Ab Initio Study of the Substituent Effects on the Relative Stability of theEandZConformers of Phenyl Esters. Stereoelectronic Effects on the Reactivity of the Carbonyl Group

Abstract

Equilibria between the Z (tau1= 0 degrees) and E (tau1= 180 degrees) conformers of p-substituted phenyl acetates 4 and trifluoroacetates 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, DeltaE(HF), was calculated to be 5.36 kcal 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 Z conformer. An excellent correlation with a negative slope was observed for both series between DeltaE of the E-Z equilibrium and the Hammett sigma constant. By using an appropriate isodesmic reaction, it was shown that electron-withdrawing substituents decrease the stability of both conformers, but the effect is higher with the E conformer. Electron-withdrawing phenyl substituents decrease the delocalization of the lone pair of the ether oxygen to the C=O antibonding orbital (nO--> pi*C=O) in both the E and Z forms and in both series studied; this effect is higher in the E conformer than in the Z conformer. The nO --> pi*C=O electron donation has a minimum value with tau1= 90 degrees and a maximum value with tau1= 0 degrees (the Z conformer), the value with tau1= 180 degrees (the E conformer) being between these two values, obviously due to steric hindrance. The effects of the phenyl substituents on the reactivity of the esters studied are discussed in terms of molecular orbital interactions. ED/EW substituents adjust the availability of the pi*C=O antibonding orbital to interact with the lone pair orbital of the attacking nucleophile and therefore affect the reactivity: EW substituents increase and ED substituents decrease it. Excellent correlations were observed between the rate coefficients of nucleophilic acyl substitutions and pi*C=O occupancies of the ester series 4 and 5.