Hans-Joachim Drexler

Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide

Hydrogen produced from renewable resources is a promising potential source of clean energy. With the help of low-temperature proton-exchange membrane fuel cells, molecular hydrogen can be converted efficiently to produce electricity. The implementation of sustainable hydrogen production and subsequent hydrogen conversion to energy is called “hydrogen economy”. Unfortunately, its physical properties make the transport and handling of hydrogen gas difficult. To overcome this, methanol can be used as a material for the storage of hydrogen, because it is a liquid at room temperature and contains 12.6 per cent hydrogen. However, the state-of-the-art method for the production of hydrogen from methanol (methanol reforming) is conducted at high temperatures (over 200 degrees Celsius) and high pressures (25–50 bar), which limits its potential applications. Here we describe an efficient low-temperature aqueous-phase methanol dehydrogenation process, which is facilitated by ruthenium complexes. Hydrogen generation by this method proceeds at 65–95 degrees Celsius and ambient pressure with excellent catalyst turnover frequencies (4,700 per hour) and turnover numbers (exceeding 350,000). This would make the delivery of hydrogen on mobile devices—and hence the use of methanol as a practical hydrogen carrier—feasible.

Selective Hydrogen Production from Methanol with a Defined Iron Pincer Catalyst under Mild Conditions

Molecularly well-defined iron pincer complexes promote the aqueous-phase reforming of methanol to carbon dioxide and hydrogen, which is of interest in the context of a methanol and hydrogen economy. For the first time, the use of earth-abundant iron complexes under mild conditions for efficient hydrogen generation from alcohols is demonstrated.

Mechanistic Investigations of the Rhodium Catalyzed Propargylic CH Activation

Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, in situ-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid. A σ-allyl complex was identified as the resting state of the catalytic transformation and characterized by X-ray crystallographic analysis. By means of ESI-MS investigations we were able to detect a reactive intermediate of the catalytic cycle.

Part III. COD versus NBD precatalysts. Dramatic difference in the asymmetric hydrogenation of prochiral olefins with five-membered diphosphine Rh-hydrogenation catalysts

Abstract Induction periods in the asymmetric hydrogenation of prochiral olefins with five-membered chelates of the type [Rh(PP)(diolefin)]BF4 originate from the parallel-running hydrogenation of the prochiral substrate and the diolefin that enters the system as a constituent of the precatalyst. Reactivities towards the most commonly used diolefins COD or NBD can differ by three powers of ten. X-ray crystal structure analyses of [Rh((S,S)-Et-DuPHOS)(NBD)]BF4 and of [Rh((R,R)-Et-DuPHOS)(COD)]BF4 imply that a recently discussed relation between the sense of rotation of the diolefin in the precatalyst (clockwise or anticlockwise twist) is not very likely.

Synthesis of Chiral 2,5-Bis(oxymethyl)-Functionalized Bis(phospholanes) and Their Application in Rh- and Ru-Catalyzed Enantioselective Hydrogenations

The synthesis of a series of chiral 2,5-bis(oxymethyl)-substituted bis(phospholanes) 13a−c and 15a,b (BASPHOS) is described, representing functionalized derivatives of the prominent DuPHOS or BPE ligands. D-Mannitol was used as the starting material for these ligands. New bisphospholanes were used as ligands in the enantioselective rhodium(I)-catalyzed hydrogenation of functionalized olefins like unsaturated α- and β-amino acid derivatives, itaconates, and an unsaturated phosphonate. A relevant ruthenium(II) catalyst was used for the reduction of prochiral β-oxo esters. The enantioselectivities, ranging from 8−99% ee, were strongly dependent on the type of the substituent on the oxymethyl group as well on the bridge connecting the phospholane units.

How Long Have Nonlinear Effects Been Known in the Field of Catalysis

The similarities and differences between nonlinear effects in asymmetric synthesis (see diagram), predicted 60 years ago by W. Langenbeck, and the long-known amplification phenomenon in stoichiometric reactions with chiral starting materials are discussed.

Homologous series of the PdCl2 and PtCl2 complexes of maleonitrile-dithiacrown ethers: synthesis, crystal structures, NMR spectroscopy and mass spectrometry

