Photochemical generation of reagents

Xor a polymer-supported reagent used for the photochemical generation of radicals in solution see DeLuca, L. Giacomelli, G. Porcu, G. Taddei, M. Org. Lett. 2001, 3, 855. 

As a matter of fact the photochemically generation of the triplet phenyl cation is synthetically equivalent to activation by metal catalysis that likewise produces a sort of phenyl cation complexes to the metal by oxidative addition of the aryl halide and elimination of the halide anion (see above). In fact, a photochemical parallel has been found for most type of metal catalyzed arylations, via reactions that occur from the same or similar reagents. 

The tetrazole is very much less sensitive to photolysis than the azide, and for a number of such systems photochemical generation of the heteroaryl nitrene has failed. This problem is important in nucleotide and nucleic acid chemistry, where azidopurines and azidopyrimidines are, at first sight, attractive candidates for photogenerated reagents. The position of the azide-tetrazole isomerization equilibrium is solvent dependent, and it is not possible to specify under what conditions (or in what kinds of receptor site) useful levels of photogenerated aryl nitrene can be produced. In general, therefore, it may be safer to avoid such systems if reasonable alternatives exist. 

This approach offers unique opportunities for the generation of multi-functionalized cyclic 2-azadiene systems. A wide variation of the substitution pattern at the positions N-1 and C-6 can be determined by an appropriate choice of the aldehyde and amine. Various substituents can easily be introduced at the C-3 position via addition/elimination reactions on the sensitive imidoyl chloride moiety [24]. Upon reaction with bi-functional reagent, an adequately AT-protected 2(lH)-pyrazinone was elaborated into C-nucleoside analogues (Scheme 8). The desired skeleton and functionalities were obtained by oxidation-cyclization reaction followed by photochemical removal of the protective o-nitrobenzyl group [25]. 

The various approaches to the generation of the active ECL reagent Ru(bpy)33+ have been reviewed by both Lee [14] and Gerardi et al. [16]. Methods of generation include purely chemical, photochemical, external electrochemical, and in situ electrochemical approaches. 

Hence, most photochemical precursors of radicals in solution (peroxides, azo compounds, most halides, etc.) are not generally suitable for in situ generation of these species in confined environments such as matrices (of course reagents that generate radicals by abstraction, such as the popular trialkyl tin hydrides, are also excluded from matrix isolation). 

The most reliable method for generating nitrenes is the thermal or photochemical elimination of nitrogen from azides. An alternative method which is useful for indole and carbazole synthesis is the deoxygenation of aromatic nitro compounds with trivalent phosphorus compounds. Triethyl phosphite is the most commonly used reagent, though more reactive compounds may be useful in special cases (B-79MI30600). 

Figure 27.18 Common configuration for postcolumn reactors with electrochemical analysis. (A) LC-chemical reaction-EC. Postcolumn addition of a chemical reagent (for example, Cu2+ or an enzyme). (B) LC-enzyme-LC. Electrochemical detection following postcolumn reaction with an immobilized enzyme or other catalyst (for example, dehydrogenase or choline esterase). (C) LC-EC-EC. Electrochemical generation of a derivatizing reagent. The response at the second electrode is proportional to analyte concentration (for example, production of Br2 for detection of thioethers). (D) LC-EC-EC. Electrochemical derivatization of an analyte. In this case a compound of a more favorable redox potential is produced and detected at the second electrode (for example, detection of reduced disulfides by the catalytic oxidation of Hg). (E) LC-hv-EC. Photochemical reaction of an analyte to produce a species that is electrochemically active (for example, detection of nitro compounds and phenylalanine). Various combinations of these five arrangements have also been used. [Reprinted with permission from Bioanalytical Systems, Inc.]...

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