Β-hydrogen Elimination Research Articles

Efficient nitrogen incorporation in GaAs using novel metal organic As–N precursor di-tertiary-butyl-arsano-amine (DTBAA)

III/V semiconductors containing small amounts of nitrogen (N; dilute nitrides) are discussed in the context of different solar cell and laser applications. The efficiency of these devices is negatively affected by carbon (C) incorporation, which comes either from the direct C–N bond in the N precursor unsymmetrical 1,1-dimethylhydrazine (UDMHy) used conventionally or from the alkyl groups of the conventional precursors for gallium (Ga), indium and arsenic (As) containing carbon. This C is incorporated together with the N due to the strength of the C–N bond. A further important issue in dilute nitride growth is the very low N incorporation efficiency in the crystal from UDMHy, which can be as little as 1% of the N supplied in the gas phase. Therefore, new metal organic chemicals have to be synthesized and their growth characteristics and suitability for dilute nitride growth have to be explored. This work presents the chemical di-tertiary-butyl-arsano-amine (DTBAA), which was synthesized, purified and tested as an N precursor for metal organic vapor phase epitaxy (MOVPE). Computational investigations show β-hydrogen and isobutane elimination to be the main reaction channel in the gas phase with high reaction barriers and absence of small fragments containing C as products. The loss of N via N2, as in UDMHy, can be excluded for unimolecular reactions of DTBAA. The Ga(NAs)/GaAs heterostructures were grown by MOVPE as initial test material and a systematic N incorporation study is presented in this paper. It is shown that high quality Ga(NAs) can be grown using DTBAA. The N incorporation was confirmed by high resolution X-ray diffraction and photoluminescence studies. All samples grown exhibit as grown room temperature photoluminescence and smooth surface morphologies. Furthermore, DTBAA shows extremely high N incorporation efficiency, which makes this molecule a very promising candidate for further research into dilute nitride material growth.

ChemInform Abstract: Iron‐Catalyzed Directed Alkylation of Alkenes and Arenes with Alkylzinc Halides.

AbstractAlkylzinc halide species are generated in‐situ from Grignard compounds and ZnCl2.

Zirconium-Catalyzed Amine Borane Dehydrocoupling and Transfer Hydrogenation

κ5-(Me3SiNCH2CH2)2N(CH2CH2NSiMe2CH2)Zr (1) has been found to dehydrocouple amine borane substrates, RR′NHBH3 (R = R′ = Me; R = tBu, R′ = H; R = R′ = H), at low to moderate catalyst loadings (0.5–5 mol %) and good to excellent conversions, forming mainly borazine and borazane products. Other zirconium catalysts, (N3N)ZrX [(N3N) = N(CH2CH2NSiMe2CH2)3, X = NMe2 (2), Cl (3), and OtBu (4)], were found to exhibit comparable activities to that of 1. Compound 1 reacts with Me2NHBH3 to give (N3N)Zr(NMe2BH3) (5), which was structurally characterized and features an η2 B–H σ-bond amido borane ligand. Because 5 is unstable with respect to borane loss to form 2, rather than β-hydrogen elimination, and 2–4 do not exhibit X ligand loss during catalysis, dehydrogenation is hypothesized to proceed via an outer-sphere-type mechanism. This proposal is supported by the catalytic hydrogenation of alkenes by 2 using amine boranes as the sacrificial source of hydrogen.

Synthesis of Zn-MOF incorporating titanium-hydrides as active sites binding H2 molecules

This paper describes the synthetic effort for a Zn-MOF imparting Ti-H as a preferential binding site potentially capturing H2 molecules via Kubas-type interaction. The formation mechanism of Ti-H innate to the final material was potentially demonstrated to follow a radical dissociation rather than a β-hydrogen elimination and a C-H reductive elimination.

β-Hydrogen Elimination Mechanism in the Absence of Low-Lying Acceptor Orbitals in EH2(t-C4H9) (E = N-Bi).

