Details

Autor(en) / Beteiligte

• Neuvonen, Helmi
• Neuvonen, Kari
• Koch, Andreas
• Kleinpeter, Erich

Titel

Ab Initio Study of the Substituent Effects on the Relative Stability of the E and Z Conformers of Phenyl Esters. Stereoelectronic Effects on the Reactivity of the Carbonyl Group

Ist Teil von

• The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 2005-07, Vol.109 (28), p.6279-6289

Ort / Verlag

United States: American Chemical Society

Erscheinungsjahr

2005

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Equilibria between the Z (τ1 = 0°) and E (τ1 = 180°) 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, ΔE(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 ΔE of the E − Z equilibrium and the Hammett σ 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 → π*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 → π*C  O electron donation has a minimum value with τ1 = 90° and a maximum value with τ1 = 0° (the Z conformer), the value with τ1 = 180° (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 π*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 π*C  O occupancies of the ester series 4 and 5.