Dramatic solvent effect on the volume of the Diels-Alder reaction between tetracyanoethylene and cyclopentadiene

Abstract

The partial molar volumes (V) and the enthalpies of dissolution (Adis/'~ for tetracyanoethylene, cyclopentadiene, and their Diels--Atder adduct were determined at 25 ~ Eleven solvents of the ~-and n-donor type were used. The use of alkylbenzenes as solvents t'or tetracyanoethylene induces pronounced changes in the enthalpy of dissolution (up to 26 kJ tool -l) and in the partial molar volume (up to I I cm 3 tool-I), whereas these parameters for the adduct change slightly. The V and ,5,jisH values for cyclopentadiene virtually do not depend on the nature of the solvent. In the case of tetracyanoethylene and the adduct in n-donor solvents, considerable variations of the Vand AdisH values are observed; they are not linear functions of the change in the partial molar volume of the adduct. Theretbre, the reaction volumes in acetonitrile (-40.69) and ethyl acetate (-45.56) differ sharply from those in o-xylene (-24.28) and mesitylene (-21.76 cm 3 mot-I).Key words: partial molar volume, reaction volume, tetrzcyanoethyIene, specific interactions, Oiels--Alder reaction.The use of elevated pressures provides data on the activation and reaction volumes of chemical reactions and allows more precise elucidation of the nature of the activated complex and its location on the reaction coordinate. I-5 A recent study is devoted to the reversible Diels--Alder reaction between tetracyanoethylene and 9-chloroanthracene. 6 It was found that the reaction volumes determined by indirect methods (from the difference between the activation volumes of the forward and reverse processes and from the pressure dependence of the equilibrium constant) and by the direct method (from the difference between the partial molar volumes of the adduct and the reagents) are equal within the accuracy of determination. For polar and ionic processes, the contribution of the solvent electrostriction effect to the overall activation volume can be commensurable with and even exceed in magnitude the structural contribution. 7 A more accurate estimation of the contribution of solvent electrostriction in the solvation of a polar or ionic activation complex requires consideration of the influence of the variable properties of the medium on the reaction rate at elevated pre~ure, a An influence of the solvent on the activation volume has been established for many processes accompanied by the formation of polar or ionic states. I-9 Testing of the process using the data on the influence of lithium perchlorate in diethyl ether on the cycloaddition rate showed that the activation complex in the Diels--Alder reaction involving tetracyanoethylene should be relatively nonpolar. 1~ This means that solvent electrostriction in the solvation shell of the activated complex should be insignificant, in this work, we studied the reaction of tetracyanoethylene with cyclopentadiene in different solvents of the rt-and n-donor type (Scheme I). Tetracyanoethylene is a strong n-acceptor (electron affinity 2.88 eVil); however, its rc-acceptor properties should not be manifested in full mea...

Why can the activation volume of the cycloadduct decomposition in isopolar retro‐Diels–Alder reactions be negative?

“…Thus, the partial molar volumes of the adduct formed from maleic anhydride with trans ‐1‐methoxybutadiene are as follows: 144.6 in nitromethane; 137.0 in acetonitrile; 142.6 in 1.2‐dichloroethane; 134.7 in 1‐chlorobutane, and 139.7 cm 3 mol −1 in dimethyl carbonate 31. Large solvent effects on the partial molar volumes of adduct 7 formed from tetracyanoethylene and cyclopentadiene were noted as well in benzene, 159.6; toluene, 158.3; o ‐xylene, 160.2; mesitylene, 159.4; chlorobenzene, 159.6; acetonitrile, 150.6; ethyl acetate, 150.3; cyclohexanone, 157.5; 1,4‐dioxane, 151.6; dichloromethane, 153.7; and 1,2‐dichloroethane, 154.7 cm 3 mol −1 32. These data illustrate the large differences in the partial molar volumes of cycloadducts, up to 10 cm 3 mol −1 , which are not related with the solvent polarity or donor–acceptor properties.…”

“…It is necessary to note that for different planar structures of dienes and dienophiles (except tetracyanoethylene), with the surfaces accessible for solvation, usually weak variety of the solvent effects on the values of partial molar volumes were observed 24,30. On the other hand, the large differences of the partial molar volumes for the branched molecular structure of adducts were detected 6–8,31,32. Thus, the partial molar volumes of the adduct formed from maleic anhydride with trans ‐1‐methoxybutadiene are as follows: 144.6 in nitromethane; 137.0 in acetonitrile; 142.6 in 1.2‐dichloroethane; 134.7 in 1‐chlorobutane, and 139.7 cm 3 mol −1 in dimethyl carbonate 31.…”

Quantifying Asymmetry in Concerted Reactions: Solvents Effect on a Diels–Alder Cycloaddition

“…Solvent effects on hetero-DA reactions have been extensively reported in the literature, showing, in general, linear correlations between various reaction properties (e.g., rate constants, endo / exo selectivity) and solvent characteristic parameters such as the acceptor number, H-bond acidity, and density . Kiselev et al − exemplified such solvent effects on the molar volume and the enthalpy of solution for several cycloaddition reactions involving TCNE. An interesting theoretical study by Yamabe et al showed that the DMB + TCNE reaction proceeds through an asymmetric TS if the reaction is carried out in methylene chloride, even though both the diene and the dienophile are substituted symmetrically.…”

Anomalies in the Change of Volumetric Parameters of the Diels–Alder Reaction in Solution

“…It is well known [1,4,[9][10][11] that the rate of isopolar DAR is weakly dependent (if any) on the polarity of the medium. Only a strong donor-acceptor interaction, which is observed, for example, for π -acceptor tetracyanoethylene in the medium of π -donors (alkylbenzenes), significantly reduces the activity of dienophile and its rate in the direct reaction [4,[11][12][13][14]. However the reaction rates of the adduct 11 decomposition are insensitive to the properties of the solvent [11].…”