Markus Drees

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

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 Shvos 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.

Highly stereoselective proton/hydride exchange: assistance of hydrogen bonding for the heterolytic splitting of H2.

The dihydrido amine complex [Ru(H)(2)PMe(3){HN(CH(2)CH(2)P(i)Pr(2))(2)}] and H(2)O exhibit highly unusual, stereoselective H(+)/H(-) exchange, as derived using (1)H 2D EXSY NMR spectroscopy. While H(RuA) rapidly exchanges with H(2)O [k = 337(20) L mol(-1) s(-1)], no direct H(RuB)/H(2)O proton exchange was detected. Methylation of the pincer amine nitrogen results in unselective slow exchange of both hydrides with H(2)O. These results emphasize the important role of hydrogen bonding of N with Brønsted acids (e.g., water) for heteroloytic H(2) activation with Ru-amide hydrogenation catalysts, which was confirmed computationally.

Hydroxy‐Functionalized Imidazolium Bromides as Catalysts for the Cycloaddition of CO2 and Epoxides to Cyclic Carbonates

Hydroxy‐functionalized mono‐ and bisimidazolium bromides were synthesized and applied as catalysts for the cycloaddition of CO2 and epoxides to cyclic carbonates. A catalyst screening showed the influence of the number of protic hydrogen atoms at the cation for the activation of epoxides. The most active catalyst operates at very mild reaction conditions (70 °C, 0.4 MPa CO2) and can be easily recycled ten times without loss of activity.

Recycling CO2? Computational Considerations of the Activation of CO2 with Homogeneous Transition Metal Catalysts

Faced with depleting fossil carbon sources, the search for alternative energy carriers and energy storage possibilities has become an important issue. Nature utilizes carbon dioxide as starting material for storing sun energy in plant hydrocarbons. A similar approach, storing energy from renewable sources in chemical bonds with CO2 as starting material, may lead to partial recycling of CO2 created by human industrial activities. Unfortunately, currently available routes for the transformation of CO2 involve high temperatures and are often not selective. With the development of more sophisticated methods and better software, theoretical studies have become both increasingly widespread and useful. This concept article summarizes theoretical investigations of the current state of the feasibility of CO2 activation with molecular transition metal catalysts, highlighting the most promising reactions of CO2 with olefins to industrially relevant acrylic acid/acrylates, and the insertion of CO2 into metal–element bonds, particularly for the synthesis of cyclic carbonates and polymers. Rapidly improving computational power and methods help to increase the importance and accuracy of calculations continuously and make computational chemistry a useful tool helping to solve some of the most important questions for the future.

Cleavage of CO Bonds in Lignin Model Compounds Catalyzed by Methyldioxorhenium in Homogeneous Phase

Methyldioxorhenium (MDO)-catalyzed C-O bond cleavage of a variety of lignin β-O-4-model compounds yields phenolic and aldehydic compounds in homogeneous phase under mild reaction conditions. MDO is in situ generated by reduction of methyltrioxorhenium (MTO) and is remarkably stable under the applied reaction conditions allowing its reuse for least five times without significant activity loss. Based on the observed and isolated intermediates, 17 O- and 2 H-isotope labeling experiments, DFT calculations, and several spectroscopic studies, a reaction mechanism is proposed.

Dynamics of the NbCl5-Catalyzed Cycloaddition of Propylene Oxide and CO2: Assessing the Dual Role of the Nucleophilic Co-Catalysts

A mechanistic study on the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO) catalyzed by NbCl5 and organic nucleophiles such as 4-dimethylaminopyridine (DMAP) or tetra-n-butylammonium bromide (NBu4 Br) is reported. A combination of in situ spectroscopic techniques and kinetic studies has been used to provide detailed insight into the reaction mechanism, the formation of intermediates, and interactions between the reaction partners. The results of DFT calculations support the experimental observations and allow us to propose a mechanism for this reaction.

