Chemistry of Dienyl Anions. IV. Geometry of Pentadienyl Anions in Solution and in the Solid State Determined by Regioselective Trimethylsilylation and NMR

Article abstract of DOI:10.1246/bcsj.54.1481

The configurational analysis of a series of alkyl substituted pentadienyl anions in THF was examined by trimethylsilylation and from the variable temperature 1H- and 13C-NMR spectra.Potassium pentadienide, -2-methylpentadienide or -2,4-dimethylpentadienide produced preferentially (Z)-2,4-pentadienyltrimethylsilane or its methyl substituted analogues to indicate that these compounds have the "U"-shaped structure in solution.By contrast, trimethylsilylation of potassium pentadienide and -2-methylpentadienide in the crystalline state produced the (E)- and the (E),(Z)-isomers, respectively, suggesting that the geometry is drastically changed to the "W"- or "S"-shaped by the medium effect.The W-shaped structure of potassium 3-methylpentadienide in THF and in the solid state is exceptional.Geometry of trapped products of potassium cycloheptadienide and -cyclooctadienide is also (Z) but the silylation occurred on the central carbon in contrast to the terminal silylation seen for the open-chain pentadienides.Geometry of these dienyl anions determined by NMR below -30 deg C was consistent with the result obtained from trimethylsilylation.Superficial conflict between MINDO/3 prediction and the experimental evidence was reasonably reconciled by considering contact ion pair for potassium pentadienide in THF and more strongly-solvated ion pair for the lithium analogues.