Olefins photocatalytic

Shiraishi, Y., Morishita, M., and Hirai, T. (2005) Acetonitrile-assisted highly selective photocatalytic epoxidation of olefins on Ti-containing silica with molecular oxygen. Chemical Communications (48), 5977—5979. 

Rhodium and cobalt carbonyls have long been known as thermally active hydroformylation catalysts. With thermal activation alone, however, they require higher temperatures and pressures than in the photocatalytic reaction. Iron carbonyl, on the other hand, is a poor hydroformylation catalyst at all temperatures under thermal activation. When irradiated under synthesis gas at 100 atm, the iron carbonyl catalyzes the hydroformylation of terminal olefins even at room temperatures, as was first discovered by P. Krusic. ESR studies suggested the formation of HFe9(C0) radicals as the active catalyst, /25, 26/. Our own results support this idea, 111,28/. Light is necessary to start the hydroformylation of 1-octene with the iron carbonyl catalyst. Once initiated, the reaction proceeds even in the... 

The question as to the existence of 17 versus 16 electron intermediates was also raised in the example of the photocatalytic hydrogenation of olefins using iron pentacarbonyl as the catalyst precursor (Equation 35). Schroeder and Wrighton studied this reaction at normal pressure, and they suggested H2Fe(C0)4 and H2Fe(CO)3> respectively, as the active catalysts /36/. 

Chlorinated olefins, particularly trichloroethylene (TCE), appear uniquely suited to destruction by photocatalytic oxidation. The photocatalytic oxidation of TCE... 

Although TCE cofeeds are unable to increase the photocatalytic oxidation of benzene, the effects on the oxidation of branched aromatics are significant. However, practical considerations will hinder the use of TCE cofeeds. First, TCE and branched aromatic contaminants are not often present in the same airstream. Adding TCE or similar chlorinated olefins to an environment or airstream containing branched aromatic contaminants is not practical, because TCE is, itself, classi-... 

Photocatalytic oxidation over illuminated titanium dioxide has been demonstrated to be effective at removing low concentrations of a variety of hazardous aromatic contaminants from air at ambient temperatures. At low contaminant concentration levels and modest humidity levels, complete or nearly complete oxidation of aromatic contaminants can be obtained in photocatalytic systems. Although aromatic contaminants are less reactive than many other potential air pollutants, and apparent catalyst deactivation may occur in simations where recalcitrant reaction intermediates build up on the catalyst surface, several approaches have already been developed to counter these potential problems. The introduction of a chlorine source, either in the form of a reactive chloro-olefin cofeed or an HCl-pretreated catalyst, has been demonstrated to promote the photocatalytic oxidation of... 

Similar redox-combined processes have been reported. For example, it has been clarified by control experiments using a photoirradiated semiconductor electrode that the photocatalytic production of indazoles from substituted azobenzenes is based on the condensation of two intermediates formed through oxidation and reduction.38 40) In the case of oxidation of substituted olefins a similar redox combined mechanism is assumed cation and anion radicals are formed by the reaction of olefin with positive hole and of 02 with excited electron, respectively, and they react to produce a 4-membered ring intermediate, a dioxethane, to undergo bond cleavages into the desired products.4l) In the photocatalytic reactions, a positive hole and excited electron must react at the neighboring surface sites of a small semiconductor particle, enabling the combination of reduction and oxidation without the addition of an electrolyte, which is an indispensable component in electrolysis. However, in the particulate system the recombination of positive hole and electron is also likely, as well as... 

At the end of 1970 s we attempted an investigation of the action of semiconductor particles on medium sized organic compounds. In this article, we would like to describe some of the work done in our laboratory on the photocatalytic action of semiconductors on the oxidation of olefins and hydrocarbons, and on the isomerization of unsaturated systems. 

Although the absolute amount of the photocurrents is governed by various factors such as the oxidation potentials of olefins and the extent of adsorption of olefins on the electrode, the above findings show that the reactive olefins in the photocatalytic oxygenation exhibit photocurrents and the olefins which do not exhibit photocurrents are unreactive in the photocatalytic oxygenation. On the other hand, the olefins which exhibit photocurrents are not always reactive. For example, stilbene shows a higher photocurrent than DPE, but is not so reactive as DPE. The electron transfer to the excited semiconductor takes place more efficiently from stilbene than from DPE due to the lower oxidation potential of the former, but in the subsequent free radical reactions, stilbene is less reactive than DPE (33). 

