Synthesis and characterization of intra- and intermolecular hydrogen bonding isomers of PH (apical) phosphoranes bearing a hydroxyl group and their thermal cyclization

Abstract

The reaction of spirophosphorane 1 with 3 equivalents of RLi (R = Me, n-Bu, t-Bu) in Et 2O at −78 °C followed by treatment with aq. HCl gave monocyclic PH phosphoranes 2–4 bearing an apical hydrogen atom in good yields. Particularly in the case of 4, an isomer 4a with intramolccular hydrogen bonding and an isomer 4b with intermolecular hydrogen bonding with a solvent molecule could be separately isolated. Monocyclic phosphoranes 2–4 cyclized intramolecularly with elimination of H 2 to form spirophosphoranes 5–7 upon heating. Kinetic examination of cyclizations implied that they proceed via four-membered ring transition states involving hexacoordinate phosphorus.