Reactivity in micellar or vesicular assemblies and in cyclodextrin complexes

Reactivity and selectivity in the hydrolytic cleavage of esters, in the presence of functionalized micelles and vesicles and within cyclodextrin receptors, are shortly reviewed. The mode of action and the factors which may affect the reactivity in these supramolecular systems are analyzed

Photoinitiated Olefin Epoxidation with Molecular Oxygen, Sensitized by Free Base Porphyrins and Promoted by Hexacarbonylmolybdenum in Homogeneous Solution

The photooxidation of various olefins in homogeneous solution under an oxygen atmosphere, using visible light, a dye sensitizer, and an oxygen-transfer catalyst has been investigated. The oxygen transfer from molecular oxygen to olefin involves the following steps: i) photoinduced singlet-oxygen formation, ii) alkylhydroperoxide formation through ene-reaction, iii) intermediacy of a reactive molybdenum-peroxide, and iv) olefin epoxidation of the remaining substrate or of a second olefin. Among the various sensitizers and catalysts tested, electron deficient free base porphyrin 5,10,15,20-tetrakis(2’,6’-dichlorophenyl)-b-octabromoporphyrin and molybdenum hexacarbonyl showed the best performances in terms of robustness and activity. Under proper conditions, a complete olefin conversion may be obtained, adopting molar ratios of sensitizer/catalyst/substrate=1/50/2000, with the formation of the corresponding epoxide up to 38% yield, which corresponds to 77% of the theoretical maximum. Quite interestingly, olefins reluctant to undergo ene-reaction may be epoxidized in the presence of a second sacrificial olefin yielding the corresponding epoxides with up to 80% total selectivity

Selective catalytic oxidations in supercritical carbon dioxide

Abstract: Conducting catalytic oxidations of organic substrates in supercritical carbon dioxide as reaction medium is one of the most promising strategy for the creation of fundamental chemical transformations that are environmentally benign. There is extensive literature on the use of supercritical carbon dioxide as solvent for unselective hydrocarbons auto-oxidations. Conversely, only a limited number of reports deal with selective oxidations carried out by molecular oxygen or by other primary oxidants such as hydrogen peroxide, alkyl hydroperoxides, monopersulfate, etc. In this paper recent developments in selective catalytic oxidations are reviewed ed in particular those processes in which is formed a metal derivatives, by action of a terminal oxidant which could be oxygen or an organic or inorganic peroxide, capable of selectively oxidizing various substrates

Manganese Porphyrin-catalysed Heterogeneous and Unusually Selective Oxidation of Sulfides by Monopersulfate in Supercritical C02

The oxidation of sulfides by Oxone\uae in the presence of catalytic amount of various manganese porphyrins in supercritical CO2 (scCO2) has been studied. The reactions have been carried out in an anhydrous two phase-system (solid Oxone\uae, solid catalyst/scCO2). In the experimental conditions employed (40 oC, 20 MPa), only the new synthesised 5,10,15,20-tetrakis(heptafluoropropyl)porphyrinate-manganese(III) chloride showed a slight solubility in scCO2. In the absence of manganese porphyrin and/or of an aromatic nitrogen base a slow stoichiometric sulfide oxidation leading mainly to sulfoxide is observed in spite of the extremely low solubility of Oxone\uae in scCO2. Conversely, in the presence of 4-tert.-butylpyridine and catalytic amount of manganese porphyrins (0.6% with respect to sulfide) enhanced reaction rates are observed. Furthermore, the selectivity of sulfide oxidation reverses and sulfone is formed in larger or comparable amount than sulfoxide even when an excess of sulfide over oxidant is employed. The results collected suggest that the local concentration of the intermediate sulfoxide around the catalyst is higher than that of the bulk supercritical phase thus determining unusual high reaction rates for sulfone formation

Highly Efficient Cascade-Oxygen-Transfer from H2O2 to Olefins Mediated by Halogenated Carbonyl Compounds and MetalloPorphyrins

