| Abstract
| - Equilibria between the Z (τ1 = 0°) and E (τ1 = 180°) conformers of p-substituted phenyl acetates 4 andtrifluoroacetates 5 (X = OMe, Me, H, Cl, CN, NO2) were studied by ab initio calculations at the HF/6-31G*and MP2/6-31G* levels of theory. The preference for the Z conformer, ΔE(HF), was calculated to be 5.36kcal mol-1 and 7.50 kcal mol-1 for phenyl acetate and phenyl trifluoroacetate (i.e., with X = H), respectively.The increasing electron-withdrawing ability of the phenyl substituent X increases the preference of the Zconformer. An excellent correlation with a negative slope was observed for both series between ΔE of theE−Z equilibrium and the Hammett σ constant. By using an appropriate isodesmic reaction, it was shown thatelectron-withdrawing substituents decrease the stability of both conformers, but the effect is higher with theE conformer. Electron-withdrawing phenyl substituents decrease the delocalization of the lone pair of theether oxygen to the CO antibonding orbital (nO → π*CO) in both the E and Z forms and in both seriesstudied; this effect is higher in the E conformer than in the Z conformer. The nO → π*CO electron donationhas a minimum value with τ1 = 90° and a maximum value with τ1 = 0° (the Z conformer), the value withτ1 = 180° (the E conformer) being between these two values, obviously due to steric hindrance. The effectsof the phenyl substituents on the reactivity of the esters studied are discussed in terms of molecular orbitalinteractions. ED/EW substituents adjust the availability of the π*CO antibonding orbital to interact with thelone pair orbital of the attacking nucleophile and therefore affect the reactivity: EW substituents increaseand ED substituents decrease it. Excellent correlations were observed between the rate coefficients ofnucleophilic acyl substitutions and π*CO occupancies of the ester series 4 and 5.
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