The β-hydrogen elimination reactions of group 15 alkyl compounds at the example of EH2(t-C4H9) (element E = N-Bi) were investigated and compared to the group 13 example of GaH2(t-C4H9). With the aid of extensive density functional theory based analysis of atomic and electronic structures at the transition state, we can derive three distinct reaction classes. The gallium compound follows the well-known β-hydride route with participation of an empty p orbital at the metal in a concerted, synchronous fashion, exhibiting a low barrier. For compounds with group 15 elements, we find highly nonsynchronous reactions with high reaction barriers. In the case of nitrogen, a proton-like H atom is transferred via attack of the nitrogen nonbonding electron pair. For the heavier homologues (P-Bi), E-Cα bond breaking occurs first and the H atom does not carry charge at the transition state. The reaction barrier in group 15 homologues is thus determined by the E-Cα bond strength down the group. The results enable a rationale for ligand design for precursors involved in chemical vapor-phase deposition processes because a good ligand needs to stabilize the positive charge at Cα.

β-Hydrogen Elimination Reactions of Nickel and Palladium Methoxides Stabilised by PCP Pincer Ligands.

Nickel and palladium methoxides [((iPr)PCP)M-OMe], which contain the (iPr)PCP pincer ligand, decompose upon heating to give products of different kinds. The palladium derivative cleanly gives the dimeric Pd(0) complex [Pd(μ-(iPr)PCHP)]2 ((iPr)PCHP = 2,6-bis(diisopropylphosphinomethyl)phenyl) and formaldehyde. In contrast, decomposition of [((iPr)PCP)Ni-OMe] affords polynuclear carbonyl phosphine complexes. Both decomposition processes are initiated by β-hydrogen elimination (BHE), but the resulting [((iPr)PCP)M-H] hydrides undergo divergent reaction sequences that ultimately lead to the irreversible breakdown of the pincer units. Whereas the Pd hydride spontaneously experiences reductive C-H coupling, the decay of its Ni analogue is brought about by its reaction with formaldehyde released in the BHE step. Kinetic measurements showed that the BHE reaction is reversible and less favourable for Ni than for Pd for both kinetic and thermodynamic reasons. DFT calculations confirmed the main conclusions of the kinetic studies and provided further insight into the mechanisms of the decomposition reactions.

Isomerization of Olefins Triggered by Rhodium-Catalyzed C-H Bond Activation: Control of Endocyclic β-Hydrogen Elimination.

Five-membered metallacycles are typically reluctant to undergo endocyclic β-hydrogen elimination. The rhodium-catalyzed isomerization of 4-pentenals into 3-pentenals occurs through this elementary step and cleavage of two C–H bonds, as supported by deuterium-labeling studies. The reaction proceeds without decarbonylation, leads to trans olefins exclusively, and tolerates other olefins normally prone to isomerization. Endocyclic β-hydrogen elimination can also be controlled in an enantiodivergent reaction on a racemic mixture.

N-Heterocycles from chromium aminocarbenes.

The initial [2 + 2]-cycloadduct between a chromium aminocarbene and a tethered alkene undergoes a β-hydrogen elimination very efficiently when triphenylphosphine is added as a ligand. The reaction gives cyclic enamines or homoenamines depending on the substitution on the alkene.

A Career in Catalysis: John E. Bercaw

On the occasion of Professor John Bercaw’s 70th birthday, we reflect and highlight his distinguished career in organometallic chemistry and homogeneous catalysis. What began as a fundamental interest in the chemistry of bis(cyclopentadienyl)titanium compounds and their interaction with molecular nitrogen evolved into a vibrant and diverse program tackling some of the most important problems in catalysis. Using well-defined organometallic compounds, fundamental insights were gained in the mechanism of CO reduction; basic transformations of organometallic chemistry, such as alkene insertion and alkyl β-hydrogen elimination; the origin of stereocontrol in metallocene-catalyzed polymerization; and in the activation of hydrocarbons by electrophilic late transition metals.

