Alkene epoxidation by ruthenium(II) phosphino–complexes. A kinetic investigation.

Coordinatively unsaturated complexes of ruthenium(I1) of the type [RUCI(LL)~]+w,h ere LL is 1,3-bis(diphenylphosphino)propane or l-(diphenylphosphino)-2-(2-pyridyl)ethane, effectively promote the epoxidation of olefins in the presence of iodosylbenzene and in low-coordinating media. Competition from oxidative cleavage and/or allylic oxidation is observed with some olefin. The kinetics of the epoxidation of 1-octene, trans-2-octene, cyclooctene, norbornene, styrene, trans-@-methylstyrenea, nd cis- and trans-stilbene have been investigated gas chromatographically, and the following conclusions are drawn. (i) The reaction order in the catalyst is 1; (ii) the reaction order in the olefins varies between 0 and 1; (iii) rates are different from each olefin examined. It is suggested that the rate-determining step consists in the breakdown of an oxo-metal intermediate containing the olefin. KM and V ,val ues are calculated from the Michaelis-Menten equation, indicating that the relative reactivity of olefins toward catalytic epoxidation falls into a relatively narrow range. Competition experiments have been performed that confirm the KM data obtained by the Lineweaver-Burk analyses. Stereospecific epoxidation is achieved with trans-olefins (stilbene, 2-hexene, and 2-octene), whereas the corresponding cis-olefins produce high yields of trans-epoxides, pointing to a substantially radicaloid character of the reaction intermediate.