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 LiAlH 4 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° (Δ G =0·8 kcal mol −1 ) gives Δ H ≈ 0·6 kcal mol −1 and Δ S ≈ 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.