Atropisomerism in 1-(2-adamantyl)naphthalene Derivatives

Two new adamantylnaphthalene derivatives 1-(2-hydroxy-2-adamantyl)-4-methoxynaphthalene (5) and 1-(2-hydroxy-2-adamantyl)-5-methoxynaphthalene (6) were synthesized and characterized by NMR spectroscopy. In addition, 5 was characterized by single crystal X-ray structural analysis and DFT calculations. For both derivatives, 5 and 6, dynamic NMR of diastereotopic spins revealed atropisomerism due to hindered rotation around the C–C bond between the adamantyl and the naphthyl moieties, giving rise to intermediate conformational exchange on the NMR timescale at room temperature. Upon decrease of the temperature to 223 K the existence of two enantiomerically related conformers in slow exchange was observed. The free energy of activation for the conformer exchange calculated using the coalescence temperature method amounts to 56.4 ± 0.3 kJ mol−1 for both derivatives. The experimental findings are corroborated by DFT calculations. The calculated NMR chemical shifts and the energy of activation for the conformer exchange are in excellent agreement with the measured values.