71215-50-4Relevant academic research and scientific papers
Synthesis and Rearrangement of Substituted Bicyclopent-2-enes. A Thermal "Walk"-Rearrangement
Klaerner, Frank-Gerrit,Adamsky, Friedhelm
, p. 299 - 322 (2007/10/02)
The synthesis and the thermal behaviour of methyl 5-methyl-, 1,5-, and 2,5-dimethylbicyclopent-2-ene-5-carboxylates 3a,b, 4a,b, 5a,b, as well as of 1,5- and 2,5-dimethylbicyclopent-2-ene-5-carbonitriles 6a, 7a,b are reported.The "walk"-rearrangement which is degenerate in the cases of 3a,b could be detected experimentally with the aid of the dimethyl derivatives 4a,b, 5a,b, 6a, and 7a,b.Electrocyclic ring opening to the correspondingly substituted 1,3-cyclopentadienes competes with the "walk"-rearrangements.Already at 0 deg C the "walk"-rearrangements 4a -> 5a, 4b -> 5b, and 6a -> 7a proceed stereospecifically with inversion at the migrating carbon atom C-5 as postulated by Woodward and Hoffmann (0 deg C: ΔG*= 21.7, 24.8, and 21.9 kcal/mol).We assume that the surprisingly low activation barriers do not only result from the resonance stabilization of an aromatic transition state but largely from the anomalously high ground-state enthalpy of the bicyclopentene system.
