Alkene cis-Dihydroxylation by [(Me₃tacn)(CF₃CO₂)Ru^VIO₂]ClO₄ (Me₃tacn = 1,4,7-Trimethyl-1,4,7-triazacyclononane):  Structural Characterization of [3 + 2] Cycloadducts and Kinetic Studies

cis-Dioxoruthenium(VI) complex [(Me₃tacn)(CF₃CO₂)Ru^VIO₂]ClO₄ (1, Me₃tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) reacted with alkenes in aqueous tert-butyl alcohol to afford cis-1,2-diols in excellent yields under ambient conditions. When the reactions of 1 with alkenes were conducted in acetonitrile, oxidative CC cleavage reaction prevailed giving carbonyl products in >90% yields without any cis-diol formation. The alkene cis-dihydroxylation and CC cleavage reactions proceed via the formation of a [3 + 2] cycloadduct between 1 and alkenes, analogous to the related reactions with alkynes [Che et al. J. Am. Chem. Soc. 2000, 122, 11380]. With cyclooctene and trans-β-methylstyrene as substrates, the Ru(III) cycloadducts (4a) and (4b) were isolated and structurally characterized by X-ray crystal analyses. The kinetics of the reactions of 1 with a series of p-substituted styrenes has been studied in acetonitrile by stopped-flow spectrophotometry. The second-order rate constants varied by 14-fold despite an overall span of 1.3 V for the one-electron oxidation potentials of alkenes. Secondary kinetic isotope effect (KIE) was observed for the oxidation of β-d₂-styrene (k_H/k_D = 0.83 ± 0.04) and α-deuteriostyrene (k_H/k_D = 0.96 ± 0.03), which, together with the stereoselectivity of cis-alkene oxidation by 1, is in favor of a concerted mechanism.