Homogeneous catalysts generated photochemically

Photoinduced catalysis means the photogeneration of a catalyst that subsequently promotes a catalyzed reaction. Photons are required to generate the catalyst only. Thus, the efficiency of such processes depends only on the activity of the catalyst produced photochemically and, in homogeneous photocatalysis, the turnover number (TON) is the useful tool. The TON is usually expressed as the number of moles of product formed per mole of catalyst and, for photoassisted catalysis, TON <1, whereas for photogenerated catalysis TON >1 and even 1 [135], Therefore, high turnovers of photochemically produced catalysts are one of the main criteria concerning efficient photocatalytic processes. Quantum yields (ratio of moles of product formed to the number of photons absorbed) >1 may occur. The same is true for photoinduced chain reactions. 

Metal oxalates often react photochemically to produce CO2 and the metal in a lower oxidation state. This reaction has recently been used to generate a coordinatively-unsaturated platinum which then functions as an efficient homogeneous catalyst, reaction (17). 

More interest, however, has been focused on the photochemistry of phosphine complexes of the second- and third-row transition metals in their lower oxidation states. This interest is primarily the result of the fact that such complexes are widely used as homogeneous catalysts in the solution phase, and it is theorized that photochemical techniques can be used to generate reactive excited states, or at least to generate reactive, coordinately unsaturated species. A primary goal of such work is the generation of a photocatalytic system whereby the photoproduct is a thermal catalyst, thereby making the transformation catalytic in the number of incident photons. Many of these ideas that have been pursued with tertiary phosphine complexes have also been followed for transition metal carbonyl complexes, with this latter photochemistry being discussed in Chapter 6. 

In earlier sections mention was made of the use of photochemical reactions to generate reactive metal carbonyls that can then act as homogeneous catalysts for a... 

Alkyne metathesis was first observed in heterogeneous catalytic systems at high temperatures. " Early homogeneous catalytic systems, which consisted of a mixture of molybdenum hexacarbonyl and a phenol, required a lower, but still high, temperature (Scheme 8.129). " Alternatively, the reaction could be photochemical. It is assumed that a carbyne complex is generated as the active catalyst. 

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