Acceleration of the Rate of the Heck Reaction through UV-and Visible-Light-Induced Palladium(II) Reduction

Irradiation with light (UV and visible) increases the rate of the Heck reaction using homogeneous (palladium(II) acetate) and heterogeneous (Pd/Al2O3 and Pd/TiO2) catalysts. The rate of the coupling of bromobenzene, chlorobenzene, and 4‐chloroacetophenone with styrene was increased under light irradiation at temperatures between 90 and 160 °C. Detailed investigations showed that light irradiation accelerates the reduction of the PdII precursor, as confirmed by 31P NMR spectroscopy, in which the in situ reduction of [Pd(OAc)2(PPh3)2] to [Pd(PPh3)n] (n=2–4) in the presence of PPh3 took only minutes under visible light irradiation and several hours in the dark. The role of light is, however, complex since it also influences other PdII reduction steps in conjunction with catalyst deactivation (Pd black formation). 31P NMR spectroscopy showed the same active species, anionic palladium halide complexes, in both irradiated and unirradiated reactions. UV/Vis absorption spectroscopy of Pd(OAc)2 and DFT calculations, theoretical UV/Vis spectra, and orbital calculations of [PdI4]2−, [PdBr4]2−, and [PdCl4]2−, showed that ligand‐to‐metal charge transfer (LMCT) is responsible for the accelerated reduction of PdII to Pd0 under light irradiation.

A Computational Study of the Stereoselective Decarboxylation in the Synthesis of Naproxen

The antiinflammatory drug Naproxen has been synthesized via several enantioselective routes. The recently published synthesis involving the asymmetric decarboxylation of 2-cyano-2-(6-methoxy-naphth-2-yl)propionic acid (H. Brunner, P. Schmidt, Eur. J. Org. Chem. 2000, 2119) was investigated by quantum-chemical calculations. It was found that the stereochemistry of the products was determined by a concerted protonation/decarboxylation reaction. According to quantum-chemical gas-phase calculations, C−H···O and N−H···O interactions between the chiral base and the substrate in the transition state stabilize the (S) enantiomer by 3.7 kcal/mol. The postulated planar ketenimine anion intermediate could be ruled out on the basis of B3LYP/6-31G(d) calculations. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Ruthenium‐Catalyzed Hydrogenation of Oxygen‐Functionalized Aromatic Compounds in Water

The application of a ruthenium complex, bearing a sulfonated bis‐N‐heterocyclic carbene (NHC) ligand as catalyst precursor for the hydrogenation of aromatic compounds is reported. The reaction proceeds under mild conditions in aqueous phase. The treatment of the Ru complex with 40 bar H2 at 60 °C in water (0.1 M KOH solution) leads to the formation of a catalytically active species, which can be stored and used in catalysis experiments. The catalyst is responsible for the hydrogenation of functionalized aromatic substrates exhibiting endo‐ and exo‐CO bonds, which are relevant products in biomass conversion. Acetophenone is quantitatively reduced to 1‐cyclohexyl ethanol with a selectivity of 100 %. Various other oxygen‐functionalized aromatic substrates were also hydrogenated in moderate to quantitative conversions. To elucidate the nature of the catalytically active species, NMR, UV/vis, and ESI‐MS experiments were undertaken, showing the presence of a mononuclear Ru hydride complex. TEM measurements of a sample of the catalyst solution did not indicate the presence of nanoparticles. Mechanistic investigations point towards a homogenous mechanism.

Reduction of carbon dioxide and organic carbonyls by hydrosilanes catalysed by the perrhenate anion

The simple perrhenate salt [N(hexyl)4][(ReO4)] acts as a catalyst for the reduction of organic carbonyls and carbon dioxide by primary and secondary hydrosilanes. In the case of CO2, this results in the formation of methanol equivalents via silylformate and silylacetal intermediates. Furthermore, the addition of alkylamines to the reaction mixture favours catalytic amine N-methylation over methanol production under certain conditions. DFT analysis of the mechanism of CO2 reduction shows that the perrhenate anion activates the silylhydride forming a hypervalent silicate transition state such that the CO2 can directly cleave a Si–H bond.