Using a tripod as a chiral chelating ligand: chemical exchange between equivalent molecular structures in palladium catalysis with 1,1,1-tris(oxazolinyl)ethane ("trisox")

Threefold symmetrical chiral podands may simplify the stereochemistry of key catalytic intermediates for cases in which they only act as bidentate ligands. This applies to systems in which chemical exchange between the different kappa2-coordinated forms takes place and in which the non-coordinated sidearm may play a direct or indirect role at some earlier or later stage in the catalytic cycle. Palladium(II)-catalysed allylic substitutions provide appropriate test reactions along these lines. A series of neutral dichloropalladium(II) complexes, [PdCl2(iPr-trisox)] (1a), [PdCl2(Ph-trisox)] (1b), [PdCl2(Bn-trisox)] (1c) and [PdCl2(Ind-trisox)] (1d) (trisox=1,1,1-tris(oxazolinyl)ethane) were synthesised by reaction of the respective trisox derivative with [PdCl2(PhCN)2] and characterised inter alia by 15N NMR spectroscopy. Direct detection of the heteronuclei without isotope enrichment and with "normal" sample concentrations was achieved with the aid of a cryogenically cooled NMR probe on a 600 MHz NMR spectrometer. Whereas the 15N nuclei of the coordinated oxazoline rings resonate at delta=160-167 ppm and appear as two singlets due to their diastereotopicity, the signal assigned to the dangling oxazoline "arm" is observed at delta=238-240 ppm. Variable-temperature NMR studies along with a systematic series of magnetisation transfer experiments established exchange between ligating and non-ligating oxazoline rings. Reaction of [Pd(allyl)(cod)]BF4 (cod=cyclooctadiene) with Ph-trisox in CH(2)Cl(2) gave the corresponding allyl complex 2, for which fast exchange between the three oxazoline heterocycles as well as between the exo and endo diastereomers was observed along with a very slow eta3-eta1-eta3 process of the allyl fragment (magnetisation transfer). Palladium(0) complexes were prepared by reaction of trisox derivatives or sidearm-functionalised BOX (BOX=bis(oxazolinyl)dimethylmethane) ligands with [Pd(nbd)(alkene)] (nbd=norbornadiene, alkene=maleic anhydride or tetracyanoethylene). X-ray diffraction studies of the iPr-trisox and Ph-trisox complexes (3a and 3b) established Y-shaped trigonal planar coordination geometries with the trisox ligand coordinated in a bidentate fashion, whilst the pi-coordinated maleic anhydride ligand adopts one of the two possible diastereotopic orientations. As the catalytic test reaction, the allylic alkylation of 1,3-diphenylprop-2-enyl acetate substrate with dimethyl malonate as nucleophile (in the presence of N,O-bis(trimethylsilyl)acetamide) was investigated for the trisox derivatives, their BOX analogues, and a series of less symmetric "sidearm" functionalised bisoxazolines. The trisoxazoline-based catalysts generally induce a better enantioselectivity compared to their bisoxazoline analogues and display significant reduction of the induction period as well as rate enhancement.     

Asymmetric organocatalytic domino reactions

The current status of organic synthesis is hampered by costly protecting-group strategies and lengthy purification procedures after each synthetic step. To circumvent these problems, the synthetic potential of multicomponent domino reactions has been utilized for the efficient and stereoselective construction of complex molecules from simple precursors in a single process. In particular, domino reactions mediated by organocatalysts are in a way biomimetic, as this principle is used very efficiently in the biosynthesis of complex natural products starting from simple precursors. In this Minireview, we discuss the current development of this fast-growing field.     

