Alkyne oxidations by cis-dioxoruthenium(VI) complexes. A formal [3+2] cycloaddition reaction of alkynes with cis-[(Cn*)(CF3CO2)(RuO2)-O-VI]ClO4 (Cn*=1,4,7-trimethyl-1,4,7-triazacyclononane)

cis-Dioxoruthenium(VI) complexes, [Cn*(CF3CO2)(RuO2)-O-VI]ClO4 (1) (Cn* = 1 ,4,7-trimethyl-1,4,7-triazacyclononane) and cis-[(Tet-Me-6)(RuO2)-O-VI](ClO 4)(2) (2) (Tet-Me-6 = N,N,N',N-tetramethy1-3,6-dimrthyl-3,6-diazaoctane-1,8 -diamine), oxidize disubstituted alkynes to 1,2-diketones selectively in go od to excellent yields under ambient conditions. The reactions proceed via the formation of dark blue [(Cn*)(CF3CO2)Ru-IV-(OC2RRO)-R-1-O-2](+) interme diates, which display a characteristic UV-visible absorption band at 550-68 0 nm. With bis(trimethylsilyl)acetylene as substrate and 1 as the oxidant, the intermediate was isolated and structurally characterized by X-ray cryst allography as a [3 + 2] cycloadduct. The kinetics of the cycloaddition of 1 with various substituted trimethylsilylacetylenes has been studied by stop ped-flow spectrophotometry. With the exception of bis(trimethylsilyl) acety lene, the second-order rate constants were found to vary over a range of le ss than an order of magnitude irrespective of a 2.3 eV change of the calcul ated I, 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 implausi ble based on product analysis. A. linear Hammett correlation was establishe d using sigma (+) and sigma (.)(JJ) parameters for the cycloaddition of 1 w ith para-substituted aryl trimethylsilylacetylenes, and the rate-limiting v inyl radical intermediate formation is proposed.