X-Ray and quantum chemical studies of strained phenanthrenes

Abstract

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" which is the largest value found for a phenanthrene hydrocarbon. A systematic yuan tum chemical investigation of strained 4,S-substit u ted phenanthrene derivatives (R, R': F, Cl, methyl, tert-butyl) at ab initiu 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 chlorinesubstituted molecules (ca. 5") 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-', respectively, and are distributed to non-bonded and aromatic ring deformation contributions.

Twisted Arenes

“…14) have also been synthesized. The X-ray structure of 48b shows a pronounced torsional angle y of 41.9 [27,53].…”

“…With a sterically hindered tert-butyl group at the C4 position and methyl groups at C1, C5, and C8 positions as depicted in 12 (Fig. 5), the torsional angle y further increases to 36.6 [27]. The total strain energy in 12 was estimated to be 34.0 kcal/mol at the ab initio HF-SCF level.…”

Photodesulfurization of 2,4‐diaryl‐1,2, 4‐triazole‐3‐thiones

“…There are a few reports [5][6][7] on the intramolecular photocyclization of amides and thioamides. Synthesis of heterocyclic systems via photodesulfurization has also been reported [8][9][10]. Photoinduced desulfurization of episulfide [11] and indoline-2-thiones to indole too are known [12].…”