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991.
Asymmetric hydrogenation plays an important role in organic synthesis, but that of the challenging substrates such as N‐unprotected imines, enamines, and N‐heteroaromatic compounds (1H‐indoles, 1H‐pyrroles, pyridines, quinolines, and quinoxalines) has only received increased attention in the past three years. Considering the interaction modes of a Brønsted acid with a Lewis base, Brønsted acids may be used as the ideal activators of C?N bonds. This Minireview summarizes the recent advances in transition‐metal‐catalyzed, Brønsted acid activated asymmetric hydrogenation of these challenging substrates, thus offering a promising substrate activation strategy for transformations involving C?N bonds. 相似文献
992.
993.
Deglmann P Ember E Hofmann P Pitter S Walter O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2007,13(10):2864-2879
Ruthenium complexes, mer-[RuX(3)(MeCN)(3)] and cis/trans-[RuX(2)(MeCN)(4)] with X=Br, Cl, were investigated as precatalysts in homogeneously catalyzed hydrosilylation of CO(2). The oxidation state of ruthenium and nature of the halide in the precatalysts were found to influence the catalytic activity in the conversion of Me(2)PhSiH to the formoxysilane Me(2)PhSiOCHO, with Ru(III) having chloride ligands being most active. Monitoring the reactions by in-situ IR spectroscopy in MeCN as the solvent indicates an interaction of the precatalyst with the silane prior to activation of CO(2). In the absence of CO(2), hydrosilylation of the MeCN solvent occurs. Catalytic activity in CO(2) hydrosilylation is enhanced by Me(2)PhSiCl, generated during reduction of Ru(III) in mer-[RuX(3)(MeCN)(3)] to Ru(II) or, when added as promoter to Ru(II) precatalysts. The reaction mechanism for the catalytic cycle has been calculated by DFT methods for the reaction of Me(3)SiH. The key steps are: Transfer of the Me(3)Si moiety to a coordinated halide ligand, resulting in an L(n)RuH(XSiMe(3)) intermediate --> CO(2) coordination --> Me(3)Si transfer to CO(2) --> reductive elimination of formoxysilane product. This reaction sequence is more favorable energetically for chloride complexes than for the analogous bromide complexes, which accounts for their differences in catalytic activity. Calculations also explain the rate increase observed experimentally in the presence of Me(2)PhSiCl. A parallel reaction pathway leads to (Me(3)Si)(2)O as a minor byproduct which arises from the condensation of two initially formed Me(3)SiOH molecules. 相似文献
994.
Yang L Mayr M Wurst K Buchmeiser MR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(22):5761-5770
The synthesis of novel ruthenium-based metathesis catalysts containing the saturated 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene ligand, that is, [RuCl2(NHC)[=CH-2-(2-PrO)-5-NO(2)-C6H3]] (1) and [Ru(CF3COO)2(NHC)[=CH-2-(2-PrO)-5-NO2-C6H3]] (2) (NHC=1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) is described. Both catalysts are highly active in ring-closing metathesis (RCM) and ring-opening cross-metathesis (ROCM). Compound 1 shows moderate activity in enyne metathesis. Compound 2 is not applicable to enyne metathesis since it shows high activity in the cyclopolymerization of diethyl dipropargylmalonate (DEDPM). Poly(DEDPM) prepared by the action of 2 consists of 95% five-membered rings, that is, poly(cyclopent-1-enevinylene)s, and 5 % of six-membered rings, that is, poly(cyclohex-1-ene-3-methylidene)s. The polymerization proceeds in a nonliving manner and results in polyenes with broad polydispersities (1.9< or =PDI< or =2.3). Supported analogues of 2 were prepared by immobilization on hydroxymethyl-Merrifield resin and a monolithic support derived from ring-opening-metathesis polymerization (ROMP). Catalyst loadings of 1 and 2.5%, respectively, were obtained. Both supported versions of 2 showed excellent reactivity. With 0.24-2% of the supported catalysts, yields in RCM and ROCM were in the range of 76-100%. Leaching of ruthenium was low and resulted in Ru contaminations of the products of less than 0.000014% (0.14 ppm). 相似文献
995.
