PI-CATION AND PI-DIPOLE-STABILIZING INTERACTIONS IN A SIMPLE-MODEL SYSTEM WITH COFACIAL AROMATIC RINGS

Four 1,8-disubstituted naphthalenes having a l-naphthyl ring cofacial to a second ring were prepared by a Suzuki-type coupling of a bromide with a borane, a boronic acid, or a boronic ester in the presence of P d(0). The second ring is either a positively charged N-methylated 3-py ridyl (4A) or a 3-(trimethylammonio)phenyl group (7A) or a neutral 3-t olyl (5) or 3-(dimethylamino)phenyl (7) ring. At ambient temperatures, all exhibit atropisomerism. The syn isomer predominates over the anti form, suggesting that pi-charge and pi-dipole electrostatic through-s pace interactions preferentially stabilize the more sterically hindere d syn isomer. The largest preference for the syn isomer, a factor of 3 .1, was found for the trimethylammonio substrate-7A in CDCl3. Semiempi rical computations (AM1 and PM3) predict that the syn isomer is lower in energy than the anti form of the positively charged compounds, but the preference for the neutral compounds is small enough to be ambiguo us.