Pushing the Limits of the Lithium Indicator Carbon Acidity Scale Using Cyclopentadiene Chemistry and 19F NMR Spectrometry

C-H bonds are easily the most common type of ordinary chemical bond and studying carbon acidity will help us understand and predict the reactivity of organic compounds. Carbon acidities are ranked using acidity scales. One of the most prominent is the Streitwieser Lithium Indicator (SLI) scale. The term "indicator scale" implies that acids have been measured against one another in sequential fashion. The SLI scale uses lithium ion as the conjugate-base counterion and THF as the solvent. Historically, the SLI scale has emphasized the characterization of weakly-acidic hydrocarbons. Prior to the work of our group, the strongest acid on the SLI scale has a pK value of about 10. Deck and Thornberry extended the scale to ca. pK = 0 by evaluating 23 perfluoroaryl-substituted cyclopentadiene and indene derivatives, using 19F NMR spectroscopy to determine the equilibrium constants of sequential acid-base reactions.
This thesis describes the further extension of the SLI scale to ca. pK = −6. To achieve this result, a set of 11 tetrasubstituted cyclopentadienes were synthesized and their acidities evaluated sequentially with the goal of reaching a low pK value while minimizing the acidity ratio at each incremental step. The four ring substituents were combinations of pentafluorophenyl, perfluoro-4-tolyl, and perfluoro-4-pyridyl, electronegative groups listed in order of increasing electron-withdrawing power. The most acidic compound in the set was 1-pentafluorophenyl-2,3,4-tetrakis(tetrafluoro-4-pyridyl)cyclopentadiene, having pK = −5.99. Trends in the acidities of tetraarylcyclopentadienes are discussed including relative electron-withdrawing power of the three selected substituents, and conformational effects among pairs of regioisomeric cyclopentadiene derivatives.