Osmium tetroxide arenes

Carbocations as electron acceptors in aromatic EDA complexes 192 Bis(arene)iron(II) complexes with arene and ferrocene donors 198 Carbonylmetallate anions as electron donors in charge-transfer salts 204 Aromatic EDA complexes with osmium tetroxide 219... 

Among oxo-metals, osmium tetroxide is a particularly intriguing oxidant since it is known to oxidize various types of alkenes rapidly, but it nonetheless eschews the electron-rich aromatic hydrocarbons like benzene and naphthalene (Criegee et al., 1942 Schroder, 1980). Such selectivities do not obviously derive from differences in the donor properties of the hydrocarbons since the oxidation (ionization) potentials of arenes are actually less than those of alkenes. The similarity in the electronic interactions of arenes and alkenes towards osmium tetroxide relates to the series of electron donor-acceptor (EDA) complexes formed with both types of hydrocarbons (26). Common to both arenes and alkenes is the immediate appearance of similar colours that are diagnostic of charge-transfer absorp-... 

Trimethylsilyldiazomethane, 327 Silyl substituted arenes Bis(trimethylsilyl)acetylene, 97 Chromium carbene complexes, 82 Titanium(IV) chloride-Diethylalu-minum chloride, 309 Other organosilanes Osmium tetroxide-Trimethylamine N-oxide-Pyridine, 223 Tributyltin chloride, 315 Di- x-carbonylhexacarbonyldicobalt, 99 Trimethylsilyl trifluoromethanesul-fonate, 329... 

The cis- 1,2-glycols, obtainable from the parent aromatic hydrocarbon by osmium tetroxide hydroxylation, can be converted to the corresponding trans-1,2-glycols by oxidation-reduction, using a mixture of dimethyl sulfoxide, sulfur trioxide, and pyridine, followed by lithium aluminum hydride reduction. The trans- 1,2-glycols can be dehydrated to arene oxides using DMF-DMA as mentioned above. Benzo[a]pyrene 4,5-oxide (28) and 7,12-dimethylbenz[a]anthracene 5,6-oxide (30) have been prepared by this method in 68 and 80% yields, respectively.18... 

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... 

Addition has also been shown to occur when the pyridinlum iodide (139) is irradiated. The product Is assigned the structure (140) and the relevance of this reaction to the photoreactivity of Kosower solvent polarity probes has been discussed. The use of osmium tetroxide to hydroxylate alkenes is a well known procedure which is often carried out in aromatic solvents. These arene solvents form charge transfer complexes with the osmium tetroxide and the photochemistry of these has now been examined. It is shown that with benzene and alkyl benzenes isolable adducts are formed that from benzene Is assigned structure (141). 

Cyclic osmic esters have long been known to be involved in the osmium tetroxide-catalyzed cis-dihydroxylation of alkenes, but not arenes. The isolation of compound (18) by Wallis and Kochi following irradiation of the charge-transfer complex between osmium tetroxide and benzene is therefore of particular interest. This suggests that the corresponding use of catalytic quantities of osmixim tetroxide in conjunction with hydrogen peroxide could lead to the formation of polyhydroxylated cyclohexenes and -anes. 

Photoinduced Charge Transfer Osmylation. The reaction of osmium tetroxide with benzenoid derivatives can only be accomplished by irradiation of the mixture. This reaction had previously been shown to work (with stoichiometric osmium tetroxide) by promotion of charge transfer between the two reactants to form an ion-pair this can then collapse to form an osmate ester of benzene diol. Subsequent dihydroxylation of this intermediate appears to follow a more conventional (and stereoselective) course. The use of catalytic osmium tetroxide (in conjunction with barium perchlorate as a reoxidant) for this reaction is noteworthy, as is the formation of both inositol and conduritol derivatives in one-pot (eq SS). The photoinduced osmylation of mono-substituted arenes was possible although the yields were lower and the amount of cyclitol-type products reduced. 

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