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ABSTRACT: I(n)organocatalysis: Neutral multidentate halogen-bond donors (halogen-based Lewis acids) catalyze the reaction of 1-chloroisochroman with ketene silyl acetals. The organocatalytic activity is linked to the presence (and number as well as orientation) of iodine substituents. As hidden acid catalysis can be ruled out with high probability, this case constitutes strong evidence for halogen-bond based organocatalysis. TBS=tert-butyldimethylsilyl.
Angewandte Chemie International Edition 05/2013; · 13.45 Impact Factor
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ABSTRACT: 3-Substituted 2-pyridones were enantioselectively (68-90% ee) converted into the respective 3-hydroxypyridine-2,6-diones by a sequence consisting of a template-mediated type II photooxygenation and an acid-catalysed rearrangement.
Chemical Communications 09/2012; 48(82):10195-7. · 6.17 Impact Factor
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ABSTRACT: A palladium-catalyzed direct 2-alkylation reaction of free N-H indoles was developed based on a norbornene-mediated regioselective cascade C-H activation. The detailed reaction mechanism was investigated by NMR spectroscopic analyses, characterization of the key intermediate, deuterium labeling experiments, and kinetic studies. The results indicate that a catalytic cycle operates, in which an N-norbornene type palladacycle is formed as the key intermediate. Oxidative addition of alkyl bromide to the Pd(II) center in this intermediate is the rate-determining step of the reaction. The synthetic utility of this indole 2-alkylation method was demonstrated by its application in natural product total synthesis. A new and general strategy to synthesize Aspidosperma alkaloids was established employing the indole 2-alkylation reaction as the key step, and two structurally different Aspidosperma alkaloids, aspidospermidine and goniomitine, were synthesized in concise routes.
Journal of the American Chemical Society 08/2012; 134(35):14563-72. · 9.91 Impact Factor
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ABSTRACT: Bi- and tridentate polycationic halogen bond donors based on 5-iodo-1,2,3-triazolium groups have been synthesized by 1,3-dipolar cycloaddition reactions. These halogen-based Lewis acids have been evaluated as activators in a halide-abstraction benchmark reaction.
Chemical Communications 08/2012; 48(74):9299-301. · 6.17 Impact Factor
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ABSTRACT: The diastereoselectivity of the Rh-catalyzed C-H amination was examined with 18 chiral open-chain substrates, which bear a benzylic methylene group in the α-position to a stereogenic center (-CHMeX), and with four chiral cyclic tetralins, in which the stereogenic center was positioned at carbon atom C2. The C-H amination was performed using trichloroethoxysulfonyl-substituted amine (H(2)NTces) as the nitrogen source, a diacyloxyiodobenzene as the oxidant, and bis[rhodium(α,α,α',α'-tetramethyl-1,3-benzenedipropionate)] [Rh(2)(esp)(2)] as the catalyst. For acyclic substrates a high syn diastereoselectivity (dr > 95/5) was found if the substituent X was Br, PO(OEt)(2), SO(2)Ph, or OOCCF(3) (eight examples). Moderate to good syn selectivities (dr = 80/20 to 91/9) were found for X = NO(2), OAc, COOMe, and CN (eight examples). Only two substrates gave a low diastereoselectivity. Kinetic isotope effect (KIE) experiments revealed that there is no secondary KIE when replacing -CHMeCOOMe by -CDMeCOOMe, but there is a significant primary KIE at the benzylic methylene position (4.8 ± 0.7). Deuteration experiments provided evidence that the reaction proceeds stereospecifically with retention of configuration. A preferred conformation is proposed, which explains the outcome of the reaction. In this conformation the X substituent is antiperiplanar to the C-H bond, which is diastereoselectively attacked, and steric strain between the remaining substituents at the stereogenic and the prostereogenic center is minimized. DFT calculations support this model. They suggest, however, that the reaction is not concerted but occurs via hydrogen atom abstraction and subsequent radical rebound. Further support for an antiperiplanar attack relative to a given substituent X = Br, COOMe, or CN was obtained with the respective 2-substituted tetralins. Attack at C1 provides almost exclusively the trans-amination product. If the size of the X substituent increases [Br < CN < COOMe < PO(OEt)(2)], attack at the carbon atom C4 prevails, delivering the respective trans-amination products at this position.
