Catalysis photochemical reactions

The chemical uses of tungsten have increased substantially in more recent years. Catalysis (qv) of photochemical reactions and newer types of soluble organometaUic complexes for industrially important organic reactions are among the areas of these new applications. 

The present volume, the thirty-fourth in the series, surveys research on organic reaction mechanisms described in the literature dated December 1997 to November 1998. In order to limit the size of the volume, we must necessarily exclude or restrict overlap with other publications which review specialist areas (e.g. photochemical reactions, biosynthesis, electrochemistry, organometallic chemistry, surface chemistry and heterogeneous catalysis). In order to minimize duplication, while ensuring a comprehensive coverage, the Editors conduct a survey of all relevant literature and allocate publications to appropriate chapters. While a particular reference may be allocated to more than one chapter, we do assume that readers will be aware of the alternative chapters to which a borderline topic of interest may have been preferentially assigned. 

Despite these apparent difficulties, there are now a number of examples for photoinduced electron transfer reactions that are significantly catalyzed. It is the purpose of this chapter to present fundamental concepts and the application of catalysis of photoinduced electron transfer reactions. The photochemical redox reactions, which would otherwise be unlikely to occur, are made possible to proceed efficiently by the catalysis on the photoinduced electron transfer steps. First, the fundamental concepts of catalysis on photoinduced electron transfer are presented. Subsequently, the mechanistic viability is described by showing a number of examples of photochemical reactions that involve catalyzed electron transfer processes as the ratedetermining steps. 

As shown above, the acid catalysis on electron transfer increases the overall efficiency of the photochemical reactions via photoinduced electron transfer without affecting the products. However, there are some cases when addition of an acid to a particular photoinduced electron transfer process results in... 

Of the oxidants described above, only [Ru(bipy)3]3+ can be efficiently prepared in a photochemical reaction and hence has potential for use in a cyclic water cleavage system. Many studies have, thus, been carried out on the Ru02 catalysis of 02 production from [Ru(bipy)3]3+ generated photochemically from [Ru(bipy)3]2+ and a sacrificial electron acceptor such as [Co(NH3)5C1]2+ or [S208]2-. 

The products formed in these reactions are very sensitive to the functionality on the carbenoid. A study of Schechter and coworkers132 using 2-diazo-1,3-indandione (152) nicely illustrates this point. The resulting carbenoid would be expected to be more electrophilic than the one generated from alkyl diazoacetate and consequently ihodium(II) acetate could be used as catalyst. The alkylation products (153) were formed in high yields without any evidence of cycloheptatrienes (Scheme 33). As can be seen in the case for anisole, the reaction was much more selective than the rhodium(II)-catalyzed decomposition of ethyl diazoacetate (Scheme 31), resulting in the exclusive formation of the para product. Application of this alkylation process to the synthesis of a novel p-quinodimethane has been reported.133 Similar alkylation products were formed when dimethyl diazomalonate was decomposed in the presence of aromatic systems, but as these earlier studies134 were carried out either photochemically or by copper catalysis, side reactions also occurred, as can be seen in the reaction with toluene (equation 36). 

The key step of the Amdt-Eistert Homologation is the Wolff-Rearrangement of the diazoketones to ketenes, which can be accomplished thermally (over the range between r.t. and 750°C, photochemically or by silver(I) catalysis. The reaction is conducted in the presence of nucleophiles such as water (to yield carboxylic acids), alcohols (to give alcohols) or amines (to give amides), to capture the ketene intermediate and avoid the competing formation of diketenes. 

Thus, the accumulation of chemical energy of the reaction in the form of highly active intermediate compounds happens with the energy consumption. For this purpose photosensibilization, light exposure (photochemical reactions), catalysis (catalytic decay) and chemical induction (couples processes) are used. 

A reactant in an enzyme catalysed reaction is known as substrate. According to the mechanism of enzyme catalysis, the enzyme combines with the substrate to form a complex, as suggested by Henri (1903). He also suggested that this complex remains in equilibrium with the enzyme and the substrate. Later on in 1925, Briggs and Haldane showed that a steady state treatment could be easily applied to the kinetics of enzymes. Some photochemical reactions and some enzymic reactions are reactions of the zero order. 

Lewis, F.D. and Barancyk, S.V. (1989) Lewis Acid catalysis of photochemical reactions. 8. Photodimerization and cross-cycloaddition of coumarin. Journal of the American Chemical Society, 111, 8653-8661. 

Catalyzed photolysis refers to catalysis of a photochemical reaction, for which there is a physical pathway for decay of the system back to its ground state (Figure 6.17). When the photocatalytic process occurs through photoexcitation of the catalyst, the physical decay may occur through recombination and/or thermal photoionization of the excited states, which ultimately leads to regeneration of the original state of the catalyst. Note that catalyzed photolysis is not catalytic in photons, contrary to photogenerated catalysis. 

One of the strongest supports for this theory of negative catalysis is found in photochemical investigations. In these cases the actual quantum yield is reduced enormously by the negative catalyst. In such cases one can see that the negative catalyst functions by breaking the chain, and it is easy to carry over this mechanism to thermal reactions. In fact, several investigations have shown that the effective inhibitors are the same for the thermochemical as for the photochemical reactions. 

The second challenge for computational chemistry concerns the timescale accessible in the simulations. Biological reactions occur on a variety of timescales, ranging from the ultra-fast photochemical reactions occurring within several hundred femtoseconds up to the millisecond or even second timescale reactions found in catalysis... 

Instead of adding two hydrogen atoms to an alkynyl silane we could add H and SiMe3 to a simple alkyne by hydrosilylation (addition of hydrogen and silicon). This is a cis addition process catalysed by transition metals and leads to a tram (E-) vinyl silane. One of the best catalysts is chloroplatinic acid (H2PtCl6) as in this formation of the E-vinyl silane from phenylacetylene. In this case photochemical isomerization to the Z-isomer makes both available. Other than the need for catalysis, this reaction should remind you of the hydroboration reactions earlier in the chapter. The silicon atom is the electrophilic end of the Si-H bond and is transferred to the less substituted end of the alkyne. 

The quantum yield 0 of a photophysical or photochemical event is a quantitative measure of the overall efficiency of this process (Braun et al., 1991). It is a unitless constant, which usually ranges from zero to one. However, some authors express 0 in units of mol einstein, which in fact is unit-less, because an einstein is defined as one mol of photons. Quantum yields greater than one indicate photo-induced chain reactions, which may involve radical species or photo-generated catalysis. Commonly used definitions of 0 are collected in Tab. 3-7. These definitions describe quantum yields of photophysical events and of photochemical reactions with regard to the reactant diminution or to the formation of the photoproduct Quantum yields may be dependent on the wavelength of the absorbed UV/VIS radiation, but many photochemical systems exist that have a constant quantum yield 0 over a defined wavelength range. Such chemical systems can be... 

---