Unconjugated Arylcyclopropanes. Acid-Catalyzed Addition of Acetic Acid to Highly Hindered Arylcyclopropanes

Article abstract of DOI:10.1021/jo01311a021

The adduct of 1-naphthylcarbene and norbornene, endo-3-(1-naphthyl)-exo-tricyclo<3.2.1.0^2,4>octane (9) has been prepared. ¹H and ¹³C NMR spectra of this system indicate that the unconjugated arylcyclopropane conformation is favored.Rotation about the naphthyl-cyclopropane bond is restricted.The temperature-dependent NMR indicates a 16.9 kcal/mol barrier to attainment of the conjugated conformation.Acetic acid adds readily under acid catalysis to the phenyl analogue exo-3-phenyl-exo-tricyclo<3.2.1.0^2,4>octane (3) and more slowly to the more strained endo-3-phenyl-exo-tricyclo<3.2.1.0^2,4>octane (4).Kinetic data suggest the involvement of a cationic intermediate with little transition-state charge development at the benzylic carbon.The rates of addition to substituted analogues of 3 and 4 correlate with Hammett ? values, giving ρ values of 2.53 and 2.35, respectively.Product-analysis data support the involvement of a benzylic cation, 23.The slower rate of reaction of the more strained endo isomers has been interpreted in terms of a barrier to attainment of the conjugated conformation which appears to be the favorable conformation for protonation of arylcyclopropanes.This suggestion is supported by the observation that the unconjugated systems, exo- and endo-3-(2,6-dimethylphenyl)-exo-tricyclo-<3.2.1.0^2,4>octanes (27 and 28), add acetic acid 480 and 6.2 x 10⁴ times, respectively, more slowly than 3.