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

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 °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 pairs for potassium pentadienide in THF and more strongly-solvated ion pairs for the lithium analogues.