Reactions of 1-substituted 2,2,3,4,4-pentamethylphosphetans with hexafluoroacetone and the 19F nuclear magnetic resonance spectra of the resulting 1,3,2-dioxaphospholans

Abstract

A series of 1-substituted 2,2,3,4,4-pentamethylphosphetans have been prepared, and converted into the corresponding 1,3,2-dioxaphospholans on treatment with hexafluoroacetone. The ¹⁹F n.m.r. spectra of these adducts over a range of temperatures have given values of the free energies of activation for the pseudorotation processes which place the four-membered ring diequatorial and the 1-substituents apical. The variation in these activation energies with the nature of the 1-substituent is accounted for in terms of the relative apicophilicity of groups being a function of both electronegativity and p_π–d_π back-bonding. Some of the consequences of this hypothesis are explored. Some unusual reactions and isomerisations of the 1,3,2-dioxaphospholans and of the related 1,2-oxa-phosphetans are described.