Osmylation electron transfer

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... 

Unfortunately, the fast rates of 0s04 addition to most alkenes preclude the observation of D/A complexes, and they are not readily characterized. However, a variety of aromatic electron donors form similar (colored) D/A complexes with Os04 that are more persistent and the observation of ArH/ 0s04 complexes forms the basis for examining the electron-transfer paradigm in osmylation reactions. 

Furthermore, kinetic analysis of the decay rate of anthracene cation radical, together with quantum yield measurements, establishes that the ion-radical pair in equation (76) is the critical reactive intermediate in osmylation reaction. Subsequent rapid ion-pair collapse then leads to the osmium adduct with a rate constant k 109 s 1 in competition with back electron-transfer, i.e.,... 

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... 

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

Electron Transfer 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... 

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]. 

Oxidation of phenol by tris(l,10-phenanthroline)osmium(III) is second order in Os(III) and phenol and inverse second order in Os(II) and acidity. A mechanism is inferred in which the phenoxyl radical is produced through a rapid proton-coupled electron transfer (PCET) pre-equilibrium, followed by rate-limiting phenoxyl radical coupling. Application of Marcus theory indicated that the rate of electron transfer from phenoxide to osmium(III) is fast enough to account for the rapid PCET preequilibrium, but it did not rule out the intervention of other pathways such as concerted proton-electron transfer or general-base catalysis DFT studies, at B3LYP/LACVP level, of the oxidation of ethylene by osmium tetroxide, osmyl hydroxide, and osmyl chloride indicated that in the reaction of osmium tetroxide, the [3 4- 2] addition pathway leading to a five-membered metallacycle intermediate is more favourable than the [24-2] addition. The reaction with osmyl hydroxide is less favourable. In the reaction with osmyl chloride, the [24-2] addition pathway is more favourable than the [3 4-2] addition. ... 

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

The use of lower temperatures in the bromate-induced monobromopenta-hydroxylation of benzene by catalytic photoinduced charge transfer osmylation favours formation of the neo diastereoisomer of the deoxybromoinositol. Photolysis of N-(diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate and N-[bis(4-methylphenyl)amino]-2,4,6-trimethylpyridinium salts induces nucleophilic addition of various 7i-nucleophiles such as electron rich alkenes to the o- and p-positions of one of the phenyl rings.These observations are thought to imply the presence of the diarylnitrenium ion (Ar2N ) as intermediate, but evidence is also presented for the involvement of radical species, as well as for the formation of indoles and indolinones. Some MO calculations are also reported. 

---