Alkyne oxidations by cis-dioxoruthenium(VI) complexes. A formal [3 + 2] cycloaddition reaction of alkynes with cis-[(Cn*)(CF3CO2)Ru(VI)O2]ClO4 (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane)

Abstract

cis-Dioxoruthenium(VI) complexes, [Cn*(CF3CO 2)Ru(VI)O2]ClO4 (1) (Cn* = 1,4,7-trimethyl-1,4,7-triazacyclononane) and cis-[(Tet-Me6)Ru(VI)O2](ClO 4)2 (2) (Tet-Me6 = 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 [(Cn*)(CF3CO2)Ru(IV)OC2R1R2O] + 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.