Catalytic Transformations With Oxorhenium and Chromium Complexes for Deoxygenation of Diols and Selective Reduction of Dioxygen
Deoxygenation of vicinal diols and polyols, common moieties in biomass-derived molecules, represents an important chemical pathway for making chemicals from renewable biomass resources. High-valent oxorhenium complexes are among the most efficient catalysts for deoxygenation reactions, and have been studies using various reductants including organic phosphines, molecular hydrogen (H2), sulfite, and alcohols. These complexes exhibit intriguing oxophilic performance, which facilitates selective C-O bond cleavage of polyols. My thesis research focuses on catalytic transformations with oxorhenium and chromium complexes for deoxydehydration of polyols and selective reduction of dioxygen. I have investigated various catalytic deoxydehydration (DODH) reactions of biomass-derived polyols with high-valent oxorhenium complexes. Cooperating with another graduate student in our group, we were able to demonstrate the use of oxo-rhenium complexes as catalysts for the transformation of glycerol and erythritol to small and useful organics (SUO). Glycerol was reduced to generate allylic alcohol, and can be continuously driven to high yields by separation of the volatile products. Meso-erythritol was reduced to dihydrofuran. The work was published in ChemSusChem (DOI: 10.1002/cssc.201200138). I further investigated the MTO-catalyzed DODH reaction with sacrificial alcohol as reductant, and elucidated the reaction mechanism based on detailed kinetics, spectroscopics (NMR, XRD, ESI-MS, UV-vis) and isotope labeling studies. Our findings revealed a new mechanism for DODH with oxorhenium based on a rhenium(V)/rhenium(III) cycle, which should stimulate future catalyst design. This mechanistic study was published in Organometallics (DOI: 10.1021/om400127z). In the spring of 2013, I had the opportunity to visit Osaka University as a visiting scholar. I was very motivated by the unique electrocatalysis properties of chromium-corrole complexes we synthesized, which emulate the natural metalloenzymes. The chromium-corrole complex catalyzed the selective two-electron reduction of O2 by ferrocene derivatives in the presence of trifluoroacetic acid (TFA) at room temperature. We further investigated the catalytic mechanism and reaction selectivities based on detailed kinetic studies of each step in the catalytic cycle as well as the overall catalytic reaction and detection of reactive intermediates. The study also revealed ultra-fast electron-transfer from ferrocene derivative to Cr(V)-oxo corrole. This work resulted in two papers, one published in Inorganic Chemistry (DOI: 10.1021/ic5013457), and the other to be submitted. My current work is a collaboration with Eli Lilly & Co. on the synthesis of green polar aprotic solvents using ruthenium catalysts, which involves dehydrogenation reactions of biomass-derived compounds followed by coupling with amines. We are able to invent a novel clean process to produce imidazolidinones and planning to scale up the reaction for real pharmaceutical application.
Abu-Omar, Purdue University.
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