Photochemically Catalyzed Processes

A primary thrust for studying the photochemistry of metal compounds is the need to develop new materials and reactions that can store solar energy in a chemical 

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Figure 8.22 shows an estimate of the contributions to the oxidation of S(IV) by H202, by the iron-catalyzed 02 oxidation, and by OH in both the gas and aqueous phases of a cloud (Jacob et al., 1989). It is seen that H202 and the iron-catalyzed process predominate at night, but the gas-phase oxidation by OH becomes significant during the day when it is formed by photochemical processes. On the other hand, the contribu-... 

In contrast to the deactivation observed in the metal-catalyzed process, the photochemical reaction tolerates the use of an excess of KCN, a relatively inexpensive cyanating agent, to generally improve the yield. 

DUiydride photochemistry is not limited to reductive elimination. CpRe(PPh3)2H2 is known to photochemically catalyze H/D exchange between CeDe and other arenes or alkanes. Photochemical studies of the mechanism of this process have ruled out loss of phosphines and instead postulate photochemical transfer of one or both hydrides to the cyclopentadienyl ligand yielding a 14-e intermediate, ( -C5H7)Re(PPh3)2, as a hkely intermediate. ... 

The cyclopropanation of a cyclopropene to produce a bicyclo[1.1.0]butane has often been achieved by dipolar addition to give an isolated 2,3-diazabicyclo[3.1.0]hexane followed by loss of nitrogen in a second step to produce the second cyclopropane. The formation of the diazabicyclo[3.1.0.]hexanes is discussed in Section 1.1.6.1.5.3.1. The decomposition of these to produce bicyclobutanes is described in Section 4.2.1.1.2. This section will only discuss those reactions in which a cyclopropene is converted directly into a bicyclo[1.1.0]butane, by thermal or photochemical methods (Table 20). Metal-catalyzed processes are discussed in Section 1.1.6.3.1.2. 

The central molecules from this saga of the 1960s are the potentially aromatic cyclodecapentaenes and bullvalene, a compound of 10/3 faces. Efforts directed to the synthesis of these compounds involved thermal, photochemical, and metal-catalyzed processes which not only accomplished the major task but also generated... 

As the key intermediates are produced photochemically under mild reaction conditions, this can make it easier to control the subsequent chemistry, such as the trapping of such intermediates, and also make photochemical reactions experimentally less demanding than comparable, metal-catalyzed processes. In fact, this is another aspect that should encourage further work in this area of research [8,... 

In terms of relay and sequential metal-organo multicatalytic systems, these catalysts are most commonly used for 1,2 (aldol type) or 1,4 (Michael type) additions to suitable electrophiles. The most pervasive approach takes advantage of some metal-catalyzed process to generate a substrate for the organocatalyst. The examples illustrated in Scheme 26.16 show how chemical [94] or photochemical [95] metal-catalyzed oxidation of amines generates imine species that then undergo organocatalyzed 1,2 addition. 

Due to the fact that the synthesis of (S)-amides by means of hydrolase-catalyzed processes can only be achieved by using proteases, which have less thermal stability, the same authors performed the racemization at a temperature that does not exceed 30 °C. With that purpose sulfanyl radicals were generated via photolysis of 2,2,2-trifluoroethanethiol (CF CH SH) xmder photochemical irradiation in the presence of AIBN using alkaline protease as biocatalyst and N-octanoyl-L-alanine trifluoroethyl ester as acyl donor in a mixture of tert-BuOH and THF as solvent for the development of the DKR, obtaining the (S)-amides in good 5delds and diastereomeric excess (Figure 9.34) [249]. A similar photochemical procedure has been applied to the... 

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

For the reason of comparison and the development of new domino processes, we have created a classification of these transformations. As an obvious characteristic, we used the mechanism of the different bond-forming steps. In this classification, we differentiate between cationic, anionic, radical, pericyclic, photochemical, transition metal-catalyzed, oxidative or reductive, and enzymatic reactions. For this type... 

Photochemical/Transition Metal-Catalyzed Domino Processes... 

To date, only one example of a combination of a photochemically induced transformation with a transition metal-catalyzed reaction has been found in the literature. This hv/Pd°-promoted process allows the synthesis of five-membered cyclic y-keto esters 5-119 from 5-iodoalkenes 5-117 in the presence of CO and an alcohol 5-118 as a nucleophile (Scheme 5.24) [41]. The yields are high, and differently substituted iodoalkenes can be employed. 

During the past few years, increasing numbers of reports have been published on the subject of domino reactions initiated by oxidation or reduction processes. This was in stark contrast to the period before our first comprehensive review of this topic was published in 1993 [1], when the use of this type of transformation was indeed rare. The benefits of employing oxidation or reduction processes in domino sequences are clear, as they offer easy access to reactive functionalities such as nucleophiles (e. g., alcohols and amines) or electrophiles (e. g., aldehydes or ketones), with their ability to participate in further reactions. For that reason, apart from combinations with photochemically induced, transition metal-catalyzed and enzymatically induced processes, all other possible constellations have been embedded in the concept of domino synthesis. 

Other photochemical reactions have been reported as in the Mercat process involving gas-phase mercury-catalyzed oxidative homo- and cross-couplings (Equation (16)) 25>2Sa>2Sb... 

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