Halo substituent effects on intramolecular cycloadditions involving furanyl amides

Intramolecular Diels-Alder reactions involving a series of N-alkenyl-substituted furanyl amides were investigated. Stable functionalized oxanorbornenes were formed in high yield upon heating at 80- 110 C. The cycloaddition reactions include several bromo-substituted furanyl amides, and these systems were found to proceed at a much faster rate and in higher yield than without substitution. This effect was observed by incorporating a halogen in the 3- or 5- position of the furan ring and appears to be general. The origin of increased cycloaddition rates for halo-substituted furans has been investigated with quantum mechanical calculations. The success of these reactions is attributed to increases in reaction exothermicities; this both decreases activation enthalpies and increases barriers to retrocycloadditions. Halogen substitution on furan increases reactant energy and stabilizes the product, which is attributed to the preference of electronegative halogens to be attached to a more highly alkylated and therefore more electropositive framework.