Β-hydride Elimination Step Research Articles

Mechanistic insights into iron catalyzed dehydrogenation of formic acid: β-hydride elimination vs. direct hydride transfer

Density functional theory calculations reveal a complete reaction mechanism with detailed energy profiles and transition state structures for the dehydrogenation of formic acid catalyzed by an iron complex, [P(CH2CH2PPh2)3FeH](+). In the cationic reaction pathway, a β-hydride elimination process is confirmed to be the rate-determining step in this catalytic reaction. A potential reaction pathway starting with a direct hydride transfer from HCOO(-) to Fe is found to be possible, but slightly less favorable than the catalytic cycle with a β-hydride elimination step.

Imparting Catalyst Control upon Classical Palladium-Catalyzed Alkenyl C–H Bond Functionalization Reactions

The functional group transformations carried out by the palladium-catalyzed Wacker and Heck reactions are radically different, but they are both alkenyl C-H bond functionalization reactions that have found extensive use in organic synthesis. The synthetic community depends heavily on these important reactions, but selectivity issues arising from control by the substrate, rather than control by the catalyst, have prevented the realization of their full potential. Because of important similarities in the respective selectivity-determining nucleopalladation and β-hydride elimination steps of these processes, we posit that the mechanistic insight garnered through the development of one of these catalytic reactions may be applied to the other. In this Account, we detail our efforts to develop catalyst-controlled variants of both the Wacker oxidation and the Heck reaction to address synthetic limitations and provide mechanistic insight into the underlying organometallic processes of these reactions. In contrast to previous reports, we discovered that electrophilic palladium catalysts with noncoordinating counterions allowed for the use of a Lewis basic ligand to efficiently promote tert-butylhydroperoxide (TBHP)-mediated Wacker oxidation reactions of styrenes. This discovery led to the mechanistically guided development of a Wacker reaction catalyzed by a palladium complex with a bidentate ligand. This ligation may prohibit coordination of allylic heteroatoms, thereby allowing for the application of the Wacker oxidation to substrates that were poorly behaved under classical conditions. Likewise, we unexpectedly discovered that electrophilic Pd-σ-alkyl intermediates are capable of distinguishing between electronically inequivalent C-H bonds during β-hydride elimination. As a result, we have developed E-styrenyl selective oxidative Heck reactions of previously unsuccessful electronically nonbiased alkene substrates using arylboronic acid derivatives. The mechanistic insight gained from the development of this chemistry allowed for the rational design of a similarly E-styrenyl selective classical Heck reaction using aryldiazonium salts and a broad range of alkene substrates. The key mechanistic findings from the development of these reactions provide new insight into how to predictably impart catalyst control in organometallic processes that would otherwise afford complex product mixtures. Given our new understanding, we are optimistic that reactions that introduce increased complexity relative to simple classical processes may now be developed based on our ability to predict the selectivity-determining nucleopalladation and β-hydride elimination steps through catalyst design.

Reductive Eliminations from Amido Metal Complexes: Implications for Metal Film Deposition

The thermal decomposition of tetrakis(dimethylamido)titanium (TDMAT) follows a number of competitive reactions, including not only the known hydrogenation and β-hydride elimination steps, to produce dimethylamine and N-methylmethaneimine respectively, but also a reductive elimination to yield tetramethylhydrazine and a more complex conversion to N,N,N′-trimethyldiaminomethane and N,N,N′,N′-tetramethyldiaminomethane. The latter reactions may account for the reduction of the metal when these compounds are used for the atomic layer deposition (ALD) of metal nitride films. Similar reactions take place with tetrakis (ethylmethylamido)titanium (TEMAT) and with pentakis(dimethylamido)tantalum (PDMAT).

A Highly Selective and General Palladium Catalyst for the Oxidative Heck Reaction of Electronically Nonbiased Olefins

A general, highly selective oxidative Heck reaction is reported. The reaction is high-yielding under mild conditions without the need for base or high temperatures, and the selectivity is excellent, without the requirement for electronically biased olefins or other specific directing groups. A preliminary mechanistic investigation suggests that the unusually high selectivity may be due to the catalyst's sensitivity to C-H bond strength in the selectivity-determining β-hydride elimination step.

ChemInform Abstract: Palladium-Catalyzed Intramolecular Cyclization of Vinyl and Aryl Triflates. Associated Regioselectivity of the β-Hydride Elimination Step.