The catalytic oxidation of cyclohexene with hydrogen peroxide promoted by manganese and iron porphyrins has been studied in the presence of halogenated carbonyl compounds as co-catalysts in a dichloromethane/water biphasic medium. The efficiency of the overall catalytic process is strictly related to the nature of the co-catalyst and to the high reactivity of its perhydrate form toward metalloporphyrins in forming an high valent oxo-porphyrin derivative. Hydrogen peroxide fixation by hexafluoroacetone hydrate provides an useful system to hamper catalyst bleaching and/or oxidant decomposition. Thus, a complete olefin conversion into oxygenated products is accomplished with a very limited excess of oxidant respect to the organic substrate

Supporting porphyrins on resin-beads by cyanuric chloride linker

A straightforward procedure for supporting porphyrin-derivatives onto resin beads has been developed by reacting the commercially available 4,6-dichloro-1,3,5-triazene polymer-bound, with 5-(4-aminophenyl)-10,15,20-triphenylporphyrins as free base and as Zn(II) and Co(II) complexes. The supported porphyrins have been characterized by on-resin UV-vis spectroscopy. Kinetic and Hammett LFER studies have shown that the SNAr reaction with the triazine-bound reagent occurs at a similar rate for free and metalated. complexes, with a nucleophilic strength comparable to that of the unsubstituted aniline (sigma(+)(p) = 0.07-0.08)

One-pot Synthesis of Cyanuric Acid-bridged Porphyrin-Porphyrin Dyads

Stepwise amination of cyanuric chloride (1) with 5-(4-aminophenyl)-10,15,20-triphenylporphyrin (2) and/or its zinc(II) complex (3) enables the synthesis of porphyrin-porphyrin dyads with predetermined free base-free base forms or free base-zinc and zinc-zinc metalation states. Furthermore, the use of aminopropyl-silanized silica gel as a scavenger for unwanted byproducts allowed the one-pot synthesis of title porphyrin compounds in high yield and purity with minimum use of preparative column chromatograph

Synthesis of a Novel Porphyrin bearing Chloroper-fluoropolyether moieties with very high Solubility in Supercritical CO2

A new porphyrin bearing chloroper-fluoropolyether substituents at the meso positions was prepared in five steps starting from chloroper-fluoropolyether carboxylic acid. Furthermore, metallation of this porphyrin was accomplished with cobalt and zinc. Both, the free base and its complex with zinc are highly soluble in supercritical CO2 under very mild conditions (40 0C and 20 MPa) thus allowing, in principle, to perform homogeneous catalytic processes in this non conventional reaction medium

Metallomicelles made of Ni(II) and Zn(II) complexes of 2-pyridinealdoxime-based ligands as catalysts of the cleavage of carboxylic acid esters

Ligands featuring a 6-alkylaminomethyl-2-pyridinealdoxime moiety (alkyl = CH(3), 2a, or n-C(12)H(25), 2b) have been synthesized, and the reactivity of their Ni(II) and Zn(II) complexes in the cleavage of p-nitrophenyl acetate (PNPA) and hexanoate (PNPH) has been investigated in the absence (2a) or in the presence (2b) of CTABr micelles. The micellar complexes are effective in promoting the cleavage of the substrate with accelerations strongly dependent on the pH, being larger in moderately acidic than in neutral solutions. At pH 5 the Ni(II)2b/CTABr micelles increase the rate of the cleavage of PNPH by 3 orders of magnitude as compared to CTABr only and by 2 orders of magnitude as compared to the nonmicellar complex of 2a. Moreover the system is truly catalytic with a turnover rate approaching that of the cleavage. Analysis of the second-order rate constants allows the conclusions that the increased reactivity of the micellar system is due to concentration and local-pH effects and not to the activation of the nucleophile. A comparison of the reactivity of the systems made of complexes of 2 with that of the analogues of the 2-pyridineketoxime ligands (1) previously investigated indicates that the insertion of the new chelating atom in the binding subsite, on one hand, increases the formation constant of the metal ion complexes and, on the other hand, decreases the nucleophilicity of the complexed oximate ion. The balance of these two effects, in the operative conditions, favors ligands 2 over ligands 1 due to the higher fraction of complexed oximes