Kinetic measurements on [4 + 21 cycloadditions of 1,a-dimethylenecyclopentane (1) as a model diene for normal DielsAlder 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 semiempirical quantumchemical calculations, we indeed obtained a good linear correlation between the lg k2 values and l/(EHomo -ELUMO). The cycloadditions of E / Z isomeric dienophiles to 1 are stereospecific. These reactions are best described in terms of concerted processes.The frontier molecular orbital (FMO) theory is the most widely accepted approach to explain reactivity in [4 + 21 cycloadditiens [',*]. While it works satisfactorily on a qualitative basis, most attempts to apply it quantitatively to Diels-Alder reactions have proved unsuccessful. Thus, rather poor quantitative correlations between Ig k2 and the first ionization potentials as a measure of the HOMO energy of the diene were obtained for cycloadditions of substituted 1,3-butadienes to maleic anhydride or tetracyanoethylener3I. A better agreement between this simplified theory and experiments was found for the classic cycloadditions of cyclopentadiene to cyano-substituted 01efins [~]. Rather poor correlations resulted for cycloadditions of substituted anthracenes to maleic anhydridet5], whereas in other examples better correlations were reportedI6I. In 1,3-dipolar cycloadditions, the other important class of [x4 + x2] cycloadditions a much better agreement between this theory and experiments is There are several reasons for the shortcomings in the FMO approach. It derives from the second-order perturbation theory and reduces the full expression for the stabilizing electronic interaction, which includes all occupied and unoccupied MOs of the reactants, to those of the frontier orbitals [']. Although these contributions should be greater than those from the interactions of other unoccupied with occupied MOs there is no a priori postulate that such a theory will work in explaining quantitatively reactivity in cycloadditions. On the contrary, it is surprising that the FMO theory functions as good as it does"'].Furthermore, the second-order perturbation theory describes only stabilizing electronic interactions. Thus, steric effects which are known to be important in many cases are not considered. More elaborate perturbation theories["] do take into account these contributions and constitute, therefore, an improvement. However, also these treatments rely on the quality of the underlying semiempirical SCF procedures like MNDO, AM1, or In the case of Diels-Alder reactions of open-chain dienes the position of the equilibrium between the synperiplanar and antiperplanar structure, i. e. a conformational situation, is another complicating factor. It is mainly influenced by the number and the size of substituents at different positions of the diene. This complex situation was present in the kinetic study of the cycloadditions of substituted 1,3-butadienes to maleic anhydr...

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ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.