Static and Dynamic Stereochemistry and Solvation of 2,2-Ditipylethenols

The effect of increased bulk of the β-aryl group in enols Ar₂C=C(OH)R from Ar = mesityl = Mes (1) to Ar = 2,4,6-i-Pr₃C₆H₂ = Tip (2) was investigated. The solid-state structure when R = H (a) does not significantly differ for 1a and 2a. The dynamic behavior of 2a resembles that of 1a, but the rotational barriers for both the threshold one-ring flip process and the two-ring flip process are higher for 2a. The one-ring flip barrier for 2a is solvent-dependent. The threshold two-ring flip barriers when R = Me (2b) and t-Bu (2c) are higher than for the mesityl analogues, but that for 2c is higher than predicted. Solvations of 2a and 1a and their associations with DMSO are similar. The C=COH conformation is syn-planar with an OH⋯π(Tip) association in non-hydrogen-bond-accepting solvents and is anti-clinal with OH⋯Solvent association in hydrogen-bond-accepting solvents. In summary, the increased bulk associated with the change Mes → Tip changes the structure and behavior in the expected direction but, except for the ΔG_c‡ values, not to a large extent.