Substituent effects on the conformational stability of allyl fluorides

Abstract

AbstractBy the B3P86/6‐311G(3d,2p) method, remote substituent effects on trans‐YCHCHCH2F were investigated by examining their conformational stabilities, molecular geometries, and stereoelectronic interactions in this paper. The cis conformer is favored for YH, Cl, Me, Vinyl, CF3, CN, CHO, and NO2, whereas the gauche is favored for YOMe, OH. A correlation of ΔH with the substituent constants σ+(Y) shows that the increasing electron‐withdrawing ability of the substituent Y increases the relative stability of the cis conformer. It was found that the substituent effect on the molecule stabilization energies (relative to CH2CHCH2F) is more significant in the gauche conformers than in the cis conformers. In agreement, molecular structures of the gauche conformers were also observed to vary more significantly with the substitution than those of the cis conformers. By the second‐order perturbation energy (E(2)) in NBO analysis, it was found that the total C2–C3 vicinal hyperconjugation is determinant in the enthalpy difference and consequently controls the conformational stability. Further analysis shows that the substituent effect on the C2–C3 vicinal hyperconjugations is much higher in the gauche conformers than in the cis conformers. The highly sensitive πCC→σ*CF interaction to the substitution in the gauche conformers, is the leading factor in variation of molecular stability and geometry. Copyright © 2010 John Wiley & Sons, Ltd.