The Ligand‐Exchange Process of P–H apical Phosphoranes and the Thermal Formation and Pseudorotation of Anti‐Apicophilic Spirophosphoranes

Monocyclic P–H apical phosphoranes 10 bearing a ring‐opened Martin ligand are formed upon treatment of the P–H equatorial spirophosphorane 7 with more than 2 equiv. of alkyllithium compounds. A ligand‐exchange process of these phosphoranes 10 involving the interconversion of the bidentate Martin ligand with the monodentate Martin ligand was found to proceed, probably through a hexacoordinate phosphorus atom. Heating of 10 in nondonating solvents furnished alkylspirophosphoranes 5 , whereas heating of 10 in donor solvents led to the formation of anti‐apicophilic phosphoranes 6 , a new class of phosphoranes in which the less apicophilic carbon substituent occupies an apical position and the more apicophilic oxygen atom occupies an equatorial position. These phosphoranes 6 were found to completely convert into their energetically more stable C equatorial ,O apical stereoisomers 5 upon heating. On the basis of kinetic examinations of the cyclization process of 10 , the formation of 6 can be rationalized by the enhanced nucleophilicity of the hydroxy group in the donor solvents. The isomer 5b was estimated to be more stable than 6b (R = n Bu) by 12 kcal mol –1 from kinetic studies on the pseudorotation of 6b to 5b . This is in good agreement with theoretical calculations with analogous 5a and 6a (R = Me), which estimate the difference in energy to be 14.1 kcal mol –1 . (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)