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 Helmi Neuvonen,* ,† Kari Neuvonen, † Andreas Koch, ‡ and Erich Kleinpeter ‡ Department of Chemistry, UniVersity of Turku, FIN-20014 Turku, Finland, and UniVersity of Potsdam, D-14415 Potsdam, POB 691553, Germany ReceiVed: December 22, 2004; In Final Form: May 9, 2005 Equilibria between the Z (τ 1 ) 0°) and E (τ 1 ) 180°) conformers of p-substituted phenyl acetates 4 and trifluoroacetates 5 (X ) OMe, Me, H, Cl, CN, NO 2 ) were studied by ab initio calculations at the HF/6-31G* and MP2/6-31G* levels of theory. The preference for the Z conformer, ∆E(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 ∆E 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 CdO antibonding orbital (n O f π* CdO ) 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 n O f π* CdO electron donation has a minimum value with τ 1 ) 90° and a maximum value with τ 1 ) 0° (the Z conformer), the value with τ 1 ) 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* CdO 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* CdO occupancies of the ester series 4 and 5. Introduction One of the conformational equilibria of carboxylic acid esters is due to the rotation around the single C(dO)-O bond. In general the most stable conformations correspond to the planar Z (τ 1 ) 0°) and E (τ 1 ) 180°) conformers; the Z conformer being clearly favored (Scheme 1). 1-3 Several different aspects have been considered to explain the higher stability of the Z conformer. These include for instance steric effects, dipole- dipole interactions, lone pair-lone pair repulsion effects, and hyperconjugative interactions. 1-6 When R is methyl or a larger group unfavorable steric interactions between R and R′ are obvious in the E conformer. However, for esters of formic acid (R ) H) the main steric interactions are between the carbonyl oxygen and R′. This is seen for instance when compared the E-Z free energy differences 2.5, 1.67, 1.36, and 0.48 kcal mol -1 , respectively, determined for methyl, ethyl, isopropyl and tert-butyl formates (in acetone-d 6 - DMF, 1:1). 7 In the gas phase the Z conformer of tert-butyl formate is highly preferred over the E conformer [5.11 kcal mol -1 (RHF)]. 8 Although the Z conformer is more stable than the E conformer in each case its preference decreases when the size of R′ increases. Increasing electronegativity of R′ in the order methyl < vinyl ≈ phenyl ≈ cyclopropyl < hydrogen < ethynyl in esters of formic acid, HCOOR′, has been suggested to favor the E relative to the Z conformer. 9 Ab initio calculations, recently performed by Noe et al. 10 for trichloromethyl formate 1 and trifluoromethyl formate 2, show that, although also for these compounds the rotational E isomer is calculated to have a higher free energy in the gas phase [the free energy difference 0.857 kcal mol -1 for 1 and 1.14 kcal mol -1 for 2 at the MP2/6-311G (df, pd) level], the E-Z free energy difference is much smaller than that for methyl formate 3 (5.16 kcal mol -1 at MP2/6-31+G** level). 11 The relatively low free-energy differences between the E and Z conformers for 1 and 2 was attributed to a combination of steric effects and the near-equality of the dipole moments for the two conformers. 10 A recent study by Uchimaru et al. 12 shows that the preference of the Z conformer over the E conformer (CBS-APNO enthalpy differences) is decreased in the series of methyl formate (5.03 kcal mol -1 ), fluoromethyl formate (3.05 kcal mol -1 ), difluoromethyl formate (2.58 kcal mol -1 ), and trifluoromethyl formate (1.14 kcal mol -1 ). Changes in geometry were discussed in terms of an interplay of orbital interactions (n O f π* CdO vs n O f σ* C-F ). 12 Despite the large amount of both the experimental and theoretical studies concerning the stability of ester conformations * Corresponding author. Fax: +358-2-3336700. E-mail: helmi.neuvonen@utu.fi † University of Turku. ‡ University of Potsdam. SCHEME 1 6279 J. Phys. Chem. A 2005, 109, 6279-6289 10.1021/jp044172k CCC: $30.25 © 2005 American Chemical Society Published on Web 06/28/2005