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  • 1
    ISSN: 0009-2940
    Keywords: Chemoselectivity ; Titanium complexes ; Niobium complexes ; Tantalum complexes ; Methylation of aldehydes and ketones ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 4 Tab.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0009-2940
    Keywords: Aldehydes ; Ketones ; Titanium complexes ; Niobium complexes ; Tantalum complexes ; Phosphonium chlorides ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 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.
    Additional Material: 2 Tab.
    Type of Medium: Electronic Resource
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