Showing papers on "Dehydrogenation published in 2013"

Applications of acceptorless dehydrogenation and related transformations in chemical synthesis.

Abstract: Conventional oxidations of organic compounds formally transfer hydrogen atoms from the substrate to an acceptor molecule such as oxygen, a metal oxide, or a sacrificial olefin. In acceptorless dehydrogenation (AD) reactions, catalytic scission of C-H, N-H, and/or O-H bonds liberates hydrogen gas with no need for a stoichiometric oxidant, thereby providing efficient, nonpolluting activation of substrates. In addition, the hydrogen gas is valuable in itself as a high-energy, clean fuel. Here, we review AD reactions selectively catalyzed by transition metal complexes, as well as related transformations that rely on intermediates derived from reversible dehydrogenation. We delineate the methodologies evolving from this recent concept and highlight the effect of these reactions on chemical synthesis.

Oxidative Dehydrogenation of Ethane: Common Principles and Mechanistic Aspects

Abstract: The increasing demand for light olefins and the changing nature of basic feedstock has stimulated substantial research activity into the development of new process routes Steam cracking remains the most industrially relevant pathway, but other routes for light-olefin production have emerged Fluid catalytic cracking only produces ethene in minor concentrations Challenged by marked catalyst deactivation, in contrast, catalytic dehydrogenation ethane up opens a more selective route to ethene The oxidative dehydrogenation (ODH) of ethane, which couples the endothermic dehydration of ethane with the strongly exothermic oxidation of hydrogen, would potentially be the most attractive alternative route because it avoids the need for excessive internal heat input, but also consumes valuable hydrogen In this Review, the current state of the ODH of ethane is compared with other routes for light-olefin production, with a focus on the catalyst and reactor system variants New catalyst systems and reactor designs have been developed to improve the industrial competitiveness of the ODH reaction of ethane The current state of our fundamental understanding of the ODH of light alkanes, in particular in terms of catalyst and reactor development, is critically reviewed The proposed mechanisms and the nature of the active site for the ODH reaction are described and discussed in detail for selected promising catalysts The reported catalytic performance and the possible limitations of these ODH catalysts will be examined and the performance of the emerging approaches is compared to the currently practiced methods

Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation and Dehydrogenation Reactions

Abstract: Cobalt(II) alkyl complexes of aliphatic PNP pincer ligands have been synthesized and characterized. The cationic cobalt(II) alkyl complex [(PNHPCy)Co(CH2SiMe3)]BArF4 (4) (PNHPCy = bis[(2-dicyclohexylphosphino)ethyl]amine) is an active precatalyst for the hydrogenation of olefins and ketones and the acceptorless dehydrogenation of alcohols. To elucidate the possible involvement of the N–H group on the pincer ligand in the catalysis via a metal–ligand cooperative interaction, the reactivities of 4 and [(PNMePCy)Co(CH2SiMe3)]BArF4 (7) were compared. Complex 7 was found to be an active precatalyst for the hydrogenation of olefins. In contrast, no catalytic activity was observed using 7 as a precatalyst for the hydrogenation of acetophenone under mild conditions. For the acceptorless dehydrogenation of 1-phenylethanol, complex 7 displayed similar activity to complex 4, affording acetophenone in high yield. When the acceptorless dehydrogenation of 1-phenylethanol with precatalyst 4 was monitored by NMR spectros...

Oxidative Dehydrogenation on Nanocarbon: Identification and Quantification of Active Sites by Chemical Titration

