Steric Effects in Fast Thermal Z, E Isomerization of Overcrowded Ethylenes. Conformational Behavior of N, N'-Dimethylbiacridans1

Israel Agranat*, Yitzhak Tapuhi

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

39 Scopus citations

Abstract

The ground-state conformation of 2, 2'-disubstituted N, N'-dimethyl biacridans (5) in solution was studied by various 1H NMR spectroscopic techniques. A folded A-type structure with N-Me groups occupying predominantly the axial positions fits the experimental findings. The existence of geometrical isomers in this series is established, the Z:E ratio in solution being dependent on the bulkiness of the 2 and 2' substituents. In the 2, 2'-di-i-Pr derivative (5d) only the E form was detected in the crystalline state. A DNMR study of 5 revealed low values of free energy of activation for thermal Z, E isomerization: ΔGQ‡~21 kcal/mol. These unusual low energy barriers are ascribed predominantly to the high energy contents of the folded ground-state conformations of 5. The increase of ΔGC‡ with the bulkiness of the 2, 2' substituents found is interpreted in terms of [R(2)-H(7')] type secondary steric interactions in the transition state for the process. An isomerization pathway in whch the transition state is not the orthogonal biradical π/2 twisted ethylene is suggested.

Original languageEnglish
Pages (from-to)5604-5609
Number of pages6
JournalJournal of the American Chemical Society
Volume100
Issue number18
DOIs
StatePublished - 1978

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