Water-soluble manganese(II1) as well as iron(II1) porphyrinates are introduced as light-sensitive precursor compounds for the photocatalytic activation of dioxygen in aqueous solutions. It is shown that in the presence of a-pinene (4) and the further cycloalkenes 11 -13 photocatalytic oxygenation reactions occur. The dependence of the selectivity of the oxygen transfer to the olefin on both the presence of water and the variation of the substrate-to-catalyst ratio is discussed. The catalyst may be conveniently separated from the substrates/products by using aqueous solvent systems.The catalytic oxygenation of hydrocarbons with dioxygen in the presence of metal porphyrinates has attracted considerable attention in the chemical as well as biochemical literature particularly with respect to the biomimetic modeling of the function of natural monooxygenases containing iron porphyrin complexes as prosthetic group. This path of the direct activation of dioxygen under mild reaction conditions is of additional interest because of the selectivity and also variability of the oxygen transfer to organic substrates. A number of papers deal with the catalytic oxygenation of various substrates in the presence of metal porphyrinates. However, instead of a direct activation of O2 monooxygen donors like iodosylbenzene or peracids are mostly used [']. Oxygenation reactions using dioxygen require additional reductants like borohyride[21 or colloidal platinum in the presence of H2 or Zn/acetic acidL31. 0x0-metal(1V) porphyrin n: cation radicals of the type [(P' .) MlV=O] are found to be the catalytically reactive intermediates by using iron and manganese porphyrinates, respectively. The n cation radical [(P'*)Felv=O] is also considered as the catalytically active species of cytochrome P-450 or appropriate metalloenzymes in biological oxygenation cycles, as detailed mechanistic investigations have However, in contrast to their natural prototypes most of the hitherto described artificial oxygenation reactions have been performed in organic solvents (e.g., benzonitrile, acetonitrile, and dichloromethane).Within the frame of our general investigations of photocatalytic systems based on light-sensitive transition-metal complexes and organometallic compounds, respecti~ely[~I, we were able to show very recently that the photocatalytic activation of molecular oxygen may be considered as an interesting alternative to the well-known catalytic thermal oxygenation reactions [6]. Here, in modeling of enzymatic oxygen transfer reactions induced by P-450 cytochromes or other metalloenzymes, we report on photocatalytic oxygenation reactions of selected cycloalkenes (selective epoxidations and hydroxylations; e.g. selective syntheses of natural products like truns-verbenol(6) from a-pinene (4), in aqueous solvent systems (watedacetone mixtures) instead of using aprotic organic solutions.Besides the avoidance of mostly toxic aprotic organic solvents (usually benzene or toluene) the use of aqueous solvent systems allows a convenient separation of cataly...
Photocatalytic Oxygenation of Cyclic Alkenes with Manganese(III)Tetraarylporphyrin Complexes
The catalytic oxygenation of cyclohexene, 1‐methylcyclohexene, α‐pinene, 1,5‐dimethylcycloocta‐1,5‐diene, and cis,trans‐cyclodeca‐1,5‐diene with (tetraarylporphyrinato)manganese(III) complexes in the presence of molecular oxygen and visible light is reported. The photocatalytic reaction results in the formation of epoxides and allylic oxygenation products due to allylic hydrogen abstraction.
Alkaloiden und bei den ubrigen Steroiden P-standige Methylgruppe an C-10 durch eine cc-standige Aldehydgruppe ersetzt. Diese anomale Konfiguration wird auch durch die Rotationsdispersionskurve des N-Methyl-neosamanonol-jodrnethylats (2) bewiesen, in dem die Aldehydgruppe frei vorliegt. Der Cotton-Effekt bei 315 mp zeigt das entgegengesetzte Vorzeichen wie der Cotton-Effekt von Corotoxigenin (5a.lOp). Durch den starken, negativen Cotton-Effekt von ('1) ist auch die absolute Konfiguration im Sinne der natiirlichen Steroide bewiesen. Darnit ist erstmalig in der Natur ein Steroid mit anomaler Konfiguration an C-10 gefunden worden. Es konnte weiter festgestellt werden, daR sich die Zusammensetzung des Alkaloidgemisches der beiden Subspecies von Salamandra maculosa unterscheidet. Wahrend in S. m . taeniata Samandarin das Hauptalkaloid ist, fehlt dieses in S. m. maculosa vollig; an seiner Stelle tritt Samandaron als Hauptalkaloid auf.
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