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

Abstract

We propose the notion that if asymmetry characterizes a concerted reaction, a quantitative treatment in terms of continuous symmetry can bridge the gap between the Woodward-Hoffmann (WH) rules, originally formulated for symmetry-idealized unsubstituted reactants, and the fact that these rules hold for a much wider scope of reactions. Instead of focusing on symmetry conservation along the minimum energy path, we suggest that the distortion with respect to the original expected symmetry must attain a certain minimal value, not necessarily zero. To demonstrate this approach we studied the effect of solvents on the symmetry and reactivity of the classical [4 + 2] Diels-Alder cycloaddition of (E,E)-1,4-dimethoxy-1,3-butadiene with tetracyanoethylene, revealing the predictive value of this approach. Calculations of the enthalpy of activation and the charge separation at the transition state (TS) predict increased reactivity with the polarity of the solvent. The symmetry measure is in excellent correlation with the enthalpy of activation and the charge separation at the TS, indicating the higher reactivity of the more symmetric case, thus quantifying the main teaching of the WH rules. The advantages of using a global structural parameter that takes into account all geometrical parameters, i.e., the symmetry measure, over specific ones (e.g., asynchronicity) are discussed.

Symmetry‐Enthalpy Correlations in Diels–Alder Reactions

“…In continuation of our previous study, [4] here too we suggest the notion that when perfect symmetry is inherently impossible in a concerted reaction, the continuous symmetry measure attains a certain minimal value, not necessarily zero. In this paper, we have demonstrated this idea on a large set of asymmetrically substituted DA reactions in seven different solvents and in the gas phase.…”

“…In a recent publication, [4] we suggested that the distorted symmetry of the transition state (TS) of the DA reaction of (E,E)-1,4-dimethoxy-1,3-butadiene with tetracyanoethylene does retain some of the original symmetry, in which the word "some" relates to the quantification of the degree of symmetry content as calculated by the continuous symmetry measures (CSM) method. [5] We briefly recall that the CSM approach extends the binary language of symmetry into a continuous scale while combining all of the geometrical parameters of a molecular structure into a single global parameter that describes the distance of that structure from the desirable symmetry.…”

Diels‐Alder Reaction in microemulsions with ionic liquid

“…Combining with taking the limited water solubility of DB and EM into account, it is the result of the reaction largely taking place at the interface, where it was much faster than that in an organic solvent. The reason is the excellent solubilization, particular orientation and local concentration gradient effects at the interfaces (or concentrating reagents in the stern layer); therefore, the rates were enhanced …”

Analyzing the electronic structure of molecules using continuous symmetry measures

“…Geometric CSMs and CShMs have been successfully applied to a variety of stereochemical problems such as the description of the coordination environement, chirality, and reactivity of chemical compounds . The usefulness of these measures is especially evident when they are applied in conjunction with a statistical analysis to a large set of structural data obtained from a search in databases such as the Cambridge Structural Database .…”