Photochemical rearrangement of dibenzo[1,4]dioxins proceeds through reactive spirocyclohexadienone and biphenylquinone intermediates

Abstract

Photochemical studies on a range of model dibenzo[1,4]dioxins were performed in aqueous and organic solutions. The compounds were found to undergo a photochemically initiated aryl-ether bond homolysis that yields reactive 2-spiro-6'-cyclohexa-2',4'-dien-1'-one and subsequent 2,2'-biphenylquinone intermediates. Under steady-state irradiation, the 2,2'-biphenylquinones were observed to participate in excited state hydrogen abstraction from the organic solvent to give the corresponding 2,2'-dihydroxybiphenyls. In the absence of continued irradiation, 2,2'-biphenylquinones with electron donating substituents thermally rearrange to the corresponding oxepino[2,3-b]benzofurans, whereas the unsubstituted 2,2'-biphenylquinone and its derivatives with electron withdrawing groups thermally rearrange to the corresponding 1-hydroxydibenzofurans.

Theoretical study of substituent and solvent effects on the thermodynamics for cis/trans isomerization and intramolecular rearrangements of 2,2′-diphenoquinones

“…1 kcal/mol, and log K isom = 11.8), supporting prior experimental studies showing these compounds can be formed via the thermal intramolecular rearrangement of various 2,2 0 -diphenoquinones [7,9,13,14]. In contrast to the trans ?…”

“…In research efforts during the 1960s through 1980s, the groups of Hewgill [1][2][3][4][5][6][7] and Becker [8,9] employed oxidative approaches to generate-and in some cases isolate-these elusive compounds. More recently, during the 1990s and 2000s, it was shown that 2,2 0 -diphenoquinones may be photochemically generated from a range of dibenzo [1,4]dioxins (including the parent system) having electron withdrawing and donating groups [10][11][12][13], as well as via traditional thermal oxidative methods with 3,3 0 ,5,5 0 -tetraaryl substituents (e.g., phenyl and 4-methoxyphenyl) [14].…”

“…1). The relative energies of these isomers are of interest, as mechanisms for the thermal intramolecular rearrangement of 2,2 0 -diphenoquinones into oxepino [2,3-b]benzofurans have been proposed that require involvement of the cis configuration [7,9,13,14]. In addition, the expected different spectral properties of the cis/trans isomers, and their potential thermal/excited state interconversions, offer possible utility as molecular switch motifs.…”

Lability of the trifluoromethyl group of trifluoromethoxybenzenes under HF/Lewis acid conditions

“…1 H and 13 C NMR was performed on a Bruker AV-400. 19 F MNR was performed on a Bruker DPX-250. MS was performed on an Agilent 6210 Time-of-Flight spectrometer.…”

“…Chlorophenols, hydroquinone and pyrocatechol were from Aldrich. Dibenzodioxin was prepared by the method of Rayne et al [19]. Trifluoromethoxybenzenes 5-8 were prepared by the method of Feiring [2,10].…”