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

✍ Scribed by Yves L. Dory; Dennis G. Hall; Pierre Deslongchamps

• Publisher: Elsevier Science
• Year: 1998
• Tongue: French
• Weight: 549 KB
• Volume: 54
• Category: Article
• ISSN: 0040-4020
• DOI: 10.1016/s0040-4020(98)00764-9

✦ Synopsis

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 zwitterioulc mechanism is possible for certain cases of Lewis acid catalysis. This conclusion was reached because the zwittexionic 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 ff 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 enai dienophiles demonstrate the extreme sensitivity in wansitinn state geometry in response to the most subtle structural changes in substrates.