Photoreactions catalysed

Several examples of aromatic hydrocarbon sensitized additions of NNP to the same arenes were demonstrated to occur if an acid is present this is in contrast to the failure of benzophenone to sensitize the photoreactions. Irradiation of anthracene in the presence of NNP and hydrochloric acid gives 308 in 70% yield and a small amount of 309 derived from the acid-catalysed elimination of piperidinium ion and addition of ethanol165 (equation 147). Anthracene possesses Es = 76.3 kcal mol 1,

[Pg.813]

Finally, in a cata lysed photoreact ion the conversion of the excited substrate molecule into the product j3 is catalysed by tt. in its electronic ground state. 

Figure 1. Simplified Jablonski-type diagram of photoinduced catalytic (I), photoassisted (II), and sensitized photoreaction (III) as well as catalysed photolysis (IV). (MLn i + L) represents coordinatively unsaturated species, free ligands as well as ligands and/or coordination compounds with changed formal oxidation number generated by photo-redox and/or photodissociation reactions.

Bahnemann, D., A. Henglein and L. Spanhel (1984b). Detection of the intermediates of colloidal Ti02-catalysed photoreactions. Faraday Discussions of the Chemical Society, 78,151-163. 

Apart from water splitting, T102 used in the form of particles or as a photoelectrode has been shown to catalyse also numerous other photoreactions. They include so different processes as, for example, photodegradation of chlorinated hydrocarbons " " hydroxylation of aromatic hydrocarbons photoreduction of dinitrogen water-gas shift reaction ... 

Intermediates in colloidal titania-catalysed photoreactions have been detected, and a study of the heterogeneous and homogeneous photooxidation of various organic compounds in both gas and liquid phases has associated the photocatalytic centre of the catalyst with the Ti-O bond. During light absorption the Ti ion changes its oxidation state and coordination number. The photocatalytic oxidation of ethane has been studied at room temperature, au d in particular the effect on the rate of ethame oxidation of the partial pressure of react ults, the UV light... 

An example of efficient chiral induction in a photoreaction is provided by the irradiation of inclusion compounds of pyridones (11) with the chiral host (10) (Tanaka et al, Chapter 2). This process afforded optically active p-lactams (12) with ee in the range 91-99%. The intramolecular 2 4-2 alkene photocycloaddition of (13) catalysed by Cu(I) afforded the tricyclic compound (14) in 89% yield,... 

Photoreaction II, the photolysis of water accompanied by Oj formation (Hill reaction), can be catalysed by isolated chloroplasts in the presence of synthetic electron acceptors, such as ferricyanides or reducible dyes. It can easily be seen if a compound has an inhibiting effect on this step of photosynthesis through measurement of the oxygen produced. 

Further papers of general synthetic interest include the full paper on the U02Cl2-Me0H-catalysed photoreactions of enones (Vol. 7, p. chlorination... 

Due to the fact that the difference between a sensitised and a catalysed photoreaction is somewhat arbitrary, due to the different and complex mechanisms involved (static and dynamic sensitisation, interaction with a photoproduct, photoinduced reactions,...), the term, photocatalysis has therefore been defined as broad as possible without the specific implication of any special mechanism, and refers then to the action of a substance whose function is activated by the absorption of a photon. A photocatalyst can be described as one involved in the quantum yield expression for a photochemical reaction without its stoichiometric involvement or more precisely, it appears in the quantum yield expression for reaction from a particular excited state to a power greater than its coefficient in the stoichiometric equation [14]. 

The light-catalysed decomposition of thietan yields ethylene and thio-formaldehyde as the primary products. Propylene, formerly considered as a primary product, is believed to arise from a secondary photoreaction of thioformaldehyde and thietan. A similar light-catalysed cleavage occurs with 2-phenyl-2-alkoxythietans. The kinetics of thermolysis at 860—1170K of thietan to thioformaldehyde and ethylene have been investigated thermolysis at 490 °C also is believed to yield thioformaldehyde. Thietan-2,4-diones decompose thermally to ketens. ... 

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