Osmylation arenes

A.6 Electron Tranter as the Common Theme in Arene Osmylation... 

Common Features in Thermal and Charge-transfer Osmylations 7.4.43 Electron Transfer in the Charge-transfer Osmylation ofArenes 7.4.4.6 Electron Transfer as the Common Theme in Arene Osmylation... 

Organometals and metal hydrides as electron donors in addition reactions 245 Oxidative cleavage of carbon-carbon and carbon-hydrogen bonds 253 Electron-transfer activation in cycloaddition reactions 264 Osmylation of arene donors 270... 

This mechanism also holds for the photochemical osmylation of arenes through irradiation of a preformed arene/0s04 CT complex. This reaction finally results in the formation of vic-diols [88]. 

The same features are observed in the osmylation of arene donors. Thus, osmium tetra-oxide spontaneously forms complexes with arenes, and the systematic spectral shift in the CT bands parallels the decrease in the arene IP [59]. The same osmylated adducts are obtained thermally on leaving mixtures to stand in the dark or upon irradiation of the CT bands at low temperature. Time-resolved spectroscopy establishes that irradiation of the CT band of the anthracene/osmium tetraoxide complex leads directly to the radical-ion pair ANT+, 0s04, which then collapses to the osmium adduct (with a rate constant fc 109 s 1) in competition with back ET [59]. 

The osmylation of arenes (Ar) with osmium tetroxide is a particularly informative system with which to illustrate the close interrelationship between the thermal and photochemical activation of electron-transfer oxidation. For example, a colorless solution of osmium tetroxide in n-hexane or dichlorometbane upon exposure to benzene turns yellow instantaneously. With durene an orange coloration develops and a clear bright red solution results from hexamethylbenzene. The quantitative effects of the dramatic color changes are illustrated in Figure 3 by the spectral shifts of the electronic absorption bands that accompany the variations in aromatic conjugation and substituents. The progressive bathochromic shift parallels the decrease in the arene ionization potentials (/F) in the order benzene 9.23 eV naphthalene... 

A4.3 Time-resolved Spectra of Arene Radical Cations in Charge-transfer Osmylation... 

Electron Transfer in the Charge-tranrfer Osmylation of Arenes... 

Molecular complexes of OSO4 with various substituted benzenes, naphthalenes, and anthracenes have been identified by their charge-transfer absorption, which follows the Mulliken correlation in Eq. 8 [114, 161], The arene-0s04 complexes are quite stable when kept in the dark and only very slowly form osmium(VI) cycloadducts by thermal osmylation (Eq. 30). 

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