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Why are olefins oxidized by RuO4 under cleavage of the carbon-carbon bond whereas oxidation by OsO4 yields cis-diols?

Quantum chemical calculations using gradient-corrected (B3LYP) density functional theory have been carried out to investigate the mechanism of the oxidative cleavage of alkenes by ruthenium tetraoxide. The initial reaction of the tetraoxide with the olefin occurs via a [3+2] cycloaddition as in the case of osmium tetraoxide. The results clearly show that the bond cleavage does not take place at the primary adduct, but much later in the reaction path. After the formation of the ruthenium(VI)dioxo-2,5-dioxolane, the reaction proceeds with the addition of a second olefin to yield ruthenium(IV)-bis(2,5-dioxolane), which in turn becomes oxidized first to rutheniumoxo(VI)-bis(2,5-dioxolane) 6(Ru) and then to ruthenium(VIII)-dioxo-bis(2,5-dioxolane) 7(Ru). Only in complexes containing the metal center in the formal oxidation state +VIII are low activation barriers for C-C bond cleavage and exothermic formation of carbonyl compounds as products calculated. The lowest activation barrier, DeltaH(++) = 2.5 kcal/mol, is calculated for the C-C bond breaking reaction of 7(Ru) which is predicted as the pivotal intermediate of the oxidation reaction. The calculations of the oxidation reaction with OsO(4) show that those reactions where the oxidation state of the metal increases have larger activation barriers for M = Ru than for M = Os, while reactions which reduce the oxidation state have a lower activation barrier for ruthenium compounds. Also, reactions which increase the oxidation state of the metal are in the case of M = Os more exothermic than for M = Ru. In this work, all important points of the potential energy surface (PES) are reported, and the complete catalytic cycle for the oxidative cleavage of olefins by ruthenium tetraoxide is presented.