Abstract The chelate complexes of maleonitrile-dithiacrown ethers mn-12S 2 O 2 –mn-21S 2 O 5 (mn=maleonitrile) and of the acyclic ligand mn-S 2 with salts, respectively, of Pd(II) and Pt(II) were studied in the solid state, in solution and in the gas phase. The structures of [MCl 2 (mn-S 2 O n )] ( n =2–5) (M=Pd, Pt) were investigated experimentally by X-ray analysis, mass spectrometry and 1D and 2D NMR spectroscopy in solution; the complex formation was studied by 1 H, 13 C and 195 Pt NMR titration experiments. As techniques the fast atom bombardment (FAB), desorption ionization (DEI) and electrospray mass spectrometry (ESMS) have been applied. The results of the extraction investigations of the maleonitrile-dithiacrown ethers prove the stability of the complex [PdCl 2 (mn-S 2 O 2 )] to be much higher than that of the other ligands with the same metal ion (H.-J. Holdt, K. Gloe, H. Bukowsky, H. Stephan, Chem. Eur. J. in preparation). The explanation for the preferred complexation of mn-S 2 O 2 regarding the salts PdCl 2 and PtCl 2 is given. For the ligands with smaller ring sizes, mn-12S 2 O 2 and mn-15S 2 O 3 , one sulfur atom was found in R and the second sulfur atom in S -configuration. In the complexes of the larger macrocycle ligands as well as that of the acyclic ligand mn-S 2 , both sulfur donor atoms have the same configuration. These differences in the configuration at the S-donor atoms of the complexes and the free ligands depending on the ring size have been never described before.

Development of an Improved Rhodium Catalyst for Z‐Selective Anti‐Markovnikov Addition of Carboxylic Acids to Terminal Alkynes

To develop more active catalysts for the rhodium-catalyzed addition of carboxylic acids to terminal alkynes furnishing anti-Markovnikov Z enol esters, a thorough study of the rhodium complexes involved was performed. A number of rhodium complexes were characterized by NMR, ESI-MS, and X-ray analysis and applied as catalysts for the title reaction. The systematic investigations revealed that the presence of chloride ions decreased the catalyst activity. Conversely, generating and applying a mixture of two rhodium species, namely, [Rh(DPPMP)2][H(benzoate)2] (DPPMP=diphenylphosphinomethylpyridine) and [{Rh(COD)(μ2-benzoate)}2], provided a significantly more active catalyst. Furthermore, the addition of a catalytic amount of base (Cs2CO3) had an additional accelerating effect. This higher catalyst activity allowed the reaction time to be reduced from 16 to 1-4 h while maintaining high selectivity. Studies on the substrate scope revealed that the new catalysts have greater functional-group compatibility.

On the enantioselective hydrogenation of isomeric β-acylamido β-alkylacrylates with chiral Rh(I) complexes—comparison of phosphine ligands and substrates

Abstract The rhodium(I)-catalyzed enantioselective hydrogenation of E - and Z -configured β-acylamido β-alkylacrylates as well as of isomeric mixtures has been investigated. As ligands 1,2-bisphospholanes like DuPHOS, BPE and Me 4 -BASPHOS have been tested, but also diphosphines forming seven-membered chelates such as DIOP. The effect of additional oxy groups in the diphosphine ligand on rate and enantioselectivity was likewise elucidated. In general, with all catalysts screened the hydrogenation is strongly sensitive to the E / Z -geometry of the substrate. E -Substrates are converted with good or excellent enantioselectivites into the desired β-amino acid derivatives. The hydrogenation of Z -substrates showed the known H 2 -pressure dependency of the ee.

The Major/Minor Concept: Dependence of the Selectivity of Homogeneously Catalyzed Reactions on Reactivity Ratio and Concentration Ratio of the Intermediates

The homogeneously catalyzed asymmetric hydrogenation of prochiral olefins with cationic Rh(I) complexes is one of the best-understood selection processes. For some of the catalyst/substrate complexes, experimental proof points out the validation of the major/minor principle; the concentration-deficient minor substrate complex, which has very high reactivity, yields the excess enantiomer. As exemplified by the reaction system of [Rh(dipamp)(MeOH)2]+/methyl (Z)-alpha-acetamidocinnamate (dipamp=1,2-bis((o-methoxyphenyl)phenylphosphino)ethane), all six of the characteristic reaction rate constants have been previously identified. Recently, it was found that the major substrate complex can also yield the major enantiomer (lock-and-key principle). The differential equation system that results from the reaction sequence can be solved numerically for different hydrogen partial pressures by including the known equilibrium constants. The result displays the concentration-time dependence of all species that exist in the catalytic cycle. On the basis of the known constants as well as further experimental evidence, this work focuses on the examination of all principal possibilities resulting from the reaction sequence and leading to different results for the stereochemical outcome. From the simulation, the following conclusions can be drawn: 1) When an intermediate has extreme reactivity, its stationary concentration can become so small that it can no longer be the source of product selectivity; 2) in principle, the major/minor and lock-and-key principles can coexist depending on the applied pressure; 3) thermodynamically determined intermediate ratios can be completely converted under reaction conditions for a selection process; and 4) the increase in enantioselectivity with increasing hydrogen partial pressure, a phenomenon that is experimentally proven but theoretically far from being well-understood, can be explained by applying both the lock-and-key as well as the major/minor principle.