Abstract: Nanostructured carbon-based materials have shown high catalytic activity in several important reactions and related chemical industrial processes, such as direct or oxidative dehydrogenation of hydrocarbons and Friedel–Crafts reactions. Nanocarbon materials exhibit significant advantages over traditional metal or metal oxide based catalysts because of their tunable acidity/basicity, electron density, and convenient recycling and reusability, and they have been shown to be potential alternatives to conventional catalysts to meet the requirements of sustainable chemistry. As a result, the field of nanocarbon catalysis has been experiencing an unparalleled development of new catalyst synthesis or their applications in new reaction systems. However, there is only slow growth of mechanistic interpretation of carbon-catalyzed reactions, which is even more urgent to advance our knowledge in related fields. Present research on the mechanism of carbon catalysis suggests that oxygen containing functional groups, especially ketonic carbonyl groups on nanocarbon, which are rich in electrons, may act as the catalytic active sites for oxidative dehydrogenation (ODH) of alkanes to corresponding alkenes. The reaction process is assumed to be similar to that for transition-metal oxide catalysts. The C H bonds of alkanes dissociate at active oxygen functional groups, and the hydrogen atoms are abstracted by Lewis base sites. After the desorption of alkene products, gas-phase O2 reacts with the abstracted hydrogen to form H2O, then the active catalytic sites are regenerated to finish one catalytic cycle. The above unspecific catalytic mechanism is only based on the qualitative characterization of carbon catalysts, while the identity of the active sites or a detailed kinetic study has never been executed with direct and convincing chemical evidence. One of the most critical problems that limits the quantitative description of the catalytic mechanism is the uncertainty of the chemical structure of nanocarbon materials. The coexistence of several kinds of surface functional groups (such as hydroxyl, carbonyl, and carboxylic acid groups) is unavoidable, as the synthesis or the following surface modification procedures of nanocarbon catalysts are normally realized by a severe physical or chemical process, such as laser irradiation and oxidation by HNO3, O2, and O3. [8] There are still lack of reliable quantification methods for the surface functional groups on nanostructured carbon materials because of their complexity in type and quantity. As a result, turnover frequency (TOF), the ultimate parameter to evaluate the intrinsic activity of heterogeneous catalysts, is also rarely reported in the case of nanocarbon catalysts, making it impossible to study the detailed reaction kinetics or compare the activity of carbon catalysts bearing different structures fairly and objectively. The quantitative surface composition analysis is also desirable for the application of nanostructured carbon as a catalyst support or electrochemical devices, which takes an even larger proportion in the field of carbon materials, as the surface structure of nanocarbon materials is essential for their physical or chemical properties (for example, affinity for a certain metal or metal ion). In view of the quantification methods of oxygen functional groups, herein we propose a chemical titration method to determine the surface concentration of three kinds of typical oxygen functional groups ( C=O, C OH, and COOH) on the surface of carbon nanotubes (CNTs) (Scheme 1). Through selective deactivation of these specific oxygen functional groups and the assessment of the catalytic activity of different CNT derivatives for ethylbenzene (EB) ODH reactions, we provided chemical evidence to show that

Boryl-mediated reversible H2 activation at cobalt: catalytic hydrogenation, dehydrogenation, and transfer hydrogenation.

Abstract: We describe the synthesis of a cobalt(I)–N2 complex (2) supported by a meridional bis-phosphino-boryl (PBP) ligand. Complex 2 undergoes a clean reaction with 2 equiv of dihydrogen to afford a dihydridoboratocobalt dihydride (3). The ability of boron to switch between a boryl and a dihydridoborate conformation makes possible the reversible conversion of 2 and 3. Complex 3 reacts with HMe_2N–BH_3 to give a hydridoborane cobalt tetrahydridoborate complex. We explore this boryl–cobalt system in the context of catalytic olefin hydrogenation as well as amine–borane dehydrogenation/transfer hydrogenation.

Remarkable enhancement in dehydrogenation of MgH2 by a nano-coating of multi-valence Ti-based catalysts

