Synthesis and reactivity of osmium-chromium oxidation catalysts and routes to a chiral analog
Osmium-chromium heterobimetallic complexes of the type (Y) (M(N)$\rm R\sb2(\mu$-O)$\rm\sb2CrO\sb2$) (Y = NBu$\sb4$, PPh$\sb4$; M = Os, Ru; R = Me, CH$\rm\sb2SiMe\sb3$, C$\rm\sb6H\sb5$) act as catalysts for the oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones using dioxygen as the secondary oxidant. These complexes also catalyze the oxidation of 1,2-bis(diphenylphosphino)ethane (dppe) to the corresponding phosphine oxide, Ph$\sb2$P(O)CH$\rm\sb2CH\sb2P(O)Ph\sb2$. None of the singly oxidized phosphine is detected. Investigations into the kinetics of the oxidation reactions showed the oxidation of benzyl alcohol to benzaldehyde is first order in both alcohol and catalyst concentration. A mechanism involving coordination of the alcohol to the osmium center, followed by proton transfer and $\beta$-hydrogen elimination of the products yields a reduced osmium-chromium complex. This reduced complex can react with dioxygen to form a bridging peroxo complex that can react with a molecule of the reduced complex to regenerate two equivalents of the original heterobimetallic complex. Dioxygen labeling studies in the oxidation of dppe are described. Based on these experiments, two potential mechanisms are proposed for the oxidation of dppe. Routes toward the synthesis of a chiral version of the heterobimetallic catalyst utilizing a chiral counterion or an alkyl group derived from S-2-methylbutane, 2,2$\sp\prime$-dimethylbinaphthyl, camphene, 9,10-dibromocamphor and 2-norbornane are described.
Chemistry, Inorganic, Chemistry, Organic
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign