Photocatalytic Activity of the Molecular Complexes of meso ‐Tetraarylporphyrins with Lewis Acids for the Oxidation of Olefins: Significant Effects of Lewis Acids and meso Substituents (original) (raw)
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Journal of Molecular Catalysis A-chemical, 2001
Oxidation of cyclohexene and cis-stilbene with dioxygen in presence of metal phthalocyanines or metal tetraphenylporphyrins complexes intercalated in ␣-zirconium phosphate and isobutyraldehyde were studied. The degradation of free metal complexes in solution in the reaction media was verified. It was observed that the matrix protects the metal complexes from degradation and the activity of the catalytic system is preserved. Oxidation of cyclohexene with intercalated complexes gave epoxide as the predominant product, while allylic oxidation products were obtained in smaller proportion and the product distributions depended on the identity of the individual metal complexes. Since the addition of a free radical inhibitor stops the reaction, a free radical mechanism should be present. Oxidation of cis-stilbene with intercalated metal complexes gives different ratios of cis-to trans-stilbene oxide and of benzaldehyde which depend on the intercalated metal complex, suggesting that in addition to the free radicals there is another active oxidizing agent.
Photochemistry and Photobiology, 1998
The tetraruthenated porphyrin, p-[meso-5,10,15,20-tetra(pyridy1)porphyrinl tetrakis[bis-(bipyridine)chloride ruthenium(II)] (TRP) is a supramolecular cationic species. The aim of the present investigation was to evaluate the photodynamic properties of TRP and Zn-TRP to damage DNA with emphasis on the mechanistic aspects. The ability for tetraruthenated porphyrin derivatives to induce photosensitization reactions has been determined using 2'-deoxyguanosine as a DNA model compound. The main photooxidation products of the targeted nucleoside were identified and classified according to their mechanisms of formation, involving either a radical pathway (type I) or a singlet oxygen-mediated mechanism (type 11). Quantification of the different oxidation products provides a means to evaluate the relative contribution of type I and type I1 pathways associated with the oxidative photosensitization of 2'-deoxyguanosine by tetraruthenated porphyrin derivatives. Results indicate that '02 plays a major role in the mechanism of photooxidation mediated by these porphyrin derivatives. In addition an increase of the photosensitizing effect in the presence of zinc is observed. For each sensitizer, the ratio between type I1 and type I photoproducts has been calculated and compared to that of other known dyes such as methylene blue and riboflavin.
The Journal of Organic Chemistry, 2009
The methyltrioxorhenium (MTO)/pyrazole-mediated H 2 O 2 oxidation of octaethylporphyrin and a number of meso-tetraarylporphyrins offers simple and good yielding access to the corresponding N-oxides, only few of which were prepared before. The crystal structure of a free base tetraarylporphyrin N-oxide demonstrates the degree to which the oxygenated pyrrole moiety is slanted with respect to the rest of the otherwise nearly planar macrocycle. The method is also suitable to the preparation of hitherto unknown chlorin N-oxides. Oxidation of meso-tetraphenyldithiaporphyrin and meso-tetraphenyldithiachlorin furnishes the corresponding novel S-oxides. The optical properties of the novel chromophores are described and rationalized.
Research on Chemical Intermediates, 2013
In this work, two new aerobic routes are introduced for the oxidation of a variety of benzoins to benzils by using molecular oxygen in the presence of porphyrins as sensitizers, using white light or sunlight in acetonitrile under mild conditions. The methods have a wide range of applications, do not involve cumbersome work-up, exhibit chemoselectivity and proceed under mild reaction conditions. The resulting products are obtained in good yields in a reasonable time.
Polyhedron, 2008
Keywords: b-Tetra-brominated meso-tetraphenylporphyrin Epoxidation Shift of the Q(0,0) and Soret bands Catalyst a b s t r a c t X-ray crystallographic studies have shown that the free base b-tetra-brominated meso-tetraphenylporphyrin (H 2 TPPBr 4 ) has a slightly ruffled structure with the dihedral angles of 70.1-79.2°between the phenyl groups and the porphyrin mean plane. The N(pyrrole)-N(pyrrolenine) distance is very similar to that of the standard planar porphyrins. The decreased N-H bond length of H 2 TPPBr 4 with respect to that of meso-tetraphenylporphyrin (H 2 TPP) seems to be due to the weaker intramolecular hydrogen bond of the former relative to the latter caused by the electron-withdrawing effects of the b-bromine substituents. The large red shifts of the Soret and Q(0,0) bands of H 2 TPPBr 4 in comparison with those of H 2 TPP, in spite of the nearly planar porphyrin core of the compound, also may be explained on the basis of the electron-withdrawing effects of the bromine atoms. Oxidation of styrene, the para-substituted derivatives and cyclooctene with tetrabutylammonium oxone in the presence of catalytic amounts of b-tetra-brominated meso-tetraphenylporphyrinatomanganese(III) acetate immediately gives the epoxide as the sole product. Terminal double bonds and unconjugated ones are less reactive than the conjugated double bonds and show lower selectivities. Catalytic activity of the electron-deficient Mn(H 2 TPPBr 4 )OAc dramatically depends on the Co-catalytic activity of the nitrogen donors as the axial base. The best axial bases are the nitrogenous donors with mixed r-and p-donor ability to the metal centre.
