Preorganized Hydrogen Bond Donor Catalysts: Acidities and Reactivities

Measured DMSO p<i>K</i>_a values for a series of rigid tricyclic adamantane-like triols containing 0–3 trifluoromethyl groups (i.e., <b>3­(0)</b>–<b>3­(3)</b>) are reported. The three compounds with CF₃ substituents are similar or more acidic than acetic acid (p<i>K</i>_a = 13.5 (<b>3­(1)</b>), 9.5 (<b>3­(2)</b>), 7.3 (<b>3­(3)</b>) vs 12.6 (HOAc)), and the resulting hydrogen bond network enables a remote γ-trifluoromethyl group to enhance the acidity as well as one located at the α-position. Catalytic abilities of <b>3­(0)</b>–<b>3­(3)</b> were also examined. In a nonpolar environment a rate enhancement of up to 100-fold over flexible acyclic analogs was observed presumably due to an entropic advantage of the locked-in structure. Gas-phase acidities are found to correlate with the catalytic activity better than DMSO p<i>K</i>_a values and appear to be a better measure of acidities in low dielectric constant media. These trends are reduced or reversed in polar solvents highlighting the importance of the reaction environment

A Preorganized Hydrogen Bond Network and Its Effect on Anion Stability

Rigid tricyclic locked in all axial 1,3,5-cyclohexanetriol derivatives with 0–3 trifluoromethyl groups were synthesized and photoelectron spectra of their conjugate bases and chloride anion clusters are reported along with density functional computations. The resulting vertical and adiabatic detachment energies span 4.07–5.50 eV (VDE) and 3.75–5.00 (ADE) for the former ions and 5.60–6.23 eV (VDE) and 5.36–6.00 eV (ADE) for the latter species. These results provide measures of the anion stabilization due to the hydrogen bond network and inductive effects. The latter mechanism is found to be transmitted through space via hydrogen bonds, and the presence of three ring skeleton oxygen atoms and up to three trifluoromethyl groups enhance the ADEs by 1.61–2.88 eV for the conjugate bases and 1.01–1.60 eV for the chloride anion clusters. Computations indicate that the most favorable structures of the latter complexes have two hydrogen bonds to the chloride anion and one bifurcated interaction between the remote OH substituent and the two hydroxyl groups that directly bind to Cl^–

Charge-Enhanced Acidity and Catalyst Activation

Acidities are commonly measured in polar solvents but catalytic reactions are typically carried out in nonpolar media. IR spectra of a series of phenols in CCl₄ and 1% CD₃CN/CCl₄ provide relative acidities. Nonprotonated charged substituents with an appropriate counterion are found to enhance their Brønsted acidities and improve catalyst performance by orders of magnitude

Power of a Remote Hydrogen Bond Donor: Anion Recognition and Structural Consequences Revealed by IR Spectroscopy

Natural and synthetic anion receptors are extensively employed, but the structures of their bound complexes are difficult to determine in the liquid phase. Infrared spectroscopy is used in this work to characterize the solution structures of bound anion receptors for the first time, and surprisingly only two of three hydroxyl groups of the neutral aliphatic triols are found to directly interact with Cl^–. The binding constants of these triols with zero to three CF₃ groups were measured in a polar environment, and <i>K</i>_CD₃CN(Cl^–) = 1.1 × 10⁶ M^–1 for the tris­(trifluoromethyl) derivative. This is a remarkably large value, and high selectivity with respect to interfering anions such as, Br^–, NO₃^– and NCS^– is also displayed. The effects of the third “noninteracting” hydroxyl groups on the structures and binding constants were also explored, and surprisingly they are as large or larger than the OH substituents that hydrogen bond to Cl^–. That is, a remote hydroxyl group can play a larger role in binding than two OH substituents that directly interact with an anionic center