On the mechanism of the Diels-Alder reaction of enal dienophiles.: Competitive reactivity and ab initio calculations using a transannular probe

A sterically unbiased macrocyclic probe has been used to study the Diels-Alder reaction mechanism of enal dienophiles. All the results, theoretical as well as kinetical, converge to the conclusion that a virtually non-concerted zwitterionic mechanism is possible for certain cases of Lewis acid catalysis. This conclusion was reached because the zwitterionic limit intermediates obtained after the first step could arise either from favored [6.11] or disfavored [7.10] bicycle formation. These two opposite effects could assist or counteract asynchronicity. The analysis of these effects showed that asynchronicity highly stabilizes the Diels-Alder reaction transition state if the latter is geometrically favorable such as in the case of a [6.11] zwitterionic limit intermediate. Altogether, these results on the Diels-Alder cycloaddition involving enal dienophiles demonstrate the extreme sensitivity in transition state geometry in response to the most subtle structural changes in substrates. (C) 1998 Elsevier Science Ltd. All rights reserved.