Kinetics and mechanism of diels-alder additions of tetracyanoethylene to anthracene derivatives—I : Substituent effects

The rates of addition of tetracyanoethylene to a number of 9- and 9,10-substituted anthracenes have been measured spectrophotometrically in solvent CCl₄. Substituent effects correlated well using the extended form of the Hammett equation. The importance of steric effects on the reaction was assessed by a systematic variation of the components of the data used in the above correlation.Activation parameters (ΔG_exp^≠, etc.) and the corresponding overall thermodynamic parameters for adduct formation (ΔG_ad°, etc.) were evaluated. ΔG_exp^≠ was found to be linearly related to ΔG_c°, the free energy of formation of the intermediate complex which confirms the role of the latter as a true reaction intermediate. From correlations between ΔG_exp^≠ and ΔG_ad°, an “early” transition state is suggested. The above thermodynamic and activation data enable detailed reaction profiles to be drawn.