A brief overview of our recent research results of thermoregulated liquid/liquid biphasic catalysis is presented. Emphasis is given to the general principles of thermoregulated phase-transfer catalysis (TRPTC) and thermoregulated phase-separable catalysis (TPSC). In addition, the applications of TRPTC and TPSC in biphasic catalysis are also discussed. The introduction of TRPTC to biphasic system is free from the shortcomings of classical aqueous/organic two-phase catalysis, in which the application scope is restrained by the water solubility of the substrate. Meanwhile, TPSC provides a very simple and reliable way to deal with the separation of catalyst in homogeneous catalysis. The common advantages of TRPTC and TPSC are characterized by homogeneous catalysis coupled with convenient biphasic separation. 相似文献
996.
Itaru Nakamura 《Tetrahedron letters》2004,45(14):2903-2906
Ytterbium-catalyzed tandem carboalkoxylation/Friedel-Crafts reaction of arylidenecyclopropanes 1 with acetals 2 afforded the corresponding indene derivatives 3 in good to high yields. For example, in the presence of 10 mol % of Yb(OTf)3 the reaction of 1-phenylbenzylidenecyclopropane 1a with the dimethyl acetals of benzaldehyde 2a, p-tolualdehyde 2b, and p-anisaldehyde 2c gave 1,3-diphenyl-2-(2-methoxyethyl)indene 3a, 2-(2-methoxyethyl)-3-phenyl-1-(p-tolyl)indene 3b, and 1-(p-anisyl)-2-(2-methoxyethyl)-3-phenylindene 3c in 82%, 80%, and 80% yields, respectively. 相似文献
997.
Pengbo Jiang Tiexin Cheng Yan Liu Xianghao Cui Yingli Bi Kaiji Zhen 《Reaction Kinetics and Catalysis Letters》2004,82(1):49-56
The novel phase transfer catalysts S-8 [4-(dimethyloctylammonium) propansultan] and DB-X [1,4-bis(triethylmethylammonium)benzene dibromide] were synthesized and employed for high conversion synthesis of dichlorocyclopropane from various olefins.This revised version was published online in December 2005 with corrections to the Cover Date. 相似文献
998.
I. B. Dzvinchuk A. V. Vypirailenko M. O. Lozinskii 《Chemistry of Heterocyclic Compounds》2004,40(9):1199-1206
Oxidative cyclocondensation of 2-aroylmethyl-1H-benzimidazoles with o-aminothiophenol gave the previously unknown 3-aryl-2-(2-benzimidazolyl)-4H-1,4-benzothiazines and (or) the isomeric 2H-1,4-benzothiazines. 2-(4-Nitrophenacyl)-1H-benzimidazole and 2-phenacylbenzothiazole gave 4H-1,4-benzothiazines which are not liable to isomerize but the reaction of the first compound is complicated by a hydrolytic fission which yields 2-[2-(4-nitrobenzoylamino)phenylthiomethyl]benzimidazole. A mixture of dimethylsulfoxide, acetic acid, and water was used as oxidant and solvent. The effect of the substituent and of the solvent on the tendency of the products to undergo prototropic isomerization in the benzothiazine ring have been studied. 相似文献
999.
Liuqing Yang Marco Hill Dr. Meitian Wang Dr. Santosh Panjikar Dr. Joachim Stöckigt Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(28):5211-5213
Cutting carbons : The three‐dimensional structure of polyneuridine aldehyde esterase (PNAE) gives insight into the enzymatic mechanism of the biosynthesis of C9‐ from C10‐monoterpenoid indole alkaloids (see scheme). PNAE is a very substrate‐specific serine esterase. It harbors the catalytic triad S87‐D216‐H244, and is a new member of the α/β‐fold hydrolase superfamily. Its novel function leads to the diversification of alkaloid structures.
1000.
Chen Zhao Yuan Kou Prof. Dr. Angeliki A. Lemonidou Prof. Dr. Xuebing Li Dr. Johannes A. Lercher Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(22):3987-3990
Oil and water : A new energy‐efficient and atom‐economical catalytic route for the production of alkanes and methanol by upgrading the phenolic fraction of bio‐oil has been developed. The one‐pot aqueous‐phase hydrodeoxygenation process is based on two catalysts facilitating consecutive hydrogenation, hydrolysis, and dehydration reactions.