Site-Selective Hydrogenation of Electron-Poor Alkenes and Dienes Enabled by a Rhodium-Catalyzed Hydride Addition/Protonolysis Mechanism.

The transition metal catalyzed hydrogenation of alkenes is a well-developed technology used on lab scale as well as on large scales in the chemical industry. Site- and chemoselective mono-hydrogenations of polarized conjugated dienes remain challenging. Instead, stoichiometric main-group hydrides are used rather than H2 . As part of an effort to develop a scalable route to prepare geranylacetone, we discovered that Rh(CO)2 acac/xantphos based catalysts enable the selective mono-hydrogenation of electron-poor 1,3-dienes, enones, and other polyunsaturated substrates. D-labeling and DFT studies support a mechanism where a nucleophilic RhI -hydride selectively adds to electron-poor alkenes and the resulting Rh-enolate undergoes subsequent inner-sphere protonation by alcohol solvent. The finding that (Ln )Rh(H)(CO) type catalysts can enable selective mono-hydrogenation of electron-poor 1,3-dienes provides a valuable tool in the design of related chemoselective hydrogenation processes.

Automated input structure generation for single-ended reaction path optimizations.

The exploration of a reaction network requires highly automated workflows to avoid error-prone and time-consuming manual steps. In this respect, a major bottleneck is the search for transition-state (TS) structures, which frequently fails and, therefore, makes (manual) revision necessary. In this work, we present a technique for obtaining suitable input structures for automated TS searches based on single-ended reaction path optimization algorithms, which makes subsequent TS searches via this method significantly more robust. First, possible input structures are generated based on the spatial alignment of the reactants. The appropriate orientation of reacting groups is achieved via stepwise rotations along selected torsional degrees of freedom. Second, a ranking of the obtained structures is performed according to selected geometric criteria. The main goals are to properly align the reactive atoms, to avoid hindrance within the reaction channel and to resolve steric clashes between the reactants. The developed procedure has been carefully tested on a variety of examples and provides suitable input structures for TS searches within seconds. The method is in daily use in an industrial setting.

Learning Design Rules for Selective Oxidation Catalysts from High-Throughput Experimentation and Artificial Intelligence.

The design of heterogeneous catalysts is challenged by the complexity of materials and processes that govern reactivity and by the fact that the number of good catalysts is very small in comparison to the number of possible materials. Here, we show how the subgroup-discovery (SGD) artificial-intelligence approach can be applied to an experimental plus theoretical data set to identify constraints on key physicochemical parameters, the so-called SG rules, which exclusively describe materials and reaction conditions with outstanding catalytic performance. By using high-throughput experimentation, 120 SiO2-supported catalysts containing ruthenium, tungsten, and phosphorus were synthesized and tested in the catalytic oxidation of propylene. As candidate descriptive parameters, the temperature and 10 parameters related to the composition and chemical nature of the catalyst materials, derived from calculated free-atom properties, were offered. The temperature, the phosphorus content, and the composition-weighted electronegativity are identified as key parameters describing high yields toward the value-added oxygenate products acrolein and acrylic acid. The SG rules not only reflect the underlying processes particularly associated with high performance but also guide the design of more complex catalysts containing up to five elements in their composition.

Phosphine-Catalyzed Vinylation at Low Acetylene Pressure.

The vinylation of various nucleophiles with acetylene at a maximum pressure of 1.5 bar is achieved by organocatalysis with easily accessible phosphines like tri-n-butylphosphine. A detailed mechanistic investigation by quantum-chemical and experimental methods supports a nucleophilic activation of acetylene by the phosphine catalyst. At 140 °C and typically 5 mol % catalyst loading, cyclic amides, oxazolidinones, ureas, unsaturated cyclic amines, and alcohols were successfully vinylated. Furthermore, the in situ generation of a vinyl phosphonium species can also be utilized in Wittig-type functionalization of aldehydes.

Revealing the structure of a catalytic combustion active-site ensemble combining uniform nanocrystal catalysts and theory insights.

