Graduate Thesis Or Dissertation
 

Mechanistic studies on Re(V) mediated C-O bond transformations

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  • In situ, reduction of Cp*ReO₃ by PPh₃ to form (Cp*ReO)₂(μ-O)₂ allows catalytic deoxygenation of epoxides, however, conproportionation between Reⱽ and Reⱽᴵᴵ species to form clusters of ((Cp*Re)₃(μ-O)₆}²⁺(ReO₄⁻)2 and new compound {(Cp*Re)₃(μ²-O)₃(μ³-O⁾³ReO₃}⁺(ReO₄⁻) leads to removal of rhenium from the catalytic cycle and loss of activity, The epoxide deoxygenation mediated by (Cp*ReO)₂(μ-O)₂ at least partially occurs through the intermediacy of the corresponding Re(V) diolates, which are unstable under conditions at which they form (72-112°C); the following cycloreversion leads to a formation of Cp*ReO₃ and alkene, Primary ¹³c and secondary ²H kinetic isotope effects (KIE) show that cycloreversion of 4-methoxystyrene from Tp'Re(0)(OCH(Ph-4-OMe)CH₂O) proceeds via a significantly asynchronous but concerted transition state. The primary KIEs are k₁₂c/ᵏ₁₃c = 1 .041 ± 0.005 at the alpha position, and 1.013 ± 0.006 at the beta position. The secondary KIEs were kH/kD = 1.076 ± 0.005 at the alpha position, and 1 .017 ± 0.005 at the beta position at 103°C. Both ¹³C and D KIEs were obtained from analysis of ¹H NMR spectra using a newly developed procedure. For the first time, D KIEs were alternatively obtained using Bayesian NMR methodology, independently confirming D KIEs derived from ¹H NMR. ¹³C and D KIEs were found to be consistent with each other in pointing to significantly more bond breaking at the alpha carbon than at the beta carbon. A Hammett study on cycloreversions of substituted styrenes from a series of Tp'Re(O)(diolato) showed dichotomous behavior for electron donors and electron acceptors as substituents: ρ = -0.65 for electron donors, but ρ = +1.13 for electron-withdrawing groups. Quantitative analysis of the KIEs coupled with the observed impact of aryl substitution on the rate of cycloreversion excludes the possibility of competing single bond-cleaving (stepwise) processes being responsible for the observed KIE values. DFT calculations locate a transition state structure for styrene extrusion that agrees with this picture. The Hammett behavior opens the possibility that, depending on a substituent, two different mechanisms might be in competition due to very well balanced stereoelectronics of the system where charge perturbation changes the mechanism,
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