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

The mechanism of epoxidation of the olefins cis- and trans-stilbene, s tyrene, and norbornene by the oxidant [Ru-IV(bpy)(2)(py)(O)](2+) has b een investigated in acetonitrile solution by both conventional product analysis (GC-MS and H-1 NMR) and newly developed global kinetic analy sis techniques. Under 1:1 stoichiometric reaction conditions (15 mM) t he organic products from the oxidations of cis- or trans-stilbene incl uded unreacted stilbene (>50%), stilbene oxide (<50%), benzophenone (s imilar to 6%) and trace amounts of diphenylacetaldehyde. In the case o f trans-stilbene, use of the O-18-labeled oxidant showed that the oxyg en atom of its Ru-IV=O2+ group was the predominant source of the oxyge n in the epoxide products and a major contributor to the oxygen conten t of benzophenone. Under similar conditions, the oxidations of styrene and norbornene gave styrene oxide and exo-norbornene oxide as product s by H-1 NMR. Kinetic studies were performed under pseudo-first-order conditions with a large excess of the olefins. Factor analysis of UV-v is spectra vs time for each reaction revealed the presence of five col ored components and four distinct kinetic processes. In the case of tr ans-stilbene, the initial reaction was well-separated from the followi ng steps, allowing a full global kinetic fit to be obtained to a multi step model. The initial stage involved net oxene insertion into the do uble 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+ an d 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-OH2was 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 furth er epoxidation. The Ru(III) epoxide intermediate appears to release ep oxide 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 so lvolysis 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 De lta S(d)ouble dagger = -46 +/- 0.4 cal deg(-1) mol(-1), and for cis-st ilbene 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 epo xide product. Overoxidation was accompanied by formation of the mu-oxo -bridged dimer, [Ru-III(bpy)(2)(py)]O-2(4+). The same products were ob served in the stoichiometric oxidation of trans-stilbene oxide by [Ru- IV(bpy)(2)(py)(O)](2+) in CH3CN.