Heavy atom effects reveal diradical intermediates. I. An aqueous Diels- Alder reaction

Article abstract of DOI:10.1016/S0040-4020(99)00903-5

The aqueous Diels-Alder reaction between 2-methylfuran and maleic acid in water is 99.9% stereospecific. Addition of heavy but not light atom salts to the retrodiene reaction reduces the degree of stereospecificity significantly. Taking into account the relatively low concentration (3.5-7 M) of heavy atoms, and the rapid fall off of the heavy atom effect with distance, these results show that a large portion, if not all of the Diels Alder occurs via diradical intermediates.