Enantiomerization of bridged 1,1′-binaphthyls

The activation energy for the isomerization of 1,1′-binaphthyl in which positions 2 and 2′ are bridged at by an -O-CH₂-O- unit was calculated at various computational levels. AM1 gave good agreement with the experimental results. The transition-state structure was found to be entirely different from that calculated for the non-bridged parent compound: whereas the latter has C₂ symmetry, the former has C_s symmetry. The C_s symmetry transition state for the non-bridged parent compound was also located and found to be ca 6 kcal mol^-1 higher than the C₂ one. However, in the bridged compound, the inclusion of the bridge counterbalanced this by raising the energy of the ground state, leaving the activation energy essentially unchanged. The isomerization of optically active bridged 1,1′-binaphthyls bearing linear polyphenyl rods of varying length, at positions 6 and 6′, was recently employed as a probe to gain information on the effect of rubber and glassy polymers on reaction rates. The model showed that the rod segments of these molecules traverse long distances in order to reach the transition state, which was consistent with a strong rod length dependence on racemization of the bridged binaphthyls in the glassy state. However, the present results demonstrate an unexpected twisting motion in the racemization process, suggesting that the appended oligophenyl rods are displaced to about half the distance previously expected. This may contribute in part to the experimental observation in the rubbery state where the microviscosity affects the racemization as a function of the appended rods far less than expected. AMl results also gave reasonable agreement with the experimental ponderal effect consistent with the prior conclusion of force constant independence of rod length for twisting about the 1,1′ bonds.