Abstract: A Ti-based multi-valence catalyst was coated on the surface of ball milled Mg powders (∼1 μm in diameter), aiming to decrease the desorption temperature and increase the kinetics of hydrogen release from MgH2 by its catalytic effect on thermodynamics. The catalysis coating was prepared by the chemical reaction between Mg powders and TiCl3 in THF solution, which is ∼10 nm in thickness and contains multiple valences in the form of Ti (0), TiH2 (+2), TiCl3 (+3) and TiO2 (+4). It is believed that the easier electron transfer among these different Ti valences plays a key role in enhancing the hydrogen recombination for the formation of a hydrogen molecule (e.g.). This recombination is generally regarded as the key barrier for hydrogen desorption of MgH2. Experimentally, temperature-programmed desorption (TPD) and isothermal dehydrogenation analysis demonstrate that the MgH2 – coated Ti based system (denoted as Mg–Ti) has excellent dehydrogenation properties, which can start to release H2 at about 175 °C and release 5 wt% H2 within 15 min at 250 °C. The dehydrogenation reaction entropy (ΔS) of the system is changed from 130.5 J K−1 mol−1 H2 to 136.1 J K−1 mol−1 H2, which reduces the Tplateau to 279 °C at an equilibrium pressure of 1 bar. A new mechanism has been proposed that multiple valence Ti sites act as the intermediate for electron transfers between Mg2+ and H−, which makes the recombination of H2 on Ti (in forms of compounds) surfaces much easier.

Acceptorless Dehydrogenation of Nitrogen Heterocycles with a Versatile Iridium Catalyst

Abstract: Catalytic dehydrogenation of a wide variety of benzo fused N-heterocycles is achieved in the presence of a cyclometalated iridium complex as the catalyst in the absence of any oxidant or acceptor for hydrogen.

Monodisperse gold-palladium alloy nanoparticles and their composition-controlled catalysis in formic acid dehydrogenation under mild conditions.

Abstract: Monodisperse 4 nm AuPd alloy nanoparticles with controlled composition were synthesized by co-reduction of hydrogen tetrachloroaurate(III) hydrate and palladium(II) acetylacetonate with a borane–morpholine complex in oleylamine. These NPs showed high activity (TOF = 230 h−1) and stability in catalyzing formic acid dehydrogenation and hydrogen production in water at 50 °C without any additives.

Effects of Si/Al ratio on the distribution of framework Al and on the rates of alkane monomolecular cracking and dehydrogenation in H-MFI.

Abstract: The aim of this study was to investigate the influence of Si/Al ratio on the locations of exchangeable cations in H-MFI and on the monomolecular cracking and dehydrogenation reactions of n-butane. On the basis of UV-visible spectroscopic analysis of Co(II) exchanged into MFI, it was inferred that the fraction of Co(II) (and, by extension, Bronsted protons) located at channel intersections relative to straight and sinusoidal channels increases with increasing Al content. Concurrently, turnover frequencies for all monomolecular reactions, and the selectivities to dehydrogenation versus cracking and to terminal cracking versus central cracking, generally increased. The changes in selectivity with Al content are consistent with the finding that the transition-state geometry for dehydrogenation is bulky and resembles a product state, and should therefore exhibit a stronger preference to occur at channel intersections relative to cracking. Increases in turnover frequencies are attributed partly to increases in intrinsic activation entropies that compensate for concurrent increases in intrinsic activation energies, most strongly for dehydrogenation and terminal cracking, resulting in increased selectivity to these reactions at higher Al content. This interpretation contrasts with the view that intrinsic activation barriers are constant. It is also observed that isobutene inhibits the rate of n-butane dehydrogenation. Theoretical calculations indicate that this effect originates from adsorption of isobutene at the channel intersections. Because cracking reaction rates are not affected by the presence of isobutene, this result suggests that the preference of dehydrogenation to occur at channel intersections is much stronger than the preference for cracking to occur at these locations.

Amine-Functionalized MIL-125 with Imbedded Palladium Nanoparticles as an Efficient Catalyst for Dehydrogenation of Formic Acid at Ambient Temperature

Abstract: Palladium nanoparticles were immobilized within the pores of metal organic frameworks MIL-125 and amine-functionalized NH2-MIL-125 using photoassisted and ion exchange deposition methods. The resulting materials showed a very high activity for hydrogen production at ambient temperature in comparison to other titanium-based porous materials. The amine groups in NH2-MIL-125 greatly increased the H2-generating activity and acted as a stabilizer of Pd NPs. The photoassisted deposition method was a more efficient method for producing highly dispersed small NPs in the MOF materials.

