The epoxide deoxygenation reaction is formally the reverse of the epoxidation reaction. Compared to epoxidation, which has reached its full maturity, epoxide deoxygenation has not been as intensively developed. Among the few deoxygenation reagents, a handful are catalytic in a metal complex, show high stereospecificity and operate under mild conditions. A common feature of all present deoxygenation reagents is that they do not perform asymmetric deoxygenation of racemic epoxides. Rhenium (VII) trioxo complexes are emerging as pliable catalysts for epoxide deoxygenation. Designing a chiral rhenium (VII) trioxo complex was our goal. Guided by the mechanism of rhenium (VII) trioxo catalyzed epoxide deoxygenation and the mechanism of the stereogenic information transfer, we have designed and prepared a chiral rhenium(VII) trioxo complex. This complex is void of stereogenic centers and the source of asymmetry is the restricted rotation around a carbon-carbon bond. Detailed conformational analysis of the new chiral complex was done by extensive NMR measurements and molecular modeling. The rotation barrier for the diolate was experimentally and computationally estimated to be 9.72 kcal/mol and 8.06 kcal/mol, respectively. Unsuccessful attempts were made to prepare a camphor based scorpionate because of the extreme steric congestion. A menthone based scorpionate was successfully prepared. The related rhenium (TII) trioxo complex with this scorpionate revealed contradicting chemical and spectroscopic features. / Graduation date: 2005

In situ reduction of hydrido-tris-(3,5-dimethylpyrazolyl)borato(trioxo) rhenium(V) with triphenylphosphine or triethylphosphite leads to a reactive rhenium(V) species that catalytically deoxygenates epoxides at 75-105��C. The reaction is stereospecific, except for trans- and cis-butene oxide which formed minor amounts of the opposite isomer. A variety of different functional groups were tolerated and even epoxides that reacted slowly could be pushed to greater than 95% conversion given extended time and/or higher temperature. The absence of clustering processes shows how the choice of ligand can have a major influence on the design of the catalytic cycle. The rhenium(V) species formed from reduction of Tp'ReO��� was identified as Tp'Re(O)(OH)���. Tp'Re(O)(OH)��� reacted with ethanol and HCl to form ethoxide and hydroxo chloride complexes, respectively. In addition, Tp'Re(O)(OH)��� was an excellent catalytic and stoichiometric reagent for the deoxygenation of epoxides and sulfoxides. Loss of water from Tp'Re(O)(OH)��� to form the catalytically active species Tp'Re02 was shown to be a necessary preequilibrium process. The kinetic behavior of the catalytic system is complex. First-order behavior in [Re][subscript T], zero-order dependence in [PPh���] and saturation behavior for epoxide were observed. The reversible formation of a coordinated epoxide complex was proposed to explain the saturation behavior. The epoxide complex was shown experimentally and computationally to engage in two separate reactions: ring expansion to form a syn-diolate complex, and direct fragmentation to alkene and trioxide. A steady-state concentration of diolate is eventually reached explaining a "burst" of alkene production prior to generation of a pseudo-zero-order catalytic system. The diolate formed is the syn-isomer, which is the kinetically formed product. Direct epoxide fragmentation is the primary source of alkene. This process was determined to be four times faster than ring expansion for cis-stilbene oxide. The synthesis and characterization of a tethered-epoxide Cp* rhenium trioxide complex has been achieved. Reduction of this complex leads to an unsaturated rhenium(V) species that is immediately complexed by the tethered epoxide. Experimental data and molecular mechanics modeling support intramolecular coordination of the epoxide to the rhenium center. These results confirm that the coordinate epoxide is a viable intermediate in rhenium-catalyzed epoxide deoxygenations. / Graduation date: 2003

Graduation date: 2002

Graduation date: 2004