Stable Cis and Trans Rotational Isomers of 1,8-Di-o-tolylnaphthalene

Studies of CPK (Corey-Pauling-Koltun) or similar space-filling models suggest that there should be a very substantial barrier to a 180° rotation about the phenyl-naphthy bonds in 1,8-diphenylnaphthalene derivatives. This leads to the expectation that such compounds with a substituent at one meta position of each phenyl ring should be expected to exist as stable cis and trans isomer pairs. However, House and co-workers have shown that several such derivatives, including 1,8-bis(3-chlorophenyl)naphthalene and 1,8-bis(3-methylcarboxyphenyl)naphthalene cannot be resolved into stable configurational isomers. Further, proton NMR studies of these derivatives indicate that ΔG^≠ for rotation is 15-16 kcal/mol, which corresponds to rather rapid rotation in solution at room temperatures.