YLIDYL-DIHALOPHOSPHANES PROVIDE STRUCTURAL SNAPSHOTS ON THEIR WAY TO DISSOCIATION

The reaction of phosphonium ylides with phosphorus trihalides has been studied for the synthesis of ylidyl-dihalophosphanes (= dihalophospha nyl ylides) Ph(3)P=CR-PX(2) 3, X = Cl, and 9, X = Br. Compounds 3, R = aryl, are readily prepared from the phosphonium bromides [Ph(3)P-CH(2 )R]Br, compounds 3, R = alkyl, SiMe(3) or PCl2, and 9 are obtained fro m silylylides Ph(3)P=CR-SiMe(3), compound 3, R = PPh(3)(+) results fro m the addition of PCl3 to the hexaphenylcarbodiphosphorane. A (beta-mo rpholinovinyl)dichlorophosphane 12 has also been prepared. Ylides 3 ar e oxidized by sulfur and selenium and are converted to ylidyl-chloroph osphenium (= chlorophosphaalkenyl-phosphonium) salts [Ph(3)P-CR=PCl]Al Cl4 10. In the P-31-NMR spectra of 3 and 9 the geminal coupling (2)J(p p) indicates the phosphorus lone pair to be synperiplanar to the phosp honio group. In one case the P(III)-C rotation barrier has been estima ted from VT-P-31-NMR spectra. By Xray crystallography the structures o f 3, R = Me, 2,6-Cl2C6H4, 4-NO2C6H5, PCl2, of 9, R = Me (two molecules ), SiMe(3), of an ylidyl-selenophosphonyl dichloride (11b), and of 12 have been analyzed. They provide representatives for the full range of rotation from the symmetric conformer with two equal P-X bonds to the conformer with one P-X bond perpendicular to the PCP plane and with t his bond being extremely elongated. Thus, they map out the pathway to P-X bond breaking. On this way the initial charge transfer from the yl idic carbon to the antibonding P-X orbital ends up in a pi donation an d P-X dissociation.