Jürgen Klankermayer

Enantioselective hydrogenation with chiral frustrated Lewis pairs.

The development of transition-metal-catalyzed asymmetric hydrogenation could be stated as the cradle of modern enantioselective catalysis. Since the early asymmetric hydrogenation example from Knowles and Sabacky in 1968, the method has rapidly advanced over the years into an important tool in academia and chemical industry. In general, for these transformations the development of effective transition-metal complexes having chiral ligands was a basic prerequisite. However, since the pioneering work of Stephan and coworkers in 2006, the field of homogenous hydrogenation has been extended to the possibility of metal-free hydrogenation based on the utilization of frustrated Lewis pairs (FLPs) for hydrogen activation. Combinations of the strong Lewis acid tris(perfluorophenyl)borane (B(C6F5)3) with a variety of sterically encumbered Lewis bases—phosphines, nitrogen bases, and carbon-derived bases—can be used to activate hydrogen at ambient conditions. The concept was subsequently broadened from variations of the Lewis base to modifications of the Lewis acid structure, which resulted in intramolecular FLPs and borane derivatives with increased activity and stability. Furthermore, these chemical peculiarities rapidly found application in catalytic hydrogenation reactions. Some of the FLPs were found to serve as catalysts for the hydrogenation of imines, nitriles, and functionalized alkenes. 6a,d, 8c,9b,c,10] In the absence of bulky Lewis bases also imine substrates could adopt the function of the FLP partner, and B(C6F5)3 was discovered to be sufficient as the catalyst for their hydrogenation. 11] Additionally, recent mechanistic investigations and preparative experiments corroborated the assumption that for asymmetric transformations, the element of chirality has to be favorably incorporated into the Lewis acid structure. In early experiments employing a-pinene-derived chiral borane, asymmetric reduction of imines was achieved, albeit with low enantioselectivity (13% ee). With these initial findings the synthesis of effective chiral Lewis acids for application in asymmetric hydrogenation reactions was envisioned. On the basis of this concept, the first example of the highly enantioselective hydrogenation of imines with chiral FLPs is demonstrated herein. The initial example with a-pinene-derived chiral borane confirmed the effectiveness of this catalyst structure. However, the stability of the Lewis acid emerged as a major drawback. For the further investigations a chiral borane derived from camphor was considered to be a more suitable structural motif. Reaction of (1R)-(+)-camphor (1) with phenylmagnesium bromide (2) resulted in the tertiary alcohol 3 (Scheme 1). Subsequent dehydration with thionyl chloride/

Selective Homogeneous Hydrogenation of Biogenic Carboxylic Acids with [Ru(TriPhos)H]+: A Mechanistic Study

Selective hydrogenation of biogenic carboxylic acids is an important transformation for biorefinery concepts based on platform chemicals. We herein report a mechanistic study on the homogeneously ruthenium/phosphine catalyzed transformations of levulinic acid (LA) and itaconic acid (IA) to the corresponding lactones, diols, and cyclic ethers. A density functional theory (DFT) study was performed and corroborated with experimental data from catalytic processes and NMR investigations. For [Ru(TriPhos)H](+) as the catalytically active unit, a common mechanistic pathway for the reduction of the C═O functionality in aldehydes, ketones, lactones, and even free carboxylic acids could be identified. Hydride transfer from the Ru-H group to the carbonyl or carboxyl carbon is followed by protonation of the resulting Ru-O unit via σ-bond metathesis from a coordinated dihydrogen molecule. The energetic spans for the reduction of the different functional groups increase in the order aldehyde < ketone < lactone ≈ carboxylic acid. This reactivity pattern as well as the absolute values are in full agreement with experimentally observed activities and selectivities, forming a rational basis for further catalyst development.

Enantioselective hydrosilylation with chiral frustrated Lewis pairs.

