Synthesis, Structure, and Resolution of Exceptionally Twisted Pentacenes

9,10,11,20,21,22-hexaphenyltetrabenzo[a,c,l,n]pentacene (2) and a dimethyl derivative (2m) were
prepared by the reaction of 1,3-diphenylphenanthro[9,10-c]furan with bisaryne equivalents generated from
1,2,4,5-tetrabromo-3,6-diarylbenzenes in the presence of n-butyllithium, followed by deoxygenation of the
double adducts with low-valent titanium. Both are bright red solids with a strong orange fluorescence in
solution. The X-ray structures of these compounds show them to be the most highly twisted polycyclic
aromatic hydrocarbons known. Compound 2 has an end-to-end twist of 144°, and the two crystallographically
independent molecules of 2m have twists of 138° and 143°. Both molecules were resolved by
chromatography on chiral supports, and the pure enantiomers have extremely high specific rotations (for
2, [α]D = 7400°; for 2m, 5600°), but the molecules racemize slowly at room temperature (ΔG⧧rac = 24
kcal/mol). Both the experimental geometry and the observed racemization barrier for 2 are in good agreement
with computational studies of the molecule at a variety of levels. Attempts to prepare compound 2 by reaction
of tetraphenylbenzyne with 9,10,12,13-tetraphenyl-11-oxacyclopenta[b]triphenylene (3, a twisted isobenzofuran) gave no adducts, and attempts to prepare tetradecaphenylpentacene by reaction of hexaphenylisobenzofuran (11) with bisaryne equivalents gave only monoadducts.