms, is predicted to give the extremely reactive methyleneberyllium CH2═Be, a ground state triplet species. All reactive intermediates generated by pyrolysis of either Me2Be or Et2Be are calculated to react with CO2 in exothermic reactions. With one possible exception, however, none of the...

Kinetics and Mechanism of Vinyl Acetate Monomer Synthesis on Pd(100) Model Catalysts

The reaction between ethylene and acetate species on Pd(100) is explored using reflection–absorption infrared spectroscopy and compared with previous results found for a similar reaction on Pd(111). As a consequence of the lower saturation acetate coverage on Pd(100) than on Pd(111), the reaction is much less selective giving rise to carbon monoxide as the predominant product. Carrying out the titration reaction with C2D4 at ~180 K results in a feature at ~1711 cm−1, which has previously been assigned to an acetoxyethyl intermediate indicting that ethylene reacts with adsorbed ethylene on Pd(100) via the Samanos pathway, which involves a direct coupling between ethylene ad acetate species for form vinyl acetate monomer by a β-hydride elimination reaction. Reactions with various ethylene isotopomers suggest that decomposition of the acetoxyethyl intermediate is responsible for the non-selective, carbon monoxide formation. A kinetic model is developed for the reaction between ethylene and acetate species that includes non-selective carbon monoxide formation that reproduces the experimental titration curves.

Fundamental organometallic chemistry under bimetallic influence: driving β-hydride elimination and diverting migratory insertion at Cu and Ni.

Bimetallic effects on stoichiometric β-hydride elimination and migratory insertion reactions were examined. Bimetallic reaction conditions drove β-hydride elimination at Cu, while bimetallic C-B elimination occurred in the absence of β-hydrogens. The inherent migratory insertion chemistry of alkynes at Ni was diverted under bimetallic reaction conditions to favor C-H deprotonation.

ChemInform Abstract: Palladium(II)‐Catalyzed Oxidative Difunctionalization of Alkenes: Bond Forming at a High‐Valent Palladium Center

AbstractReview: 67 refs.

Reactivity Studies of [(thf)2Mg{μ-C(CH3)2C2H4C(CH3)2}]2: Scrambling Reactions and Diverse Reactions with Dichlorophenylphosphane

In THF solution [{(thf)2Mg{μ-C(CH3)2C2H4(CH3)2C}}2] (1) exchanges the alkanediide ligand with [{(thf)2Mg{μ-CH2)5}}2] in an equilibrium leading to the formation of [{(thf)2Mg}2{μ-C(CH3)2C2H4C(CH3)2}{μ-(CH2)5}] (2). Depending on the crystallization temperature, homoleptic 1 or heteroleptic 2 crystallizes from THF solutions, verifying a temperature-dependent Schlenk equilibrium. Irradiation of a solution of 1 in [D8]THF with UV light yields magnesium hydride and alkene via a β-hydride elimination reaction. In a metathetical approach dichlorophenylphosphane reacts with 1 in THF to give the intermediate “PhP(Cl){C(CH3)2C2H4(CH3)2CMgCl” (3-MgCl), which forms three subsequent products. In order to ease handling and characterization of these compounds, hydrolysis and oxidation with sulfur has been performed. This product mixture was separated by column chromatography, yielding the chlorophosphane sulfide [Ph(S)P(Cl){C(CH3)2C2H4(CH3)2CH)}] (3-S), the cyclic phosphane sulfide [Ph(S)P{C(CH3)2C2H4(CH3)2C)}] (4-S), an...

Palladium(II)-Catalyzed Oxidative Difunctionalization of Alkenes: Bond Forming at a High-Valent Palladium Center