Growth of Ge Homoepitaxial Films by Metal-Organic Chemical Vapor Deposition Using t-C4H9GeH3

Ge homoepitaxial films are grown at low growth temperature of 320°C by metal-organic chemical vapor deposition (MOCVD) using tertiarybutylgermane (t-C4H9GeH3). We also performed ab initio calculations in order to reveal the chemical reaction for the epitaxial growth. As the result, it was revealed that the t-C4H9GeH3 was most likely decomposed into germane (GeH4) and isobutene [CH2=C(CH3)2] through the β-hydrogen elimination. We considered that this chemical nature allowed the growth temperature as low as that obtained by GeH4 precursor with sufficiently suppressed C impurity incorporation.

ChemInform Abstract: Alkenyl Complexes of Platinum Group Metals: A Platform for Diverse Reactivity Patterns

AbstractReview: 132 refs.

A facile method for generating Michael acceptor thin films via amine substituted poly(vinyl methyl ketone)

Abstract Alpha-morpholine substituted poly(vinyl methyl ketone) (PVMK) was determined to undergo rapid self-crosslinking by short-term vacuuming (i.e., 30–60 min) or curing at 70–100 °C. The chemical transformations involved were monitored by TGA, DSC, FT-IR, 1HNMR and solid-state 13C NMR techniques. These experiments revealed an “unzipping-like” mechanism yielding multi conjugated enone structure via consecutive deamination by β-hydrogen elimination and simultaneous Diels–Alder cyclization resulting in crosslinking. This chemistry was exploited for generating Michael acceptor thin film coatings on silica nanoparticles, in which the silica was impregnated with morpholine substituted poly(vinyl methyl ketone) and simply heated at 100 °C. The dense polyene functional films so formed were demonstrated to add amines and thiols at room temperature. The resulting silica nanoparticles were employed to form homogenous composite films using poly(butyl acrylate-stat-methylmethacrylate) as carrier matrix. Morpholine substituted poly(vinyl methyl ketone) (MSP) seems to be generally applicable for surface modification of different inorganic surfaces with organics.

Fate of Aryl/Amido Complexes of Rhodium(III) Supported by a POCOP Pincer Ligand: C–N Reductive Elimination, β-Hydrogen Elimination, and Relevance to Catalysis

Rhodium complexes supported by the aryl/bis(phosphinite) POCOP pincer ligand undergo reactions that constitute a RhI/RhIII synthetic cycle for C–N coupling analogous to the classical Pd0/PdII Buchwald–Hartwig chemistry. (POCOP)Rh(Ar)(X) complexes (X = Cl, Br) can be readily obtained by oxidative addition of ArX to the (POCOP)Rh fragment generated in situ from (POCOP)Rh(H)(Cl) (1) and NaOtBu. (POCOP)Rh(Ar)(X) complexes react with anilines and diphenylamine in the presence of an equimolar amount of NaOtBu to give RhIII aryl/amido complexes (POCOP)Rh(Ar)(NHAr′) and (POCOP)Rh(Ar)(NPh2). The intermediate (POCOP)Rh(p-F3CC6H4)(OtBu) (7) was isolated and shown to react irreversibly with p-MeC6H4NH2 to give (POCOP)Rh(p-F3CC6H4)(NHC6H4Me-p) (5). The latter undergoes reductive elimination of the diarylamine product p-F3CC6H4NHC6H4Me-p upon heating. The kinetics of this reaction point to a first-order process, and DFT calculations located a transition state for concerted C–N reductive elimination. Complex 1 effected ...

Bis(imino)pyridine cobalt-catalyzed dehydrogenative silylation of alkenes: scope, mechanism, and origins of selective allylsilane formation.

The aryl-substituted bis(imino)pyridine cobalt methyl complex, ((Mes)PDI)CoCH3 ((Mes)PDI = 2,6-(2,4,6-Me3C6H2-N═CMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.

Silyl ligand mediated reversible β-hydrogen elimination and hydrometalation at palladium.

