The ground-state conformation of 2,2'-disubstituted dixanthylenes (3) in solution was investigated by 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 2' substituents. A DNMR study of 3 revealed low values of free energy of activation for thermal E, Z isomerization: AG*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 2,2'-diisopropyldixanthylene (3c) a conformational inversion process could be followed in the E isomer, apart from the E, Z isomerization. The AG*C values of both dynamic processes are equal, implying a common transition state which is not the orthogonal biradical /2 twisted ethylene. The observed increase in AG*C values with the bulkiness of the 2,2' substituents is interpreted in terms of [R(2)-H(7')] 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.
a solution of 4 (2.10 g, 10 mmol) in dry DMF (100 mL) and a solution of 8 (crude, 3.77 g, 5 mmol) in dry DMF (100 mL). The addition lasted 4 h, and stirring was continued for 18 h. The reaction mixture was decomposed and worked up in the usual manner (vide infra). The crude product was chromatographed on a silica gel column (100 g) using CeHe as eluent. PLC chromatography of the eluted product on silica PLC plates using hexane-benzene (7:1) afforded la (Rf 0.28) as a pale yellow solid, identical [UV, IR, NMR (270 MHz), TLC] with a sample of la prepared by method A. The yield was 0.3% (0.008 g)• Acknowledgment. We are grateful to our colleague Professor H. J. G. Hayman of The Hebrew University of Jerusalem for stereoscopic drawings of la and lb. We also would like to thank The Weizmann Institute of Science, Rehovot, Israel, for use of a Brüker WH-270 NMR spectrometer.
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