Steric Effects and Threshold Rotational Mechanisms in 1-Substituted 2,2-Dimesitylethenols

The static and dynamic stereochemistry of 1-alkyl-(and 1-H) 2,2-dimesitylethenols (Mes₂C=C(OR₂)R₁) is studied and analyzed. The low-temperature ¹H NMR data for the enols are in agreement with a propeller conformation in solution. A ¹H dynamic NMR study shows that 1 (R₁= R₂= H) behaves differently from the other enols: enol 1 shows two different processes for the exchange of groups at each mesityl ring (ΔG_C= 10.4 and 14.2 kcal mol^-1)-The 1-alkyl-substituted enols 2–5 (R₁= Me, Et, i-Pr, t-Bu; R₂= H) display a single measurable barrier, which decreases with the increase of the bulk of the R₁ substituent. Substitution of the enolic hydrogen of 1 by an isopropyl group affords the enol ether 6 (R₁= H; R₂= i-Pr). From comparison of the enantiomerization barrier and the mesityl rotational barrier in either 4 or 6 it is concluded that the threshold rotational mechanism is the one-ring flip for 1 and 6 and the two-ring flip for 2–5. These conclusions are strengthened by molecular mechanics calculations on 1 and 2 that satisfactorily reproduce their experimental rotational barriers. The rotational barriers for the two-ring process of 1–5 are linearly correlated with Taft's E_s values, with ϕ₂(the torsional angle of the ring cis to R₁), and with a₄(the C=C—R₁ bond angle). The two latter relationships with the structural parameters represent dynamics-structure correlations, and from that with 0₂a crude estimate of this 0₂ torsional angle in the transition state of the two-ring flip process is obtained.