The mechanism and origin of the facile β-hydrogen elimination and hydrometalation of a palladium complex bearing a phenylene-bridged PSiP pincer ligand are clarified. Experimental and theoretical studies demonstrate a new mechanism for β-hydrogen elimination and hydrometalation mediated by a silyl ligand at palladium, which enables direct interconversion between an ethylpalladium(II) complex and an η(2) -(Si-H)palladium(0) complex without formation of a square-planar palladium(II) hydride intermediate. The flexibility of the PSiP pincer ligand enables it to act as an efficient scaffold to deliver the hydrogen atom as a hydride ligand.

Copper-catalyzed alkyl-alkyl cross-coupling reactions using hydrocarbon additives: efficiency of catalyst and roles of additives.

Cross-coupling of alkyl halides with alkyl Grignard reagents proceeds with extremely high TONs of up to 1230000 using a Cu/unsaturated hydrocarbon catalytic system. Alkyl fluorides, chlorides, bromides, and tosylates are all suitable electrophiles, and a TOF as high as 31200 h(-1) was attained using an alkyl iodide. Side reactions of this catalytic system, i.e., reduction, dehydrohalogenation (elimination), and the homocoupling of alkyl halides, occur in the absence of additives. It appears that the reaction involves the β-hydrogen elimination of alkylcopper intermediates, giving rise to olefins and Cu-H species, and that this process triggers both side reactions and the degradation of the Cu catalyst. The formed Cu-H promotes the reduction of alkyl halides to give alkanes and Cu-X or the generation of Cu(0), probably by disproportionation, which can oxidatively add to alkyl halides to yield olefins and, in some cases, homocoupling products. Unsaturated hydrocarbon additives such as 1,3-butadiene and phenylpropyne play important roles in achieving highly efficient cross-coupling by suppressing β-hydrogen elimination, which inhibits both the degradation of the Cu catalyst and undesirable side reactions.

ChemInform Abstract: A Novel and Diastereoselective Construction of H‐Pyrazolo[3,2‐a]isoquinoline Fused Spirooxindoles via [3 + 2] Cycloaddition.

Abstract A one-pot, highly efficient and diastereoselective construction of 1,2-dihyro-10′bH-spiro[indole-3,1′-pyrazole[3,2-a]isoquinoline]-2′-carboxylates via silver triflate-catalyzed [3+2] cycloaddition of N′-(2–alkynylbenzylidene)hydrazide with methyleneindolinones in good to excellent yield has been described. The transformation involves initially a condensation followed by 6-endo-intramolecular cyclization forming dipole which then undergoes [3+2]-cycloaddition with methyleneindolinones and finally, β-hydrogen elimination.

Formic acid dehydrogenation with bioinspired iridium complexes: a kinetic isotope effect study and mechanistic insight.

Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H2 O)]SO4 (TH4BPM: 2,2',6,6'-tetrahydroxyl-4,4'-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39 500 h(-1) at 80 °C in a 1 M aqueous solution of HCO2 H/HCO2 Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is β-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions.

ChemInform Abstract: Amidinates, Guanidinates and Iminopyrrolidinates: Understanding Precursor Thermolysis to Design a Better Ligand

AbstractReview: 48 refs.

Synthesis of petaloid graphene/polyethylene composite nanosheet produced by ethylene polymerization with metallocene catalyst adsorbed on multilayer graphene

We report the synthesis of a multilayer graphene (MLG)/polyethylene composite nanosheet via in situ ethylene polymerization with an (n-BuCp)2ZrCl2-adsorbed MLG system. The shape of the MLG/PE composite nanosheet resembles the petal of a chrysanthemum. According to TEM, SEM images and XRD results, the layers of the graphene sheets are exfoliated and peeled from the MLG flake because the PE mass increases on the surface of the graphene sheet during the PE polymerization. The exfoliated graphene/PE nanosheets are sliced into many parts and become petaloid in shape. Graphene plays a role not only as a support for adsorption but also as an additional ligand to (n-BuCp)2ZrCl2 during ethylene polymerization. In the π–π stacking between graphene and (n-BuCp)2ZrCl2, the rich electron environment and bulky surface of graphene increase the electron density of (n-BuCp)2ZrCl2 and inhibit the β-hydrogen elimination, resulting in a higher MW and PDI of PE than those of homogenous (n-BuCp)2ZrCl2.