Unraveling the Catalytic Cycle of Tertiary Phosphine Oxides Reduction with Hydrosiloxane and Ti(O i Pr) 4 through EPR and 29 Si NMR Spectroscopy
Résumé
The reduction of tertiary phosphine oxides using tetramethyldisiloxane (TMDS) as a mild reducing agent and,Catalytic amount of Ti-IV isopropoxide has been studied in detail. An extensive EPR study has revealed the presence of at least five Ti-III species, and structures have been proposed. Thus, a single electron transfer (SET) mechanism, consisting of a back and forth oxido-reduction of Ti from the IV to the III oxidation state could be proposed. Reduction of Ti-IV produces a Si-center dot that undergoes a O2- abstraction from P-V compounds leading to P-III compounds and Si-O-center dot species. Reoxidation of Ti-III by the latter gives silanol species. This mechanism was further probed by Si-29 NMR analysis of the reaction mixture as a function of time and by the reduction of optically active P-stereogenic tertiary phosphine oxides. A practical reduction protocol of Ph3PO with these environmentally benign reagents on a 100 g scale has been developed.