Solvent Effects on the 9-Hydroxymethylanthracene + N-Ethylmaleimide Diels−Alder Reaction. A Theoretical Study

The origin of the high reaction rates of the 9-hydroxymethylanthracene + N-ethylmaleimide Diels−Alder reaction in fluorous solvents and supercritical carbon dioxide is analyzed through a combination of regression analyses and theoretical calculations. Both approaches allow the solvent effects on the activation barrier (a decrease by solvophobic interactions, an increase by dipolarity−polarizability) to be attributed to the existence of a hydrogen bond between the two reactants in the transition state, refuting a previous hypothesis based on strong solvophobic interactions.