TY - JOUR
T1 - Steric Effects in Fast Thermal Z, E Isomerization of Overcrowded Ethylenes. Conformational Behavior of N, N'-Dimethylbiacridans1
AU - Agranat, Israel
AU - Tapuhi, Yitzhak
PY - 1978
Y1 - 1978
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0000969625&partnerID=8YFLogxK
U2 - 10.1021/ja00486a004
DO - 10.1021/ja00486a004
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AN - SCOPUS:0000969625
SN - 0002-7863
VL - 100
SP - 5604
EP - 5609
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 18
ER -