Modulating the reactivity of phosphanylidenephosphoranes towards water with Lewis acids

Abstract

Phosphanylidenephosphoranes of the type R–P(PR′₃), also known as phospha-Wittig reagents, can be utilized in a variety of bond activation reactions exploiting their phosphinidenoid reactivity. Herein, we thoroughly show that a facile PMe₃ for H₂O exchange gives access to various primary phosphine oxides of the general formula RP(H)₂O (R = Mes*, ^MesTer, ^DipTer) and the molecular structure of ^DipTerP(O)H₂ was determined. However, phosphanylidenephosphoranes are described to be highly nucleophilic as well. We show that the attachment of main group Lewis acids such as GaCl₃ and GaI₃ to R–P(PMe₃) yielded the highly sensitive, yet stable coordination compounds [(RPGaX₃)PMe₃] (R = Mes*, ^DipTer) or [(RPPMe₃)₂GaCl₂]GaCl₄ (R = ^MesTer). In contrast to the free phosphanylidenephosphoranes, these species reacted differently with H₂O, which was demonstrated for [(Mes*PPMe₃)GaI₃]. Here the formation of the phosphino-phosphonium cation [Mes*P(H)PMe₃]⁺ and different anions was observed with combined NMR spectroscopic and SC-XRD (SC-XRD = single crystal X-ray diffraction analysis) studies. This work demonstrates that the ambiphilic character of phosphanylidenephosphoranes can be utilized to manipulate the reactivity of R–P(PMe₃) towards water, giving primary phosphine oxides, whereas the Lewis acid adducts [(RPGaX₃)PMe₃] gave phosphino-phosphonium species.