Unraveling the Catalytic Cycle of Tertiary Phosphine Oxides Reduction with Hydrosiloxane and Ti(O<i>i</i>Pr)₄ through EPR and ²⁹Si NMR Spectroscopy

Abstract

The reduction of tertiary phosphine oxides using tetramethyldisiloxane (TMDS) as a mild reducing agent and catalytic amount of TiIV isopropoxide has been studied in detail. An extensive EPR study has revealed the presence of at least five TiIII 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 TiIV produces a Si• that undergoes a O2– abstraction from PV compounds leading to PIII compounds and Si–O• species. Reoxidation of TiIII by the latter gives silanol species. This mechanism was further probed by 29Si 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.