Ylidyl‐dihalogenphosphane – Strukturbilder einer sich anbahnenden Dissoziation

Abstract

Ylidyl‐dihalophosphanes Provide Structural Snapshots on Their Way to DissociationThe reaction of phosphonium ylides with phosphorus trihalides has been studied for the synthesis of ylidyl‐dihalophos‐phanes (= dihalophosphanyl ylides) Ph3PCRPX2 3, X = Cl, and 9, X = Br. Compounds 3, R = aryl, are readily prepared from the phosphonium bromides [Ph3PCH2R]Br, compounds 3, R = alkyl, SiMe3 or PCl2, and 9 are obtained from silylylides Ph3PCRSiMe3, compound 3, R = PPh3+ results from the addition of PCl3 to the hexaphenylcarbodiphosphorane. A (β‐morpholinovinyl)dichlorophosphane 12 has also been prepared. Ylides 3 are oxidized by sulfur and selenium and are converted to ylidyl‐chlorophosphenium (= chloro‐phosphaalkenyl‐phosphonium) salts [Ph3PCRPCl]AlCl4 10. In the 31P‐NMR spectra of 3 and 9 the geminal coupling 2JPP indicates the phosphorus lone pair to be synperiplanar to the phosphonio group. In one case the P(III)C rotation barrier has been estimated from VT‐31P‐NMR spectra. By X‐ray crystallography the structures of 3, R = Me, 2,6‐Cl2C6H4, 4‐NO2C6H5, PCl2, of 9, R = Me (two molecules), SiMe3, 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 this bond being extremely elongated. Thus, they map out the pathway to PX bond breaking. On this way the initial charge transfer from the ylidic carbon to the antibonding PX orbital ends up in a π donation and PX dissociation.