Facile Bifunctional Addition of Lactones and Esters at Low Temperatures. The First Intermediates in Lactone/Ester Hydrogenations

The 2-(aminomethyl)pyridine (ampy) ligand is known to activate ruthenium complexes for the catalytic hydrogenation of ketones. Here we prepare well-defined catalysts using the new ligand 2-amino-2-(2-pyridyl)propane (appH) in order to elucidate the role of the pyridyl group. The ligand has two methyl groups on the α-carbon to block β-hydride elimination reactions. It reacts with RuHCl(S-binap)(PPh3) to produce the orange-yellow complex RuHCl(S-binap)(appH) (2). In the presence of a strong base (KOtBu), complex 2 is converted into an active catalyst for the H2-hydrogenation of acetophenone in benzene under mild conditions (20 °C, 5 atm H2). Solutions of 2 rapidly react with KOtBu under an argon atmosphere to produce a deep red amidohydrido complex RuH(S-binap)(app) (3), which is an active catalyst. A crystal structure determination of 3 represents the first structure of a Ru-binap hydrido-amido complex. It reveals a five-coordinate Ru(II) center with a short Ru−N(amido) distance (1.962(3) Å) and a trigonal planar geometry at the amido nitrogen. The kinetic experiments using 3 as a catalyst and acetophenone as a substrate in benzene show that the rate of 1-phenylethanol production is dependent on both catalyst and H2 concentrations. These results parallel the behavior of the conventional Noyori-type Ru(II) catalysts with diamine ligands. However, unique features of catalysis with 3 are as follows: (1) the formation of a dihydride is thermodynamically unfavorable at 1 atm H2, 20 °C; (2) the rate shows a dependence on the product concentration since it increases as the product builds up during the reaction in an autocatalytic fashion. A significant increase in the initial rate is observed when a critical concentration of rac-1-phenylethanol is present at the beginning of the reaction. The addition of 2-propanol in benzene raises the rate as well, and the fastest H2-hydrogenation is achieved if 2-propanol is used as a solvent. This “alcohol effect” is favored by the pyridyl ligand app since it was not observed for the similar catalyst RuH(NHCMe2CMe2NH2)(binap). While 3 is an exceptional catalyst for H2-hydrogenation in 2-propanol (TOF > 6700 h-1 at 20 °C, 5 atm H2), it has a lower activity in transfer hydrogenation from the same solvent under comparable conditions (TOF 110 h-1 at 20 °C, 1 atm Ar). DFT calculations on the model amido complex Ru(H)(PH3)2(HNCH2C5H4N) (4) confirm that the splitting of H2 to give the trans dihydride is the turnover-limiting step and lies 9 kcal/mol in free energy above the transition state for the ketone hydrogenation step. The formation of the dihydride is entropically unfavorable. The theoretical activation barrier for H2 splitting is lowered by 5 kcal/mol by an alcohol-assisted mechanism but still remains higher in energy than the ketone hydrogenation step. This latter step can also be alcohol-assisted and can result in a different ee in the product alcohol than without alcohol assistance, as observed experimentally for reactions using 2-propanol versus benzene as the solvent. With alcohol present, an alkoxohydridoruthenium(II) complex is calculated to be the catalyst resting state.

Xylitol is one of the most famous chemicals known to people as the essential ingredient of chewing gum and as the sugar alternative for diabetics. Catalytic hydrogenation of biomass-derived xylose with H2 to produce high-value xylitol has been carried out under harsh reaction conditions. Herein, we exhibit the combination of Ru NPs with an environmentally benign MOF (ZIF-67) to afford a heterogeneous composite catalyst. Complete conversion of xylose with 100% selectivity to xylitol was achieved at 50°C and 1 atm H2. This is the first successful attempt to produce xylitol with ambient pressure H2 as well as the first time to achieve a 100% selectivity of xylitol for applicable catalysts. We also proved the universality of the Ru@ZIF-67 towards other hydrogenation processes. Under 1 atm H2, we achieved 100% conversion and >99% selectivity of 1-phenylethanol at 50°C for the hydrogenation of acetophenone. This is also the first report of hydrogenating acetophenone to 1-phenylethanol under 1 atm H2, which confirms that our result not only contributes to enhance the industrial yields of xylitol and reduces both the economical and energy costs but also provides new perspectives on the other hydrogenation process with H2.

