Phosphonous acid dichlorides and tricyclic phosphoranes from hexafluoro- and trifluoropentane-2,4-dione

1,1,1,5,5,5-Hexafluouo-2-hyduoxy-2-pentene-4-one reacted diastereospecifically with phosphonous acid dichlorides, RPCl2 (R = Me, Et, iPr, tBu, Me3SiCH2, PhCH2, Ph) to give in a concerted mechanism thermally stable tricyclic lambda(5) sigma(5)P phosphoranes containing two five-and one six-membered ring. In one case, hydrolysis gave 3,5-dihydroxy-2-oxo-1, 2 lambda(5) sigma(4)-oxaphosphalane, whereas methanol added to the double bond in the six-membered ring furnishing two isomeric phosphoranes. When 1,1,1-trifluro-2-hydroxy-2-pentene-4-one was reacted with RPCl2 (R = Et, Me3SiCH2, PhCH2, Ph), diastereomerically pure regioisomeric phosphoranes were obtained. The solid-state molecular structures of three lambda(5) sigma(5)P species exhibited two oxygen atoms in the axial position of a slightly distorted trigonal-bipyramidal geometry at phosphorus. Surprisingly, the two CF3 groups bonded to an sp(3)-hybridized carbon were in a cisoid arrangement, having closest nonbounding F ... F distances of 301.4 or 273.5 pm. These findings reflect the "through-space" F-F coupling constants of the tricyclic phosphoranes (J(FF) = 4.0-7.0 Hz) in solution. The solid-state structure of the phosphorane revealed the two hydroxy groups to be directed to one side of the five-membered ring. (C) 1998 John Wiley & Sons, Inc.