Photosensitized Generation of Singlet Molecular Oxygen by Aryloxazinones
2008
The photophysical properties, the capacity to produce singlet molecular oxygen by sensitization and the photostability of a series of 20 aryloxazinones and one quinoxalinone have been evaluated using a combination of methods including steady-state and time-resolved spectroscopic techniques. The photophysics of aryloxazinone derivatives and consequently its capacity to produce singlet molecular oxygen is very dependent on the structure. Benzoxazinone derivatives substituted in para position to the 2-phenyl group (1)-(3) have low fluorescence quantum yields and produce singlet oxygen in moderate quantum yields. Inclusion of electron donors groups in position 7 of the aromatic fused moiety to the heterocyclic ring (4)-(6), increases significantly the emission from the excited singlet state with a concomitant diminution in the ability to produce singlet oxygen. An exception corresponds to the 7-methoxy-2phenylbenzoxazinone (4) able to generate larger quantities of excited oxygen and photodecompose in less than a 2% under large energy doses in aerobic irradiation conditions. In general, 2-phenyl and 2-methyl naphthoxazinones derivatives (7)-(17) are more fluorescent than the compounds of the corresponding benzo series, and generate singlet molecular oxygen in low to moderate yields. Exceptions are the 2-methyl (13) and the 9-methoxy-2-methyl (14) derivatives that produce excited oxygen efficiently, however, with appreciable decomposition. The most promising compounds to be employed as singlet oxygen sensitizers are the anthracene like 3-phenyl-2H-naphtho[2,3-b][1,4]-oxazin-2-one (19) and the 1-methyl-3-phenylquinoxalin-2(1H)-one (21). These compounds are photostable in the absence and in the presence of electron donor additives under large doses of irradiation, accomplishing all requisites to be good sensitizer and to produce singlet oxygen in high yields, almost independently on solvent polarity.
The Journal of Physical Chemistry A, 2009
The cytochromes P450 are versatile enzymes involved in various catalytic oxidation reactions, such as hydroxylation, epoxidation and dehydrogenation. In this work, we present combined experimental and theoretical studies on the change of regioselectivity in cyclohexadiene oxidation (i.e., epoxidation vs dehydrogenation) by oxoiron(IV) porphyrin complexes bearing different porphyrin ligands. Our experimental results show that meso-substitution of the porphyrin ring with electron-withdrawing substituents leads to a regioselectivity switch from dehydrogenation to epoxidation, affording the formation of epoxide as a major product. In contrast, electron-rich iron porphyrins are shown to produce benzene resulting from the dehydrogenation of cyclohexadiene. Density functional theory (DFT) calculations on the regioselectivity switch of epoxidation vs dehydrogenation have been performed using three oxoiron(IV) porphyrin oxidants with hydrogen atoms, phenyl groups, and pentachlorophenyl (ArCl 5 ) groups on the meso-position. The DFT studies show that the epoxidation reaction by the latter catalyst is stabilized because of favorable interactions of the substrate with halogen atoms of the meso-ligand as well as with pyrrole nitrogen atoms of the porphyrin macrocycle. Hydrogen abstraction transition states, in contrast, have a substrate-binding orientation further away from the porphyrin pyrrole nitrogens, and they are much less stabilized. Finally, the regioselectivity of dehydrogenation versus hydroxylation is rationalized using thermodynamic cycles. † Part of the "Walter Thiel Festschrift". * Corresponding authors.
Synthesis of meso-tetraarylporphyrins using SeO2 as oxidant
Tetrahedron Letters, 2011
The oxidative step of the two-step, one-flask synthesis of meso-tetraarylporphyrins is herein conducted with heterogeneous oxidant SeO 2 instead of the usual quinones DDQ or p-chloranil. Evaluation of BF 3 O(Et) 2 or I 2 amount for the condensation of the first step combined with the excess of SeO 2 defined porphyrin synthesis conditions employing benzaldehydes and pyrrole (or 5-phenyldipyrromethane) as starting materials. The simplicity of the workup, allied with reaction mild conditions, makes this method a good option for the synthesis of this kind of compound.
Kinetics of Triplet Oxidation of Metallo-Porphyrin Compounds to Their Corresponding Radical Cations
Photochemistry and Photobiology, 1979
The kinetics of photooxidation of triplets of metalloporphyrin compounds to their corresponding radical cations was investigated. Zn-tetraphenyl porphyrin (ZnTPP) and Mg-tetraphenylporphyrin (MgTPP) triplets were oxidized by europium salt with rate constants of 4.8 x to5 M-' s-' and 2.1 x lo6 M-' s-' , respectively. The high rate constant of oxidation of MgTPP triplet might be related to the ground state oxidation potential, being 0.54V (SCE) for the Mg complex and 0.71 (SCE) for the Zn complex. The rate constant of oxidation of ZnTPP excited singlet is in the order of diffusion control, i.e.-10" M-' s-l. Excitation of ferric, cupric, cobaltic, and vanadyl tetraphenylporphyrin did not result in a long-lived triplet state that would allow oxidation studies using flash photolysis.