Hindered Rotations in 8-Halo and 8-Cyano-1-Pyrrolidinyl Naphthalenes

Three 1-pyrrolidinyl 8-halo and one 1-pyrrolidinyl 8-cyano-substituted naphthalenes are prepared. The rate of the rotation of the pyrrolidinyl group is determined through variable temperature proton NMR using line-shape analysis and EXSY. The potential energy of the rotation is modeled through DFT calculations using the aug-cc-pVDZ-PP basis set. The activation energies are compared with the experimental values. The strain components of the transition states are obtained from molecular mechanics analysis using MM3. The activation energies follow the steric footprint of the halogen/nitrile: I > Br > Cl > CN. Rotation requires significant twisting and splaying of the naphthalene framework and inversion of the pyrrolidine nitrogen atom. The twisted naphthalene snaps backwards as the barrier is traversed, giving the opposite helicity.