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  • 1
    Electronic Resource
    Electronic Resource
    Organometallic Compounds of Titanium and Zirconium as Selective Nucleophilic Reagents in Organic SynthesisTaken in part from the Dissertation of B. Weidmann, ETH Zürich (1982), Diss. No. 7203. (1983)
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 22 (1983), S. 31-45 
    Details
    ISSN: 0570-0833
    Keywords: Nucleophilic reactions ; Synthetic methods ; Selectivity ; Organotitanium compounds ; Organozirconium compounds ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The addition of carbanionic organometallic compounds (usually RLi or RMgX) to a carbonyl group - a key step in numerous syntheses - is not always straightforward. Depending on the substrate, various complications and problems may arise, but in many cases these can be remedied by addition of (RO)3TiCl, (RO)3ZrCl or (R2N)3TiX to the classic lithium and Grignard reagents. This usually leads to formation of stable organo-titanium and -zirconium compounds which react highly selectively with carbonyl groups. For example, CH3Ti(OiPr)3 reacts five orders of magnitude faster with benzaldehyde than with acetophenone at room temperature; reagents of the type RTi(OiPr)3 add smoothly to nitro-, ido-, or cyano-subsituted benzaldehyde, and the reactions may be performed in chlorinated solvents or acetonitrile; the zirconium analogues have particularly low basicity and add in high yield to α- and β-tetralones or to substrates containing a nitroaldol group; the inclusion of chiral OR* groups gives enantioselective reagents (up to 90% ee); allylic (RO3)Ti- derivatives react only at the more highly substituted carbon atom and, in addition, react diastereoselectively (up to 98% ds) with unsymmetrical ketones. Finally, titanium reagents have also been found to effect novel transformations such as direct geminal dialkylation (C=O→CMe2) and alkylative amination [C=O→CR(NR2′)]. The modification and finetuning (“taming”) of carbonyl reactivity obtainable by use of the new reagents is not dearly bought; starting materials are the cheap and harmless “titanates”, “zirconates” and the corresponding tetrachlorides.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/anie.198300311
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  • 2
    Electronic Resource
    Electronic Resource
    Homolytic Cyclizations of N-ChloroalkenylaminesDedicated to Professor Jean-Marie Surzur on the occasion of his 50th birthday. (1983)
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 22 (1983), S. 337-350 
    Details
    ISSN: 0570-0833
    Keywords: Cyclization ; Chloroamines ; Alkenylamines ; Synthetic methods ; Heterocycles ; Free radicals ; Natural products ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Many N-chloroalkylamines with double bond(s) dissociate homolytically to give chlorine atoms and aminyl radicals which undergo intramolecular cyclization reactions. Here, the nature of the aminyl radicals (neutral, protonated, or complexed to metal salts) appears to be important. The reactivity varies strongly from one species to another, and their selectivities can be different towards addition, substitution, abstraction, or dimerization reactions. A good synthetic tool appears to be the “complexed radical”, which is generated under mild conditions. A variety of β-functionalized, substituted, fused, or bridged azaheterocycles with alkaloid part structures thus become easily accessible in high yields, in a regio- and, possibly, stereoselective manner. Many examples are discussed illustrating the synthetic possibilities, limitations, and mechanistic features of this cyclization process.
    Additional Material: 7 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/anie.198303373
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  • 3
    Electronic Resource
    Electronic Resource
    Complex Reducing Agents (CRA's) - Versatile, Novel Ways of Using Sodium Hydride in Organic Synthesis (1983)
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 22 (1983), S. 599-613 
    Details
    ISSN: 0570-0833
    Keywords: Complex reducing agents ; Reduction ; Sodium hydride ; Synthetic methods ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Why do we hardly use the simplest and, at the same time, inexpensive reducing agent sodium hydride in organic chemistry? To this question the answer is invariably: “It is too basic”. In this progress report we describe work we have performed aimed at controlling the basicity of NaH using sodium alcoholates and metal salts. The complex reducing agents (CRA's) developed (symbolized NaH-RONa-MXn) allow organic halides, alkenes, alkynes and ketones to be reduced selectively. Highly regioselective 1,4- and 1,2-reductions of α,β-unsaturated ketones are easily performed using appropriate metal salts. Modified CRA's have proved to be excellent hydrosilylating reagents for carbonyl groups, non-pyrophoric heterogeneous hydrogenation catalysts, coupling reagents for aryl and vinyl halides, and reagents for the carbonylation of organic halides under very mild conditions. The study of these reactions opened up the field to phase-transfer-catalyzed photostimulated carbonylations as well as to SRN1 reactions of metalates.-Thus, starting from the simple sodium hydride a large number of useful reagents have become accessible.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/anie.198305991
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