Effects of CH–O and CH–π interactions on the conformational preference of a crownophane core unit

Abstract

Relative energies of the rotamers of prop-2-enyl o-methoxyphenyl ether and related compounds (CH₂CH–CH₂–X–R, X = O or CH₂, R = CH₃, C₆H₅ or o-CH₃OC₆H₄) were calculated at the MP2/6-311G**//HF/6-311G** level as models of the crownophane core unit [1,1-bis(aryloxymethyl)ethylene]. The calculations show that CH–O and CH–π interactions play important roles in determining the conformational preference of the core unit. The terminal methylene unit of the crownophane, which consists of the core unit and a –O–(CH₂–CH₂–O)_n– chain (n = 4), points towards the inside of the ring cavity (in-conformation) in the crystal. The CC–C–O and C–C–O–C bonds of the crownophane have an eclipse-trans conformation. Conformational analysis of model compounds shows that the eclipse-trans conformation is stabilized by a CH–O interaction. The methylene unit of the crownophane which has a shorter oxyethylene chain (n = 3) points toward the outside (out-conformation) in the crystal. The CC–C–O and C–C–O–C bonds of this crownophane adopt a skew-gauche′ conformation that is stabilized by a CH–π interaction. Conformational analysis of model compounds shows that the CC–C–O and C–C–O–C bonds of the core unit prefer the eclipse-trans conformation and that the skew-gauche′ conformation is slightly less stable. Calculations on the in- and out-conformations of the crownophane (n = 3) show that the out-conformation is more stable and that the in-conformation has significant strain due to the short oxyethylene chain, suggesting that this strain is the cause of the observed out-conformation in the crystal.