Difunctionalization of alkenes to incorporate two functional groups across a double bond has emerged as a powerful transformation to greatly increase molecular complexity in organic synthesis with improved efficiency. Historically, palladium-catalyzed difunctionalization of alkenes has suffered from difficulties with introducing a second functional group through reductive elimination of a Pd(II) intermediate and competing β-hydride elimination reactions. To overcome these challenges, one strategy involves utilizing a steric bulky ligand to promote the reductive elimination steps from the Pd(II) center and impeding the β-hydride elimination reactions, which are beyond the scope of this Account. Alternatively, strong oxidants have been utilized to generate high-valent palladium species, which are prone to undergo reductive elimination to form a second C-X bond. This new strategy has been extensively applied to explore the difunctionalization of alkenes with enriched functional group diversity over the past decade. In this Account, we discuss our exploration and application of a "high-valent palladium strategy" for the synthesis of fluorine-containing organic molecules that are typically inaccessible from other methods. These studies were focused on the difunctionalization of alkenes that was initiated by nucleopalladation to form the alkyl C-Pd(II) species in high exo/endo regioselectivity. In the presence of nucleophilic fluorine-containing reagents (e.g., AgF, TMSCF3, and AgOCF3) and strong oxidants (hypervalent iodine and electrophilic fluorinating reagents), the in situ generated fluorine-containing high-valent Pd(IV) intermediates undergo reductive elimination to provide the corresponding alkyl C-F, C-CF3, and C-OCF3 bonds. Using these methods, we synthesized a variety of heterocycles containing fluorine, trifluoromethyl, and trifluoromethoxyl moieties from alkene substrates under mild reaction conditions. Besides hypervalent iodine reagents and electrophilic fluorinating reagents, our group has demonstrated that hydrogen peroxide, which is an environmentally friendly oxidant, can oxidize alkyl C-Pd(II) species to form high-valent alkyl C-Pd intermediates, and based on this observation, several catalytic difunctionalizations of alkenes, such as aminochlorination, aminoacetoxylation, and aminohydroxylation reactions, have been successfully developed. In addition, water was the only waste derived from the oxidant. All of these studies provide attractive methods for the stereoselective introduction of C-N and C-O bonds across double bonds via high-valent palladium intermediates. To gain a deeper understanding of this "high-valent palladium strategy", systematic mechanistic studies were performed to illustrate the stereochemistry of aminopalladation and reductive elimination. These results are summarized in the final section and serve as a guide for further exploration of novel alkene transformation as well as in other areas, such as Pd-catalyzed C-H bond functionalization reactions.

Synthesis, structure and thermolysis of cis-dialkylplatinum(II) complexes – Experimental and theoretical perceptions

The formation of new CC bonds by metal complexes always stimulates great interest because these fundamental reaction types possess numerous potential applications in organic synthesis. These reactions are well documented for a variety of transition metal complexes. Herein we report synthesis and characterization of a series of platinum-dialkyl complexes (1–10) of the type [Pt(L2)R2], (where L2 = dppp (1,3-bis(diphenylphosphino)propane or L = PPh3; R = n-butyl to n-nonyl) with a view to understand the organic product distribution patterns on thermolysis. The single crystal X-ray structures of the complexes [Pt(dppp){CH2(CH2)3CH3}2] (1) and [Pt(dppp){CH2(CH2)6CH3}2] (7) are reported. Thermal decomposition studies of these complexes show interesting behaviour; the longer chain dialkyls i.e. C7C9 complexes undergo reductive elimination whereas the shorter chain dialkyl complexes and C3C6 prefer only the β-hydride elimination reaction. Possible mechanistic aspects are discussed. Theoretical calculations reveal the strongest delocalizations in both complexes involve the interaction of PtC bond pair electron density with the trans positioned PtP antibonding orbital and vice-versa.

Factors Controlling β‐Elimination Reactions in Group 10 Metal Complexes

Trends in reactivity of β-chloride and β-hydride elimination reactions involving Group 10 transition-metal complexes have been computationally explored and analyzed in detail by DFT. These reactions do not require the initial formation of a vacant coordination site; they proceed concertedly without a prior ligand-dissociation step. Whereas β-chloride elimination is associated with relatively moderate activation barriers, the high barriers calculated for analogous β-hydride eliminations suggest that the latter process is unfeasible for this type of compounds. This differential behavior is analyzed within the activation strain model, which provides quantitative insight into the physical factors controlling these β-elimination reactions. The effects of the nature of the Group 10 transition metal (Ni, Pd, Pt), as well as the substituents attached to the β-eliminating fragment (R2 CCR2 X; R, X=H, Cl) on the transformation have also been considered and are rationalized herein.

Alkali Metal Hydride Complexes: Well-Defined Molecular Species of Saline Hydrides

The first examples of well-defined alkali metal hydride complexes have been synthesised and characterised in recent years, and their properties and underlying principles for their generation and stabilisation are emerging. This article gives an account of the hydrides of the alkali metals (Group 1 metals) and selected ‘-ate’ complexes containing hydrides and alkali metals, and reviews the chemistry of well-defined alkali metal hydride complexes including their syntheses, structures, and characteristics. The properties of the alkali metal hydrides LiH, NaH, KH, RbH, and CsH are dominated by their ionic NaCl structure. Stable, soluble, and well-defined LiH and NaH complexes have been obtained by metathesis and β-hydride elimination reactions that require suitable ligands with some steric bulk and the ability to coordinate to several metal ions. These novel hydride complexes reward with higher reactivity and different properties compared with their parent ionic solids.

