Fluorine for Hydrogen Exchange in the Hydrofluorobenzene Derivatives C₆H_<i>x</i>F_{(6-<i>x</i>),} where <i>x</i> = 2, 3, 4 and 5 by Monomeric [1,2,4-(Me₃C)₃C₅H₂]₂CeH: The Solid State Isomerization of [1,2,4-(Me₃C)₃C₅H₂]₂Ce(2,3,4,5-C₆HF₄) to [1,2,4-(Me₃C)₃C₅H₂]₂Ce(2,3,4,6-C₆HF₄)

Abstract

The reaction between monomeric bis(1,2,4-tri-tert-butylcyclopentadienyl)cerium hydride, Cp′2CeH, and several hydrofluorobenzene derivatives is described. The aryl derivatives that are the primary products, Cp′2Ce(C6H5-xFx) where x = 1,2,3,4, are thermally stable enough to be isolated in only two cases, since all of them decompose at different rates to Cp′2CeF and a fluorobenzyne; the latter is trapped by either solvent when C6D6 is used or by a Cp′H ring when C6D12 is the solvent. The trapped products are identified by GC/MS analysis after hydrolysis. The aryl derivatives are generated cleanly by reaction of the metallacycle, Cp′((Me3C)2C5H2C(Me2)CH2)Ce, with a hydrofluorobenzene, and the resulting arylcerium products, in each case, are identified by their 1H and 19F NMR spectra at 20 °C. The stereochemical principle that evolves from these studies is that the thermodynamic isomer is the one in which the CeC bond is flanked by two ortho-CF bonds. This orientation is suggested to arise from the negative charge that is localized on the ipso-carbon atom due to Co(δ+)Fo(δ−) polarization. The preferred regioisomer is determined by thermodynamic rather than kinetic effects; this is illustrated by the quantitative, irreversible solid-state conversion at 25 °C over two months of Cp′2Ce(2,3,4,5-C6HF4) to Cp′2Ce(2,3,4,6-C6HF4), an isomerization that involves a CeC(ipso) for C(ortho)F site exchange.