Epoxidation of olefins by [RuIV(bpy)2(py)(O)]2+ in acetonitrile solution. A global kinetic analysis of the epoxidation of trans-stilbene

Article abstract of DOI:10.1021/ja00114a015

The mechanism of epoxidation of the olefins cis- and trans-stilbene, styrene, and norbomene by the oxidant [Ru^Iv(bpy)₂(py)(O)]²⁺ has been investigated in acetonitrile solution by both conventional product analysis (GC-MS and ¹H 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 (～6%) and trace amounts of diphenylacetaldehyde. In the case of trans-stilbene, use of the ¹⁸O-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 ¹H 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 × 10^-3 s^-1 at 25 °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 × 10^-3 s"1 for cis-stilbene at 25 °C. Activation parameters in CH₃CN for trans-stilbene were ΔH^? = 4.4 ± 0.1 kcal mol^-1 and ΔS^? = -46 ± 0.4 cal deg^-1 mol^-1, and for cis-stilbene ΔH^? = 11.9 ± 0.1 kcal mol^-1 and ΔS^? = -30.4 ± 0.3 cal deg^-1 mol^-1. Additional, non-epoxide products are formed under stoichiometric or near-stoichiometric conditions because of overoxidation of the epoxide product. Overoxidation was accompanied by formation of the μ-oxo-bridged dimer, [Ru^III(bPy)₂(Py)]₂O⁴⁺. The same products were observed in the stoichiometric oxidation of trans-stilbene oxide by [Ru^IV(bpy)₂(py)(O)]²⁺ in CH₃CN.