The hydrogenation of cinnamaldehyde under mild conditions (293–343 K; 1 atm H2) by using Pd-based catalysts supported on different metal oxides (SiO2, Co3O4, Fe2O3) has been investigated. Compared to other supports, cobalt oxide was found to promote the catalytic properties of palladium effectively for the hydrogenation of the conjugated carbonyl group. The effect of the addition of Sn to Pd/Co3O4 and reaction temperature on the selectivity to cinnamyl alcohol was also evaluated. A low reaction temperature (293 K) and a proper Sn loading (1 wt%) further enhance the selectivity of Pd/Co3O4 towards the desired unsaturated alcohol (up to 18.9 % at 55 % conversion of the substrate).

The synthesis and characterization of two new square-planar (COD)M(κ2-P,O-phosphinoenolate) complexes (M = Rh, 3a; M = Ir, 3b; COD = η4-1,5-cyclooctadiene) are described. Complex 3b represents an unusual example of a neutral (not cationic) square-planar Ir(I) complex that is capable of mediating the reduction of alkenes under mild benchtop conditions (∼1 atm of H2, 22 °C) in the presence of high-polarity (THF, CH2Cl2, CH3CN) and low-polarity solvents (hexanes, benzene).

Polyfluorinated arenes are increasingly used in industry and can be considered emerging contaminants. Environmentally applicable degradation methods leading to full defluorination are not reported in the literature. In this study, it is demonstrated that the heterogeneous catalyst Rh/Al2O3 is capable of fully defluorinating and hydrogenating polyfluorinated benzenes in water under mild conditions (1 atm H2, ambient temperature) with degradation half-lives between 11 and 42 min. Analysis of the degradation rates of the 12 fluorobenzene congeners showed two trends: slower degradation with increasing number of fluorine substituents and increasing degradation rates with increasing number of adjacent fluorine substituents. The observed fluorinated intermediates indicated that adjacent fluorine substituents are preferably removed. Besides defluorination and hydrogenation, the scope of the catalyst includes dehalogenation of polychlorinated benzenes, bromobenzene, iodobenzene, and selected mixed dihalobenzenes. Polychlorobenzene degradation rates, like their fluorinated counterparts, decreased with increasing halogen substitution. In contrast to the polyfluorobenzenes though, removal of chlorine substituents was sterically driven. All monohalobenzenes were degraded at similar rates; however, when two carbon-halogen bonds were in direct intramolecular competition, the weaker bond was broken first. Differences in sorption affinities of the substrates are suggested to play a major role in determining the relative rates of transformation of halobenzenes by Rh/Al2O3 and H2.

A polymeric hydrogen carrier with reversible H2 storage capability was developed by incorporating a quinoxaline/tetrahydroquinoxaline redox couple into an aliphatic polymer chain. The quinoxaline unit was designed with a view to significantly increase the mass H2 storage density in the polymer up to 2.6 wt % calculated for the repeating unit, compared with the previously reported density of 0.9 wt % for poly(vinylfluorenone). An iridium complex-catalyzed hydrogenation of quinoxaline was characterized by its reversibility according to temperature, giving rise to tetrahydroquinoxaline under mild conditions (60 °C and 1 atm of H2), which released H2 gas by simply warming in common organic solvents. Its polymer extension allowed quinoxaline to be a polymeric hydrogen carrier in a solid state, which was characterized by the inherent advantages of safety, moldability, and handling easiness. Poly(vinylquinoxaline) was prepared by the radical polymerization of vinylquinoxaline. Poly(vinylquinoxaline) swollen with γ-butyrolactone became a gel-like solid, which quantitatively released and then fixed H2 gas under mild conditions.

In the context of advancing the use of metal-based building blocks for the construction of mechanically interlocked molecules, we herein describe the preparation of late transition metal containing [2]rotaxanes (1). Capture and subsequent retention of the interlocked assemblies are achieved by the formation of robust and bulky complexes of rhodium(iii) and iridium(iii) through hydrogenation of readily accessible rhodium(i) and iridium(i) complexes [M(COD)(PPh3)2][BArF4] (M = Rh, 2a; Ir, 2b) and reaction with a bipyridyl terminated [2]pseudorotaxane (3·db24c8). This work was underpinned by detailed mechanistic studies examining the hydrogenation of 1 : 1 mixtures of 2 and bipy in CH2Cl2, which proceeds with disparate rates to afford [M(bipy)H2(PPh3)2][BArF4] (M = Rh, 4a[BArF4], t = 18 h @ 50 °C; Ir, 4b[BArF4], t < 5 min @ RT) in CH2Cl2 (1 atm H2). These rates are reconciled by (a) the inherently slower reaction of 2a with H2 compared to that of the third row congener 2b, and (b) the competing and irreversible reaction of 2a with bipy, leading to a very slow hydrogenation pathway, involving rate-limiting substitution of COD by PPh3. On the basis of this information, operationally convenient and mild conditions (CH2Cl2, RT, 1 atm H2, t ≤ 2 h) were developed for the preparation of 1, involving in the case of rhodium-based 1a pre-hydrogenation of 2a to form [Rh(PPh3)2]2[BArF4]2 (8) before reaction with 3·db24c8. In addition to comprehensive spectroscopic characterisation of 1, the structure of iridium-based 1b was elucidated in the solid-state using X-ray diffraction.

