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

Article abstract of DOI:10.1016/0040-4039(96)01923-5

The reaction of spirophosphorane 1 with 3 equivalents of RLi (R=Me, n-Bu, t-Bu) in Et₂O 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 intramolecular 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 H₂ to form spirophosphoranes 5-7 upon heating. Kinetic examination of cyclizations implied that they proceed via four-membered ring transition states involving hexacoordinate phosphorus.