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

Silvio E. Biali*, David A. Nugiel, Zvi Rappoport

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

36 Scopus citations


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 (ΔGC= 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 Es 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.

Original languageAmerican English
Pages (from-to)846-852
Number of pages7
JournalJournal of the American Chemical Society
Issue number3
StatePublished - Feb 1989


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