Catalysts photoproducts

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

On photolyzing CoziCOg in the matrix (20), a number of photoproducts could be observed. The results of these experiments are summarized in Scheme 4, which illustrates the various species formed. Of particular interest is the formation of Co2(CO)7 on irradiation of Co2(CO)g in CO (254 nm), as this species had not been characterized in the metal-atom study of Hanlan et al. (129). Passage of Co2(CO)g over an active, cobalt-metal surface before matrix isolation causes complete decomposition. On using a less active catalyst, the IR spectrum of Co(CO)4 could be observed. An absorption due to a second decomposition product, possibly Co2(CO)g, was also noted. 

Production of hydrogen from an inexhaustible somce, water, by a cheaper route has been under extensive investigation in recent years (Koca and Sahin, 2003). The requirement for the photoproduction of hydrogen using a semiconductor is the need for a hydrogen evolution catalyst on a semiconductor surface as reported by marty... 

Chemical utilization of the photoproducts has been accomplished by the introduction of synthetic catalysts or natural enzymes into the photochemical systems. 

The design of such artificial photosynthetic systems suffers from some basic limitations a) The recombination of the photoproducts A and S+ or D+ is a thermodynamically favoured process. These degra-dative pathways prevent effective utilization of the photoproducts in chemical routes, b) The processes outlined in eq. 2-4 are multi electron transfer reactions, while the photochemical reactions are single electron transformations. Thus, the design of catalysts acting as charge relays is crucial for the accomplishment of subsequent chemical fixation processes. 

A different approach for utilization of the photoproducts in chemical routes involves the introduction of natural enzymes as catalysts in the photochemical system. In nature, dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide dinucleotide phosphate (NADPH) participate as reducing cofactors in a variety of enzymatic reduction processes. Thus, the development of photochemical NADH and NADPH regeneration cycles is anticipated to allow a variety of reduction processes by inclusion of substrate specific NAD(P)H dependent enzymes. 

Different aspects involved in the design of artificial photosynthetic systems have been discussed. Charged colloids and water-oil microemulsions provide effective organized media for controlling photosensitized electron transfer processes. Development of catalysts capable of utilizing the photoproducts in chemical routes, particularly in multi-electron fixation processes is of major... 

An important advance was made when it was observed that photolysis of the (3-enamido ketone 165, which was readily available from the indoline 163 by Birch reduction followed by N-aryloylation, delivered the lactam 168 as the only photoproduct (Scheme 17) (125). Reduction of 168 with LiAlH4 gave ( )-a-anhydrodihydrocaranine (143), which was then converted to ( )--y-lycorane (93) on hydrogenation over Adams catalyst in acetic acid. In a similar fashion, irradiation of the bromo or iodo enaminones 166 (Z = Br, I), which were obtained by alkylation of the intermediate imino ether formed on Birch reduction of 163, afforded a mixture (approximately 3 2) of the lactam 168 together with the photoreduction product 167 (126). 

The homogeneous complex RhCl(dpm)3 acts also as hydroformylation catalyst [159], Upon illumination of the catalytic photosystem Ru(bpy) +/ascorbic acid/RhCl(dpm)3- in the presence of ethylene and carbon monoxide, propionaldehyde is obtained as photoproduct. Similarly, propene yields the hydroformylation product butyraldeyde. The facts that no hydrogenation products are produced in this assembly, and that hydridocarbonyl-tris-(diphenylphos-phinobenzene-3-sulphonate) rhodium(I), RhHfCOXdpm) -, substitutes RhCl(dpm)3- as catalyst in the photosystem to yield the hydroformylation products at similar efficiency, suggest that the homogeneous catalyst RhClfdpmJj -is transformed into a new catalytic species under CO. A possible route for the interconversion of RhCl(dpm)3 into the hydroformylation catalyst is provided in Scheme 4. 

Fig. 1 Schematic drawing of hydrogen peroxide photoproduction by the biological photosynthetic apparatus with electrons either from water or from an exogenous electron donor. A redox catalyst (RC) transfers electrons from the terminal acceptor of photosystem I to molecular oxygen.

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