Synthesis of an Extremely Crowded Naphthalene via a Stable Norbornadienone

Computational studies at the HF/3-21G and B3LYP/6-31G(d) levels suggest that 5,6,8-tri(tert-butyl)-1,2,3,4-tetraphenylnaphthalene (2) is perhaps the most crowded naphthalene derivative that will show normal
stability; more highly congested naphthalenes will prefer to exist as the corresponding Dewar isomers. Initial
attempts to prepare 2 by reacting 3,4,5,6-tetraphenylbenzyne with 2,3,5-tri(tert-butyl)cyclopentadienone at 83
°C gave instead a stable norbornadienone, 1,2,4-tri(tert-butyl)-5,6,7,8-tetraphenyl-9-oxo-1,4-dihydro-1,4-methanonaphthalene (3), which was characterized by X-ray crystallography. The experimental and calculated
(HF/3-21G) activation energies (Ea) for the decarbonylation of 3 were quite high:  39 and 46 kcal/mol,
respectively, a manifestation of the substantially increased strain in the transition state as the tert-butyl groups
are forced together. The naphthalene 2 was obtained in good yield by heating 3 in refluxing toluene, and its
X-ray structure showed exceptional distortions from a normal naphthalene geometry. However, 2 is not
completely stable, and it decomposes upon prolonged heating in xylenes.