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Relative stability of crowded isomeric enols: 2‐Mesityl‐2‐phenylethenols and their methyl ethers
Author(s) -
Nadler Ella B.,
RÖCk Maik,
Schmittel Michael,
Rappoport Zvi
Publication year - 1993
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610060407
Subject(s) - chemistry , steric effects , mesityl oxide , hydrogen bond , intramolecular force , chlorobenzene , medicinal chemistry , yield (engineering) , stereochemistry , molecule , organic chemistry , catalysis , materials science , metallurgy
Abstract 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.