The development of transition-metal-catalyzed asymmetric hydrogenation could be stated as the cradle of modern enantioselective catalysis. Since the early asymmetric hydrogenation example from Knowles and Sabacky in 1968, the method has rapidly advanced over the years into an important tool in academia and chemical industry. In general, for these transformations the development of effective transition-metal complexes having chiral ligands was a basic prerequisite. However, since the pioneering work of Stephan and coworkers in 2006, the field of homogenous hydrogenation has been extended to the possibility of metal-free hydrogenation based on the utilization of frustrated Lewis pairs (FLPs) for hydrogen activation. Combinations of the strong Lewis acid tris(perfluorophenyl)borane (B(C6F5)3) with a variety of sterically encumbered Lewis bases—phosphines, nitrogen bases, and carbon-derived bases—can be used to activate hydrogen at ambient conditions. The concept was subsequently broadened from variations of the Lewis base to modifications of the Lewis acid structure, which resulted in intramolecular FLPs and borane derivatives with increased activity and stability. Furthermore, these chemical peculiarities rapidly found application in catalytic hydrogenation reactions. Some of the FLPs were found to serve as catalysts for the hydrogenation of imines, nitriles, and functionalized alkenes. In the absence of bulky Lewis bases also imine substrates could adopt the function of the FLP partner, and B(C6F5)3 was discovered to be sufficient as the catalyst for their hydrogenation. Additionally, recent mechanistic investigations and preparative experiments corroborated the assumption that for asymmetric transformations, the element of chirality has to be favorably incorporated into the Lewis acid structure. In early experiments employing a-pinene-derived chiral borane, asymmetric reduction of imines was achieved, albeit with low enantioselectivity (13% ee). With these initial findings the synthesis of effective chiral Lewis acids for application in asymmetric hydrogenation reactions was envisioned. On the basis of this concept, the first example of the highly enantioselective hydrogenation of imines with chiral FLPs is demonstrated herein. The initial example with a-pinene-derived chiral borane confirmed the effectiveness of this catalyst structure. However, the stability of the Lewis acid emerged as a major drawback. For the further investigations a chiral borane derived from camphor was considered to be a more suitable structural motif. Reaction of (1R)-(+)-camphor (1) with phenylmagnesium bromide (2) resulted in the tertiary alcohol 3 (Scheme 1). Subsequent dehydration with thionyl chloride/

Enhancement of near-IR emission by bromine substitution in lanthanide complexes with 2-carboxamide-8-hydroxyquinoline

Three novel 2-carboxamide-8-hydroxyquinoline derivatives wrap helically around trivalent lanthanide ions to form monometallic 3 : 1 complexes possessing strong NIR emission.

Asymmetric hydrogenation of imines with a recyclable chiral frustrated Lewis pair catalyst

A camphor based chiral phosphonium hydrido borate zwitterion was synthesised and successfully applied in the enantioselective hydrogenation of imines with selectivities up to 76% ee. The high stability of the novel chiral FLP-system enables effective recycling of the metal-free catalyst.

Trimethylenemethane-Ruthenium(II)-Triphos complexes as highly active catalysts for catalytic C—O bond cleavage reactions of lignin model compounds

