9, 10-DICYANOANTHRACENE-SENSITIZED PHOTOOXYGENATION OF α,α'-DIMETHYLSTILBENES. MECHANISM AND KINETICS OF THE COMPETING SINGLET OXYGEN AND ELECTRON TRANSFER PHOTOOXYGENATION REACTIONS

Abstract— The 9, lodicyanoanthracene-sensitized photooxygenation of 2-methyl-2-butene and (+)-limonene proceeds via the singlet oxygen pathway in carbon tetrachloride as well as in acetonitrile, although the fluorescence of the sensitizer in acetonitrile is quenched by these olefins in an electron transfer quenching mechanism. The 9, 10-dicyanoanthracene-sensitized photooxygenation of cis- and trans-ä, ä′-dimethylstilbenes occurs exclusively via the singlet oxygen pathway in carbon tetrachloride; in acetonitrile, however, singlet oxygen and electron transfer photooxygenation reactions compete with one another. Addition of tetra-n-butyl ammonium bromide and increasing oxygen concentrations favor the formation of the singlet oxygen product, whereas addition of anisole, increasing substrate concentrations and decreasing oxygen concentrations favor the electron transfer photooxygenation products. In carbon tetrachloride, exciplexes of the sensitizer and the dimethylstilbenes are formed which give rise to cidrrans-isomerization of the substrates. In acetonitrile, neither exciplex formation nor cisltrans-isomerization are observed. A mechanism is proposed which allows us to calculate product distributions of the competing singlet oxygen/electron transfer photooxygenation reactions and thus to determine the efficiencies with which encounters between the singlet excited sensitizer and the substrates finally result in electron transfer photooxygenation products. Using (I) these efficiencies, (2) the β-value obtained from singlet oxygen photooxygenation sensitized by rose bengal, and (3) the appropriate k-values determined from fluorescence quenching of 9, 10-dicyanoanthracene in MeCN by oxygen and the stilbene, allows the calculation of the quantum yield of oxygen consumption by this stilbene. The quantum yield thus calculated is strictly proportional to the rate of oxygen consumption experimentally obtained; this result is considered as convincing evidence for the mechanism proposed.     

Formation of a 1,2-dioxane by electron-transfer photooxygenation of 1,1-di(p-anisyl)ethylene

A new mode of electron-transfer photooxygenation is shown to occur with the title compound (4). With this electron-rich ethylene derivative, DCA-sensitization in acetonitrile gives rise to the quantitative formation of a cyclic peroxide (5) by cycloaddition of 2 molecules of 4 and 1 molecule of O₂.A mechanism is outlined for this reaction.     

Electron-transfer photooxygenation of tetraphenylallene formation of 1,3-dihydroperoxy-1, 1,3,3-tetraphenyl-2-propanone and its decomposition under chemiluminescence

DCA-sensitized electron-transfer photooxygenation of tetraphenylallene ($\text{1}$) in acetonitrile yields benzophenone ($\text{3}$) and polymeric material. In acetone, the yield of $\text{3}$ is better than twice the amount obtained in acetonitrile and very little of polymeric material is observed. If the acetone solution is worked-up immediately after the oxygen consumption ceased, 1,3-dihydroperoxy-1,1,3,3-tetrapheny]-2-propanone ($\text{8}$) is isolated. Its formation is proposed to occur via the peroxyallyl zwitterion $\text{4}$ and the tetraphenylcyclopropanone ($\text{7}$) (Scheme 1). $\text{8}$ decomposes slowly into $\text{3}$, and CO + CO₂ (3:1) in neutral solution; in the presence of a base, decomposition is fast, resulting in the formation of two molecules of $\text{3}$, one molecule of water, and one molecule of CO₂ . Decomposition of $\text{8}$ in the presence of various fluorescers and a base yields a bright fluorescence of the additives.     

Photooxygenation of tetramethylallene competing (2+2) cycloaddition and ene-reactions with singlet oxygen

TPP-sensitized photooxygenation of tetramethylallene (4) in carbon tetrachloride yields acetone (5), 2,4-dimethyl-4-hydroxy-1-penten-3-one (8) and 2,4-dimethyl-1,4-pentadien-3-one (9) in a ratio of 35:20:45, besides minor amounts of resinous products and carbon dioxide. Isomerization of 4 to 2,4-dimethyl-1,3-pentadiene (6) does not occur under the reaction conditions. DABCO quenches the photooxygenation, whereas 2,4,6-tri-t-butylphenol (10) enhances the oxygen consumption rate but leaves the ratio of 5:8:9 unchanged. These results indicate that 4 is oxygenated by singlet oxygen. A mechanism is proposed according to which acetone is generated via a (2+2) cycloaddition whereas 8 and 9 are formed via an ene-reaction between 4 and singlet oxygen.     

Formation of 1,2-dioxanes by electron-transfer photooxygenation of 1,1-disubstituted ethylenes

Electron-rich 1,1-diarylethylenes (1a–e) afford 3,3,6,6-tetraaryl-1,2-dioxanes (3a–e) in high yields (>907%) when subjected to electron-transfer photooxygenation in the presence of DCA. Whereas 1,1-diphenyl-ethylene (1f) and 1,1-di(p-chlorophenyl)ethylene (1h) yield the 1,2-dioxanes 3f and 3h at 30% and less than 10%, respectively, there is negligible (if any) 1,2-dioxane formation with 1,1-di(m-anisyl)ethylene (1i). 1,2-Dioxane formation proceeds in a chain reaction (Scheme 1). N-Vinylcarbazol (1g), however, yields the 1,2-dioxane 3g via the cyclobutane derivative 7 (Scheme 2).