Dynamic Stereochemistry and Steric Effects in Overcrowded Ethylenes. Conformational Behavior of Dixanthylenes

The ground-state conformation of 2, 2ʹ-disubstituted dixanthylenes (3) in solution was investigated by¹H NMR spectroscopic techniques. The experimental data are consistent with a folded structure. The existence of geometrical isomers in this series is established; the E : Z ratio in solution depends on the bulkiness of the 2 and 2ʹ substituents. A DNMR study of 3 revealed low values of free energy of activation for thermal E, Z isomerization: ΔG^≠_c~18 kcal/mol. These unusual low energy barriers, the lowest yet found in the bistricyclic ethylenes series (1), are ascribed predominantly to the high energy contents of the folded ground-state conformations of 3. In 2, 2ʹ-diisopropyldixanthylene (3c) a conformational inversion process could be followed in the E isomer, apart from the E, Z isomerization. The ΔG^≠_c values of both dynamic processes are equal, implying a common transition state which is not the orthogonal biradical π/2 twisted ethylene. The observed increase in ΔG^≠_c values with the bulkiness of the 2, 2ʹ substituents is interpreted in terms of [R(2)-H(7ʹ)] type secondary steric interactions in the transition state for the process. Isomerization and topomerization pathways and free-energy profiles, including the postulated folded-twisted transition state, are submitted/The role of the steric interactions in the ground state and in the transition states is discussed.