Ionic effects on the stereoselectivities of diels-alder reactions: the cyclopentadiene-methyl acrylate reaction in aqueous LiCl, NaCl, NaBr, CaCl₂, LiClO₄, and guanidinium chloride

Abstract

The variations in the endo- and exo-stereoselectivities for the reaction of cyclopentadiene (CP) with methyl acrylate (MA) have been investigated in salt-solvent reaction media (SSRM) at different concentrations of salts. The SSRM employed in the reaction include aqueous solutions of LiCl, NaCl, NaBr, CaCl2, guanidinium chloride, and LiClO4. The endo/exo (N/X) ratios have been noted to be sensitive to the ionic concentrations. Salts such as LiCl, NaCl, NaBr, and CaCl₂ enhance the N/X ratios, while the solutions of guanidinium chloride and LiClO₄ decrease them. The results have been analyzed in light of the salting-out and -in coefficients. The scaled particle theory has also been employed to calculate the contributions from the cavitation and solvation effects. The correlations of the N/X ratios with the surface tension of the SSRM are presented, which bring out clearly the impact of the surface tension of reaction media on the stereoselectivities of the above reaction. An attempt has also been made to interpret the results in terms of internal pressure of the SSRM in conjunction with the activation parameters. With the help of internal pressure, it is possible to suggest a division of zones to describe the salting-out and -in effects by different SSRM for the CP + MA reaction.