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
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/31653 |
| Date | 31 October 2002 |
| Creators | Brown, Eric C. |
| Contributors | Gable, Kevin P. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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