Alkene <i>cis</i>-Dihydroxylation by [(Me<sub>3</sub>tacn)(CF<sub>3</sub>CO<sub>2</sub>)Ru<sup>VI</sup>O<sub>2</sub>]ClO<sub>4</sub> (Me<sub>3</sub>tacn = 1,4,7-Trimethyl-1,4,7-triazacyclononane):  Structural Characterization of [3 + 2] Cycloadducts and Kinetic Studies

<i>cis</i>-Dioxoruthenium(VI) complex [(Me<sub>3</sub>tacn)(CF<sub>3</sub>CO<sub>2</sub>)Ru<sup>VI</sup>O<sub>2</sub>]ClO<sub>4</sub> (<b>1</b>, Me<sub>3</sub>tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) reacted with alkenes in aqueous <i>tert</i>-butyl alcohol to afford <i>cis</i>-1,2-diols in excellent yields under ambient conditions. When the reactions of <b>1</b> with alkenes were conducted in acetonitrile, oxidative CC cleavage reaction prevailed giving carbonyl products in >90% yields without any <i>cis</i>-diol formation. The alkene <i>cis</i>-dihydroxylation and CC cleavage reactions proceed via the formation of a [3 + 2] cycloadduct between <b>1</b> and alkenes, analogous to the related reactions with alkynes [Che et al. <i>J. Am. Chem. Soc</i>. <b>2000</b>, <i>122</i>, 11380]. With cyclooctene and <i>trans</i>-<i>β</i>-methylstyrene as substrates, the Ru(III) cycloadducts (<b>4a</b>) and (<b>4b</b>) were isolated and structurally characterized by X-ray crystal analyses. The kinetics of the reactions of <b>1</b> with a series of <i>p</i>-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-<i>d</i><sub>2</sub>-styrene (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 0.83 ± 0.04) and α-deuteriostyrene (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 0.96 ± 0.03), which, together with the stereoselectivity of <i>cis</i>-alkene oxidation by <b>1</b>, is in favor of a concerted mechanism.