Klaus Gollnick

Singlet oxygen photooxygenation of furans : Isolation and reactions of (4+2)-cycloaddition products (unsaturated sec.-ozonides)

Tetraphenylporphin-photosensitized oxygenations of furan (19), 2-methylfuran (26), 2-ethylfuran (39), furfurylalcohol (24), 2-acetylfuran (40), 2-methoxyfuran (42), 2,5-dimethylfuran (30), furfural (25) and 5-methylfurfural (41) in non-polar aprotic solvents yield the corresponding monomeric unsaturated secondary ozonides due to a (4+2)-cycloaddition of singlet oxygen to these furans. With the exception of the ozonide derived from 25, the ozonides were isolated and characterized (1H- and 13C-NMR spectra, etc.). In non-polar aprotic solvents, the ozonides derived from 19, 26 and 39 undergo thermal rearrangements to the corresponding cis-diepoxides and epoxylactones. Ozonide 31, derived from 30, however, dimerizes, only above about 60° is a cis-diepoxide formed from either 31 or its dimer. Rose bengal-photosensitized oxygenations of the furans in alcohols (MeOH, EtOH, i-PrOH) also produce the corresponding ozonides as the primary products of (4+2)-cycloadditions of singlet oxygen to these furans. However, reactions of the alcohols with the ozonides are too fast to allow the ozonides to be isolated. Instead, the same products are obtained as are isolated from reactions carried out by dissolving the ozonides in the alcohols. Depending on the structure of the ozonide, three pathways are available to ozonide/alcohol (ROH) interactions:(1) addition of ROH to yield alkoxy hydroperoxides; one out of several possible isomers is formed in a completely stereoselective and regiospecific reaction; (2) elimination of a bridgehead proton by ROH as a base, as observed with the ozonide derived from 19 to give hydroxy butenolide (78) in yields between 20 and 60%, and (3) ROH-attack on a carbonyl side-chain under elimination of the corresponding alkyl ester, as observed with furfural photooxygenation which yielded hydroxy butenolide (78) in high yields (95%). Interaction of ozonide 31 with tert.-butyl alcohol (t-BuOH) yields quantitatively cis-3-oxo-1-butenylacetate (81) by a Baeyer-Villiger-type rearrangement with vinyl group migration Hydrogenbonding between the alcohol and the peroxy group of the ozonides assist the heterolysis of the C—O bonds in the ozonides; the most stabilized cation develops. Front-attack of ROH on this cation explains the stereoselectivity as well as the regiospecificity of the alkoxy hydroperoxide formation; with a bulky alcohol like t-BuOH, ROH-attack on the cation is sterically hindered thus allowing a rearrangement to occur. 1,3-Dipolar cycloaddition of p-nitrophenyl azide to ozonide 31 proceeds stereoselectively to one of the isomers 87a/87b. Finally, kinetic results of furan photooxygenation in methanol show the following order of furan-reactivity towards singlet oxygen: 30 > 42 > 26 > 19 > 41 > 25, with absolute rate constants ranging from 1.8 × 108 (with 30) to 8.4 × 1O4 M-1P-1 (with 25).

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

Abstract 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 O 2 .A mechanism is outlined for this reaction.

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

Abstract 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).

Solvent dependence of singlet oxygen / substrate interactions in ene-reactions, (4+2)- and (2+2)-cycloaddition reactions

Abstract Product formation of singlet oxygen reactions with simple olefins occurring as ene-reactions, (4+2)- and (2+2)-cycloaddition reactions is independent on solvent polarity. Thus, 2,3-dimethyl-2-butene (1) and 2-methy]-2-butene (3), 1,3-cyclohexadiene (6), and benzvalene (8) yield allylic hydroperoxides (2) and(4) (54%) + (5) (46%), endoperoxide (7), and dioxetane (9), respectively. The rates of the ene-reactions and (4+2)-cycloaddition reactions are only slightly dependent, those of the (2+2)-cycloaddition reaction, however,are clearly dependent on solvent polarity. “Physical” quenching of singlet oxygen by the olefins is negligible, but substantial by the sensitizer tetraphenylporphin (TPP) in chlorinated solvents.