Journal of the American Chemical Society 08/2012; 134(32):13524-31. · 9.91 Impact Factor
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ABSTRACT: The crystal structure of the title compound, tetra-aqua-tris-(nitrato-κ²O,O')gadolinium(III) dihydrate, was redetermined from single-crystal X-ray data. In comparison with the first determination [Ma et al. (1991 ▶). Wuji Huaxue Xuebao, 7, 351-353], all H atoms could be located, accompanied with higher accuracy and precision. The Gd(III) atom shows a ten-coordination with three nitrate ligands behaving in a bidentate manner and the other positions being occupied by four water mol-ecules, forming a distorted bicapped square anti-prism. Two nitrate ions coordinate to the metal atom with similar bond lengths while the third shows a more asymmetric bonding behaviour. An intricate network of O-H⋯O hydrogen bonds, including the lattice water mol-ecules, stabilizes the crystal packing.
Acta Crystallographica Section E Structure Reports Online 07/2012; 68(Pt 7):i56-i57. · 0.35 Impact Factor
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ABSTRACT: In the crystal structure of the title compound, [CoCl(2)(C(7)H(12)N(2))(2)], molecular units are formed by coordination of the unsubstituted N atoms of two tert-butyl-substituted imidazole molecules and two chloride ligands, which distinguishes the complex from structures of imidazolium-based dications with tetrachloridocobaltate dianions. There are two crystallographically independent molecules in the asymmetric unit, related by a noncrystallographic inversion centre.
Acta crystallographica. Section C, Crystal structure communications 06/2012; 68(Pt 6):m166-8. · 0.78 Impact Factor
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ABSTRACT: Two [2+2] photocycloaddition routes were evaluated as possible ways to access the tricyclic core structure found in the terpene monoglycoside lactiflorin. While the first route via γ-substituted cyclopentenones was quickly discarded, the reactions of racemic (5R*)-3-benzyloxy-5-but-3'-enyl-4-methoxycarbonylfuran-2(5H)-ones proceeded in high yields and with perfect diastereoselectivity. However, it turned out that the regioselectivity was strongly dependent on the substitution pattern within the but-3'-enyl chain, which connects the terminal olefinic double bond to the photoexcited butenolide chromophor. If the chain was unsubstituted or if a tert-butyldimethylsilyloxy group was placed at the 2' position in a syn-relationship to the existing stereogenic center (5R*,2'S*), the crossed product prevailed with regioselectivities of 89:11 to 69:31. If the tert-butyldimethylsilyloxy group was positioned at 2' in an anti-relationship to the existing stereogenic center (5R*,2'R*), the desired straight products were obtained in regioselectivities of 74:24 to 55:45 (61-83% yield). Following this route, the aglycon part of lactiflorin was obtained by an intramolecular [2+2] photocycloaddition and a subsequent hydrogenolysis in 53% yield. Its further conversion into the natural product after glycosylation included a methyl addition to the lactone carbonyl group, which was optimized to give the desired key intermediate in a yield of 70%. The further conversion to lactiflorin was achieved in four steps and with an overall yield of 49%.
Chemistry - An Asian Journal 05/2012; 7(8):1947-58. · 4.50 Impact Factor
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ABSTRACT: We have conducted isothermal calorimetric titrations to investigate the halogen-bond strength of cationic bidentate halogen-bond donors toward halides, using bis(iodoimidazolium) compounds as probes. These data are intended to aid the rational design of halogen-bond donors as well as the development of halogen-bond-based applications in solution. In all cases examined, the entropic contribution to the overall free energy of binding was found to be very important. The binding affinities showed little dependency on the weakly coordinating counteranions of the halogen-bond donors but became slightly stronger with higher temperatures. We also found a marked influence of different solvents on the interaction strength. The highest binding constant detected in this study was 3.3 × 10(6) M(-1).
