Preparative irradiation

Organized and constrained media may provide cavities and surfaces, sometimes called microreactors or nanoreactors,171 that can control the selectivity of photochemical reactions of reactants. There are many types of microreactors, for example, molecular aggregates of micelles or monolayers, macrocyclic host cavities of crown ethers or cyclodextrins and microporous solid cavities and/or surfaces of zeolites, silica or 

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Small scale test runs prior to preparative irradiation experiments may be carried out in tubes which are either taped to the lamp housings (immersion wells) depicted in Figures 13-1 and 13-2 or placed in turntable reactors ( merry-go-rounds ). These arrangements permit the simultaneous irradiation of several samples, but only a fraction of the available light emission is used. In Figure 13-4 a simple reactor is shown which focusses almost all the emitted light into one sample which can be scaled up also to semi-preparative volumes. In this way the necessary irradiation time can be reduced sharply. 

The issue of parallel versus sequential synthesis using multimode or monomode cavities, respectively, deserves special comment. While the parallel set-up allows for a considerably higher throughput achievable in the relatively short timeframe of a microwave-enhanced chemical reaction, the individual control over each reaction vessel in terms of reaction temperature/pressure is limited. In the parallel mode, all reaction vessels are exposed to the same irradiation conditions. In order to ensure similar temperatures in each vessel, the same volume of the identical solvent should be used in each reaction vessel because of the dielectric properties involved [86]. As an alternative to parallel processing, the automated sequential synthesis of libraries can be a viable strategy if small focused libraries (20-200 compounds) need to be prepared. Irradiating each individual reaction vessel separately gives better control over the reaction parameters and allows for the rapid optimization of reaction conditions. For the preparation of relatively small libraries, where delicate chemistries are to be performed, the sequential format may be preferable. This is discussed in more detail in Chapter 5. 

Benorylate (315) [4 -(acetamido)phenyl-2-acetoxybenzoate] is another example. It is the ester between two well-known antiinflamatory drugs, aspirin and paracetamol, and is employed in rheumatoid arthritis therapy. In view of the chemical structure with three photolabile groups (two esters and one amide), its possible phototoxicity has been investigated. From the preparative irradiations, it has been concluded that the PFR takes place with breaking of the central C—O bond to yield 5-acetamido-2 -acetoxy-2-hydroxybenzophenone (316). This product undergoes transacetylation to 5 -acetamido-2 -acetoxy-2-hydroxy-benzophenone (318) (Scheme 80) [300]. 

To achieve these goals, the analysis scheme which involved sample preparation, irradiation, sample counting, and data reduction was optimized to achieve maximum sample output with minimum manpower expended. 

The methods used to achieve these goals involved analyzing samples in large numbers carefully planning and scheduling sample preparation, irradiation, and counting complete computer automation of the data reduction scheme and strict attention by the work team to small details... 

Stewart, E.M., Stevenson, M.H. and Gray, R. (1992). Detection of irradiation in scampi tails - effects of sample preparation, irradiation dose and storage on ESR response in the cuticle, lnt. J. Food Sci. Technol. 27. 125. 

Figure 10.28 Principal head-end steps in preparing irradiated LMFBR core and blanket assemblies for Purex process. F.P. = fission products S.S. = stainless steel.

From all these examples it becomes evident that S-heterocyclic unsaturated carbonyl compounds are convenient compounds for preparative irradiations. 

The basic methods of sample preparation. Irradiation and analysis have been described elsewhere (25. 26. Mi. 5). Recent experiments have incorporated several refinements as detailed below. 

Most of the rhodium-carbene complexes reported during the period 1984-1994 were half-sandwich-based complexes, as described in COMC (1995). The number of carbene complexes was somewhat scarce in the last decade, but it has increased enormously, widening the variety of carbenes that we can now find in the literature. Some new halfsandwich carbene complexes have been prepared. Irradiation of CpRh(PMe3)(C2H4) in pentafluoroanisole generated a metallacycle that evolved to a cyclic carbene in the presence of an electrophile (Scheme 40). ... 

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