Bis(silylation) of Tris(dimethylamino)phosphonium Methylide

Abstract

Abstract Treatment of (Me2N)3PCHSiMe3 with Me3SiOTf (OTf = triflate) affords the phosphonium salt [(Me2N)3PCH(SiMe3)2]OTf (2a), which can be converted into the correspond­ing ylide (Me2N)3PC(SiMe3)2 (3a) in high yield using NaN(SiMe3)2. The compounds with SiMe2Ph substituents (2, 3b) are obtained following an analogous procedure. The ylide (Me2N)3PC[Si(OMe)3]2 (3c) is available in 50% yield by direct transylidation of (MeO)3SiOTf with (Me2N)3PCH2 in the molar ratio 2:3, with [(Me2N)3PMe]OTf as the by-product. Com­pound 3c is a distillable liquid, but 3a,b are crystalline solids the structure of which has been determined by single crystal X-ray diffraction. Both compounds feature a trigonal planar ylidic carbon atom and short ylidic P-C and Si-C bonds. The nitrogen atoms are also in a planar configuration with non-standard orientations of the NMe2 groups at the phosphorus atoms. These data are in agreement with the results of recent quantum chemical calculations of simple prototypes which show very flat energy profiles for internal motions (rotation, inversion) of dimethylamino groups at the phosphorus atoms in phosphines and phosphoranes.