The scope and limitations of the Diol-to-Alkene Reaction catalyzed by MTO and its corresponding mechanism

Andrew James Evans, Purdue University

Abstract

Oxidation of olefins to diols and epoxides using high valent metal oxo complexes has been known for decades; however, the corresponding deoxydehydration of diols and deoxygenations of epoxides to alkenes is relatively an unexplored reaction. This Diol-to-Alkene Reaction (DARe) could have significant value for use with biological compounds, in that it could be utilized in transforming low energy density biomass into a fuel source or into Highly Valuable Organics (HVO). Methyltrioxorhenium (MTO) has been shown to be a highly versatile catalyst, able to catalyze the epoxidation of olefins, the polymerization of olefins, and, more recently, the reduction of epoxides and diols to alkenes with a variety of reducing agents. The objective of my research is to examine the scope of the DARe catalyzed by MTO using model epoxide and diol compounds under varying conditions of temperature, time, and amount of H2. MTO catalyzes the reduction of 2, 3-dimethylepoxybutane, pinacol, trans-2-epoxy-1-hexanol, and trans-3-epoxy-1-hexanol. However, it was highly inefficient and yielded a large amount of mass loss. MTO catalyzed a highly interesting reaction with 1, 2, 3-butanetriol and 1, 2, 6-hexanetriol to form a mixture of alcohols and cyclic ether products. It was also discovered that in systems like hydrobenzoin and cyclohexane diol, the starting material can act as the reductant to produce a mixture of olefinic and ketonic products. Overall, molecular hydrogen is not a good reductant for use with DARe. Different combinations of the MTO/Pd/C were also investigated for deoxydehydration of diols. The products were always hydrocarbons and alcohols rather than alkenes. It is believed that the Pd/C component of the heterogeneous catalyst is responsible for the subsequent hydrogenation of the initial olefin product. MTO/Pd/C yielded less conversion of starting material, but it prevented charring of substrates as well as the cyclization of triols to form ethers. In systems more complex than linear hydrocarbons, the reactions cleanly yielded large quantities of hydrocarbons and alcohols in comparison to MTO alone. The mechanism of DARe was explored by analyzing the products formed by isomerically pure chiral diols upon reacting with MTO under differing reductants. (R, R)-(+)-hydrobenzoin and (S, S)-(-)-hydrobenzoin were reacted with MTO under an excess of molecular hydrogen and upon analysis of the products, the trans-stilbene isomer was observed as the major product. However, reaction with meso-hydrobenzoin yields a mixture of cis-stilbene and trans-stilbene products. This suggests that the diols form a diolate adduct that can undergo a competing SN2 reaction with water to cause isomerization of products. When using alcohols as reductants, there is an increase in yield of the stereospecific alkene. A mechanism for the DARe using alcohols and hydrogen was proposed involving a hydride transfer to the oxo group on a MTO-diolate, followed by a proton transfer to the alkoxy oxygen. The resulting bis-hyroxy MTO-diolate complex will then undergo dehydration to form a MDO-diolate complex observed in the work of Nicholas et al. Finally, the kinetic resolution of a racemic mixture of chiral diols was determined by incomplete reaction with MTO. A racemic mixture of R, R and S, S-hydrobenzoin was reacted with MTO, and the reaction was stopped before completion. After separation and analysis of the products, it was determined that MTO had no kinetic selectivity between the diastereomers of chiral diols.

Degree

M.S.

Advisors

Abu-Omar, Purdue University.

Subject Area

Inorganic chemistry|Organic chemistry

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