Supported metal catalysts are extensively used in industrial and environmental applications. To improve their performance, it is crucial to identify the most active sites. This identification is, however, made challenging by the presence of a large number of potential surface structures that complicate such an assignment. Often, the active site is formed by an ensemble of atoms, thus introducing further complications in its identification. Being able to produce uniform structures and identify the ones that are responsible for the catalyst performance is a crucial goal. In this work, we utilize a combination of uniform Pd/Pt nanocrystal catalysts and theory to reveal the catalytic active-site ensemble in highly active propene combustion materials. Using colloidal chemistry to exquisitely control nanoparticle size, we find that intrinsic rates for propene combustion in the presence of water increase monotonically with particle size on Pt-rich catalysts, suggesting that the reaction is structure dependent. We also reveal that water has a near-zero or mildly positive reaction rate order over Pd/Pt catalysts. Theory insights allow us to determine that the interaction of water with extended terraces present in large particles leads to the formation of step sites on metallic surfaces. These specific step-edge sites are responsible for the efficient combustion of propene at low temperature. This work reveals an elusive geometric ensemble, thus clearly identifying the active site in alkene combustion catalysts. These insights demonstrate how the combination of uniform catalysts and theory can provide a much deeper understanding of active-site geometry for many applications.

Ru0 or RuII: A Study on Stabilizing the "Activated" Form of Ru-PNP Complexes with Additional Phosphine Ligands in Alcohol Dehydrogenation and Ester Hydrogenation.

The complex Ru-MACHO has been previously shown to undergo uncontrolled degradation subsequent to base-induced dehydrochlorination in the absence of a substrate. In this study, we report that stabilization of the dehydrochlorinated Ru-MACHO with phosphines furnishes complexes whose structures depend on the phosphines employed: while PMe3 led to the expected octahedral RuII complex, PPh3 provided access to a trigonal-bipyramidal Ru0 complex. Because both complexes proved to be active in base-free (de)hydrogenation reactions, thorough quantum-chemical calculations were employed to understand the reaction mechanism. The calculations show that both complexes lead to the same mechanistic scenario after phosphine dissociation and, therefore, only differ energetically in this step. According to the calculations, the typically proposed metal-ligand cooperation mechanism is not the most viable pathway. Instead, a metal-ligand-assisted pathway is preferred. Finally, experiments show that phosphine addition enhances the catalyst's performance in comparison to the PR3-free "activated" Ru-MACHO.

Synergistic effects of binary oil mixtures on the solubility of sunscreen UV absorbers.

Sunscreens for the photoprotection of human skin often are prepared as emulsions, containing organic UV-absorber molecules dissolved in the oil phase. The solubility of such oil-soluble UV-absorbers can be a limiting factor when aiming for high protection against UV-radiation. Possible synergistic effects of combinations of oil components toward UV-absorber solubility are therefore of great interest. Since a multitude of different combinations of oil components are possible, it would be desirable to predict synergistic effects by computational methods. As a model system, the solubility of a hydroxyphenyl triazine type UV-absorber was studied in several binary oil mixtures, experimentally and also by using a computational procedure based on density functional theory (DFT) and the continuum solvation model COSMO-RS. We have found good agreement of experimental and computational results. Computational methods may thus be employed to predict synergistic behaviour of solubility for systems containing two or more solvents.

Technical Synthesis of 1,5,9-Cyclododecatriene Revisited: Surprising Byproducts from a Venerable Industrial Process.

The synthesis of 1,5,9-cyclododecatriene by selective trimerization of butadiene catalyzed by TiCl4 and ethylaluminum sesquichloride has been commercially used since 1965. Although thoroughly investigated, not all details of the mechanism are completely understood. The recent development of a new process to produce cyclododecanone involving oxidation of 1,5,9-cyclododecatriene with N2O has led to the serendipitous discovery of an array of hitherto unknown byproducts, formed in the trimerization of butadiene: eleven tricyclic C12H20 and one tetracyclic C12H18 hydrocarbons, three of which had never been described before. The identification of these byproducts became possible by using a combination of chemical enrichment, high-resolution distillation, 13C-2D-INADEQUATE NMR, and comparison with ab initio calculated spectra, thus demonstrating the power of these combined techniques. The identification of these byproducts contributes to a better understanding of the mechanism of this centrally important reaction.

Crystallization Velocity and UV Performance of Formulations With Oversaturated UV-Filter Content.

