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  • 1
    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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  • 2
    Electronic Resource
    Electronic Resource
    FMO Approach in [4 + 2] Cycloadditions. Kinetic Studies with 1,2-Dimethylenecyclopentane (1993)
    Weinheim : Wiley-Blackwell
    Berichte der deutschen chemischen Gesellschaft 126 (1993), S. 1241-1245 
    Details
    ISSN: 0009-2940
    Keywords: Diels-Alder reactions, kinetics of ; Cycloaddition ; FMO analysis ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Kinetic measurements on [4 + 2] cycloadditions of 1,2-dimethylenecyclopentane (1) as a model diene for normal Diels-Alder reactions are described. This diene was selected in order to check whether a synperiplanar diene is a good test case for the FMO model. Using HOMO and LUMO energies from semi-empirical quantumchemical calculations, we indeed obtained a good linear correlation between the lg k2 values and 1/(EHomo - ELUMO). The cycloadditions of E/Z isomeric dienophiles to 1 are stereospecific. These reactions are best described in terms of concerted processes.
    Additional Material: 3 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/cber.19931260525
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  • 3
    Electronic Resource
    Electronic Resource
    Mechanistic Aspects of Diels-Alder Reactions: A Critical SurveyDedicated to Professor Rolf Huisgen on the occasion of his 60th birthday (1980)
    Weinheim : Wiley-Blackwell
    Angewandte Chemie International Edition in English 19 (1980), S. 779-807 
    Details
    ISSN: 0570-0833
    Keywords: Diels-Alder reaction ; Cycloaddition ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The question of concerted or consecutive bond formation arises in all types of cycloaddition reactions. Stereochemical investigations and studies of regiospecificity as well as intensive kinetic investigations with regard to substituent and solvent effects, and the dependence of the reaction on temperature and pressure permit in many cases mechanistic conclusions concerning the Diels-Alder reaction. Efforts towards a theoretical interpretation, ab initio and semiempirical calculations, the application of frontier molecular orbital theory (FMO) as well as thermochemical measurements permit a description of the energy hypersurface of these [4+2]-cycloadditions. An attempt is made here to draw a line of distinction between the mechanistic alternatives - one-step reaction versus two-step reaction - considering all experimental and theoretical criteria.
    Additional Material: 13 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/anie.198007791
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