Long-range metal–ligand bifunctional catalysis: cyclometallated iridium catalysts for the mild and rapid dehydrogenation of formic acid

Abstract: Formic acid (HCO2H) is an important potential hydrogen storage material, which, in the presence of appropriate catalysts can be selectively dehydrogenated to give H2 and CO2. In this work, well defined N^C cyclometallated iridium(III) complexes based on 2-aryl imidazoline ligands are found to be excellent catalysts for the decomposition of HCO2H–NEt3 mixtures to give H2 and CO2 under mild conditions with high turnover frequencies (up to 147 000 h−1 at 40 °C) and essentially no CO formation. The modular structures of these catalysts have allowed for the construction of structure–activity relationships for the complexes, leading to the rational optimisation of the catalyst structure with respect to both the rate of H2 production and catalyst lifetime. In particular, the presence of the remote γ-NH unit in the ligand is shown to be essential for catalytic activity, without which no reaction occurs. Mechanistic studies suggest that the dehydrogenation is rate-limited by the step of hydride protonation, which is made feasible by the γ-NH unit via an unusual form of long-range metal–ligand bifunctional catalysis involving formic acid-assisted proton hopping.

Aerobic Dehydrogenation of Cyclohexanone to Phenol Catalyzed by Pd(TFA)2/2-Dimethylaminopyridine: Evidence for the Role of Pd Nanoparticles

Abstract: We have carried out a mechanistic investigation of aerobic dehydrogenation of cyclohexanones and cyclohexenones to phenols with a Pd(TFA)2/2-dimethylaminopyridine catalyst system. Numerous experimental methods, including kinetic studies, filtration tests, Hg poisoning experiments, transmission electron microscopy, and dynamic light scattering, provide compelling evidence that the initial PdII catalyst mediates the first dehydrogenation of cyclohexanone to cyclohexenone, after which it evolves into soluble Pd nanoparticles that retain catalytic activity. This nanoparticle formation and stabilization is facilitated by each of the components in the catalytic reaction, including the ligand, TsOH, DMSO, substrate, and cyclohexenone intermediate.

Aerobic Dehydrogenation of Cyclohexanone to Cyclohexenone Catalyzed by Pd(DMSO)2(TFA)2: Evidence for Ligand-Controlled Chemoselectivity

Abstract: The dehydrogenation of cyclohexanones affords cyclohexenones or phenols via removal of 1 or 2 equiv of H2, respectively. We recently reported several PdII catalyst systems that effect aerobic dehydrogenation of cyclohexanones with different product selectivities. Pd(DMSO)2(TFA)2 is unique in its high chemoselectivity for the conversion of cyclohexanones to cyclohexenones, without promoting subsequent dehydrogenation of cyclohexenones to phenols. Kinetic and mechanistic studies of these reactions reveal the key role of the dimethylsulfoxide (DMSO) ligand in controlling this chemoselectivity. DMSO has minimal kinetic influence on the rate of Pd(TFA)2-catalyzed dehydrogenation of cyclohexanone to cyclohexenone, while it strongly inhibits the second dehydrogenation step, conversion of cyclohexenone to phenol. These contrasting kinetic effects of DMSO provide the basis for chemoselective formation of cyclohexenones.

Effect of Ti Intermetallic Catalysts on Hydrogen Storage Properties of Magnesium Hydride

Abstract: Magnesium hydride is a promising candidate for solid-state hydrogen storage and thermal energy storage applications. A series of Ti-based intermetallic alloy (TiAl, Ti3Al, TiNi, TiFe, TiNb, TiMn2, and TiVMn)-doped MgH2 materials were systematically investigated in this study to improve its hydrogen storage properties. The dehydrogenation and hydrogenation properties were studied by using both thermogravimetric analysis and pressure–composition–temperature (PCT) isothermal to characterize the temperature of dehydrogenation and the kinetics of both desorption and absorption of hydrogen by these doped MgH2. Results show significant improvements of both dehydrogenation and hydrogenation kinetics as a result of adding the Ti intermetallic alloys as catalysts. In particular, the TiMn2-doped Mg demonstrated extraordinary hydrogen absorption capability at room temperature and 1 bar hydrogen pressure. The PCT experiments also show that the hydrogen equilibrium pressures of MgH2 were not affected by these additives.

