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

1,1,1,5,5,5-Hexafluoro-2-hydroxy-2-pentene-4-one reacted diastereospecifically with phosphonous acid di-chlorides, RPCl₂ (R = Me, Et, iPr, tBu, Me₃SiCH₂, PhCH₂, Ph) to give in a concerted mechanism thermally stable tricyclic λ⁵σ⁵P phosphoranes containing two five- and one six-membered ring. In one case, hydrolysis gave 3,5-dihydroxy-2-oxo-1,2λ⁵σ⁴-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 λ⁵σ⁵P 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 sp³-hybridized carbon were in a cisoid arrangement, having closest nonbonding 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 phospholane revealed the two hydroxy groups to be directed to one side of the five-membered ring. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:173–181, 1998