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 Cl aisen rearrangement from 1,1-bis(phenyloxymethyl)ethylene derivatives. Alth ough the conformational preference of this unit is important to understand, the three-dimensional structures of the crownophanes and their binding aff inities with guest molecules and the relative energies of rotamers of 1,1-d ibenzylethylene and its related compounds were not known. MP2/6-311G**//HF/ 6-311G** calculations showed that the C+C-C-C bonds of 1,1- dibenzylethylen e had a skew-skew conformation in the most stable rotamer. The skew-skew co nformation had been observed in the crystal form of the water-crownophane c omplex. The calculated relative energies of rotamers suggested that nonbond ing interaction between the benzene rings (quadrupole-quadrupole interactio n) 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, whi le the second most stable rotamer, in which the two methoxy groups are on t he same side, was only 0.38 kcal mol(-1) less stable.