The mechanism of borane-amine dehydrocoupling with bifunctional ruthenium catalysts.

Abstract: Borane-amine adducts have received considerable attention, both as vectors for chemical hydrogen storage and as precursors for the synthesis of inorganic materials. Transition metal-catalyzed ammonia-borane (H3N-BH3, AB) dehydrocoupling offers, in principle, the possibility of large gravimetric hydrogen release at high rates and the formation of B-N polymers with well-defined microstructure. Several different homogeneous catalysts were reported in the literature. The current mechanistic picture implies that the release of aminoborane (e.g., Ni carbenes and Shvo's catalyst) results in formation of borazine and 2 equiv of H2, while 1 equiv of H2 and polyaminoborane are obtained with catalysts that also couple the dehydroproducts (e.g., Ir and Rh diphosphine and pincer catalysts). However, in comparison with the rapidly growing number of catalysts, the amount of experimental studies that deal with mechanistic details is still limited. Here, we present a comprehensive experimental and theoretical study about the mechanism of AB dehydrocoupling to polyaminoborane with ruthenium amine/amido catalysts, which exhibit particularly high activity. On the basis of kinetics, trapping experiments, polymer characterization by (11)B MQMAS solid-state NMR, spectroscopic experiments with model substrates, and density functional theory (DFT) calculations, we propose for the amine catalyst [Ru(H)2PMe3{HN(CH2CH2PtBu2)2}] two mechanistically connected catalytic cycles that account for both metal-mediated substrate dehydrogenation to aminoborane and catalyzed polymer enchainment by formal aminoborane insertion into a H-NH2BH3 bond. Kinetic results and polymer characterization also indicate that amido catalyst [Ru(H)PMe3{N(CH2CH2PtBu2)2}] does not undergo the same mechanism as was previously proposed in a theoretical study.

Boron-based hydrides for chemical hydrogen storage

Abstract: SUMMARY The development of the hydrogen economy is hampered by many issues connected with production, storage, distribution, and end-use. Although the hydrogen storage problem is particularly difficult, there are several attractive solutions under investigation, and chemical hydrogen storage (involving hydrogen-rich materials) has shown much promising properties. The boron-based materials are typical examples. They have high hydrogen densities, with up to four reactive B − H bonds. Most of the works have focused on dehydrogenation by hydrolysis or thermolysis so that it takes place in high extent in mild conditions. The first materials studied have been lithium borohydride, sodium borohydride, and ammonia borane. However, their development has been hindered by technical issues such as very high dehydrogenation temperatures, incomplete reaction, and purity of the produced hydrogen. To get round such problems, new materials have been proposed since the mid-2000s. Interestingly, those materials present attractive attributes, but also drawbacks. This is illustrated in the present review. We believe that boron-based hydrides have a significant potential in chemical hydrogen storage, but their implementation depends on the recyclability of the solid by-products; this seems to be the key factor. Copyright © 2013 John Wiley & Sons, Ltd.

Aerobic oxidative Heck/dehydrogenation reactions of cyclohexenones: efficient access to meta-substituted phenols.