ChemInform Abstract: Palladium-Catalyzed Intramolecular Cyclization of Vinyl and Aryl Triflates. Associated Regioselectivity of the β-Hydride Elimination Step.

The Stereoselectivity of the Dehydrogenation of Alkyl Groups on Pt(111) Single-Crystal Surfaces

The thermal chemistry of erythro- and threo-2-butyl-3-d1 species adsorbed on Pt(111) single-crystal surfaces was studied under ultrahigh vacuum (UHV) by temperature-programmed desorption (TPD). The alkyl intermediates were prepared via thermal activation of the appropriate erythro- and threo-2-iodobutane-3-d1 precursors, which were made in house via a three-step synthesis starting with cis- and trans-2,3-epoxybutanes, respectively. Several products were detected in the TPD experiments, including butenes and butanes with different degrees of isotope substitutions and hydrogen from deep dehydrogenation reactions. At least two distinct temperature regimes were identified at higher coverages for the β-hydride elimination steps that produce the olefins, the focus of this work, but one single high-temperature regime could be isolated by working with low coverages of the iodoalkanes. On clean Pt(111), an inverse kinetic isotope effect was observed for the conversion of 2-butyls to 2-butenes, and a slight preference for trans- (rather than cis-) 2-butene production. In addition, a reversal in relative rates for 2-butene-d0 versus 2-butene-d1 was detected in the high-temperature side of the TPD peaks obtained with the two alkyl epimers, an observation that we explain by a high degree of reversibility of the alkyl-to-alkene conversion. Coadsorption of hydrogen or deuterium on the surface reverses the kinetic isotope effect, but not the selectivity for trans olefin production. However, these effects, in particular, the preference for trans olefin formation, are small, indicating an early transition state for the β-hydride elimination step and a thermodynamic (rather than kinetic) control of selectivity in olefin isomerization reactions.

Theoretical and Experimental Studies on Reaction Mechanism for Aerobic Alcohol Oxidation by Supported Ruthenium Hydroxide Catalysts

The experimentally proposed reaction mechanism for the aerobic alcohol oxidation by supported ruthenium hydroxide catalysts (Ru(OH)x/support, support = TiO2 or Al2O3) is theoretically investigated by means of ab initio quantum chemistry calculations with model catalysts “Ru(OH)3(OH2)3” and “RuCl3(OH2)3” for Ru(OH)x/support and RuClx/support, respectively. The experimentally proposed alcoholate formation and β-hydride elimination steps can be verified. In the case of 2-butanol (as a model substrate), the calculated activation energy for the alcoholate formation step with Ru(OH)3(OH2)3 (27.7 kJ mol−1) is much smaller than that with RuCl3(OH2)3 (123.2 kJ mol−1), showing that the alcoholate formation with Ru(OH)x/support much more easily proceeds than that with RuClx/support. The Ru(OH)x/support catalysts possess both Lewis acid (Ru center) and Bronsted base (OH− species) sites on the same metal site. Therefore, the alcoholate formation step can be promoted by the “concerted activation” of an alcohol by the L...

Origin of the High Activity of Porous Carbon-Coated Platinum Nanoparticles for Aerobic Oxidation of Alcohols

In an attempt to clarify the origin of the high activity and durability for aerobic oxidation of alcohols over a platinum (Pt)−carbon composite, i.e., Pt nanoparticles embedded in microporous carbon (nPt@hC), the catalytic reaction mechanism and microstructure of Pt nanoparticles were investigated in detail. By means of kinetic analyses, catalytic oxidation on nPt@hC was found to proceed through the formation of Pt-alcoholates, the β-hydride elimination to form Pt-hydride species (Pt−H), and oxidation of Pt−H with molecular oxygen. It was also revealed that the β-hydride elimination step was a rate-determining step in this reaction. These findings and results of structural studies indicate that the achievement of high catalytic activity on nPt@hC is due to stabilization of its transition state of a positively charged carbocationic component by the electron-rich carbon matrix surrounding Pt nanoparticles, leading to lowering activation energy. Moreover, detailed investigation of the surface characteristics of Pt nanoparticles in nPt@hC after catalytic reactions by using various analytical methods revealed that the durability of nPt@hC for aerobic oxidation of alcohols is due to the suppression of aggregation of Pt nanoparticles and prevention of chemical poisoning of Pt surfaces.