Journal of the American Chemical Society 05/2012; 134(20):8507-12. · 9.91 Impact Factor
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ABSTRACT: The title compounds were prepared by aldol reaction of anisaldehyde and the respective N,N-dibenzyl glycinates. Deprotection of the nitrogen atom with Pearlman's catalyst delivered the unprotected β-hydroxytyrosine esters, which were further N-protected as N,N-phthaloyl (Phth) and N-fluorenylmethylcarbonyloxy (Fmoc) derivatives. The Friedel-Crafts reaction with various arenes was studied employing these alcohols as electrophiles. It turned out that the facial diastereoselectivitiy depends on the nitrogen protecting group and on the ester group. The unprotected substrates (NH(2)) gave preferentially syn-products but the anti-selectivity increased when going from NHFmoc over NPhth to NBn(2). If the ester substituent was varied the syn-preference increased in the order Me < Et < iPr. The reactions were shown to be fully stereoconvergent and proceeded under kinetic product control. A model is suggested to explain the facial diastereoselectivity based on a conformationally locked benzylic cation intermediate. The reactions are preparatively useful for the N-unprotected isopropyl ester, which gave Friedel-Crafts alkylation products with good syn-selectivity (anti/syn=21:79 to 7:93), and for the N,N-dibenzyl-protected methyl ester, which led preferentially to anti-products (anti/syn=80:20 to >95:5). Upon acetylation of the latter compound to the respective acetate, Bi(OTf)(3) -catalyzed alkylation reactions became possible, in which silyl enol ethers served as nucleophiles. The respective alkylation products were obtained in high yield and with excellent anti-selectivitiy (anti/syn≥95:5).
Chemistry - An Asian Journal 03/2012; 7(6):1372-82. · 4.50 Impact Factor
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ABSTRACT: Perfect polypropylene: A versatile synthesis route leading to 2-, 4-, and 7- substituted bisindenyl metallocenes (Zr, Hf) affording rigid and thus highly stereo- and regioselective polymerization catalysts, even at elevated temperatures, is presented. In particular, the hafnocene is able to produce isotactic polypropylene with extremely high melting transitions, exceeding 170 °C with molecular weights above 5 000 000 g mol(-1) .
Chemistry 03/2012; 18(14):4174-8. · 5.93 Impact Factor
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ABSTRACT: The crystal structure of the industrially relevant tetrahydroxy ether 2,2,2',2'-(tetrahydroxymethyl)-dibutylether (technically known as di(trimethylol)propane, Di-TMP; C12H26O5) was determined from single-crystal X-ray data at 123 K: monoclinic, space group C2/c (no. 15), a = 20.1202(13), b = 5.8169(4), c = 13.0323(8) angstrom, beta = 114.296(3)degrees, V = 1390.17(16) angstrom(3) and Z = 4. The adjacent molecules assemble into a two-dimensional framework in the solid state, linked by two intermolecular O-H center dot center dot center dot O hydrogen bonds. The compound is characterized via spectroscopic methods and mass spectrometry.
Zeitschrift fur Naturforschung B 02/2012; 67(2):181-184. · 0.86 Impact Factor
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Chemistry 01/2012; 18(5):1306-10. · 5.93 Impact Factor
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ABSTRACT: Nine new complexes of the type fac-[(CO)(3)ReBr(L)], where L represents a chelating bis-NHC ligand consisting of two alkylene (propylene and butylene) bridged (R') N-substituted (R) imidazoline moieties (1,1'-di-R-3,3'-R'-diimidazolin-2,2'-diylidene), were synthesized. Spectroscopic methods, elemental analysis, mass spectrometry and X-ray crystallography were employed to characterize the complexes both in solution and in solid state. The complexes were compared to the analogous complexes with smaller bridge lengths in order to reveal differences and similarities. Depending on the bridge length and the ligand size in some cases dinuclear complexes were obtained as small scale side products.