Homopolymerization of Ethylene by Palladium Phosphine Sulfonate Catalysts: The Role of Structural and Environmental Factors

Palladium phosphine sulfonate catalysts mediate the homopolymerization of ethylene and copolymerization with polar monomers very efficiently. Due to their great potential, a number of theoretical studies have complemented earlier experimental work in the quest of rationally improving their performance. In particular, structural features and reaction conditions are observed to tune the outcome of the polymerization reaction. In this work, we aim at understanding the contribution of factors such as the temperature, the structure of the catalyst, and the concentration of reactants toward production of polyethylene. The influence of each of these factors is studied separately, and trends in the reactivity that explain the experimental results are found. By pursuing a more realistic representation of the system, we traced back the length of the polymer chain obtained to the influence the temperature, the reactants’ concentration, and the catalyst structure have on the β-hydride elimination reaction that terminates the chain propagation. Finally, guidelines for the optimization of the catalyst performance are suggested.

ChemInform Abstract: Gold‐Catalyzed Cyclization Reactions of Allenol and Alkynol Derivatives

AbstractReview: 53 refs.

Disentangling ensemble, electronic and coverage effects on alloy catalysts: Vinyl acetate synthesis on Au/Pd(1 1 1)

The activation energies for the reaction of gas-phase ethylene with adsorbed acetate species on Au/Pd(111) alloys are measured as a function of gold coverage in the alloy to probe the elementary steps in gold–palladium alloy catalyzed vinyl acetate monomer (VAM) synthesis. The activation energy decreases with increasing gold coverage in the alloy up to a gold coverage of ∼0.25ML and then increases. The reaction exhibits a kinetic isotope effect when using C2D4 for gold coverages below ∼0.25ML, and no isotope effect for alloys with larger gold coverages. This is ascribed to a change in rate-limiting step for the reaction, which is postulated to proceed via an initial coupling between ethylene and adsorbed acetate species to form an acetoxyethyl intermediate, which decomposes via a β-hydride elimination reaction to form VAM. For low gold coverages, the second step is rate limiting, while at higher coverage, the coupling step becomes rate limiting. This allows the reaction energies for each of the steps to be obtained as a function of gold coverage, where it is found that both energies decrease with increasing gold coverage, causing an increase in overall reaction rate. The activation energy for β-hydride elimination decreases more rapidly than that for the coupling step. The contributions to the energy change are estimated, and it is found that the effect of coverage on the reaction energies is comparable to the more commonly considered ensemble and electronic effects.

Gold-Catalyzed Cyclization Reactions of Allenol and Alkynol Derivatives

Although gold is chemically inert as a bulk metal, the landmark discovery that gold nanoparticles can be effective catalysts has opened up new and exciting research opportunities in the field. In recent years, there has been growth in the number of reactions catalyzed by gold complexes [gold(I) and gold(III)], usually as homogeneous catalysts, because they are soft Lewis acids. In addition, alkynes and allenes have interesting reactivities and selectivities, notably their ability to produce complex structures in very few steps. In this Account, we describe our work in gold catalysis with a focus on the formation of C-C and C-O bonds using allenes and alkynes as starting materials. Of these, oxa- and carbo-cyclizations are perhaps the best known and most frequently studied. We have divided those contributions into sections arranged according to the nature of the starting material (allene versus alkyne). Gold-catalyzed carbocyclizations in allenyl C2-linked indoles, allenyl-β-lactams, and allenyl sugars follow different mechanistic pathways. The cyclization of indole-tethered allenols results in the efficient synthesis of carbazole derivatives, for example. However, the compound produced from gold-catalyzed 9-endo carbocyclization of (aryloxy)allenyl-tethered 2-azetidinones is in noticeable contrast to the 5-exo hydroalkylation product that results from allenyl sugars. We have illustrated the unusual preference for the 4-exo-dig cyclization in allene chemistry, as well as the rare β-hydride elimination reaction, in gold catalysis from readily available α-allenols. We have also observed in γ-allenols that a (methoxymethyl)oxy protecting group not only masks a hydroxyl functionality but also exerts directing effects as a controlling unit in a gold-catalyzed regioselectivity reversal. Our recent work has also led to a combined experimental and computational study on regioselective gold-catalyzed synthetic routes to 1,3-oxazinan-2-ones (kinetically controlled products) and 1,3-oxazin-2-one derivatives (thermodynamically favored) from easily accessible allenic carbamates. In addition, we discuss the direct gold-catalyzed cycloketalization of alkynyldioxolanes, as well as aminoketalization of alkynyloxazolidines. We performed labeling studies and density functional calculations to gain insight into the mechanisms of the bis-heterocyclization reactions. We also describe the controlled gold-catalyzed reactions of primary and secondary propargylic hydroperoxides with a variety of nucleophiles including alcohols and phenols, allowing the direct synthesis of β-functionalized ketones. Through computations and (18)O-labeling experiments, we discovered various aspects of the controlled reactivity of propargylic hydroperoxides with external nucleophiles under gold catalysis. The mechanism resembles a Meyer-Schuster rearrangement, but notably, the presence and geometry characteristics of the OOH functional group allow a new pathway to happen, which cannot apply to propargylic alcohols.