Therefore, it can be concluded that for the photocatalytic oxygenation to occur, the electron transfer from the olefin to the positive hole has to take place, but the overall reactivity of the olefins is governed by the efficiency of free radical processes as exemplified in Table II. 

In view of the above results, in photocatalytic oxidation of a series of 4-substituted diphenylethylenes, an increase in reactivity with decreasing Hammett s sigma constants (31) seems to arise not only from the lowering of the oxidation potentials of the olefins in this sequence but also from the general trend of the increase in the reactivity of olefins toward peroxyl radicals with increasing the electron donating ability of olefins (33). 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

The third type of photochemical reaction, photocatalytic hydrogenation of olefins, was pursued because of the possibility that the bent, four coordinate, formally 16 electron excited state of RhfPPh IgNO could act in a manner similar to... 

Table II. Photocatalytic Reactions of Fe(CO) [1with Olefins and Trialkylsilanesa...

Olefin isomerization has also been mediated by the photolysis of Fe(C0)s.144 Recently, a detailed study of alkene isomerization by photolysis of Fe(CO)5 has shown that the reaction is truly photocatalytic.14S The very high quantum yields ( 1.0), Table 24, and the fact that the pentenes are ultimately equilibrated to the thermodynamic ratio support the notion that the role of the light is to generate a thermally active catalyst. A mechanism similar to that in reactions (53)-(57) involving Fe(CO)3 as the repeating unit can be used to account for the results. 

An interesting but under explored method for the formation of 1,2-dioxetanes employing the photocatalytic oxygenation of olefins with dioxygen via selective radical coupling using 9-mesityl-10-methylacridinium ion as an electron-transfer photocatalysis has also appeared (Equation 6) <2004JA15999>. 

Kotani, H., Ohkubo, K. and Fukuzumi, S. (2004) Photocatalytic oxygenation of anthracenes and olefins with dioxygen via selective radical coupling using 9-mesityl-10-mefhylacridinium ion as an effective electron-transfer photocatalyst. Journal of the American Chemical Society, 126 (49), 15999-16006. 

The coexistence of various pollutants does not have to be deleterious, but, in certain cases, can be quite beneficial. The first evidence for this claim came probably from the work of Lichtin et al. (1994) who found that the edition of 0.03% by volume to an air-stream containing 0.1% iso-octane caused an enhancement in the photocatalytic oxidation of the latter. Likewise, a significant rate enhancement was recorded in the photocatalytic degradation of chloroform and dichloromethane in the presence of TCE. Similar effects were recorded also with other chlorinated olefins, such as perchloroethylene (PCE) and trichloropropene (TCP), which enhanced the photooxidation of toluene in a manner similar to that of TCE (Sauer et al., 1995). 

Samuel (30) recently reported that irradiation of complexes of the type TiCp2R2 (R = CH3 or CH2Ph) under H2 generates species capable of effecting the rapid and photocatalytic hydrogenation of linear and cyclic olefins under mild conditions [Eq. (22)]. Irradiation of dimethyltitanocene... 

Many fewer photocatalytic organic reductions have been reported. Reductions of organic substrates are less thoroughly studied, largely because of the early emphasis on the use of organic compounds as oxidizable source for the production of hydrogen gas. Nonetheless, some examples do exist, such as the hydrogenation of olefins, vinyl ethers, and a, S-unsaturated enones and alkynes [166, 167]. Similarly, other multiple bonds can be reduced, e.g., the N=N double bond of diaryl azo compounds [168] or carbonyl C=0 bonds [169, 170]. 

The most important photocatalytical reactions are oxidation reactions which include the oxygenation of unsaturated systems, but also oxidations of saturated carbons with or without incorporation of oxygen (C-H activation). The photooxygenation of olefins in the presence of Ti or Mo catalysts leads one-pot to epoxy alcohols (17) via singlet oxygen (eq. (9)) [82]. 

RSCOCH, — RSOtsH. Swem and co-workers " developed an efficient synthesis of alkylsuBbnic acids involving photocatalytic addition of thiolacetic acid to a terminal olefin and oxidation of the adduct with 90% hydrogen peroxide in acetic... 

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