Disilaferra- and disilaruthenacyclic complexes containing mesityl isocyanide as a ligand, 3' and 4', were synthesized and characterized by spectroscopy and crystallography. Both 3' and 4' showed excellent catalytic activity for the hydrogenation of alkenes. Compared with iron and ruthenium carbonyl analogues, 1' and 2', the isocyanide complexes 3' and 4' were more robust under the hydrogenation conditions, and were still active even at higher temperatures (∼80 °C) under high hydrogen pressure (∼20 atm). The iron complex 3' exhibited the highest catalytic activity toward hydrogenation of mono-, di-, tri-, and tetrasubstituted alkenes among currently reported iron catalysts. Ruthenium complex 4' catalyzed hydrogenation under very mild conditions, such as room temperature and 1 atm of H2. The remarkably high catalytic activity of 4' for hydrogenation of unfunctionalized tetrasubstituted alkenes was especially notable, because it was comparable to the activity of iridium complexes reported by Crabtree and Pfaltz, which are catalysts with the highest activity in the literature. DFT calculations suggested two plausible catalytic cycles, both of which involved activation of H2 assisted by the metal-silicon bond through σ-bond metathesis of late transition metals (oxidative hydrogen migration). The linear structure of M-C≡N-C (ipso carbon of the mesityl group) played an essential role in the efficient hydrogenation of sterically hindered tetrasubstituted alkenes.

Doing it on the cheap: An iron-based homogeneous catalyst for ketone hydrogenation was reported, thus avoiding the use of a precious metal. Excellent yields and chemoselectivity for hydrogenation are found under mild conditions (25 °C, 3 atm H2). An ionic hydrogenation mechanism allows the delivery of a proton from the OH group and a hydride from the metal center (see picture).

The [OsH(CO)(NCMe)2(PPh3)2]BF4 complex (1) is an efficient and regioselective precatalyst for the hydrogenation of the nitrogen-containing ring of quinoline (Q), isoquinoline (iQ), 5,6- and 7,8-benzoquinoline (BQ), and acridine (A) under mild reaction conditions (125 °C and 4 atm H2). Kinetic studies of the hydrogenation of Q and iQ to give tetrahydroquinoline (THQ) and tetrahydroisoquinoline (THiQ), respectively, lead to the rate law r = K1k2/(1 + K1[H2])[Os][H2]2, which becomes r = K1k2[Os][H2]2, at low hydrogen concentrations (below 1 atm H2); the catalytically active species is of the type [OsH(CO)(L)(η1-N)(PPh3)2]BF4 [(2a): L = NCMe, N = Q; (2b): L = N = iQ]. The generic mechanisms involve a rapid and partial hydrogenation of the coordinated substrate (N) of complex (2) to yield the corresponding dihydroderivative (DHN) species [OsH(CO)(L)(η1-DHN)(PPh3)2]BF4 [(3a): L = NCMe, DHN = DHQ; (3b): L = iQ or THiQ, DHN = DHiQ], followed by the rate-determining second hydrogenation of the DHN ligand, which yield [OsH(CO)(L)(η1-THN)(PPh3)2]BF4 [(4a): L = NCMe, THN = THQ; (4b): L = iQ or THiQ, THN = THiQ]; substitution of the THN ligand by a new molecule of the respective substrate regenerates the active species and restarts the catalytic cycle. For the hydrogenation of acridine to give 9,10-dihidroacridine (acridane), the rate law was r = k1[Os][H2]; the mechanism involves the hydrogenation of the active species [OsH(CO)(NCMe)(η1-A)(PPh3)2]BF4(2c) to yield acridane and the unsaturated species [OsH(CO)(NCMe)(PPh3)2]BF4 as the rate-determining step.

