X-ray and quantum chemical studies of strained phenanthrenes

The synthesis of the highly strained helical 4-tert-butyl-1,5,8-trimethylphenanthrene 6 was achieved by photochemically induced dehydrocyclization. The helical deformation of the aromatic carbon skeleton in the reported X-ray structure is found to be 36.6 degrees which is the largest value found for a phenanthrene hydrocarbon. A systematic quantum chemical investigation of strained 4,5-substituted phenanthrene derivatives (R, R': F, Cl, methyl, tert-butyl) at ab initio Hartree-Fock and semiempirical AM1 theoretical levels is presented. The geometrical parameters describing the strain in these helical molecules are compared with X-ray data from the literature and are found to be generally in good agreement. Substantial deviations between theory and experiment for the helical deformation angles are observed in the case of the chlorine-substituted molecules(ca. 5 degrees) which indicates the importance of crystal packing effects. A prediction of the structure of the unknown 4,5-di-tert-butylphenanthrene 5, that could be detected by GC-MS, but has not been isolable so far is also given. The total strain energies of 5 and 6 are calculated to be 49.6 and 34.0 kcal mol(-1), respectively, and are distributed to non-bonded and aromatic ring deformation contributions.