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  • Electronic Resource  (3)
  • Allylic alkylation  (2)
  • Hydrogen bonding  (1)
  • 1
    Electronic Resource
    Electronic Resource
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 1998 (1998), S. 25-27 
    ISSN: 1434-1948
    Keywords: Bite angle ; Catalysis ; P ligands ; Palladium ; Allylic alkylation ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The effect of the natural bite angle (βn) of diphosphane ligands on catalyst selectivity and activity in the palladium-catalyzed allylic alkylation was investigated. The selectivity and rate of the reaction are mainly determined by steric hindrance induced by the diphosphane ligands. The steric hindrance at the palladium center increases as the natural bite angle of the ligand becomes larger. This results in an increasing selectivity at larger bite angles, but at very large bite angles the rate of the reaction drops. The ligand with the largest calculated bite angle, Xantphos, induced 100% selectivity but the reaction rate became low.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-1948
    Keywords: Supramolecular chemistry ; Metallamacrocycle ; Hydrogen bonding ; Phosphane ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two novel phosphane ligands 3 and 4 based on the rigid diphenylglycoluril molecule have been synthesized and characterised. Binding studies with 3 and 4, using 1,3-dihydroxybenzene derivatives reveal that ligands 3 and 4 behave similarly to clip molecule 5, which has the same binding site as ligands 3 and 4. The size of the flexible spacers in the ligands has been varied and the effect of this variation on the association constant of resorcinol derivatives has been determined. These cavity-containing ligands are able to coordinate two transition metal centres, leading to bimetallic macrocycles. The metallamacrocycles formed from 4 containing platinum or rhodium bind the guest, olivetol (5-pentylbenzene-1,3-diol), almost four times as strongly as the free tetrapodant 4. Complexes of 4 having palladium centres display similar or reduced binding affinities for resorcinol derivatives, when compared to free 4. Metal complexes of ligand 3 do not form host-guest complexes, probably because of a too small a ring-size of the metallamacrocycle.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1434-1948
    Keywords: Palladium ; Phosphane ligands ; Bite angle ; Allylic alkylation ; Molecular modelling ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The natural bite angle of bidentate phosphane ligands influences the isomer distribution (syn and anti) in (1-methylallyl)(bisphosphane)Pd OTf complexes. It was found (31P- and 1H-NMR studies) that the syn/anti ratio changes from 12 (dppp) to 1.3 (sixantphos). Molecular orbital calculations [PM3(tm) level] indicate that for ligands inducing a large bite angle, the phenyl rings of the ligand embrace the allyl moiety, thus influencing the syn/anti ratio. This bite-angle effect on the syn/anti ratio is transferred to the regioselectivity in stoichiometric allylic alkylation. Ligands inducing large bite angles direct the regioselectivity towards the formation of the branched product 2. Catalytic alkylation of (E)-2-butenyl acetate showed that for ligands with a small bite angle the regioselectivity of the catalytic and stoichiometric alkylation are in good agreement. This correspondence is worse for ligands with a larger bite angle, which is rationalised in terms of the relative rates of syn/anti isomerisation and alkylation. The ligand with the largest bite angle (sixantphos) gives the most active catalytic species.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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