Density Functional Theory Study on β-Hydride Elimination as Thermal Decomposition Process of Diethylzinc

The β-hydride elimination of diethylzinc (DEZn) is investigated as a decomposition process of DEZn from the viewpoint of quantum chemistry. There are two steps in the β-hydride elimination of DEZn, as we determined from experimental results. Transition states are fully optimized at each β-hydride elimination step, and the energies of these transition states are about 45 kcal/mol higher than their reactants. The products of the two β-hydride elimination steps have higher energies of about 20 kcal/mol than their reactants. These energy profiles agree well with our experimental results indicating that DEZn is decomposed by the two β-hydride elimination steps at 300 and 650 °C and that such decomposition requires thermal energy for the reactions to proceed. On the other hand, about 55 kcal/mol is required to eliminate an ethyl radical from DEZn. Therefore, the main decomposition process of DEZn is β-hydride elimination.

Coupling Pd-Catalyzed Alcohol Oxidation to Olefin Functionalization: Hydrohalogenation/Hydroalkoxylation of Styrenes.

A hydrochlorination reaction of styrenes catalyzed by Pd(II) in combination with Cu(II) was developed, which was followed by an in situ conversion of electron-rich products to an ether in the presence of an alcohol. Mechanistic experiments indicate that olefin functionalization is coupled to an alcohol oxidation, wherein a Pd hydride formed in the β-hydride elimination step of the alcohol oxidation was incorporated into the product.

A Theoretical Study of the Heck Reaction: N-Heterocyclic Carbene versus Phosphine Ligands

The reaction mechanism of the Heck reaction has been studied using theoretical chemistry. Our main focus has been the comparison between phosphine ligands and N-heterocyclic carbenes (NHC) and between the normal C2-NHC versus the “abnormal” C5-NHC. Our overall free-energy landscape shows that oxidative addition involves a significant energy barrier, while the phosphine system is relatively favored. In the olefin insertion and β-hydride elimination steps, we considered both the neutral and ionic pathways. For the ionic pathway, the rate-determining step in addition to oxidative addition is olefin migration. For the neutral pathway, the second rate-determining step in addition to oxidative addition is also olefin insertion for NHC systems, while for the phosphine system the energy profile is much flatter. For the phosphine system, our result suggests that the neutral pathway is favored. Our study demonstrates that the catalytic capacity of Pd0L2 involving the “abnormal” C5-NHC ligand is comparable to the regular C2-NHC ligand. For the mechanisms examined in this study, there is no obvious advantage of using NHC ligands.

Selective oxidation of alcohols with oxygen on Ru–Co-hydroxyapatite: A mechanistic study

The chemical nature of the active ruthenium species and the mechanism of the oxidation of alcohols on Co-promoted Ru-hydroxyapatite have been investigated by in situ and ex situ EXAFS and kinetic analysis (reaction order of alcohol and oxygen, competing hydrogenation of primary and secondary alcohols, dehydrogenation in the absence of oxygen, kinetic isotope effect, Hammett study). It is concluded that the probable active sites are dihydroxo-ruthenium species (instead of RuCl 2+ as suggested earlier) and only about half of them are accessible to the reactant benzyl alcohol. The oxidative dehydrogenation reaction obeys the Mars–van Krevelen mechanism and the reduced hydrido-ruthenium species is inactive in alcohol dehydrogenation without reoxidation by molecular oxygen. In the catalytic cycle, the rate limiting step is either the β-hydride elimination step from the alcoholate or reoxidation of the ruthenium-hydride species, depending on the reaction conditions.

Reversible Beta-Hydrogen Elimination of Three-Coordinate Iron(II) Alkyl Complexes: Mechanistic and Thermodynamic Studies

High-spin organometallic complexes have not received extensive mechanistic study, despite their potential importance as unsaturated intermediates in catalytic transformations. We have found that, with a suitably bulky bidentate ligand, three-coordinate, high-spin alkyl complexes of iron(II) are stable. They undergo isomerization and exchange reactions of the alkyl group through β-hydride elimination and reinsertion, and the β-hydride elimination step is rate-limiting. The alkyl complexes transfer a β-hydrogen atom to CC, CN, and CO double bonds and undergo deprotonation by Bronsted acids. The reversible β-hydride elimination reactions can be used to explore relative M−C bond energies. Competition experiments and density functional calculations demonstrate an enthalpic preference for alkyl isomers with iron bound to the terminal carbon of the alkyl fragment. This preference arises from steric and electronic effects. The steric preference could be overcome with a phenyl substituent, which steers iron to the...