Catalysis is expected to play an important role for valorization of lignocellulosic biomass by means of selective and sustainable synthetic pathways to chemicals and fuels. The gradual conversion of the biomass feedstock to the envisaged target structures requires the development of specialized and robust catalysts adapted to the challenging defunctionalization reactions of the complex and the diverse raw materials and intermediates involved. Although there are various established routes for the conversion of the carbohydrate constituents of lignocellulose, access to lignin-based chemicals remains challenging because of the complex, heterogeneous structure of the aromatic biopolymer. Within the multifaceted lignin network, b-[O]-4’-glycerolarylether linkages represent the most abundant connection and therefore provide a promising target for selective depolymerization and defunctionalization. Recently, R. B. Bergman and J. A. Ellman reported that 2-aryloxy-1-arylethanol derivatives, mirroring the basic b-O-4-linkage in lignin, can be effectively disconnected by applying a ruthenium catalyzed tandem hydrogen transfer/C O cleavage reaction. During their evaluation of catalytic systems based on various phosphine ligands and ruthenium precursors, the combination of [H2Ru(PPh3)3CO] and the bidentate phosphine ligand (9,9-dimethyl-9H-xanthene-4,5-diyl)bis(diphenylphosphine) XANTPHOS (A) proved to be the only system providing efficient catalytic turnover. In the present study, we report on a new ruthenium-phosphine catalyst that exhibits comparable activity for this cleavage reaction. It is based on the combination of ruthenium with tripodal phosphine ligands of the TRIPHOS-type, 6] and the unprecedented formation of a trimethylenemethane (TMM) ligand in the coordination sphere of ruthenium. The model substrate for the initial screening was 2-phenoxy1-phenylethanol (1), leading to phenol (2) and acetophenone (3) as products after C O-bond cleavage by means of internal hydrogen transfer. Combinations of the metal precursor [Ru(cod)(methallyl)2] (COD = 1,5-cyclooctadiene, methallyl =h C4H7) with various phosphine ligands were tested under a standard set of reaction conditions. Selected results are shown in Table 1.

Enantioselective Hydrogenation with Racemic and Enantiopure Binap in the Presence of a Chiral Ionic Liquid

There is growing interest in the use of chiral ionic liquids (cILs) in asymmetric catalysis as the reaction media or as an additive. Whereas chiral solvents have shown limited success in enantioselective synthesis, the use of cILs have recently resulted in generating significant enantioselectivity in organocatalysis, heterogeneous catalysis, and transition-metal-catalyzed reactions. As part of our interest in this area, we investigated the Rh-catalyzed homogeneous hydrogenation in amino-acid-derived cILs. Product enantioselectivities up to 69% ee were obtained by using rhodium catalysts derived from tropoisomeric phosphine ligands in combination with cILs as the only source of fixed chirality. Herein we report for the first time that cILs can be used to induce high levels of enantioselectivity when combined with racemic catalysts; the product enantioselectivites obtained are as high as those obtained with the corresponding enantiomerically pure ligand. We provide experimental evidence that the key role of the cIL is to effectively block the catalytic cycle for one of the two enantiomers of the catalyst (chiral poisoning). In addition, the cIL can amplify and even reverse the enantioselectivity of a given enantiopure ligand in comparison to the reaction in organic solvents. Binap (2,2’-bis(diphenylphosphanyl)-1,1’-binaphthyl) was selected as a prototypical ligand as it has a broad range of possible applications. The rhodium-catalyzed hydrogenation of dimethyl itaconate (S1) and methyl N-acetamido acrylate (S2) were chosen as benchmark reactions (Scheme 1). Under conventional conditions, enantiomerically pure (S)-binap leads to only moderate enantioselectivities (P1: 67% ee, (S); P2 : 21–25% ee, (R)) in these transformations, thus providing a sensitive diagnostic tool for the effectiveness of the cIL. Themethyl ester of (S)-proline was used as the source of chirality in the cIL ([MeProl][NTf2]), which has already proved successful in case of the tropoiosmeric ligands. The hydrogenation of S1 was carried out under a set of standard reaction conditions employing a 5:1 mixture of CH2Cl2 and [MeProl][NTf2] as the reaction medium. By using a rhodium catalyst, formed in situ from [Rh(acac)(cod)] (A ; acac= acetylacetonate, cod= 1,5-cyclooctadiene) and racemic binap, (S)-2-methyl-succinic acid dimethyl ester ((S)-P1) was obtained quantitatively with an enantioselectivity of 67% ee (Table 1, entry 1). Almost the same enantioselectivity was achieved with complex B as the rhodium source (Table 1, entry 2). These results demonstrate that an identical level of enantiodifferentiation can be achieved with the combination of racemic binap and [MeProl][NTf2], compared to that obtained with a single enantiomer of the chiral ligand. In the case of substrate S1, the presence of the cIL does not affect the principle mode of enantiodifferentiation of the chiral ligand. This is demonstrated by the observation that the use of enantiomerically pure (R)-binap leads to enantioselectivities of 66–71% for (R)-P1 in the presence of [MeProl][NTf2] (Table 1, entry 3 and 4). The use of (S)-binap results in (S)-P1 having almost identical enantioselectivities of 64–70%

