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  • 1
    ISSN: 0009-2940
    Schlagwort(e): Chemoselectivity ; Titanium complexes ; Niobium complexes ; Tantalum complexes ; Methylation of aldehydes and ketones ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Transition Metal-Activated Organic Compounds, XXXVIII[1].  -  Chemoselective Nucleophilic Methylation Reactions by In-Situ-Blocking of Aldehyde Groups by α1-Phosphonioalkoxide FormationHerrn Professor Ulrich Schöllkopf zum 65. Geburtstag gewidmet.For the selective methylation of a keto group in the presence of an aldehyde group the complex MeTiCl3·PPh3 and especially the reagent system [TiC]4·PPh3 + 0.5 Me2Zn] proved to be favourable. For instance, 6-oxoheptanal (8) was methylated by [TiCl4·PPh3 + 0.5 Me2Zn] at the keto group with 89% yield and 99:1 selectivity, whereas it was methylated by [TiCl4 + 0.5 Me2Zn] at the aldehyde group with 81% yield and 96:4 selectivity. A selective methylation of benzaldehyde in the presence of heptanal was achieved with MeNbCl4·PPh3 or [NbCl5·PPh3 + 0.75 Me2Zn] to give (1-chloroethyl)benzene (14; yield 92 or 53%; selectivity in each case 93:7) or with [TaCl5·PPh3 + 1.5 Me2Zn] to give mainly 1-phenylethanol (15) besides 16. A 96:4-selective benzylation (yield 74%) of a ketone in the presence of an aliphatic aldehyde was possible with [TiCl4·PPh3 + 1 PhCH2MgBr].  -  The high chemoselectivity is caused by irreversible blocking of aliphatic aldehyde groups by α1-phosphonioalkoxide formation[1], whilst the corresponding reaction of keto groups[1] and of benzaldehyde is reversible.
    Zusätzliches Material: 4 Tab.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 2
    ISSN: 0009-2940
    Schlagwort(e): Aldehydes ; Ketones ; Titanium complexes ; Niobium complexes ; Tantalum complexes ; Phosphonium chlorides ; Chemistry ; Inorganic Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Transition Metal-Activated Organic Compounds, XXXVII[1].  -  A Novel Reaction of Aldehydes and Ketones: Synthesis and Spectroscopic Investigation of α1-Phosphonioalkoxy Transition Metal ComplexesHerrn Professor Ulrich Schöllkopf zum 65. Geburtstag gewidmet.By treatment of aldehydes and ketones with one mole equivalent of a Lewis acid (TiCl4, NbCl5, TaCl5) and one mole equivalent of PPh3, or in one case of Ph2PCH2CH2PPh2, the aldehyde derivates [R1CH(P+Ph2R2)-OMCln]Cl- (4-8; M=Ti, Nb, Ta; R2=Ph or CH2CH2PPh2) and the ketone derivatives [R1R2C(P+Ph3)-ONbCl4]Cl- (9-10) were prepared. Complexes 4-10 were isolated as air-and water-sensitive solids. According to the NMR spectra some of the obtained aldehyde derivatives 4-8 exist in solution as a mixture of isomers or oligomers, whereas the ketone derivatives 9 and 10 form in solution an equilibrium with the free ketone and the complex NbCl5 · PPh3. The chloride ion of 4-10 is perhaps coordinated to the metal.
    Zusätzliches Material: 2 Tab.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
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  • 3
    Digitale Medien
    Digitale Medien
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 13 (1974), S. 291-305 
    ISSN: 0570-0833
    Schlagwort(e): C-C coupling ; Organocopper compounds ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: The type of reaction described here, which achieved preparative importance before the turn of the century in the form of the Glaser reaction, has found new applications in recent years: Syntheses of 1,3-dienes, 2,3,6,7-tetraaza-1,3,5,7-tetraenes, 2,3-diaza-1,3-dienes, hydrazine derivatives, bis(heteroallyl) compounds, hetarenes, polyhetarenes, cyclopolyarenes, protophanes, phanes, and heteraprotophanes. The reaction, which consists in the metalation of the starting substance followed by oxidative coupling of the metal derivative, merits special preparative interest when several successive coupling processes are possible (e. g. polyyne, polyarene, cyclopolyarene, and protophane syntheses). The main factors limiting its applicability are the high thermal stability of some organic copper compounds and the disproportionation of the ligands that frequently occurs as a competing reaction. Selective asymmetric linkages are not generally possible by this method.
    Zusätzliches Material: 1 Ill.
    Materialart: Digitale Medien
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  • 4
    ISSN: 0570-0833
    Schlagwort(e): C—C coupling ; carbonylmethylenation ; μ-methylene complexes ; molybdenum ; tungsten ; Carbonyl methylenation ; C-C coupling ; Bridging ligands ; Molybdenum ; Tungsten ; Chemistry ; General Chemistry
    Quelle: Wiley InterScience Backfile Collection 1832-2000
    Thema: Chemie und Pharmazie
    Notizen: Hitherto there was no reaction known that permits transformations of R1R2-CO → 0.5 R1R2R3C-CR1R2R3 in one step. This type of additive-reductive carbonyl dimerization is now possible using alkoxy(alkyl)tungsten(v) complexes with aromatic, heteroaromatic or α,β-unsaturated aldehydes and ketones. When a corresponding phenyl complex was employed in a test experiment, it was revealed that an aliphatic ketone could be used as the substrate in this reaction. A second interesting type of reaction is the transformation of CH3 ligands into μ-CH2 ligands, which occurs during the treatment of MeLi or Me3Al with molybdenum or tungsten chlorides (oxidation states VI and V, for Mo additionally IV) at low temperatures with liberation of CH4. Here, the question arises as to whether the intermediate involved has a terminal CH2 ligand (Schrock carbene complex) or a μ-CH3 ligand (CH3 bound by a two-electron three-center bond to two metal atoms). Of all the μ-CH2 complexes obtained, those which were synthesized by the action of MeLi on molybdenum chlorides can be recommended as reagents for carbonylmethylenation of aldehydes and ketones. They display high selectivity, very low basicity, a surprising resistance to protons, they are readily available, can be easily modified and, as regards their selective behavior, they have been investigated more thoroughly than other readily accessible carbonylmethylenation reagents of comparable selectivity. The results of NMR spectroscopic investigations on the structure of the μ-CH2 complexes, and associated reaction mechanisms are discussed. A survey of carbonylmethylenation reagents, which have been reported in the literature, permits comparisons to be made with carbonylmethylenating molybdenum and tungsten complexes.
    Zusätzliches Material: 9 Tab.
    Materialart: Digitale Medien
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