2,4,4,6-Tetrabromocyclohexa-2,4-dienone: a new electron acceptor in the photosensitized oxidation of unsaturated substrates

Photoinduced electron transfer oxygenations

“…TBCHD, usually used as a mild and selective brominating agent [189,190], acts as an electron acceptor in the sensitized photooxygenation of several unsaturated organic substrates such as 8, 10a, 22a, 25, 64a [90,111], affording reaction products similar to those previously reported by Foote [84], Farid [99], and Schaap [98] in the DCA-sensitized reactions on the same substrates. The exceedingly low reduction potential (E rca = 0.29 V vs SCE), together with its excitation energy…”

“…(AEo-0 = 2.94 eV) and the oxidation potentials of the donors substantiate, on the basis of Weller's equation [59], that an exothermic electron-transfer process between the donors and the singlet excited sensitizer ITBCHD* is in agreement with the physical properties of the reagents [90,111]. The TBCHD-sensitized reaction conditions closely resemble the methylene blue (MB+)-sensitized photooxidation of 10a [124], a case in which the reduced sensitizer is unable to reduce molecular oxygen to superoxide anion.…”

“…In this regard, clear chemical and spectroscopic evidence for the disproportionation of the intermediate radical cations, photochemicaUy and/or thermally generated, were achieved on 2,3-diphenyl-5,6-dihydro-l,4-dioxin, and derivatives 22a-c [90,111,. In fact, the 2,4,4,6-tetrabromo-2,5-cyclohexadien-l-one (TBCHD)-sensitized photooxygenation of 22a affords the corresponding 1,2-ethanedioldibenzoate, 67, the cleavage product of the intermediate 1,2-dioxetane, 24a, together with minor amounts of 4a,8a-diphenyl-2,3,4a,6,7,8a-hexahydro-pdioxino [ Experimental proofs suggest that ~-diketones could arise from a mechanism still involving dioxene radical cations as intermediates.…”

“…However, to better investigate these sensitized reactions leading to the oxidation products through a chain electron-transfer mechanism, it was desirable to use sensitizers which neither produce superoxide ion, nor singlet oxygen, for this purpose TCA [99], 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+BP4 -) [185], and 2,4,4,6-tetrabromocyclohexa-2,5-dien-l-one, TBCHD [90,112], were found appropriate sensitizers.…”

“…Interestingly, as reported in the previous section, two unconnected mechanisms seem to operate in the TBCHD-sensitized photooxidation of several substituted 2,3-diaryl-5,6-dihydro-l,4-dioxins 22a-c, and substituted 2,3,5,6-tetraphenyl-l,4-dioxin, 64a-d, yielding, as major products, 1,2-ethanedioldibenzoates 24a-c, and (Z)-stilbenediol dibenzoates 66a-d respectively, together with variable amounts of the corresponding ct-diketones [90,111] (Eqs. (27)].…”