Dalton Transactions 12/2011; 41(7):2110-21. · 3.84 Impact Factor
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ABSTRACT: The treatment of the dimeric paddle-wheel (PW) compound [Mo(2)(NCCH(3))(10)][BF(4)](4)1 with oxalic acid (0.5 equiv.), 1,1-cyclobutanedicarboxylic acid (1 equiv.), 5-hydroxyisophthalic acid (1 equiv.) (m-bdc-OH) or 2,3,5,6-tetrafluoroterephthalic acid (0.5 or 1 equiv.) leads to the formation of macromolecular dicarboxylate-linked (Mo(2))(n) entities (n = 2, 3, 4). The structure of the compounds depends on the length and geometry of the organic linkers. In the case of oxalic acid, the dimeric compound [(CH(3)CN)(8)Mo(2)(OOC-COO)Mo(2)(NCCH(3))(8)][BF(4)](6)2 is formed selectively, whereas the use of 2,3,5,6-tetrafluoroterephthalic acid affords the square-shaped complex [(CH(3)CN)(6)Mo(2)(OOC-C(6)F(4)-COO)](4)[BF(4)](8)3. Bent linkers with a bridging angle of 109° and 120°, respectively, lead to the formation of the molecular loop [(CH(3)CN)(6)Mo(2)(OOC-C(4)H(6)-COO)](2)[BF(4)](4)4 and the bowl-shaped molecular triangle [(CH(3)CN)(6)Mo(2)(m-bdc-OH)](3)[BF(4)](6)5. All complexes are characterised by X-ray single crystal diffraction, NMR ((1)H, (11)B, (13)C and (19)F) and UV-Vis spectroscopy.
Dalton Transactions 09/2011; 40(43):11490-6. · 3.84 Impact Factor
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ABSTRACT: Copolymerization of epoxides and CO(2) with heterogeneous zinc dicarboxylates is prominent since the early days of this area of chemistry. However, in over 30 years of research, the efficiency of this catalyst system could not be improved significantly. Furthermore, a huge activity difference between zinc glutarate and its lower homologue zinc succinate exists, which could not be explained so far. A detailed investigation of the underlying copolymerization mechanisms on heterogeneous catalysts is therefore necessary. Such investigations are so far lacking, which renders logical improvements of the catalysts difficult. We therefore decided to conduct a detailed investigation on the different zinc-dicarboxylic catalysts, their copolymerization efficiency, solid state structure and supplemented the results with theoretical calculations. The results imply that the widely discussed bimetallic mechanism (for homogeneous catalysts) is in place for heterogeneous zinc dicarboxylates as well. Theoretical calculations conducted to identify an "ideal" Zn-Zn distance suggest an optimal separation of Zn atoms in the range of 4.3-5.0 Å. The combined copolymerization experiments and calculated models give a consistent explanation for the difference in activity of the different zinc-dicarboxylate catalysts and give a hint why the activity of the heterogeneous zinc-dicarboxylate system is limited.
Journal of the American Chemical Society 08/2011; 133(33):13151-61. · 9.91 Impact Factor
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ABSTRACT: From simple to complex: Starting from easily accessible isoquinolones 1 (X=Br, OH), complex cyclobutane photoproducts such as compound 2 can be obtained with high enantioselectivity (88-96 % ee) through the use of a chiral template. Compound 3, which was isolated in 53 % ee starting from a racemic substrate, is the product of a unique, unprecedented kinetic resolution process.
Angewandte Chemie International Edition 07/2011; 50(36):8416-9. · 13.45 Impact Factor
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ABSTRACT: Squaring the circle: The novel dienamido pincer ligand N(CHCHPtBu(2) )(2) (-) affords the isolation of the unusual square-planar iridium(II) and iridium(III) amido complexes [IrCl{N(CHCHPtBu(2) )(2) }](n) (n=0 (1), +1 (2)). In contrast, the corresponding iridium(I) complex of the redox series (n=-1) is surprisingly unstable. The diamagnetism of 2 is attributed to strong N→Ir π donation.
Angewandte Chemie International Edition 07/2011; 50(35):8184-7. · 13.45 Impact Factor
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Angewandte Chemie International Edition 07/2011; · 13.45 Impact Factor
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Angewandte Chemie International Edition 06/2011; 50(31):7187-91. · 13.45 Impact Factor
