Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
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.
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