Effects of CH-O and CH-pi interactions on the conformational preference ofa crownophane core unit

Relative energies of the rotamers of prop-2-enyl o-methoxyphenyl ether and related compounds (CH2=CH-CH2-X-R, X = O or CH2, R = CH3, C6H5 or o-CH3OC6H 4) 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 sh ow that CH-O and CH-pi 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)- c hain (n = 4), points towards the inside of the ring cavity (in-conformation ) in the crystal. The C=C-C-O and C-C-O-C bonds of the crownophane have an eclipse-trans conformation. Conformational analysis of model compounds show s 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 C =C-C-O and C-C-O-C bonds of this crownophane adopt a skew-gauche' conformat ion that is stabilized by a CH-pi interaction. Conformational analysis of m odel compounds shows that the C=C-C-O and C-C-O-C bonds of the core unit pr efer the eclipse-trans conformation and that the skew-gauche' conformation is slightly less stable. Calculations on the in- and out-conformations of t he crownophane (n = 3) show that the out-conformation is more stable and th at the in-conformation has significant strain due to the short oxyethylene chain, suggesting that this strain is the cause of the observed out-conform ation in the crystal.