The Rearrangement of 2,2-Diphenyl-1-[(E)-2-phenylethenyl]cyclopropane to 3,4,4-Triphenylcyclopent-1-ene: a DFT Analysis

Willi Sicking, Reiner Sustmann, Johann Mulzer, Rolf Huisgen

A computational study on the rearrangement of 2,2-diphenyl-1-[(E)-2-phenylethenyl]cyclopropane (1) is presented, using density functional theory (DFT), (U)B3LYP with the 6-31G* basis set (DFT1) and (U)M05-2X with the 6-311+G** basis set (DFT2). In agreement with a biradical character of the transition structure (TS) or intermediate, the potential-energy hypersurface is lowered by the influence of three conjugated Ph groups. Surprisingly, two conformations of the geminal diphenyl group (different twist angles) induce two different minimum-energy pathways for the rearrangement. Independent of the functional used, the first hypersurface harbors true biradical intermediates, whereas the second energy surface is a flat, slightly ascending slope from the starting material to the TS. The functional (U)M05-2X with the basis set 6-311+G** provides realistic energies which seem to be close to experiment. The activation energy for racemization of enantiomers of 1 is lower than that of rearrangement by 2.5 kcal mol⁻¹, in agreement with experiment.