Normal, neutral, and inverse-type Diels-Alder reactions can be observed in [4+2) cycloadditions of polyhalogenated cyclopentadienes la-l i with various aryl-substituted cyclic and open-chain dienophiles, and characteristic gradations of reactivity associated with the different mechanistic types are often displayed. Reaction rate constants and semiempirically derived HOMO and LUMO values for the reactants suggest that neutral [4+2] cycloadditions should be regarded as a transition between normal and inverse Diels-Alder reactions.
The electronic modes of Diels-Alder reactions of a number of polyhalogenated cyclopentadienes with various aryl-substituted cyclic and open-chain dienophiles are investigated. For example, the cycloaddition of the cyclopentadiene (Ia) to styrene (IIa) is found to proceed with normal electron demand, whereas the reactions of (Ib) with the dienophiles (II) proceed with neutral electron demand, and the reactions of (Ic) with (II) proceed with inverse electron demand. The neutral cycloaddition can be regarded as a transition between normal and inverse Diels-Alder reaction. -(EIBLER, E.; HOECHT, P.; PRANTL, B.; ROSSMAIER, H.; SCHUHBAUER, H. M.; WIEST, H.; SAUER, J.; Liebigs Ann.
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