The Surface Chemistry of Hydrocarbon Partial Oxidation Catalysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

The surface chemistry of hydrocarbon partial oxidation catalysis

The catalytic oxidation of simple alkyl moieties and alcohols on nickel surfaces has been studied by a combination of surface-science and catalytic techniques on increasingly more complex model systems. Initial work was carried out on oxygen-treated nickel single-crystal surfaces under ultrahigh vacuum conditions. The emulation of sub-stoichiometric nickel oxides was corroborated by a number of physical and chemical means, in particular by the use of carefully chosen probe molecules. The mechanism for partial oxidation was then determined. In the case of alkyl groups, an initial facile oxygen insertion was determined to lead to the formation of surface alkoxides, and to a subsequent dehydrogenation of those intermediates at the beta position to yield aldehydes or ketones. Alcohols were found to dehydrogenate by the same β-hydride elimination step, and surface OH groups to enhance that partial oxidation pathway. High selectivity towards dehydrogenation was observed on most nickel-based catalysts, except when alumina was used as a support, where the new acid sites favored dehydration instead. Total oxidation appears to be a secondary reaction on the initial dehydrogenation products, not a parallel pathway.

Thermal Chemistry of C3Metallacycles on Pt(111) Surfaces

The thermal chemistry of 1,3-diiodopropane on a Pt(111) single-crystal surface was investigated by temperature programmed desorption (TPD) and reflection−absorption infrared spectroscopy (RAIRS). It was found that the first decomposition steps of the chemisorbed diiodo compound are the scissions of its C−I bonds, and that that takes place sequentially and results in the initial formation of an iodopropyl surface intermediate which subsequently decomposes to a C3 metallacycle. Upon further heating of the crystal, the metallacycle intermediate dehydrogenates via a β-hydride elimination step to form an allylic moiety. This allylic species then hydrogenates to propene, a product that subsequently desorbs in two different temperature regimes (around 240 and 330 K) or dehydrogenates to surface propylidyne. The metallacyclic surface intermediate displays additional chemistry at high coverages. Specifically, some of the hydrogen made available by β-hydride elimination from a few of the C3 metallacycles is incorpo...

Partial oxidation of hydrocarbons on nickel: from surface science mechanistic studies to catalysis

The mechanistic details of hydrocarbon partial oxidation reactions were studied with both ultra-high vacuum (UHV) modern surface-sensitive techniques and a micro-batch reactor. The focus of this work was to characterize the reactivity of alkyl species and of alcohols on nickel substrates. In the UHV experiments the chemistry of oxides was mimicked via the oxidation of metal surfaces under controlled conditions, and alkyl surface moieties were prepared by thermal excitation of adsorbed alkyl halides. The oxidation of 2-iodopropane on Ni(100) in particular was found to yield several products in a distribution dependent on oxygen pre-coverage, with partial oxidation being favored at low oxygen coverages and total oxidation dominating on thin oxide films. X-ray photoelectron (XPS) I 3d core level spectra indicate that the adsorption of 2-iodopropane below 100 K is always molecular, and ion-scattering spectra (ISS) data strongly suggest preferential bonding to Ni (not O) sites. Annealing the alkyl iodide adsorbed on O/Ni(100) surfaces below 200 K leads to the dissociation of the C–I bond, and generates 2-propyl groups bonded to nickel atoms, the same as on the clean nickel metal. For submonolayer oxygen coverages the 2-propyl groups then follow one of two reaction pathways: they either undergo hydrogenation–dehydrogenation on the nickel sites to form propane, propene, and hydrogen, or, in the case of the moieties adsorbed near the oxygen sites, they incorporate an oxygen atom to form 2-propoxide groups. 2-Propoxide moieties, which can also be prepared by decomposition of 2-propanol, are stable on the surface up to ∼325 K, at which point they follow a β-hydride elimination step to yield acetone. Additional temperature-programmed desorption (TPD) experiments indicated that propene does not convert directly to acetone on these O/Ni(100) surfaces, and that the presence of hydroxo groups greatly enhances the yield for ketone production. Lastly, preliminary atmospheric pressure experiments proved that alcohols can be oxidized catalytically to aldehydes or ketones on nickel surfaces under controlled conditions, a result that validates the knowledge developed by the mechanistic surface-science studies.

