The cyclooctatetraenoquinones, a computational study

Tests for aromaticity applied to the pentalenoquinones — A computational study

“…The curves for planar (these are hilltops) 1,2-and 1,4-cyclooctatetraenoquinones 4 and 5 (Fig. 5d) are as expected for nonaromatic molecules, implying that the actual, nonplanar quinones are nonaromatic (17).…”

“…Cyclobutadienoquinones are well-known, but the question of their aromaticity does not appear to have been exhaustively studied (16). The history of the cycloctatetraenoquinones has been briefly reviewed (17). The 1,2-quinone was reported in 1977 (18) and the 1,4-quinone in 1975 (19), but the question of their putative aromaticity was not specifically addressed experimentally, and apparently the only subsequent work on these compounds has been the computational study by Jakins and Lewars (17); this indicated that these quinones are nonplanar and nonaromatic.…”

The Quinones of Benzocyclobutadiene: A Computational Study

“…The study of quinones corresponding to less prosaic polyenes made its debut in 1971 with the synthesis of benzocyclobutenedione ( 1 ) and cyclobutenedione ( 7q ), necessarily more arduous and creative, because the “obvious” hydrocarbon precursors, or suitable derivatives, evaded synthesis. Subsequently, quinones of other nonbenzenoid compounds have been examined: the azulenoquinones, the cyclooctatetraenoquinones ( 9q1,4 and 9q1,2 ), − and the pentalene quinones ( 10 , 11 , 12 , 13 ) (Figure ). Again, indirect synthetic methodologies had to be used, as suitably functionalized derivatives of the parent hydrocarbons remain absent.…”

“…Again, indirect synthetic methodologies had to be used, as suitably functionalized derivatives of the parent hydrocarbons remain absent. Indeed, an important impetus for the study of the quinones of cyclooctatetraene and pentalene was their putative aromaticity, the theoretical basis for, and computational probing of which, has been discussed. , Briefly, a quinone formed from a polyene with an even number of CC double bonds is potentially aromatic if its carbonyl groups are sufficiently polarized to endow the molecule with significant dication character, so that it then has a Hückel aromatic 4 n + 2 number of π electrons, and if the energy to be gained by flattening, whenever necessary, is enough to allow atomic p−p overlap around the perimeter that the species can overcome incurred strain. A corollary of this is that a quinone from a cyclic polyene with an odd number of CC double bonds is potentially antiaromatic, although here there would seem to be no incentive for carbonyl polarization and ring flattening.…”

“…The principle is outlined in Figure for a quinone of cyclooctatetraene and one of pentalene. Cyclooctatetraene, 14 , has the nonplanar tub conformation, and calculations indicate both its 1,4- and 1,2-quinones to be nonplanar and nonaromatic, and NICS values − indicate that even the hypothetical planar conformations would be nonaromatic . Pentalene, 15 , is planar and its quinones 10 − 13 are (almost perforce) planar, yet homodesmotic ring-opening reactions (a kind of isodesmic reaction − ) showed 10 and 11 to be nonaromatic, while 12 and 13 were antiaromatic, evidently because their cyclopentadienone moieties usurp truly peripheral electron delocalization …”