Restricted rotation. Part 3. Barrier to rotation in methyl 2-(8-quinolyl)-6-oxocyclohex-1-enylacetate: comparative study of the effective bulk of a nitrogen lone pair in quinoline and of a naphthalene hydrogen

The rotational free energy barrier of methyl 2-(8-quinolyl)-6-oxocyclohex-1-enylacetate (2) about the aryl-cyclohexenone bond is found to be 75.7 kJ mol-1 by dynamic n.m.r. The value is lower than that (95.4 kJ mol-1) for the corresponding 1-naphthyl derivative (1) indicating that the effective bulk of the nitrogen lone pair in quinoline is less than that of a naphthalene hydrogen. A similar study with methyl 2-(1-naphthyl)-5-oxocyclopent-1-enylacetate (4) shows that the energy barrier in this case is lower than 37.0 kJ mol-1