Joule-Thomson coefficients and Joule-Thomson inversion curves for pure compounds and binary systems predicted with the group contribution equation of state VTPR

In this work the accuracy of the prediction of Joule-Thomson coefficients for the gases CO₂ and Ar and the binary systems CO₂-Ar and CH₄-C₂H₆ was examined using the group contribution equation of state VTPR. Furthermore the experimental and correlated data of Joule-Thomson inversion curves of a few compounds including carbon dioxide, nitrogen, benzene, toluene, methane, ethane, ethylene, propyne, and SF₆ were compared with the results of the group contribution equation of state VTPR, the Soave-Redlich-Kwong (SRK), the Peng-Robinson (PR) and the Helmholtz equation of state (HEOS). Moreover, Joule-Thomson inversion curves for pure fluids, binary (CH₄-C₂H₆, N₂-CH₄, CO₂-CH₄), and ternary systems (CO₂-CH₄-N₂, CH₄-C₂H₆-N₂, CO₂-CH₄-C₂H₆) were calculated with VTPR and compared to the results of SRK, PR, HEOS and the molecular simulation results of Vrabec et al. It was found that the calculated values for the Joule-Thomson coefficients and Joule-Thomson inversion curves are in good agreement with the experimental findings.     

Enantioselective organocatalytic domino synthesis of tetrahydropyridin-2-ols

The asymmetric synthesis of tetrahydropyridin-2-ols from enals and enaminones is described. The organocatalytic domino reaction involves a Michael addition-hemiaminalization sequence using the J?rgensen-Hayashi catalyst. Dehydration or oxidation leads to the corresponding 1,4-dihydro-pyridines or 3,4-dihydropyridin-2-ones in a one-pot fashion.     

Hydrogen-bonded perylene/terthiophene-materials: synthesis and spectroscopic properties

The synthesis of layered donor/acceptor-materials based on perylenes (1a–c) and ter(thiophen)es (2a, 2b), ordered by hydrogen bonding moieties is reported. Based on the successful (selective) chlorination of 3,4:9,10-perylene tetracarboxylic dianhydride (3) to obtain a perylene derivative with only four chlorine atoms, subsequent functionalization with different hydrogen-bonding moieties is achieved via the azide/alkyne click reaction as proven by extensive ESI-TOF measurements. The perylene- (1a–c) and terthiophene- (2a, 2b) compounds are useful as acceptor and donor parts, respectively, in organic solar cells as proven via UV–vis and fluorescence measurements. Charge transfer between donor and acceptor parts (2a/1b) was determined as 41% via fluorescence resonance energy transfer (FRET), proving the association of the two components via the attached hydrogen bonding moieties. These measurements indicate that the mixture 2a/1b displays large potential for use as a layered ordered material with controlled spacings for organic solar cells based on a thereby facilitated charge-transfer.     

Quadruple domino organocatalysis: an asymmetric aza-michael/michael/michael/aldol reaction sequence leading to tetracyclic indole structures with six stereocenters

Quadruple domino/Wittig-one pot: An organocatalytic, asymmetric, three-component quadruple cascade, which leads to tetracyclic double-annulated indole derivatives, is described. The domino products were further derivatized by aldehyde olefination under one-pot conditions to obtain complex isoindolo[2,?1-a]indole derivatives bearing six stereogenic centers in a highly stereoselective fashion.     

Asymmetric Michael addition of N-boc-protected oxindoles to nitroalkenes catalyzed by a chiral secondary amine

New mission for prolinol ethers: A secondary-amine-catalyzed Michael addition of N-Boc-protected oxindoles to nitroalkenes through a Br?nsted base activation mode has been developed, furnishing the products in excellent yields (88-98?%), diastereoselectivities (d.r.=98:2->99:1) and high to excellent enantioselectivities (82->99?%?ee; see scheme).     

Synthesis and optical properties of diaza- and tetraazatetracenes

A series of functionalized diaza- and tetraazatetracenes was synthesized, either by condensation of an aromatic diamine with an ortho-quinone/diethyloxalate followed by chlorination with POCl(3) to give diazatetracenes or by palladium-catalyzed coupling of a phenylenediamine with various 2,3-dichloroquinoxalines to give tetraazatetracenes (after oxidation with MnO(2)). Representative examples included halogenated and nitrated derivatives. The optical properties of these azatetracenes were discussed with respect to their molecular structures and substitution patterns. The diazatetracenes and tetraazatetracenes formed two different groups that had significantly different electronic structures and properties. Furthermore, 1,2,3,4-tetrafluoro-6,11-bis((triisopropylsilyl)ethynyl)benzo[b]phenazine was synthesized, which is the first reported fluorinated diazatetracene. Single-crystal X-ray analysis of this compound is reported.