Unraveling the Catalytic Cycle of Tertiary Phosphine Oxides Reduction with Hydrosiloxane and Ti(OiPr)4 through EPR and 29Si NMR Spectroscopy

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 • that undergoes a O 2– abstraction from P V compounds leading to P III compounds and Si–O • species. Reoxidation of Ti III by the latter gives silanol species. This mechanism was further probed by 29 Si 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 Ph 3 PO with these environmentally benign reagents on a 100 g scale has been developed.