Conformational analysis of the core unit of crownophanes by ab initio calculations: 1,1-dibenzylethylene and related compounds

1,1-Dibenzylethylene is the core unit of crownophanes obtained by tandem Claisen rearrangement from 1,1-bis(phenyloxymethyl)ethylene derivatives. Although the conformational preference of this unit is important to understand, the three-dimensional structures of the crownophanes and their binding affinities with guest molecules and the relative energies of rotamers of 1,1-dibenzylethylene and its related compounds were not known. MP2/6-311G**//HF/6-311G** calculations showed that the CC–C–C bonds of 1,1-dibenzylethylene had a skew–skew conformation in the most stable rotamer. The skew–skew conformation had been observed in the crystal form of the water–crownophane complex. The calculated relative energies of rotamers suggested that nonbonding interaction between the benzene rings (quadrupole–quadrupole interaction) stabilized the skew–skew rotamer. The calculations of 1,1-bis(2-hydroxy-3-methoxyphenylmethyl)ethylene showed that the most stable rotamer had the conformation in which the two methoxy groups are on the opposite sides, while the second most stable rotamer, in which the two methoxy groups are on the same side, was only 0.38 kcal mol−1 less stable.