[3.3]Metacyclophane: A Novel Synthesis and a Study of the Structure through X-ray Diffraction, Molecular Mechanics, and Solution NMR Analysis

Coordination of 3-phenylpropionitrile with chromium hexacarbonyl followed by treatment of the resulting arene- chromium complex with lithium diisopropylamide and then iodine produced 1,12-dicyano[3.3]metacyclophane in a remarkable 84% yield. The process involves intermolecular nucleophilic addition to the coordinated arene, followed by cyclization of the dimer; iodine completes the addition/oxidation procedure for nucleophilic aromatic substitution for hydrogen. Reductive cleavage of the cyano groups produces the parent hydrocarbon, [3.3]metacyclophane. In the crystals, the molecule assumes a syn geometry with the bridging chains in a chair-chair conformation; however, the arene rings are tilted with respect to one another and slightly twisted. Molecular mechanics calculations find the same conformer as the energy minimum, with a similar tilt of the arene rings, but perfect C_2vsymmetry (no twist). Two other conformations, syn(chair-boat) and syn(boat-boat), are within 1–2 kcal/mol of the lowest energy structure, while all conformers with the anti geometry are more than 6 kcal/mol higher. In solution, dynamic behavior is observed by variable-temperature ¹H NMR and ¹³C NMR spectroscopy, attributed to interconversion of two syn conformers via a chair-boat motion of one of the three-carbon bridges. The barrier to isomerization is found to be 10–11 kcal/mol from both ¹H NMR and ¹³C NMR data sets.