Interactions of singlet oxygen with 2,5-dimethyl-2,4-hexadiene in polar ano non-polar solvents evidence for a vinylog ene-reaction

Abstract 2,5-Dimethyl-2,4-hexadiene ( 1 )was studied as a singlet oxygen acceptor in various solvents. 1 undergoes concomitantly the three well-known modes of singlet oxygen reactions: (1) the ene-reaction to give the allylic hydroperoxide 3 , (2) the (4+2)-cycloaddition to give the endoperoxide 4 , and (3) the (2+2)-cycloaddition to give the dioxetane 2 . Beyond that (and in contrast to simple olefins), there are (4) “physical” quenching and (5) a “vinylog ene-reaction” to give the twofold-unsaturated hydroperoxide 5 . The latter reaction represents a novel mode of singlet oxygen interaction with a substituted 1,3-diene. - Kinetic analysis shows that “physical” quenching, endoperoxide and vinylog ene-product formations proceed with solvent-inde pendent rates; the rates of dioxetane and ene-product formations, however, are solvent-dependent. - A mechanism (Scheme 3) is proposed, according to which endoperoxide formation is due to a concerted singlet oxygen reaction with the s-cis-conformational isomer 1b ; with the s-trans-isomer 1a , “physical” quenching and the vinylog ene-reaction proceed via a non-polar singlet diradical intermediate, whereas the ene-product formation occurs via a per epoxide-like transition state. In aprotic solvents, the dioxetane is mainly formed via a “tight-geometry intermediate”, in methanolic solution via a solvent-stabilized zwitterion; the latter is also responsible for the formation of the methanol-addition product 6 .

Studien in der caranreihe—III : Zur darstellung des (+) (1R:6S)-Δ4(10)-carens [“β-caren”]

Zusammenfassung Die Bichromat-Oxydation des (-) (1R:6S)-Δ4(10)-Caren-trans-3-ols (I) in Benzol fuhrt zu (-) (1R:6S)-Δ3-Caren-10-al (II), das durch Reduktion nach Huang-Minlon zum (+) (1R:6S)-Δ4(10)-Caren (V) reduziert wird.

[4+2] - cycloaddition of singlet oxygen to conjugated acyclic hexadienes : evidence of singlet oxygen induced cis ⇌ trans - isomerization

Abstract Addition of singlet oxygen to trans , trans -2,4-hexadiene ( 1 ) occurs stereospecifically to give endoperoxide 2 . With cis , trans -2,4-hexadiene ( 4 ), however, a mixture of diastereomeric endoperoxides, 2 + 5 , is observed. Evidence of a singlet oxygen - induced cis ⇌ trans - isomerization is gained by competitive Diels-Alder reaction of 4 with singlet oxygen / diethyl diazenedicarboxylate.

Studien in der caranreihe—II : Über die pyrolyse der acetate von (+) (1r:4r:6s)-caran-trans-4-ol und (+) (1r:4s:6s)-caran-cis-4-ol; (+) (1r:6s)-Δ4-caren und (+) (1R:4R)-trans-Δ2,8-p-menthadien

Zusammenfassung Die Pyrolyse des Acetats von (+) (1R:4R:6S)-Caran- trans -4-ol (I) fuhrt bei 10 Torr und Temperaturen zwischen 386 und 604° unter Abspaltung von Essigsaure zu einem Kohlenwasserstoff-Gemisch aus (+) (1R:6S)-Δ 4 -Caren (III), (+) (1R:6S)-Δ 3 -Caren (IV) und (+) (1R:4R)- trans -Δ 2,8 0 p -Menthadien (VI), dessen Zusammensetzung von der Temperatur abhangt. Dieselben Kohlenwasserstoffe entstehen auch bei der Pyrolyse des Acetats von (+) (1R:4S:6S)-Caran- cis -4-ol (II).

Thiaozonide formation by singlet oxygen cycloaddition to 2,5-dimethylthiophene

Singlet oxygen reacts with 2,5-dimethylthiophene (1) exclusively by (4+2)-cycloaddition to yield the thiaozonide 2. The structure of this product is inferred from its 1H- and 13C NMR spectra. Neat thiaozonide 2 decomposes violently at room temperature. In aprotic non-polar and in protic polar solvents it is slowly transformed into cis-sulfine 3c and cis-2,5-dione 4c, which rearranges to the trans-isomer 4t. A mechanism for the transformation reaction is proposed.