RU(III)(HEDTA) AS AN OXYGEN ATOM TRANSFER CATALYST IN THE EPOXIDATION OF STILBENES

Ru(III)(hedta) and Ru(III)((CH3)2edda)+ (hedta3- =N-hydroxyethylethylenediaminetriacetate; (CH3)2edda2- = N,N'-dimethethylethylenediamine-N,N'-diacetate) catalyze the epoxidation of cis-stilbene and trans-stilbene using tert-butylhydroperoxide (t-BuOOH) as the oxygen source. Prior spin-trapping studies have documented the existence of LRu(III)O <-> LRu(IV)O.- <-> LRu(V)O2- character in the species obtained from Ru(III)L and to-BuOOH (L=polyaminopolycarboxylate ligands related to edta4-). The O-atom complex, LRu(III)O, appears responsible for the epoxidation of stilbenes. Yields as high as 63.5% cis-stilbene oxide plus 11.0% trans-stilbene oxide from cis-stilbene and 65.1% cis-stilbene oxide from trans-stilbene (with no trans-stilbene oxide) are formed in the epoxidation reactions. Secondary oxidations of the epoxide products produce between 4 to 8% benzaldehyde depending on conditions. The product distribution using the Ru(III)L/t-BuOOH catalyst requires at least three epoxidation pathways: (i) concerted transfer of the oxenoid oxygen to the stilbene nucleophile; this process is favored for cis-stilbene; (ii) an outer-sphere electron transfer from the stilbene to LRu(III)O forming a carbon-centered cation radical adjacent to LRu(III)O.-; this radical pair may couple directly for cis-stilbene or after a rapid isomerization of the trans-stilbene radical; (iii) an acyclic pathway which has both free radical and carbocation resonant character; this allows for isomerism of cis-stilbene to trans-stilbene oxide products. Ru(III)O(hedta) is also observed to cleanly oxidize benzaldehyde to benzoic acid, sec-phenetyl alcohol to acetophenone, and benzyl alcohol to benzaldehyde and benzoic acid. Cyclohexene is hydroxylated and further oxidized to 2-cyclohexene-1-one.