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  • 1
    Electronic Resource
    Electronic Resource
    Conformational effects in 2-deoxyglucopyranos-1-yl radicals (1990)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 123 (1990), S. 1891-1898 
    Details
    ISSN: 0009-2940
    Keywords: ESR spectroscopy ; Conformational analysis ; Carbohydrate radicals ; Anomeric effect ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: 2-Unsubstituted and 2-substituted (fluoro, tosylamino, n-propyl) 3,4,6-tri-O-acetyl-2-deoxypyranosyl radicals are obtained by reaction of the corresponding pyranosyl bromides or phenylselenides with photolytically generated trimethyltin radicals in benzene solution. Analysis of the ESR hyperfine splittings reveals that the 2-deoxy-2-fluoroglucopyranosyl radical exists in a boat-like conformation at room temperature, whereas the others retain the 4C1 chair conformation of the starting material. The observed conformational effects are explained by a “quasi-homo-anomeric” frontier orbital interaction of the nx lone pair of the ring oxygen atom with the σ* MO of the β-C-O bond, in which the singly occupied p orbital at C-1 acts as a mediator for this 1,3-anomeric interaction.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19901230922
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  • 2
    Electronic Resource
    Electronic Resource
    Are Charge-Transfer Complexes Intermediates in Diels-Alder Reactions? A Case Study of the Reaction of 1,2-Dimethylenecyclopentane with Tetracyanoethylene (1991)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 124 (1991), S. 2811-2817 
    Details
    ISSN: 0009-2940
    Keywords: Cycloaddition ; Diels-Alder reaction ; Charge-transfer complex ; Kinetics ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Diels-Alder reaction of 1,2-dimethylenecyclopentane (1) and tetracyanoethylene (TCNE) is kinetically studied by conventional and stopped-flow UV/Vis absorption-spectroscopic techniques. A transient charge-transfer (CT) absorption band with λmmax=490 nm (half-life at 20°C ca. 3.4 × 10-2 s) is detected in dichloromethane solution. For the molar extinction coefficient a value of e490=310 M-1 cm-1 has been determined from measurements of the optical densitity at zero time as a function of excess diene concentration. With the aid of computer simulation the combined kinetic analysis of the disappearance of reactants and CT complex reveals that the data obtained from experiments at equimolar concentration and at a single temperature (20°C) can be interpreted equally well by both the model where the CT complex is an intermediate on the reaction path and where it is in a nonreactive side equilibrium with the reactants. Activation parameters have been determined for the disappearance of the starting compounds and of the CT complex. Contrary to what is expected for a CT complex being an intermediate in a concerted Diels-Alder reaction a more negative activation entropy for the decay of the CT complex than for the disappearance of the reactants is found. This suggests that the CT complex is unlikely to exhibit a transition-state-like structure as is required for a concerted cycloaddition. Experiments under pseudo-first-order conditions have led to an extension of the reaction scheme by a second-order reaction of the CT complex with the excess diene, the rate constant of which is virtually identical to that for the direct bimolecular cycloaddition. The conclusion is drawn for our case that there are more arguments against than arguments for the CT complex being a preorientation complex in the [4 + 2] cycloaddition.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19911241225
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  • 3
    Electronic Resource
    Electronic Resource
    Conformation of C-1-Substituted Pyranos-1-yl and Pyranosan-5-yl Radicals Evidence for a Quasi-Homo-Anomeric Stabilization Effect (1990)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 123 (1990), S. 1155-1160 
    Details
    ISSN: 0009-2940
    Keywords: Carbohydrate radicals ; Conformational analysis ; ESR spectroscopy ; Anomeric effect ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Acetylated 1-cyano- and 1-chloro-pyranos-1-yl radicals and 5-acetoxycarbonyl-, 5-methoxycarbonyl-, and 5-unsubstituted pyranosan-5-yl radicals were generated from the corresponding bromides by bromine abstraction with trimethyltin radicals. The conformation of these radicals, as deduced from the ESR hyperfine splittings, is explained by the combined action of a quasi-anomeric and a homo-anomeric stabilization effect. A captodative substitution pattern of the radical center does not influence the conformations.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19901230532
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