Carina Merkens

Asymmetric Synthesis of Pyrroloindolones by N‐Heterocyclic Carbene Catalyzed [2+3] Annulation of α‐Chloroaldehydes with Nitrovinylindoles

NHC-enolate plus 3: N-heterocyclic carbenes (NHCs) serve as organocatalysts for the [2+3] annulation of nitrovinylindoles with α-chloroaldehydes via an intermediate azolium enolate. The method provides trans-disubstituted pyrroloindolones with good yields and excellent diastereo- and enantioselectivities. Further transformations lead to tetracyclic pyrrolo[1,2-a]indoles with potential psychotropic and other bioactivities.

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.

Organocatalytic One-Pot Asymmetric Synthesis of 4H,5H-Pyrano[2,3-c]pyrazoles

An efficient one pot asymmetric synthesis of tetrahydropyrano[2,3-c]pyrazoles has been developed. This class of biologically active heterocycles can be obtained via a secondary amine catalyzed asymmetric Michael/Wittig/oxa-Michael reaction sequence. Remarkably, the title compounds were accessible in good to very good yields and very good to excellent enantioselectivities after a single purification step.

3-(4-Pyridyl)-2,4-pentanedione – a bridge between coordinative, halogen, and hydrogen bonds

3-(4-Pyridyl)-2,4-pentanedionate, an established ditopic ligand for metal complexes, offers additional interaction modes which may be exploited for crystal engineering: we have assembled structures in which halogen bonds coexist with coordinative and hydrogen bonds. An adduct of the neutral 3-(4-pyridyl)-2,4-pentanedione to tetrafluorodiiodobenzene proves the suitability of the pyridine N as a nucleophile in halogen bonds. This feature is preserved in Fe and Al complexes of the substituted acetylacetonate. For the latter, an experimental charge density based on high resolution diffraction data gives insight into the halogen bonds.

Bimetallic coordination networks based on Al(acacCN)3: a building block between inertness and lability

The Al(III) complex of 3-cyanopentane-2,4-dionate (acacCN) features peripheric nitrile groups which may coordinate to silver cations. As the Al(acacCN)(3) building block ranges between inertness and lability, its reactivity towards Ag(I) salts depends on the solvent and the weakly or non-coordinating counter anions; an impressive range of different extended structures has been encountered. With AgPF(6), the original building block is retained and hexafluorophosphate remains uncoordinated. A highly symmetric 3D crystalline solid forms in the presence of trichloromethane, and with benzene a tetrasolvate with large solvent-filled voids is obtained. Two different classes of reaction products with silver triflate have been observed. In addition to networks incorporating Al(acacCN)(3), partial solvolysis may lead to a dinuclear methoxide-bridged derivative. The resulting Al(2)(μ-OMe)(2) core may be perceived as a four-connected node in a self-interpenetrating 3D network. Earlier studies reported transmetalation for the reaction of Al(acacCN)(3) with AgNO(3) and we find the same reactivity for silver tetrafluoroborate. Full degradation of the Al(III) building block with formation of [Ag(acacCN)] is observed.

Bifunctional Ruthenium(II) Hydride Complexes with Pendant Strong Lewis Acid Moieties: Structure, Dynamics, and Cooperativity

The synthesis of a novel class of bifunctional ruthenium hydride complexes incorporating Lewis acidic BR(2) moieties is reported. Determination of the molecular structures in the solid state and in solution provided evidence for tunable interaction between the two functionalities. Cooperative effects on the reactivity of the complexes were demonstrated including the activation of small Lewis basic molecules by reversible anchoring at the boron center.

Crosslinking of the Pd(acacCN)2 building unit with Ag(I) salts: dynamic 1D polymers and an extended 3D network

After deprotonation, the acetylacetonate moiety of the ditopic ligand 3-cyanoacetylacetone (HacacCN) acts as a chelating ligand towards Pd(II). The resulting square-planar complex Pd(acacCN)2 represents a suitable building unit for extended structures via coordination of Ag(I) cations to the peripheral nitrile groups. These target compounds have been structurally characterized: with silver salts of the anions BF4−, ClO4−, PF6− and CF3SO3−, chain polymers with an alternating sequence of Pd(II) and Ag(I) are obtained. Solvent molecules and counter anions fill voids close to the silver cations; more weakly coordinating anions are engaged in longer, the triflate anion in a shorter interaction to Ag(I). In contrast to powder samples, larger crystals of these one-dimensional polymers are rather stable with respect to desolvation. Two isomorphous 1D structures undergo a fully reversible k2 phase transition which can be monitored by single crystal diffraction. The phase transition temperature depends on the nature of the counter anion and may therefore be tuned as a function of chemical composition. The formation of chain polymers by linking Pd(acacCN)2 building blocks with Ag(I) salts of BF4−, ClO4−, PF6− and CF3SO3− follows chemical intuition whereas its reaction with silver nitrate leads to an unexpected and close-packed 3D structure in which layers of composition Ag(NO3) are connected by Pd(acacCN)2 linkers.