Thermal Chemistry of 1,6-Diiodohexane on Ni(100) Single-Crystal Surfaces: Mimicking Cyclization Reactions

The thermal chemistry of 1,6-diiodohexane, 6-bromo-1-hexene, 1,5-hexadiene, methyl cyclopentane, methylene cyclopentane, and 1-methyl-1-cyclopentene on Ni(100) surfaces has been studied under ultrahigh vacuum conditions by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The thermal activation of the diiodo alkane leads to the initial scission of the C−I bonds around 160 K, the same as with other iodoalkanes, and presumably results in the formation of a surface metallacyclic intermediate. Further heating of that system induces the desorption of hexene, hexane, iodohexane, methylene cyclopentane, benzene, and cyclohexene. The formation of the latter two cyclic products through 5-hexen-1-yl, methyl cyclopentane, methylene cyclopentane, or 1-methyl-1-cyclopentene intermediates was ruled out in this case because direct activation of those compounds does not lead to the desorption of any C6-cyclic molecules at all. TPD experiments with 1,5-hexadiene, on the other hand, did show the formation of benzene, suggesting that such a molecule could be involved in the conversion of the diiodohexane. Additional results from studies with cyclohexane, iodocyclohexane, and cyclohexene indicate that the first cyclic intermediate from the reaction of the C6−Ni metallacycle is likely to be cyclohexene, since both cyclohexane and cyclohexyl moieties yield much more cyclohexane than the diiodo compound. On the basis of these data, a mechanism is proposed for the cyclization reaction of nickelacycloheptane where two initial β-hydride elimination steps at both ends of the hydrocarbon moiety result in the formation of adsorbed 1,5-hexadiene and where that is followed by insertion of one of the double bonds into the metal−carbon bond at the other end to yield cyclohexene.

Selectivity among Dehydrogenation Steps for Alkyl Groups on Metal Surfaces: Comparison between Nickel and Platinum

The thermal chemistry of ethyl and neopentyl iodides on Pt(111) surfaces was investigated by temperature-programmed desorption and reflection−absorption infrared (RAIRS) spectroscopies. The analysis using RAIRS of the isotopic composition of the ethylidyne formed from adsorption of CH3CD2I at different temperatures provided a reasonable estimate for the difference in activation energies between α- and β-hydride elimination steps from alkyl groups adsorbed on Pt(111). A study of the reactivity of neopentyl groups on the same surface using selective deuterium labeling yielded additional information on the relative rates of α- versus γ-hydride eliminations. It was determined that C−H bond-scission steps at the α and γ positions display comparable rates and that both are several orders of magnitude slower than dehydrogenation at the β carbon. A comparison is also presented here to similar chemistry on nickel substrates, where dehydrogenation reactions are usually much faster and where α-elimination dominates over the γ counterpart. The implications of these results to catalysis are discussed in the text.

A Surface Science Study of the Hydrogenation and Dehydrogenation Steps in the Interconversion of C6Cyclic Hydrocarbons on Ni(100)

The thermal chemistry of C6cyclic hydrocarbons (cyclohexane, cyclohexene, benzene, 1,3- and 1,4-cyclohexadienes, 1-methyl-1-cyclohexene, and toluene) and halo hydrocarbons (iodo cyclohexane, iodo benzene, and 3-bromo cyclohexene) on Ni(100) surfaces has been studied under ultrahigh vacuum conditions by using temperature-programmed desorption (TPD). Cyclohexane was found to only desorb molecularly from the surface, but this is because of the dynamic nature of TPD experiments, and is therefore not an indication of the inability of nickel to activate the adsorbed molecules. Indeed, cyclohexyl groups, which are the first expected intermediates after the initial C–H bond scission and which were prepared via the thermal treatment of adsorbed iodo cyclohexane, were shown to undergo a facile β-hydride elimination step to yield first cyclohexene and ultimately benzene, and to concurrently follow a reductive elimination path with surface hydrogen to produce cyclohexane. Cyclohexene was found to dehydrogenate easily to benzene and to not hydrogenate to any significant extent even if atomic hydrogen is present on the surface (again, because the dynamic nature of TPD favors molecular desorption instead). The allylic intermediate expected to form during the dehydrogenation of cyclohexene to cyclohexadiene was prepared by thermal decomposition of 3-bromo cyclohexene and studied by TPD as well, and the remaining hydrogenation and dehydrogenation steps were probed by characterizing the thermal chemistry of the other compounds listed above.