Stereoselectivities and reaction rates in Diels-Alder reactions promoted by non-aqueous solvents and their aqueous mixtures: Correlations with non-adjustable parameters

Abstract

Stereoselectivities and rates of Diels-Alder reactions in different solvents were correlated with a parameter, Q, defined based on internal pressure, which is normalized with respect to a polarity scale. The polarity scale or index, although purely empirical in nature, can be calculated from the refractive index of the solvent. These correlations were linear for both stereoselectivities and reaction rates. Using expressions derived in this study, it is possible to represent the kinetic profiles of several Diels-Alder reactions in about 75 pure and mixed solvent media. The activation volume of a reaction can be directly estimated from the regression of data on rates and Q in various solvents using the proposed correlations.

Ionic Effects on the Stereoselectivities of Diels−Alder Reactions: The Cyclopentadiene−Methyl Acrylate Reaction in Aqueous LiCl, NaCl, NaBr, CaCl2, LiClO4, and Guanidinium Chloride

“…Correlation of any pressure term with the rates requires knowledge of the activation volume, Δ V # of a reaction, which is in general negative 26 for the Diels−Alder reactions. For this reaction, the estimated Δ V # is reported 4k to be −30.1 cm 3 mol -1 , which is in good agreement with our value (−27.54 cm 3 mol -1 ) calculated from the use of reaction rates and internal pressures …”

“…Not disagreeing with the existing explanations to describe the kinetic profiles of these reactions, we had proposed an alternate concept to quantify the correlation between the rate enhancement and internal pressure of SSRM. ,,, Internal pressure of an ionic solution varies due to the electrostriction process …”

“…Since there are no side products of the reaction, it is safe to presume that log(N/X) is equivalent to log( k N / k X ), where k denotes the rate constant. The ratios of activation volumes for the endo- and exo-stereoisomers, i.e., Δ( V N # )/Δ( V X # ), can be calculated from k N / k X and pressure terms with the procedure outlined elsewhere . The calculated Δ( V N # )/Δ( V X # ) being 1.17 indicates that the transition state for the endo-form is more compact than that for the exo-form (note Δ( V N # ) < Δ( V X # ), where Δ( V N # ) or Δ( V X # ) is negative).…”

“…In this direction, our efforts have been prompted toward the possible application of the solution properties of SSRM for explaining the rate enhancement of Diels−Alder reactions. This property, namely, internal pressure P i , of SSRM caused by the ion−solvent interactions was shown to be an alternative explanation for describing the rate enhancements in both aqueous and nonaqueous ionic media.…”

Experimental Evidence to Support Viscosity Dependence of Rates of Diels−Alder Reactions in Solvent Media

“…Though the dependence of rates of Diels−Alder reactions on the solvent viscosity has been supported and criticized, , we strongly feel that this issue, when supported by experimental data, should receive attention of organic chemists through a more appropriate vehicle of communication. In view of the current work from this laboratory in identifying the origin of forces responsible for rate enhancement of Diels−Alder reactions in different solvents and their salt solutions, we set out to assess the existing data as a step to resolve the issue of the role of viscosity on rates of Diels−Alder reactions. The main objective of this work, therefore, is to examine the rate−viscosity data to ascertain whether the viscosity of the solvent plays any effective role in governing the kinetics of Diels−Alder reactions.…”