Solid-State Structures of 1-Alkyl-2,2-dimesitylethenols. Application of the Principle of Structural Correlation to Ring-Flip Processes in 1,1-Diarylvinyl Systems

The solid-state structures of 1-R-2,2-dimesitylethenols (R = H, Me, Et, i-Pr, t-Bu (1a-e)) and of 1,1-dimesitylethylene (4) were determined by X-ray diffraction. Enol la displayed tetramers of four crystallographically independent molecules in the unit cell, and lb crystallized with an EtOH molecule. As the bulk of R is increased, the C=C bond length increases, the R—C═C bond angle (α₄) opens from 118.1° (1a) to 133.2° (1e), the RCO bond angle closes from 118.7° (1a) to 107.4° (1e), and the torsional angles ϕ₁ (of the Ar group cis to the OH) and ϕ₂ (of the aryl group trans to the OH) increase. α₄ is linear in Taft's E_s steric parameter. These trends are reproduced by MM2(85) calculations. Intermolecular enol-enol and enol-EtOH and intramolecular π (Ar)-HO hydrogen bonding are observed. The potential energy surface for 1,1-diphenylethylene (8) was calculated by molecular mechanics, and a propeller conformation with ϕ₁= ϕ₂= 40° is the lowest energy conformer. The calculated enantiomerization barriers for correlated rotations of zero-, one-, and two-ring flips are 12.9, 1.2, and 3.0 kcal mol^-1. In the calculated transition state for the zero-ring flip, both rings are puckered. The Cambridge Structural Database gave 116 crystallographically independent molecules with the Ar₂C═CR¹R² subunit. The ϕ₁ vs ϕ₂ angles for the 1,1-diarylethylenes were superimposed on the calculated surface for 8 in a conformational map. Experimental points concentrate around the calculated minimum and are absent in the vicinity of the (0°, 0°) region. When R¹ and R² differ much in bulk, the points prefer to concentrate around (0°, 90°) and (90°, 0°) diagonal whereas when the bulk of R¹ resembles that of R², many points concentrate along the diagonal between (40°, 40°) and (90°, 90°). By the crystal structure correlation principle, the one-and the two-ring flips are clearly favored over the zero-ring flip. Trimesityl-substituted systems are displaced toward the (90°, 90°) region compared with other triarylvinyl systems. Correlations between the bond angle ArCAr and the sum ϕ₁ + ϕ₂ and between the C—Ar bond length and the bond angle ArC═C were found.