Abstract: Phenol derivatives are common and important structural motifs in bioactive natural products and pharmaceuticals,[1] and the selective synthesis of substituted phenols is facilated by the strong ortho/para-directing effect of the hydroxyl group. The same directing effect, however, limits access to analogous meta-substituted derivatives. In recent years, considerable efforts have targeted C–H functionalization reactions that enable preparation of meta-substituted arenes via steric[2] or directing-group[3] control over the site selectivity. The overall efficiency of these methods is often limited by functional group interconversions or installation/removal of directing groups needed to access the final product.[4] Moreover, in molecules with more than one electronically or sterically active substituent, competition between the directing groups can lead to product mixtures. Following our recent development of Pd-catalyzed aerobic dehydrogenation reactions of ketones,[5,6,7] we envisioned that meta-substituted phenols could be accessed efficiently via an aerobic oxidative Heck/dehydrogenation sequence with cyclohexenone (Scheme 1).[8] Cyclohexenone is a convenient and inexpensive phenol precursor, and the proposed strategy exploits the intrinsic regioselectivity of additions to electron-deficient alkenes to enable functionalization of the "meta" C–H bond. Here, we describe a new Pd catalyst and reaction conditions compatible with this sequence, and we showcase their utility in the synthesis of a pharmaceutically active phenol derivative. Scheme 1 Strategy for the synthesis of meta-substituted phenols. The proposed sequence in Scheme 1 faces several challenges. The oxidative Heck reaction must be more facile than the dehydrogenation step in order to avoid direct conversion of the cyclohexenone starting material to unsubstituted phenol. Furthermore, while aerobic oxidative Heck reactions have extensive precedent with terminal alkenes,[9, 10] analogous reactions with cyclohexenone tend to be more difficult.[11,12] With this substrate, the PdII-enolate must isomerize to place the Pd atom on the opposite side of the ring in order to undergo β-hydride elimination (Scheme 2.[13] Finally, the catalyst and conditions must be compatible with both reactions in the sequence. The only general method for dehydrogenation of cyclohexenones to phenols employs a strong-acid additive (p-TsOH; Scheme 3),[5a] which interferes with oxidative Heck reactions.[14] Scheme 2 Mechanistic steps highlighting the requirement for isomerization of the PdII-enolate intermediate in Heck reactions of cyclohexenone. Scheme 3 Previously reported aerobic dehydrogenation conditions for the synthesis of phenols. Our initial studies targeted the identification of non-acidic reaction conditions for aerobic dehydrogenation of 3-methylcyclohexenone. Upon screening diverse PdX2 sources, ligands, additives and solvents (see Supp. Info. for full screening data), we found that the dicationic PdII complex [Pd(CH3CN)4](BF4)2 was particularly effective as a catalyst (Table 1). Formation of Pd black and gradual loss of catalytic activity during the reaction prompted us to test ancillary ligands to stabilize the catalyst. Most of the ligands tested inhibited the reaction (cf. Table 1 and Supp. Info.); however, 4,5-diazafluorenone L4[15] and 6,6'-dimethyl-2,2'-bipyridine L5 enabled good product yields to be obtained. While screening of numerous additives, including Bronsted bases, CuII and AgI salts, and quinones showed little beneficial effect, nearly quantitative yield of the phenol product (95%) was obtained when 9 mol % AMS (anthraquinone-2-sulfonic acid sodium salt) was included in the reaction with ligand L5.[16] The optimal result was obtained upon addition of water (20 vol %) to enhance the solubility of AMS. Table 1 Dehydrogenation of 3-Cyclohexenones: Screening Results.[a] The optimized conditions proved to be effective with a number other substituted cyclohexenones, including those with heteroatom substituents (Table 2). These neutral reaction conditions revealed some advantages over the previously reported conditions in Scheme 3. For example, 6-phenylcyclohexanone underwent dehydrogenation to o-phenyl phenol in only 33% yield under the previous conditions, but this product is obtained in excellent yields under the present conditions (entries 1 and 2). The successful reaction of 3-arylcyclohexenones, prepared via oxidative Heck reactions with cyclohexenone (entries 9–11), provided a useful starting point for the investigation of oxidative Heck and tandem oxidative Heck/dehydrogenation reactions. Table 2 Dehydrogenation of Substituted Cyclohexenones,[a] Preliminary experiments showed that this catalyst was quite effective for the oxidative Heck coupling of 4-methoxyphenylboronic acid and cyclohexenone. Moveover, the reaction could take place at 50 °C, a temperature at which no conversion of cyclohexenone to phenol was observed. In DMSO, the oxidative Heck reaction proceeded in 65% yield. Upon heating of this reaction mixture to 80 °C, nearly complete in situ conversion to the 3-aryl phenol was observed (i.e., 64% yield of the phenol; Table 3, entry 1). Several other solvents, including DMF, N-methylpyrrolidone (NMP) and 1,4-dioxane, proved to be better for the oxidative Heck reaction (entries 2–7); however, they proved less effective for the tandem sequence (e.g., entry 2). Further studies revealed that an effective one-pot sequence could be achieved by performing the oxidative Heck reaction in NMP at 50 °C, followed by addition of DMSO and heating to 80 °C for the dehydrogenation step. This protocol enabled a good yield of the phenol to be obtained (84%, entry 12). Table 3 Optimization of Conditions for the Oxidative Heck and one-pot Oxidative Heck/Dehydrogenation Reactions.[a] [Pd(CH3CN)4](BF4)2/L5 proved to be very effective as a standalone catalyst for oxidative Heck reactions with cyclohexenone. Good yields of the 3-arylcyclohexenones were obtained with diverse arylboronic acids (Table 4). Reactions with the electron-rich arylboronic acids typically led to higher yields than electron-deficient substrates as the latter substrates were more susceptible to the formation of homocoupling products. Halogenated arylboronic acids (X = F, Cl, Br) were tolerated in the oxidative Heck reaction, with yields ranging from 68% to 86% (entries 4–6 and 18). The same arylboronic acids were then employed in the one-pot oxidative Heck/dehydrogenation to afford the 3-substituted phenol derivatives. In most cases, the phenol yields correlate closely with the yields of the 3-aryl cyclohexenones in the independent oxidative Heck reaction. Table 4 Oxidative Heck and One-Pot Oxidative Heck/Dehydrogenation Reactions to Prepare Substituted Cyclohexenones and Phenols.[a] In order to demonstrate the potential utility of the aerobic oxidative Heck/dehydrogenation sequence and further test its functional group compatibility, we investigated the synthesis of URB597 from cyclohexenone and the commercially available benzamide-derived boronic acid 1 (Scheme 4). URB597 is a potent inhibitor of fatty acid amide hydrolase (FAAH) and an important focus of efforts to treat pain, anxiety and depression.[17,18] The phenol intermediate 3 was prepared via stepwise oxidative Heck coupling of 1 and cyclohexenone, followed by catalytic dehydrogenation of the isolated intermediate 2, and in a direct, one-pot process. The [Pd(CH3CN)4](BF4)2/L5 catalyst was employed for each of these steps, and both pathways led to the phenol product 3 in good yield (approx. 72%, in each case). Scheme 4 Application of one-pot oxidative Heck/dehydrogenation reactions in the synthesis of URB597. The results above highlight a new catalyst system that mediates both aerobic oxidative Heck reactions with cyclohexenone and aerobic dehydrogenation of cyclohexenones. The one-pot sequence developed for these reactions represents an efficient strategy for the preparation of meta substituted of phenols, which should be advantageous or highly competitive with other approaches based on C–H functionalization of an aromatic ring.