3-(4-pyridyl)-acetylacetone--a fully featured substituted pyridine and a flexible linker for complex materials.

3-(4-Pyridyl)-acetylacetone (HacacPy) acts as a pyridine-type ligand towards CdX2 (X = Cl, Br, I). Chain polymers with six-coordinated metal cations are obtained from CdCl2 and with alternating five- and six-coordinated Cd centers from CdBr2. In either case, the formation of these compounds does not depend on the precise stoichiometry. In contrast, two different reaction products form with the heavier congener CdI2, namely a ligand-rich molecular complex CdI2(HacacPy)2 and a ligand-deficient one-dimensional polymer [CdI2(HacacPy)](1)(∞). Interconversion between these two iodo derivatives is possible via thermal degradation and mechanochemical synthesis. The acetylacetone moiety in HacacPy may be deprotonated and chelated to Fe(III), and the resulting complex Fe(acacPy)3 reacts analogously to a bridging polypyridine ligand towards the same Cd halides as the molecule HacacPy itself. With CdCl2 and CdBr2, isomorphous chain polymers are obtained in which the Cd cations adopt distorted octahedral coordination and one of the peripheric pyridyl groups remains uncoordinated. With CdI2, the iron complex acts as a mu3Fe(acacPy)3 bridge between tetrahedral Cd centers and gives rise to a ladder structure.

Interplay of ligand chirality and metal configuration in mononuclear complexes and in a coordination polymer of Cr(III)

In order to investigate the influence of ligand chirality on the configuration of the coordinated metal, five pseudooctahedral Cr(III) complexes with one or two chelating R,R-1,2-diaminocyclohexane ligands have been synthesized. The mononuclear complexes [Cr(R,R-chxn)2Cl(DMSO)]Cl2, [Cr(R,R-chxn)2Cl2]Cl, [Cr(acacCN)(R,R-chxn)2](NO3)2, [Cr(acacCN)2(R,R-chxn)]NO3, [Cr(acacCN)2(R,R-chxn)]PF6. (R,R-chxn = R,R-1,2-diaminocyclohexane; acacCN = deprotonated 3-cyanoacetylacetone and DMSO = dimethyl sulfoxide) have been obtained as crystalline solids, mostly solvates, and the potential chirality transfer from the enantiopure ligand to the configuration at the Cr(III) center has been investigated. The cationic complex [Cr(acacCN)2(R,R-chxn)]+ has been synthesized as exclusively Λ configured at the metal. In this complex, the dangling nitrile groups of the ditopic acacCN ligands may coordinate to Ag(I): the chiral-at-metal building block has thus been converted to the 2D network Ag[Cr(acacCN)2(R,R-chxn)]2(PF6)3 under retention of the stereochemistry at Cr(III). With respect to topology, the polycations in this mixed-metal coordination polymer correspond to two interpenetrated {4,4} nets.

Isolation, Optical Properties and Core Structure of a Water-soluble, Phosphine-stabilized [Au9]3+ Cluster

A water-soluble, crystalline Au9 cluster compound stabilized with sodium (m-sulfonatophenyl) diphenylphosphine (TPPMSNa) was obtained during the NaBH4 reduction of [Au(TPPMSNa)Cl] H2O in ethanol. Characterization by optical extinction spectroscopy, singlecrystal X-ray diffraction in combination with energy dispersive X-ray diffraction led to the preliminary formula Na(8-x)[Au9(TPPMS)8]Cl(3-x) nH2O (0 ≤ x ≤ 3, n ≈ 16) with the cluster core in crown conformation. 31P{1H} nuclear magnetic resonance spectroscopy revealed one singlet at 57:3 ppm. This is the first example where gold clusters stabilized by TPPMSNa formed crystals which where suitable for core structure determination Graphical Abstract Isolation, Optical Properties and Core Structure of a Water-soluble, Phosphine-stabilized [Au9]3+ Cluster