Osmylation thermal

Thermal osmylation. Upon standing in the dark, the purple CT color of anthracene complex [ANT, 0s04] slowly diminishes to afford the 1 2 (insoluble) osmium adduct as the sole product, i.e.,... 

Photochemical osmylation. The irradiation of the charge-transfer bands (Fig. 13) of the EDA complex of 0s04 with various benzenes, naphthalenes, anthracenes, and phenanthrene yields the same osmylated adducts as obtained in the thermal reactions. For example, irradiation of the purple solution of anthracene and 0s04 in dichloromethane at k > 480 nm yields the same 2 1 adduct (B) together with its syn isomer as the sole products, i.e.,... 

The identical stoichiometries and the color changes that are observed in thermal and photochemical aromatic osmylations point to the ion-radical pair Ar+, OsO T as the seminal intermediate in both activation processes. It is similarly possible that the osmylation of olefinic donors may proceed via the same types of reactive intermediates as delineated for the aromatic osmylation. 

Recently, Wallis and Kochi reported on the photochemical and thermal osmylation of benzene, substituted benzenes, naphthalene, and anthracene (331, 332). Complexes of the form Ar-0s04 are formed and were studied by laser flash photolysis. In the presence of donor... 

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]. 

Common Features in Thermal and Charge-tranter Osmylations... 

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... 

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... 

The osmylation of l-[Af-methylphenylsulfoximinomethyl]-l-hydroxy-2-cyclopentenols and -2-cyclohexenols, obtained in turn from the corresponding 2-cycloalkenones by the addition of lithiated, V,.S -dimethyl-5-phenylsulfoximine, occurs from the same side as the sulfoximino functionality with complete Ik topicity. Subsequent thermal elimination of the sulfoximine group allows the synthesis of optically pure 2,3-dihydroxycycloalkanones. This method can, therefore, be regarded as proceeding via an auxiliary-controlled osmylation92-93. 

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). 

Most importantly, both in the thermal and the charge-transfer osmylation of anthracene, identical cycloadducts on the terminal ring are observed, which underscores the close relationship between the two reaction modes. Thus, a unifying electron-transfer mechanism is proposed for both thermal and photoactivated osmylation, which reveals the ion-radical pair [Ar+ , 0s04 ] as the common (primary) reactive intermediate [161]. 

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