Charge‐Transfer Complexes and Reactivity in Diels‐Alder Cycloadditions

The reactivity of 2,3‐bis(methylene)norbornane ( 1 ) towards the highly reactive dienophiles 2,3‐dicyano‐ p ‐benzoquinone ( 2 ), tetracyanoethylene ( 3 ),2,3‐dicyanomaleimide ( 4 ), and p ‐benzoquinone‐2,3‐dicarboxylic anhydride ( 5 ) was analysed kinetically. CT complexes were observed for 1 with 2 – 5 and equilibrium constants for the formation of the CT complexes between 1 and 2 – 4 were determined kinetically. The relationship between relative rates, CT‐excitation energies, HOMO‐LUMO separations, and CT‐complex equilibrium constants is discussed. The results do not require the CT complexes to be intermediates but strongly support the importance of CT interactions in the transition state of these cycloadditions.