Association of α-Alkyl-β,β-Dimesitylethenols with Hydrogen Bond Accepting Solvents

The δ(OH) values for α-alkyl-β,β-dimesitylethenols (Mes₂C=C(OH)R, 2, R = Me, Et, i-Pr, t-Bu) were measured in eight solvents and in several binary CCl₄-DMSO-d₆ mixtures. Large shifts to a lower field were observed on increasing the hydrogen bond accepting ability of the solvent or the mixture. The δ(OH) value for 2 in the various solvents were linear with δ(OH) for Mes₂C=CHOH (1) and approximately linear (with slopes C) with Kamlet-Taft’s hydrogen bond accepting parameter β of the solvent. Long-range couplings ⁴J(HCCOH) were observed for 2, R = i-Pr in several solvents but not in DMSO. The change in δ(OH) in the CCl₄-DMSO-d₆ mixtures was analyzed in terms of formation of a 1:1 association complex of the enol with DMSO and assuming that a syn-type conformerin CCl₄ and an anti-type conformer in DMSO are present in a rapid equilibrium. The δ(OH) for the anti-type conformer (δ_anti-DMSO) was calculated, but information on its exact geometry is not available. Complete association with DMSO is not achieved even in the pure solvent. The log K_assoc values are linearly correlated with the σ* values and decrease nonlinearly with the increase in the steric parameter E_g. They are linear with the slopes C and with δ_anti-DMSO The polar effect on K_assoc is in the expected direction. The small sensitivity to steric effects is in contrast with the large sensitivity to steric effects found for other mechanistic phenomena on increasing the bulk of R in 1 and 2. This is presumably due to the occurrence of the association with the hydrogen bond accepting solvent on the relatively unhindered side of the crowded enol molecules.