Some aspects of phosphetan chemistry.

The alkaline hydrolysis of optically active l-benzyl-2,2,3,3- tetramethyl-l-phenylphosphetanium iodide is shown to proceed with predominant retention of configuration. Pseudorotation of intermediate phosphoranes is suggested to explain the partial inversion observed during hydrolysis and the base-catalysed racemisation of this salt. The alkaline hydrolyses of 2- and 3-phospholenium salts are studied and the role of pseudorotation in determining the product distribution is discussed. The ring opening of 1,2,2,3,3-pentamethyl-l-phenylphosphetanium iodide by the cyanide ion is suggested to involve a carbonium ion intermediate. A similar process may be involved in the reaction of chlorine with 2,2,3,3-tetramethyl-l-phenylphosphetan to give chloro-(2,2,3-trimethylbut-3-enyl) phenylphosphine. Cyclisation of this phosphine occurred on heating or on treatment with chlorine to give 2,2,3-trimethyl-1-phenyl-2-phospholen 1-oxide, while treatment with aluminium chloride gave a mixture of phospholen and phosphetan 1-oxides. The generality of these cyclisation reactions are investigated. A variety of cyclic oxyphosphoranes of the phosphetans were prepared and a study made of the high energy pseudorotations available to these adducts. The process whereby the five-membered ring was placed diequatorial in these phosphoranes was not observed, however the equivalent process for the phosphetan ring was found to occur at temperatures accessible for study by n.m.r. spectroscopy. The energetic data so obtained is suggested to provide a limited scale of relative apicophilicities of the substituents on phosphorus in phosphoranes. Deviations from the ordering expected on the basis of the electronegativities of the substituents are explained in terms of preferential equatorial [special character omitted]--bonding between ligand 7[special character omitted]--orbitals and the phosphorus d-orbitals. Applications of an apicophilicity scale are discussed. Attempts are made to observe an enhanced phosphorus-ligand bond rotational barrier due to the suggested [special character omitted]-bonding from aryl substituents and to modify this [special character omitted]--bonding by substituent effects. Possible mechanisms for two reactions of the phosphetan oxyphosphoranes are discussed, nucleophilic substitution being favoured.