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

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 ΔG_c* 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 ΔG_c* 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 ΔG_c* values of the three rings of l-(9-anthryl)-2,2-dimesitylethenol (25). The ΔG_c* 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 ∆G_c* 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.