Unusual stability of N-methylmaleimide cycloadducts: characterization of isobenzofuran retro-Diels-Alder reactions

Many years ago Diels and Thiele noted the unusual stability of maleic anhydride cycloadducts and employed the feature in scavenging anthracene generated in a retro-[4+2] reaction. N-methyl-maleimide, although nearly identical to maleic anhydride in rates of Diels-Alder reactions, forms even more stable cycloadducts. The rate constants for the four reactions that characterize the furan/N-methylmaleimide system were measured under the conditions used by Lee and Herndon to study the furan/maleic anhydride system. Comparison of the two dienophiles indicates that N-methylmaleimide adducts are ca. 3 kcal.mol -1 more stable than the corresponding exo- and endo-maleic anhydride adducts. Somewhat larger (ca- 4±0.5 kcal.mol -1 ) differences in stability were found for the cycloadducts of the much more reactive diene isobenzofuran