Cosmetic oils are used to dissolve crystalline lipophilic UV filters; however, little knowledge exists about the effect of other formulation ingredients. This study investigates the influence of emulsifiers on the recrystallization speed of 4 UV filters and the impact of UV-filter crystal formation on delivered performance. The crystallization pattern of studied UV filters was assessed using X-ray diffractometry, whereas their recrystallization speed in formulations with various emulsifiers was monitored microscopically. UV-filter concentration was above the saturation level to promote recrystallization. Furthermore, to understand the kinetics of recrystallization, the conformer number of each UV-filter was calculated. For the impact on performance, the absorbance of a sunscreen was measured before and after recrystallization of the contained UV filter. This study confirmed the crystallinity of tested UV filters. The emulsifier was shown to influence the UV-filter recrystallization speed in emulsions. Continuous oil phase sunscreens were critical; all UV filters recrystallized promptly in oils and water-in-oil emulsions. Large molecule UV filters showed slowest recrystallization speed explained by a higher number of possible conformers. Finally, this work confirmed the negative impact of crystal formation on the delivered photoprotection of a sunscreen.

Direct Asymmetric Ruthenium-Catalyzed Reductive Amination of Alkyl-Aryl Ketones with Ammonia and Hydrogen.

The asymmetric ruthenium-catalyzed reductive amination employing ammonia and hydrogen to primary amines is described. Here we demonstrate the capability of our catalyst to perform a chemo- and enantioselective process while using simple ammonia gas as a reagent, one of the most attractive and industrially relevant nitrogen sources. The presence of a catalytic amount of ammonium iodide was essential for obtaining good yields and enantioselectivities. The mechanism of this reaction was investigated by DFT and we found a viable pathway that also explains the trend and magnitude of enantioselectivity through the halide series in good agreement with the experimental data. The in-depth investigation of substrate conformers during the reaction turned out to be crucial in obtaining an accurate prediction of the enantioselectivity. Furthermore, we report the crystallographic data of the chiral [Ru(I)H(CO)((S,S)-f-binaphane)(PPh3)] complex, which we identified as the most efficient catalyst in our investigation.

Synthesis of Mono- and Dinuclear Vanadium Complexes and Their Reactivity toward Dehydroperoxidation of Alkyl Hydroperoxides.

Several vanadium(V) complexes with either dipic-based or Schiff base ligands were synthesized. The complexes were fully characterized by elemental analysis, IR, 1H, 13C, and 51V NMR spectroscopy, as well as mass spectrometry and X-ray diffraction. Furthermore, they were tested toward their catalytic deperoxidation behavior and a significant difference between 4-heptyl hydroperoxide and cyclohexyl hydroperoxide was observed. In the case of 4-heptyl hydroperoxide, the selectivity toward the corresponding ketone was higher than with cyclohexyl hydroperoxide. DFT calculations performed on the vanadium complex showed that selective decomposition of secondary hydroperoxides with vanadium(V) to yield the corresponding ketone and water is indeed energetically feasible. The computed catalytic path, involving cleavage of the O-O bond, hydrogen transfer, release of ketone/water, and finally addition of hydroperoxide, can proceed without the generation of radical species.

The first catalytic synthesis of an acrylate from CO2 and an alkene-a rational approach.

For more than three decades the catalytic synthesis of acrylates from the cheap and abundantly available C(1) building block carbon dioxide and alkenes has been an unsolved problem in catalysis research, both in academia and industry. Herein, we describe a homogeneous catalyst based on nickel that permits the catalytic synthesis of the industrially highly relevant acrylate sodium acrylate from CO(2), ethylene, and a base, as demonstrated, at this stage, by a turnover number of greater than 10 with respect to the metal.

Prediction of the vapor pressure and vaporization enthalpy of 1-n-alkyl-3-methylimidazolium-bis-(trifluoromethanesulfonyl) amide ionic liquids.

The vapor pressures and vaporization enthalpies of a series of 1-n-alkyl-3-methylimidazolium-bis-(trifluoromethanesulfonyl) amide ionic liquids have been predicted with two different approaches using the COSMO-RS method and quantum chemical gas phase calculations. While the calculated enthalpies are in good agreement with the experimental data, COSMO-RS seems to underestimate the vapor pressures by roughly 0.5-4 log units dependent on the IL and approach used.

Zinc Silicates: Very Efficient Heterogeneous Catalysts for the Addition of Primary Alcohols to Alkynes and Allenes.

There is an astonishing parallel between the mechanism generally accepted for the addition of water to CO2 catalyzed by the enzyme carbonic anhydrase and the mechanism calculated for the addition of methanol to allene catalyzed by the naturally occurring zinc silicate hemimorphite. The latter reaction was investigated in detail following the observation that hemimorphite as well as an amorphous zinc silicate prepared in situ are excellent heterogeneous catalysts for the addition of primary alcohols to alkynes and allenes [Eq. (1)].