Osmylation charge-transfer

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.,... 

Note that the charge-transfer irradiation of the same anthracene/Os04 complex in hexane solution yields only a small amount of the adduct (B) together with considerable amounts of anthraquinone. The latter probably arises from osmylation at the (9,10) positions followed by decomposition of the unstable osmium adduct.218... 

Such a charge-transfer osmylation of aromatic donors constitutes the critical... 

Charge-transfer Osmylation cf Benzene, Naphthalene and Anthracene... 

Electron Tranter in the Charge-transfer Osmylation cfArenes... 

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

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

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 first interaction in alkene osmylation is believed to involve a weak charge transfer complex (/ max 400-500 nm) between the 71-donor alkene and the acceptor metal oxide, which usually collapses instantaneously. When the substrate is the bulky adamantylineadamantane or an aromatic hydrocarbon, breakdown of the complex becomes a slow or forbidden process and its detection is possible1. Further decomposition of the charge transfer complex is believed to go through the corresponding metastable ion pair which eventually collapses to the final adduct. 

In close analogy with the osmylation reaction, kinetic data are consistent with a mechanism involving initial reagent-substrate interaction to form a charge-transfer complex with subsequent breakdown via oxametallacyclobutane 1 to the metastable cyclic manganate(V) 2. This diester is believed to lead to the desired diol as well as to other alkene oxidation products. 

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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