Photosubstitution

Photosubstitution.—The primary process in the photosubstitution of [Cr(CO)s] and of [W(CO)8] by pyridine has been studied, in particular to account for the quantum deficit (for example, d A=0.66 0.05 for [py] = 2.4x 10 M and [Cr(CO)6l = 3.5 X 10- M 292 nm irradiation in cyclohexane). Results in viscous solvents eliminated cage recombination as a possibility, and excited singlet to excited triplet intersystem crossing was shown to occur with unit efficiency so that the triplet state must dissociate and the deficit is due to a radiationless decay which competes with dissociation. 

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Cyanopyrazoles are formed by irradiation of pyrazoles in the presence of cyanide ions in photosubstitution reactions. 

Only two topics are of importance for this section the reduction of pyrazolium salts and 3-pyrazolin-5-ones by complex hydrides, and the nucleophilic photosubstitution of pyrazoles and indazoles. 

Figure 2.47 The limiting photosubstitution mechanism for rhodium(III) ammine complexes. (Reprinted from Coord. Chem. Rev., 94, 151, 1989, with kind permission from Elsevier Science S.A., P.O. Box 564, 1001 Lausanne, Switzerland.)...

Mechanistic aspects of the photosubstitution and photoisomerisation reactions of d6-metal complexes. P. C. Ford, D. Wink and J. Dibenedetto, Prog. Inorg. Chem., 1983.30, 213-271 (182). 

The ligand field photosubstitution reactions of d6 hexacoordinate metal complexes. P. C. Ford, Coord. Chem. Rev., 1982, 44, 61-82 (44). 

Holmium, tris(2,2,6,6-tetramethyl-3,5-heptanedione)-photosubstitution, 1,408 Holmium complexes 1,3-diketones, 2,387 phenanthroline, 3,1069 Homoazaporphyrins synthesis, 2, 817 Homolytic cleavage catalysis... 

Nucleophilic aromatic photosubstitution reactions in aqueous solutions and in micellar media has been investigated extensively.42... 

The positional specificity of these nucleophilic and electrophilic photosubstitutions can be readily understood by examination of the calculated charge densities of some of these molecules in their ground and first excited singlet states. For example, 4-nitrocatechol has the charge densities shown in (82) and (83).<139> Thus on this basis one would predict that 4-nitroveratrole... 

The failure to observe photosubstitution in the presence of a sensitizer in which the latter is the principal absorber, the invariance of product quantum yield with wavelengths shorter than 350 nm (onset of n -> -n absorption), and the observation that chloride and bromide ions (known to catalyze S-+T intersystem crossing) strongly diminish the quantum yields of these reactions, strongly points to the lowest excited ir- n singlet state as the reactive species in these transformations. Excitation into the n->ir absorption band results in little product formation. A triplet state may, however, be involved in the photoamination of nitrobenzene.a41)... 

These studies have recently been extended to the naphthalene system.(142) In alkaline media 2-methoxy-5-nitro-, l-methoxy-6-nitro-, and 2-methoxy-7-nitro-naphthalenes all undergo photosubstitutions to produce naphthols ... 

This chapter contains discussions of photoelimination, photoaddition, and photosubstitution. Although there may appear to be some degree of overlapping between the first two topics in that the species produced by photo-elimination may undergo addition to another substrate, our approach will be to concentrate on the reactions brought about by light absorption rather than subsequent dark reactions. 

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