The ligand-exchange process of P-H apical phosphoranes and the thermal formation and pseudorotation of anti-apicophilic spirophosphoranes

Abstract

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 phos-phoranes 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 = nBu) 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, 2006.