Advanced Biofuels and Beyond: Chemistry Solutions for Propulsion and Production

Sustainably produced biofuels, especially when they are derived from lignocellulosic biomass, are being discussed intensively for future ground transportation. Traditionally, research activities focus on the synthesis process, while leaving their combustion properties to be evaluated by a different community. This Review adopts an integrative view of engine combustion and fuel synthesis, focusing on chemical aspects as the common denominator. It will be demonstrated that a fundamental understanding of the combustion process can be instrumental to derive design criteria for the molecular structure of fuel candidates, which can then be targets for the analysis of synthetic pathways and the development of catalytic production routes. With such an integrative approach to fuel design, it will be possible to improve systematically the entire system, spanning biomass feedstock, conversion process, fuel, engine, and pollutants with a view to improve the carbon footprint, increase efficiency, and reduce emissions.

Optically active transition-metal complexes ☆: Part 124. Chiral 1-phospha[1]ferrocenophanes and 1,12-diphospha[1.1]ferrocenophanes — synthesis, characterization and ring-opening polymerization

Abstract The phosphorus(III)-bridged [1]ferrocenophanes 1,1′-ferrocenediylphenylphosphine ( 1 ), (−)-1,1′-ferrocenediylmenthylphosphine ( 2 ) and (−)-bornyl-1,1′-ferrocenediylphosphine ( 3 ) have been synthesized via the reaction of 1,1′-dilithioferrocene (TMEDA adduct) and Cl 2 PR (R=Ph, Men, Bor). Compounds 1 and 2 have been used as ligands in the preparation of the complexes Cp*Mn(CO) 2 [Fe(η 5 -C 5 H 4 ) 2 PPh] ( 4 ) and (−)- trans -PdCl 2 [Fe(η 5 -C 5 H 4 ) 2 PMen] 2 ( 5 ). The new compounds 2 – 5 were characterized by multinuclear NMR, by MS, and 2 , 4 and 5 by single-crystal X-ray diffraction. Remarkably, the cyclic dimer anti - exo,exo -1,12-dimenthyl-1,12-diphospha[1.1]ferrocenophane ( 6 ) could be isolated and structurally characterized. The thermal ring-opening polymerization of 1 , 2 and 3 yielded the poly(ferrocenediylphosphines) 7 , 8 and 9 . Compounds 2 and 8 were used as chiral ligands in the Rh-catalyzed diastereoselective hydrogenation of folic acid.

Asymmetric Autocatalysis with Organozinc Complexes; Elucidation of the Reaction Pathway

This review describes the development of mechanistic understanding of amplifying asymmetricautocatalysis. After a brief description of kinetics, the main body of the work discusses theapplication of a variety of NMR techniques to the structure of the resting state in solution.The results are consistent with a dominant square Zn–O bonded dimer at ambient temperature.Furthermore, the energies of homo- and heterochiral dimers is comparable; they exchange slowly onthe NMR timescale but fast enough for the lifetime of an individual molecule to be established. Theassociation of alkylzinc with this dimer can be quantified and located, and dynamic alkyl exchangesdefined. DFT calculations have been carried out, which underpin the dimer structure and provide furtherinsight into the steric control of autocatalysis by the bulk of diisopropylzinc. NMR, kinetics andcomputation converge in supporting the role of dimers of the indicated structure, and in pointingto a mechanism whereby the unique reactivity of the homochiral dimer is the driving force, atleast at ambient temperature.