Electrophilic substitution with rearrangement. Part 9. Dienones derived from brominations of o-, m-, and p-cresol

Abstract

Regiospecific protodebrcmination of ring-substituted bromophenols derived from 2-, 3-, or 4-methylphenol can be effected by heating them with aqueous hydrogen iodide ; the synthetic scope of this reaction has been explored. These di-and poly-bromophenols can generally be converted by further bromination in aqueous acetic acid into dienones, which have now been shown to have the 4-bromo-2,5-dienone rather than the 2-bromo-3,5-dienone structure. The rearrangements of these dienones to ring-substituted polybromophenols by treatment with sulphuric acid have been investigated ; where more than one product is formed, the regioselectivity differs from that prevailing in the corresponding direct bromination of the phenol with liquid bromine. The alternative rearrangements of these dienones in aprotic solvents with and without illumination have been compared with results obtained by reaction of the methylphenols with bromine under the same conditions. Characteristic differences between the behaviours of 2-, 3-, and 4-methyl-substituted compounds reflect the specific reactions available to the particular dienones.

Aspects of the chemistry of halophenols and halodienones

“…3,5-Di-r-butyl-2,2-dichlorocyclohcxa-3,5-dienone reacted with sodium methoxide in methanol to give 3,5-di-r-butyl-2,4-dichlorophenol, and it was suggested that the pathway was as shown in sequence (54). A reaction which could be a unimolecular example of this kind was rcportcd by Crozier and HcwittI4'.…”

Chapter 10. Aromatic compounds

“…The unstable 2,3-naphthoquinone has been trapped by cyclopentadiene during its formation from 2,3-dihydro~ynaphthalene. '~' The synthesis of anthraquinones by the use of isobenzofulvalene (33), analogous to the behaviour of isobenzofuran, is described,121 and oquinones (e.g. phenanthraquinone) afford a good general annelation reaction through the formation of the diacetylene diol (34) and ringclosure of the derived diallyl diol (35) (Scheme 9).12* Amongst new and interesting systems, Paquette and his colleague^^^^^ have reported the preparation of three derivatives of 11 4sopropylidenedibenzonorbornadiene in which the two benzene rings are dissimilarly substituted.…”

13C nuclear magnetic resonance spectra of some bromo- and methyl-substituted cyclohexa-2,5-dienones

“…Details concerning some of them have been given, or outlined, in earlier papers. [6][7][8] Here the earlier results are amplified with information relating to a number of other compounds, and the chemical shifts accompanying replacements by bromo substituents in various positions are compared with those for other cyclohexa-2,5-dienones 9-1 and for related unsaturated systems. The single-resonance ('undecoupled') spectra have been analysed as far as possible to give coupling constants (lJ('H, 13C), 'J('H, 13C), 3J('H, l3C), 'J('H, 13C)), and these also have been compared with cognate values available in the literature.…”