BIS(SILYLATION) OF TRIS(DIMETHYLAMINO)PHOSPHONIUM METHYLIDE

Treatment of (Me2N)(3)PCHSiMe3 with Me3SiOTf (OTf = triflate) affords the phosphonium salt [(Me2N)(3)PCH(SiMe3)(2)] OTf (2a), which can be c onverted into the corresponding ylide (Me2N)(3)PC(SiMe3)(2) (3a) in hi gh yield using NaN(SiMe3)(2). The compounds with SiMe2Ph substituents (2, 3b) are obtained following an analogous procedure. The ylide (Me2N )(3)PC[Si(OMe)(3)](2) (3c) is available in 50% yield by direct transyl idation of (MeO)(3)SiOTf with (Me2N)(3)PCH2 in the molar ratio 2:3, wi th [(Me2N)(3)PMe] OTf as the by-product. Compound 3c is a distillable liquid, but 3a,b are crystalline solids the structure of which has bee n determined by single crystal X-ray diffraction. Both compounds featu re 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. Thes e data are in agreement with the results of recent quantum chemical ca lculations of simple prototypes which show very hat energy profiles fo r internal motions (rotation, inversion) of dimethylamino groups at th e phosphorus atoms in phosphines and phosphoranes.