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 title complexes, 1 and 2, have
been
prepared and their interconversion has been studied by
1H NMR spectroscopy; the activation enthalpies
and
entropies as well as the equilibrium constants for these
reversible intramolecular first-order processes have been
determined. The iron tricarbonyl complex 3 of
3-bromotropone has also been prepared, but this does not
isomerize to regio-isomer 4.
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