Graduate Thesis Or Dissertation
 

Direct atom transfer vs. ring expansion in reaction of rhenium oxo complexes with cyclooctene epoxides and episulfides

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  • The rhenium (V) complex Tp'ReO₂ (Tp' = hydrido-tris-(3,5- dimethylpyrazolyl)borate), generated in situ from PPh₃ reduction of Tp'Re0₃, reacts with small ring heterocycles such as epoxides and episulf ides. The strained trans-cyclooctene ring provides an unusual thermodynamic bias against direct atom transfer, but trans-cyclooctene oxide still produces substantial amounts of trans-cyclooctene. Episulf ides also react under the same conditions. These are more reactive than epoxides and show a greater propensity for direct atom transfer; under catalytic conditions the eventual products are dithiolate complexes. The differences in reaction outcome point to important differences in selectivity for O- vs. S-atom transfer. The rate constant for trans-cyclooctene production was obtained using a parallel reaction model since an epoxide coordinated to rhenium as the intermediated undergoes direct O-atom transfer to give alkene and Tp'Re0₃ and ring expansion to give the diolate. The ratio of diolate/alkene is 2:1. The observed rate constant for ring expansion is (1.24 ± 0.03)x10⁻⁵ s⁻¹ and the rate constant for direct atom transfer is (6.02±O.02)x10⁻⁶ s⁻¹. cis- Cyclooctene showed a higher preference to give alkene (24.5:1) than transcyclooctene. The observed rate constant for direct atom transfer is 5.07 x 10⁻⁵ s' and the rate constant for ring expansion is 2.07 x 10⁻⁶ s⁻¹. Desulfidation of trans- and cis-cyclooctene showed the reaction proceeds faster than deoxygenation to give the thermally stable dithiodiolate species and alkene at 75 °C. cis-Cyclooctene episulfide gave a higher alkene/diolate ratio (10:1) than trans-episulfide (5:1). However, monothiodiolate formation in a phosphine free system (Tp'ReO(OH)(EtOH) as a Tp'ReO₂ source) was observed, in addition to dithiodiolate. Tp'ReOS or Tp'O₂ were suggested to serve as S-atom transfer agents. Tp'ReO₃ was also detected in the phosphine free system by formation of diolate from excess trans-cyclooctene Combining the kinetic and thermodynamic data, the order of reactivity of substrates is: cis-cyclooctene episulfide> trans-cyclooctene episulfide> cis-cyclooctene oxide> trans-cyclooctene oxide.
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