INORGANIC PERCHLORATES AND PERICYCLIC-REACTIONS .4. RATE ENHANCEMENT BY SPECIFIC CATION-SUBSTRATE INTERACTION OR BY INCREASED INTERNAL-PRESSURE

The kinetic behaviour of Diels-Alder and ene reactions was investigated in several inorganic perchlorate-organic solvent solutions. The observed rate accelerations are the result either of specific solute-cation interactions or of the increased internal pressure of the media, depending on substrates, solvents, and metal cations. Small rate accelerations (up to 10 times) are observed in those reactions where the solute does not give specific interactions with the cation (reactions of nitrosoarenes) or when the solvent strongly solvate the metal perchlorate. This small rate increase is largely due to the increasing of the medium internal pressure. Large rate enhancements (from 10(2) to 10(5) times depending on substrate, solvent, and salt) are observed in Diels-Alder and ene reactions showing electrophilic solvent effects and are consistent with a catalysis of the metal cation behaving as a Lewis acid.