Alkyne Oxidations by cis-Dioxoruthenium(VI) Complexes. A Formal [3 + 2] Cycloaddition Reaction of Alkynes with cis-[(Cn*)(CF₃CO₂)RuVIO₂]ClO₄ (Cn* = 1,4,7-Trimethyl-1,4,7-triazacyclononane)

cis -Dioxoruthenium(VI) complexes, [Cn*(CF 3 CO 2 )Ru VI O 2 ]ClO 4 ( 1 ) (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane) and cis -[(Tet-Me 6 )Ru VI O 2 ](ClO 4 ) 2 ( 2 ) (Tet-Me 6 = N,N,N ‘,N ‘ -tetramethyl-3,6-dimethyl-3,6-diazaoctane-1,8-diamine), oxidize disubstituted alkynes to 1,2-diketones selectively in good to excellent yields under ambient conditions. The reactions proceed via the formation of dark blue intermediates, which display a characteristic UV−visible absorption band at 550−680 nm. With bis(trimethylsilyl)acetylene as substrate and 1 as the oxidant, the intermediate was isolated and structurally characterized by X-ray crystallography as a [3 + 2] cycloadduct. The kinetics of the cycloaddition of 1 with various substituted trimethylsilylacetylenes has been studied by stopped-flow spectrophotometry. With the exception of bis(trimethylsilyl)acetylene, the second-order rate constants were found to vary over a range of less than an order of magnitude irrespective of a 2.3 eV change of the calculated I p of the alkynes; therefore, a rate-limiting single electron-transfer mechanism is unlikely. The participation of oxirene (oxene insertion) and metallaoxetene ([2 + 2] cycloaddition) intermediates appears to be implausible based on product analysis. A linear Hammett correlation was established using σ + and σ JJ • parameters for the cycloaddition of 1 with para-substituted aryl trimethylsilylacetylenes, and the rate-limiting vinyl radical intermediate formation is proposed.