EPOXIDATION OF OLEFINS BY [RU-IV(BPY)(2)(PY)(O)](2+) IN ACETONITRILE SOLUTION - A GLOBAL KINETIC-ANALYSIS OF THE EPOXIDATION OF TRANS-STILBENE

The mechanism of epoxidation of the olefins cis- and trans-stilbene, styrene, and norbornene by the oxidant [Ru-IV(bpy)(2)(py)(O)](2+) has been investigated in acetonitrile solution by both conventional product analysis (GC-MS and H-1 NMR) and newly developed global kinetic analysis techniques. Under 1:1 stoichiometric reaction conditions (15 mM) the organic products from the oxidations of cis- or trans-stilbene included unreacted stilbene (>50%), stilbene oxide (<50%), benzophenone (similar to 6%) and trace amounts of diphenylacetaldehyde. In the case of trans-stilbene, use of the O-18-labeled oxidant showed that the oxygen atom of its Ru-IV=O2+ group was the predominant source of the oxygen in the epoxide products and a major contributor to the oxygen content of benzophenone. Under similar conditions, the oxidations of styrene and norbornene gave styrene oxide and exo-norbornene oxide as products by H-1 NMR. Kinetic studies were performed under pseudo-first-order conditions with a large excess of the olefins. Factor analysis of UV-vis spectra vs time for each reaction revealed the presence of five colored components and four distinct kinetic processes. In the case of trans-stilbene, the initial reaction was well-separated from the following steps, allowing a full global kinetic fit to be obtained to a multistep model. The initial stage involved net oxene insertion into the double bond of the olefin to form the Ru(II) epoxide complex, [Ru-II(bpy)(2)(py)(epoxide)](2+), without evidence for an intermediate. This was followed by a competition between its rapid oxidation by Ru-IV=O2+ and solvolysis by CH3CN. in the oxidation step both the Ru(III) epoxide and [Ru-III(bpy)(2)(py)(OH)](2+) are formed. Once formed, Ru-III-OH2+ was found to react further via initial disproportionation to Ru-IV=O2+ and Ru-II-OH22+. The aqua complex undergoes irreversible solvolysis (k = 1.66 x 10(-3) s(-1) at 25 degrees C), and Ru-IV=O2+ produces further epoxidation. The Ru(III) epoxide intermediate appears to release epoxide and undergo reduction to form [Ru-II(bpy)(2)(py)(NCCH3)](2+) via a pathway first order in complex. The details of the reduction and solvolysis remain unknown. For the initial step to form Ru(II) epoxide, k=0.28 M(-1) s(-1) for trans-stilbene, and k = 2.5 x 10(-3) s(-1) for cis-stilbene at 25 degrees C. Activation parameters in CH3CN for trans-stilbene were Delta H double dagger = 4.4 +/- 0.1 kcal mol(-1) and Delta S(d)ouble dagger = -46 +/- 0.4 cal deg(-1) mol(-1), and for cis-stilbene Delta H(d)ouble dagger = 11.9 +/- 0.1 kcal mol(-1) and Delta S(d)ouble dagger = -30.4 +/- 0.3 cal because of overoxidation of the epoxide product. Overoxidation was accompanied by formation of the mu-oxo-bridged dimer, [Ru-III(bpy)(2)(py)]O-2(4+). The same products were observed in the stoichiometric oxidation of trans-stilbene oxide by [Ru-IV(bpy)(2)(py)(O)](2+) in CH3CN.