Stable Simple Enols. 3. Static and Dynamic NMR Behavior of Crowded Triaryletnenols and Related Compounds. Three-Ring Flip as the Threshold Mechanism for Enantiomerization of Crowded Triarylvinyl Propellers

Silvio E. Biali, Zvi Rappoport*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

51 Scopus citations


The static and dynamic stereochemistry of vinyl propellers Ar3Ar2C=C(X)Ar1 (2) was analyze and compared with those for the Ar1Ar2Ar3Z (3) and Ar1Ar2Ar3ZX (4) molecular propellers. The number of the maximum labeled propellers for each configuration of 2 is smaller than for 4, but permutations leading to positional and E,Z isomers of 2 increase the overall number when Ar1 ≠ Ar2 ≠ Ar3. Five possible stereoisomerization routes were analyzed: (i) rotations around the C(sp2)-C(Ar) bonds including various types of “flip” mechanisms; (ii) rotation around the double bond; (iii) addition-rotation-elimination; (iv) ketonization-enolization; (v) ionization-recombination with or without β-aryl rearrangement across the double bond. The DNMR behavior of several triarylvinyl propellers, which exist only as a pair of (helicity) enantiomers since the three rings have local C2 symmetry, was investigated. Trimesitylethenol (Mes2C=C(OH)Mes, 1) shows at room temperature 16 separate singlets in the 1H NMR spectrum, one each for the methyl groups, the aromatic protons, and the OH, in line with a propeller conformation in solution. By use of analogues with α-and β-2,4,6-(CD3)3C6H2, α-2,6-Me2C6H3, and β-4-r-Bu-2,6-Me2C6H2 rings and by application of aromatic-solvent-induced shifts, off-resonance, and saturation transfer techniques, all the 1H NMR signals and most of the 13C NMR signals were identified. A 1H dynamic NMR study of the coalescence of the three pairs of methyl protons and one pair of aromatic protons and a 13C DNMR study of one pair of aromatic carbons were conducted in C6D5NO2 and identical ΔGc* values (18.4 ± 0.1 kcal mol−1) were found for the three rings. These values are nearly independent of the solvent and insensitive to addition of CF3COOH. Analysis of the enantiomerization routes suggested that the process followed is a correlated rotation involving a three-ring flip. This is the mechanism for the enantiomerization as shown by the identical ΔGc* values for the three rings and for the isopropyl doublets of trimesitylvinyl isopropyl ether (19), where the isopropyl group serves as an enantiomerization probe. The case for a three-ring flip is strengthened by the identical ΔGc* values of the three rings of l-(9-anthryl)-2,2-dimesitylethenol (25). The ΔGc* values for Mes2C=C(X)Mes are 18.4, 19.0, 15.8, and 16.4 kcal mol−1 when X = OH (1), OAc (18), OCHMe2 (19), and Cl (20), respectively, and 16.2 for 25. The lower barrier for 25 is ascribed to a higher torsional angle of the α-ring in the ground state, as verified by X-ray diffraction of solid 25 and to a lower transition-state energy since a mesityl group is wider than an anthryl group. X-ray diffraction data for 18 indicate a propeller conformation. The low-temperature 1H NMR of 1 and 19 shows only one signal for X, but that of 18 at 212 K shows a 3.4:1 ratio of two conformers due to a different orientation of the acetate group. The ∆Gc* for their interconversion is 12.1 ± 0.3 kcal mol−1, and possible stereoisomerization routes were suggested. Structural requirements for the preparation of conformationally stable optically active vinyl propellers were discussed.

Original languageAmerican English
Pages (from-to)477-496
Number of pages20
JournalJournal of the American Chemical Society
Issue number3
StatePublished - 1 Dec 1984


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