Effect of Tetra-m-bromo and Tetra-m-methyl Buttressing on the Ground-State Structures, Rotational Barriers, and Keto ⇌ Enol Equilibria of 2,2-Dimesityl-1-R-ethenols

The stable enols (3,5-Br₂-2,4,6-Me₃C₆)₂C=C(OH)R (3a–d) and (Me₅C₆)₂C=C(OH)R (9a–d), R = H, Me, Mes, or t-Bu, were prepared. The effect of buttressing by four m-Br or m-Me groups was investigated by comparison with the 2,2-dimesityl analogs Mes₂C=C(OH)R (1). No buttressing effect on the ease of formation of 3 and 9 was observed. Differences due to buttressing in torsional and bond angles in the solid-state structures were mostly moderate or small, the largest being when R = H. The threshold rotational mechanisms for 1, 3, and 9 are a one-ring flip when R = H, a two-ring flip when R = Me or t-Bu, and a three-ring flip when R = Mes. The one-ring flip barrier is higher for 3a and 9a than for 1a due to increased buttressing in the transition state for the rotation. The two- and three-ring-flip barriers are lower for 3 and 9 than for 1. These differences are partially accounted for by solvent effects and by electronic and torsional angle effects on the extent of ground-state Ar—C=C conjugation for most of the enols but not when R = t-Bu. Enols 3a and 3d do not isomerize to the keto-enol mixtures even after prolonged heating in hexane/CF₃COOH. This is ascribed to increased kinetic stability over enols 1 due to reduced nucleophilicity by the electron-withdrawing bromines. Enols 9a, 9b, and 9d and the isomeric ketones of 9b and 9d isomerize in hexane/CF₃COOH at 80 °C to the equilibrium mixtures. The higher K_enol values of 185 (9a), 3.6 (9b), and 0.021 (9d) compared to those for the corresponding enols 1, were attributed to increased crowding. In conclusion, after accounting for electronic effects, the buttressing effect by four m-Br or m-Me groups on the solid-state structures, rotational barriers, and keto ⇌ enol equilibria is mostly moderate.