Topological resonance energies of conjugated ions, radicals, and ion radicals

o-Quinodimethanes: Efficient Intermediates in Organic Synthesis

“…50 The use of ultrasound has been recommended to improve the efficiency of these reactions. 51 Hofmann elimination from (o-methylbenzyl)trimethylammonium hydroxide (30), induced iodide debromination of o-xylylene dibromides (28), 1,52 and elimination from o-(trimethylsilyl)methylammonium salts (29), triggered by fluoride ion, 53 have been also used to generate oQDM (Scheme 12).…”

“…Formation of 80 may be explained as being the result of oQDM having considerable radical character. In a series of experiments, Errede 262 prepared oQDM itself by the flash pyrolysis of o-methylbenzyltrimethylammonium hydroxide (30), the oQDM being quenched soon after it was formed by cooling to -78 °C. The product trapped out under these conditions was an approximately 25:75 mixture of a 1,2,5,6-dibenzocyclooctadiene 80 and the spiro oquinodimethane dimer 299 (Scheme 69).…”

“…Despite the great amount of research focused on the 8π-electron oQDM system, the 9π-electron quinodimethane radical anion and its derivatives have remained elusive. Although resonance-energy calculations suggest that the oQDM radical anion should be well stabilized, it has eluded unequivocal experimental observation. , It has been only recently that the oQDM radical anion of α,α‘-bis(trimethylsilyl)- o -quinodimethane ( 16 ) was prepared by photolysis of a suitable precursor (Scheme ) at −120 °C with a high-pressure mercury lamp (500 W), which leads to a strong ESR signal ( g = 2.0029) showing hyperfine splittings of 0.698 (2H) and 0.192 mT (4H) 5 …”

Aromaticity of Heterocirculenes

“…According to REPEs, cyclooctatetraene shows a highly antiaromatic nature (REPE is −0.074 β), but its dianion and dication possess an aromatic nature, with REPE values of 0.019 and 0.031 β, respectively [60,61,69].…”

General rules for predicting the local aromaticity of carbon polyhedra

“…Accordingly, the NICS(0) criteria cannot be used for predicting the aromaticity of these compounds. In general, those monocyclic -electron systems which contain (4n + 2) -electrons, following the Hückel rule, are aromatic; whereas those monocyclic systems containing 4n electrons are strongly antiaromatic; and those monocyclic systems containing either a (4n + 1) or a (4n + 3) number of -electrons are weakly antiaromatic [32,33]. In a polycyclic -electronic system, the contributions of individual circuits to aromaticity obey the Hückel rule [34,35].…”