PHOSPHONOUS ACID DICHLORIDES AND TRICYCLIC PHOSPHORANES FROM HEXAFLUOROPENTANE-2,4-DIONE AND TRIFLUOROPENTANE-2,4-DIONE

1,1,1,5,5,5-Hexafluouo-2-hyduoxy-2-pentene-4-one reacted diastereospec ifically with phosphonous acid dichlorides, RPCl2 (R = Me, Et, iPr, tB u, Me3SiCH2, PhCH2, Ph) to give in a concerted mechanism thermally sta ble tricyclic lambda(5) sigma(5)P phosphoranes containing two five-and one six-membered ring. In one case, hydrolysis gave 3,5-dihydroxy-2-o xo-1, 2 lambda(5) sigma(4)-oxaphosphalane, whereas methanol added to t he double bond in the six-membered ring furnishing two isomeric phosph oranes. When 1,1,1-trifluro-2-hydroxy-2-pentene-4-one was reacted with RPCl2 (R = Et, Me3SiCH2, PhCH2, Ph), diastereomerically pure regioiso meric phosphoranes were obtained. The solid-state molecular structures of three lambda(5) sigma(5)P species exhibited two oxygen atoms in th e axial position of a slightly distorted trigonal-bipyramidal geometry at phosphorus. Surprisingly, the two CF3 groups bonded to an sp(3)-hy bridized carbon were in a cisoid arrangement, having closest nonboundi ng 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 ph osphorane revealed the two hydroxy groups to be directed to one side o f the five-membered ring. (C) 1998 John Wiley & Sons, Inc.