Relative stability of crowded isomeric enols: 2‐Mesityl‐2‐phenylethenols and their methyl ethers

The structure of 2‐mesityl‐2‐phenylethenol (7) obtained by reduction of mesityl phenyl ketene with LiAlH4 and by acid‐catalysed rearrangement of 1‐mesityl‐2‐phenylethylene glycol was determined by x‐ray crystallography to be Z [(Z)‐7]. In contrast with a literature report, the reduction of 2‐acetoxy‐2‐mesityl‐2‐phenylacetaldehyde did not provide the E isomer [(E)‐7], but a mixture of (Z)‐7 and 2‐mesityl‐2‐phenylethenol. An (E)‐7–(Z)‐7 mixture of 1:5 was obtained starting from pure (Z)‐7 at 80°C in dimethyl sulphoxide. The lower stability of (E)‐7 was ascribed to higher steric effects due to a smaller Ph–C=C compared with Mes–C=C torsional angle and a preferred intramolecular π(Mes)–OH in (Z)‐7 over π(Ph)–OH hydrogen bonding. In order to dissect the effects, the corresponding 2‐mesityl‐2‐phenylvinyl methyl ethers (E)‐15 and (Z)‐15, where hydrogen bonding is absent, were prepared and equilibrated in chlorobenzene. The (Z)‐15: (E)‐15 ratio of ca 3:1 between 58° and 132° (ΔG=0·8 kcal mol−1) gives ΔH ≈ 0·6 kcal mol−1 and ΔS ≈ 0·5 e.u. It was concluded that steric effects contribute ca 1 kcal mol−1 and hydrogen bonding ca 1·5 kcal mol−1 to the higher stability of (Z)‐7 over (E)‐7. The unknown mesitylphenylacetaldehyde 16 was obtained from (Z)‐7 at 135°C in 31% yield.