Selectivity in propene dehydrogenation on Pt and Pt3Sn surfaces from first principles

Abstract: Propene can be produced via dehydrogenation of propane on Pt-based catalysts; however, the catalysts are plagued by low selectivity toward propene and high coke formation. The selectivity can be improved and the coke formation reduced by alloying Pt with Sn. The alloying is known to weaken the binding of propene, which in part explains the improved performance. We conducted density functional theory calculations to study the dehydrogenation of propene on flat and stepped Pt and Pt3Sn surfaces. The steps on Pt dehydrogenate propene readily, whereas, on Pt3Sn, the steps are inert because they are decorated with Sn. Our results indicate that the high selectivity and low coking on the Pt–Sn catalyst can result from a lack of active Pt step sites.

How Strain Affects the Reactivity of Surface Metal Oxide Catalysts

Abstract: Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepared by anion exchange resulting in a series of catalysts with changing Mo densities (0.2–2.5 Mo atoms nm−2). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temperature-programmed reduction and a steep increase in the catalytic activity observed in metathesis of propene and oxidative dehydrogenation of propane at 8 % of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calculations, NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.

Oxidant-free conversion of primary amines to nitriles.

Abstract: An amide-derived NNN-Ru(II) hydride complex catalyzes oxidant-free, acceptorless, and chemoselective dehydrogenation of primary and secondary amines to the corresponding nitriles and imines with liberation of dihydrogen. The catalyst system tolerates oxidizable functionality and is selective for the dehydrogenation of primary amines (-CH2NH2) in the presence of amines without α-CH hydrogens.