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.

Hydrophobic Effects Are Dominant over Secondary Orbital Interactions for a Simple Diels−Alder Reaction in Salt Solutions

“…The salts that promote Diels−Alder reactions are prohydrophobic and those which inhibit these reactions are antihydrophobic in nature. 3g,h The former class of salts enhances hydrophobic effects, while the latter decreases them. The possible origin of forces includes hydrophobic packing, solvent pressure, hydrogen bonding, hydrophobic hydration, and salting-out (S-O) and salting-in (S-I) effects, etc. 2a, As part of our studies to delineate and quantify the origin of these forces, we have investigated the reactions of cyclopentadiene 1 with methyl acrylate 2a and with methyl methacrylate 2b (Scheme ) in aqueous salt solutions. , Original contributions from Breslow and his group on the spectacular role of water and its salt solutions 1,3 and subsequent investigations from our group on the salt effect on the kinetics of Diels−Alder reactions have established that salts such as LiCl, NaCl, NaBr, KCl, MgCl 2 , CaCl 2 , and Na 2 SO 4 enhance the rates and endo/exo ratios of Diels−Alder reactions in water, whereas guanidinium chloride (GnCl) and LiClO 4 lower them. − Accordingly, LiCl, NaCl, CaCl 2 , MgCl 2 , etc. are the S-O agents, while GnCl, LiClO 4 , urea, etc.…”

Reaction Mechanisms. Part 2. Pericyclic Reactions

“…Pawar, Phalgune and Kumar have examined the in£uence of the addition of salts on the stereoselectivity of the aqueous Diels^Alder reaction between cyclopentadiene and methyl acrylate. 18 Guanidinium chloride and lithium perchlorate were found to cause a decrease in the endo:exo product ratio, while LiCl, NaCl, NaBr and CaCl 2 cause this ratio to increase. These results were explained by salting-in and -out effects.…”

Convenient NaCl-Catalysed Multicomponent Synthesis of Spiro[indoline-3,4′-Pyrano[2,3-c]Pyrazole]-3′-Carboxylate Derivatives in Aqueous Medium

“…It is clear that all reactants are brought close together in water due to the hydrophobic effect. 20 This effect is increased by the presence of NaCl because of the salting-out effect. 21 To explore the scope and limitation of the reaction, various isatins with different substituents 1 and either dimethyl or diethyl acetylenedicarboxylate 2 were utilised under the same reaction conditions.…”