Detlef Heller

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.

New carbohydrate bisphosphites as chiral ligands

Abstract The synthesis of the 2,3-bisphosphite derivatives of phenyl 4,6- O -benzylidene-β-D-glucopyranoside leads to new chelating ligands. Their rhodium(I) and platinum(II) complexes have been tested as catalysts for the asymmetric hydroformylation of vinyl acetate, allyl acetate and p -methoxystyrene. Good regioselectivity (>90% branched product), but an enantioselectivity of only ≤36% ee were found under mild reaction conditions (25–40°C, 40–70 bar syngas).

Synthesis of the first water-soluble chiral tetrahydroxy diphosphine Rh(I) catalyst for enantioselective hydrogenation

The synthesis of the first chiral polyhydroxy diphosphine-Rh(I) catalyst for asymmetric hydrogenation is described. Based on a simple synthetic pathway starting from d-mannitol the new metal complex is constructed. A special feature of the approach is the protection of the air-sensitive phosphine groups by the catalytically active metal in the final synthetic steps. Therefore, tedious P-protection-deprotection procedures could be advantageously avoided. The new water-soluble catalyst hydrogenates functionalized olefins by up to 99.6% ee in water as solvent.

Application of the term “relative enantioselectivity” as useful measure for comparison of chiral catalysts, demonstrated on asymmetric hydrogenation of amino acid precursors☆

Abstract The application of the quotient “relative enantioselectivity” Q = q/q′ in which q and q′ are enantiomeric ratios R/S (or S/R) for two comparable cases of asymmetric synthesis experiments is recommended for comparison of two or more catalysts or other variables like solvents, substrates and cofactors. The importance of this term Q lies in the possibility to form and compare values of enantioselectivity over the whole range from 99,9 % ee (R) to 99,9 % ee (S) in a mathematically correct way and is demonstrated on multiple examples of asymmetric hydrogenation of partly new N-acyl-dehydroamino acid derivatives with a couple of catalysts [Rh(Pb-β-glup)(COD)]BF41 and [Rh(Ph-β-glup-OH)(COD)]BF42. Interesting inversions of Q could be discovered for changes of the type of substrates and in dependence of the polarity of solvents.

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.

Reduction of the rotational ambiguity of curve resolution techniques under partial knowledge of the factors. Complementarity and coupling theorems

Multivariate curve resolution techniques allow to uncover from a series of spectra (of a chemical reaction system) the underlying spectra of the pure components and the associated concentration profiles along the time axis. Usually, a range of feasible solutions exists because of the so‐called rotational ambiguity. Any additional information on the system should be utilized to reduce this ambiguity.

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.

Catalytic and Kinetic Studies of the Dehydrogenation of Dimethylamine Borane with an iPr Substituted Titanocene Catalyst

In recent years, amine borane adducts have been widely discussed as promising candidates for the storage of hydrogen, owing mainly to the high gravimetric capacity (ideally up to 19.6 % in ammonia borane) and the facile release of the energy carrier under mild conditions. A significant amount of work has been reported towards the catalytic dehydrogenation of such compounds using transition metal complexes, as well as in the efficient regeneration of spent fuels. However, some of the main drawbacks for general applicability remain, such as the production of amines and boranes using environmentally benign processes and the fact that most amine borane adducts are solid and thus cannot serve as a liquid “fuel” without dilution and loss of hydrogen storage capacity. In this context, Baker et al. reported on the use of amine borane fuel blends, which remain liquid throughout the catalytic process. Moreover, addition of an ionic liquid hindered the formation of insoluble dehydrogenation products such as linear poly(aminoborane). The use of early transition metal complexes, especially of group 4 metals for the catalytic dehydrogenation of dimethylamine borane (1) was demonstrated by Manners and Chirik and co-workers for titanocene and zirconocene compounds. A study of dimethylamine borane dehydrogenation using cationic zirconocene-phosphinoaryloxide complexes was published by Wass et al. and revealed high activities (TOF up to 600 h ) and reaction times of only several minutes. Very recently, our group reported on the efficient dehydrogenation of dimethylamine borane using well-defined metallocene bis(trimethylsilyl)acetylene complexes of the type [Cp’2M(L)(h Me3SiC2SiMe3)] [M = Ti, Cp’= cyclopentadienyl (Cp) or pentamethylcyclopentadienyl (Cp*), no L; M = Zr, Cp’= Cp, L = pyridine; M = Zr, Cp’= Cp*, no L] , which eliminate the alkyne under mild conditions and thus generate the catalytically active 14 electron fragment “Cp2M”. [7] It became evident that complexes bearing Cp ligands are active whereas fully methylated Cp* species are completely inactive for dehydrogenation reactions. However, a full study of the influence of steric and electronic parameters of substituents at the Cp ligands is not available to date. The titanocene bis(trimethylsilyl)acetylene complex [(h-iPrC5H4)2Ti(h -Me3SiC2SiMe3)] (2) appeared to be worth investigating for the catalytic dehydrogenation of dimethylamine borane (Scheme 1), as it possesses the iPr group as a single sterically demanding yet moderately electron donating substituent. Complex 2 was first described by our group several years ago and reported to be isolable as a dark yellow oily liquid.

Hydroxyalkylphosphines in asymmetric hydrogenations

Abstract The synthesis of chiral bisphosphines bearing a hydroxyl group in a rigid backbone is described. These compounds which are analogues of the prominent ligand DIOP formed definite rhodium complexes. IR spectra provided evidence that the hydroxyl groups do not participate in the complexation to the metal or to one of the acetal oxygen atoms. The complexes have proven successful in the asymmetric hydrogenation of prochiral functionalized olefins in different solvents. In methanol only small effects could be detected in comparison to parent complexes which do not carry a hydroxyl group. However, when hydrogenations were performed in methylene chloride or toluene significant differences in the enantioselectivities (by up to 17 %ee) were observed, especially in the reaction with itaconic acid or its dimethyl ester, respectively. In the latter cases the effect is contingent on the spatial orientation of the hydroxyl group in the catalyst.