Selective hydrogenation of fatty esters into high-value-added fatty alcohols over non-noble-metal (e.g., Ni and Co) catalysts is highly desirable for the utilization of natural oils. Although these catalysts offer high hydrogenation activity and are reasonably economical, they suffer from low hydrodeoxygenation activity at low temperatures while promoting certain side reactions (e.g., C–C bond hydrogenolysis and decarbonylation/decarboxylation) at high temperatures, which are undesirable for the production of fatty alcohols. In this study, we tested and optimized the performance of Ni metal catalysts with various metal–oxide modifiers and supports for the selective hydrogenation of methyl palmitate to cetyl alcohol. The most efficient catalytic system (Ni-VOx/TiO2, Ni:V ≈ 1:1, mol/mol) achieved almost quantitative conversion with 90% selectivity for cetyl alcohol under mild conditions (220 °C and 4.0 MPa of H2), which was ascribed to the ternary synergistic action of Ni, oxygen vacancies, and Lewis acid sites. The catalytic synergy of the Ni metal and oxygen vacancies (formed by the reduction of TiO2) was primarily responsible for C–O bond cleavage in methyl palmitate to form palmitic aldehyde as the main intermediate. Furthermore, the V species engaged in metal–Lewis acid cooperation at the metal–oxide interface to efficiently promote the C═O bond hydrogenation of palmitic aldehyde and suppress the undesired C–C bond cleavage, thereby facilitating the highly selective production of cetyl alcohol. The obtained insights into the active phase of Ni-VOx/TiO2 catalysts are expected to facilitate the development of highly efficient catalysts for the selective hydrogenation of fatty esters and other biomass-derived compounds containing C═O/C–O groups.

A cobalt-rhodium heterobimetallic nanoparticle-catalyzed reductive cyclization of 2-(2-nitroaryl)acetonitriles to indoles has been achieved. The tandem reaction proceeds without any additives under the mild conditions (1 atm H2 and 25 °C). This procedure could be scaled up to the gram scale. The catalytic system is significantly stable under these reaction conditions and could be reused more than ten times without loss of catalytic activity.

Stabilizers and reductants of soluble metal nanoparticle (SMNP) catalysts have particular properties that differ from those of heterogeneous catalysts and can dramatically influence the activity of catalysts. To achieve better performances of SMNP catalysts, the functions of stabilizers and reductants have to be understood. Herein, we prepared a batch of SMNP catalysts by adjusting the amount of stabilizers and the type of reductants and found that the stabilizers and reductants will affect not only the size of catalysts but also the electronic states and accessibility of active metal sites, which determine the activity of catalysts. The SMNP catalyst prepared with a suitable stabilizer and reductant presents significantly higher catalytic activity with complete conversion in the selective hydrogenation of benzoic acid to cyclohexane carboxylic acid under mild conditions (30 °C and 1 atm H2) than that of the conventional heterogeneous catalyst (Pt/C). Besides, one-step selective hydrogenation of various benzoic acid derivatives was carried out over the SMNP catalyst. These conclusions on the functions of stabilizers and reductants provide new insight into the preparation of high-efficiency SMNP catalysts.

In this study, we investigated the influence of polymer nature and support characteristics on the performance of Pd-based heterogeneous catalysts. Catalysts were prepared via sequential adsorption of poly(4-vinylpyridine) (P4VP) or chitosan (CS) and palladium ions onto MgO and SBA-15 supports under ambient conditions. The resulting hybrid materials were characterized by IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectra (XPS). TEM analysis revealed that Pd nanoparticles with an average size of 2–3 nm were well-dispersed on P4VP/MgO, while larger and less uniformly distributed particles (8–10 nm) were observed on SBA-15-based systems. Catalytic tests in the hydrogenation of 2-propen-1-ol, phenylacetylene, and 2-hexyn-1-ol under mild conditions (40 °C, 1 atm H2, ethanol) demonstrated that both the support and polymer type significantly influence activity and selectivity. P4VP-modified catalysts outperformed CS-containing analogs in all reactions. MgO-based systems showed higher activity and selectivity in 2-propen-1-ol hydrogenation compared to SBA-15-based catalysts. The 1%Pd–P4VP/MgO catalyst exhibited the best performance, with a reaction rate of 5.2 × 10−6 mol/s, 83.4% selectivity to propanol, and stable activity over 30 consecutive runs. In phenylacetylene and 2-hexyn-1-ol hydrogenation, all catalysts showed high selectivity to styrene (93–95%) and cis-2-hexen-1-ol (96–97%), respectively. The incorporation of P4VP polymer into the Pd/MgO catalyst leads to smaller and better-distributed palladium particles, resulting in enhanced catalytic activity and stability during hydrogenation reactions. These results confirm that the choice of polymer modifier and inorganic support must be tailored to the specific reaction, enabling the design of highly active and selective polymer-modified Pd catalysts for selective hydrogenation processes under mild conditions.

Multinary metal oxide semiconductors - A study of different material systems and their application in thin-film transistors