Ruthenium-catalyzed cis-dihydroxylation of alkenes: Scope and limitations

Oxidative ruthenium catalysis (0.07 mol-equiv RuCl3 . (H2O)(3), 1.5 mol-equiv NaIO4, EtOAc/CH3CN/H2O 3:3:1), beyond the usual C-C bond cleavage to give dicarbonyls, has been shown to syn-dihydroxylate a wide range of alkenes (except for strained bicyclic alkenes, sterically hindered trisubstituted alkenes, and most tetrasubstituted alkenes) to give vicinal diols rapidly (within minutes) and efficiently. The minor products are the usual oxidative fission products, namely, ketones and aldehydes or carboxylic acids, and sometimes ketols. Longer reaction times lower the yields of most diols, probably owing to oxidative glycol cleavage. Reactions with substrates containing one or more electron-withdrawing groups in conjugation with or adjacent to the alkene moiety are generally slower but give better yields. The diastereoselectivity of the present ''flash'' dihydroxylation, anti to the existing alpha-stereogenic center, with cycloalkenes is excellent whereas that with acyclic alkenes is moderate to poor. Sodium metaperiodate is still the best co-oxidant for the catalytic reaction. Aqueous acetonitrile (approximately 86%) as an alternative solvent system was found to give better yields of 1,2-diols than the original solvent system in some cases.