Facile Reduction of Phosphine Oxides by O-Silylated Hydrazide Supported Hydrosilanes

Abstract

The present work describes the investigations on a metal-free reduction of pentavalent tetracoordinate phosphine oxides to the corresponding trivalent phosphines using inexpensive hydrosilanes (Ph3SiH, Ph2SiH2, Et2SiH2, and Cl2SiMeH) in the presence of various activators including O-silylated hydrazide derivative. The presence of hydrazide precursor with CH3SiHCl2 leading to a hypercoordinate silicon(IV) complex has proved more effective in the reduction of phosphine oxides. Various phosphine oxides with substituents like alkyl, aryl, or N-alkyl on central phosphorus atom were reduced with notable features of chemoselectivity in the presence of other active reducible groups. They exhibited good to excellent yields under the safe reaction conditions. A generalized reduction mechanism has been proposed involving the formation of hypercoordinate silicon(IV) species as the key intermediates. According to the DFT calculations, the reactivity of silanes as reducing agents is influenced by the oxophilic/electrophilic nature of silicon atom present therein. On comparison, the order of reactivity calculated on the basis of NPA charge distribution appears as: pentacoordinate silicon(IV) complex > Ph3SiH > Cl2SiMeH > Ph2SiH2 ~ Et2SiH2 > PhSiH3 > SiH4.