Photocatalytic Oxygenation of Selected Cycloalkenes in Aqueous Solutions Induced by Water‐Soluble Metal Porphyrin Complexes

Abstract

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...

New Functionalized Polycycles Obtained by Photocatalytic Oxygenation Using Mn(III) Porphyrins in Basic Media

“…This has been exemplified in earlier works [1,9] and more recent studies [10][11][12][13], regarding the photocatalytic oxygenation of cyclohexene. In aqueous systems, photo-induced homolysis of the bond between the metal center and chloride or hydroxide axial ligands led to the formation of (P)Mn IV =O [9,13,14]. The oxidized ligand is a very reactive oxidative agent, e.g., hydroxyl radical in the latter case (Equation (1)).…”

Functionalization of the benzobicyclo[3.2.1]octadiene skeleton via photocatalytic oxygenation of thiophene and furan derivatives: The impact of the type and position of the heteroatom

“…11,12 In the latter case the Mn(II) species formed in the primary photochemical step (Eq. 1) undergoes an oxidation with the dissolved O2 (Eq.…”

“…2 represents an overall reaction comprising several steps. 11 Our experiments were carried out in water-acetone solvent mixture, thus hydroxide or water was axially coordinated to the Mn(III) center. Hence, hydroxyl radicals generated in the primary photochemical step most probably react with the organic solvent.…”

“…They can be utilized in autooxidation reactions, hydroxylations or direct oxygen transfer yielding epoxides. 10,11 Cationic manganese(III) porphyrins were applied as efficient catalysts for oxygenation of -pinene. Its selective epoxidation was achieved in aqueous systems at relatively low substrate:catalyst ratio (S/C=500), while in aprotic organic solvents, such as benzene or toluene, allylic hydroxylation products were formed.…”

Functionalization of the benzobicyclo[3.2.1] octadiene skeleton possessing one isolated double bond via photocatalytic oxygenation

“…2 several steps are comprised. 18 In water-acetone solvent mixture (applied in our experiments), hydroxyl radicals produced in the primary photochemical step most probably react with the organic solvent. Disproportionation of the Mn(IV) complexes formed generates highly reactive manganese(V)-oxo species (Eq.…”