Dynamic Stereochemistry in Overcrowded Ethylenes. Conformational Behavior of Bianthrones

The ground-state conformation of bianthrone derivatives in solution was investigated by ¹H NMR spectroscopic techniques. The experimental data are consistent with a folded structure. The existence of geometrical isomers in this series is established, and their chromatographic separation at low temperatures is described. A DNMR study of 2, 2ʹ-disubstituted bianthrones (2) and their benzannelated analogues (5 and 6) revealed low values of the free energy of activation (ΔG≠_c) for thermal E, Z isomerization.ΔG≠_c values were not lowered by benzannelation of the parent system. The low ΔG≠_c values are ascribed predominantly to the high-energy content of the folded ground-state conformations of these systems. An isomerization pathway implying a double-minimum free-energy profileand a nonorthogonal, partly twisted and folded highest energy transition state is discussed in terms of stericandelectronic effects in the ground state and in the transition state.